ABSTRACT. Magnetic islands are regions of reconnected magnetic field that appear frequently in magnetized fusion plasmas as well as in astrophysical plasmas. While they can be directly created by the current-driven "tearing" instability, in tokamak plasmas they frequently appear in situations where the tearing mode is stable. Therefore, the ubiquity of micro turbulencein tokamaks plasmas has prompted many studies seeking to clarify the possible mechanisms through which turbulence may generate magnetic island. In the meantime, magnetic islands (regardless of their origin) perturb heat and particle transport, because they connect field lines across equilibrium magnetic surfaces. In addition to the associated degradation of confinement, this can lead to the explosive growth of so-called Neoclassical Tearing Modes. However, while the effect on collisional transport of single tearing-driven islands in simple MHD models is rather well understood, the situation with more detailed models, turbulence and/or multiple islands is an other matter. Therefore, a consistent view of turbulence, transport and magnetic islands together is necessary.

Several different processes, discovered in the last decade, allow turbulence at small scales to drive large-scale magnetic islands. First, we show how the competition between those processes determines the size and shape of the generated large-scale magnetic islands, depending on the thermal power injected into the plasma [1]. Next, with use a 6-feld model [2, 3] to present a newly identified mechanism whereby the turbulence-generated "zonal" mean flows nonlinearly change the parity of the turbulent modes and, after a zonal flow-mediated coalescence process, drives large-scale islands. While this process depends on terms that are usually neglected in usual 2- or 3-field MHD models, it is dominant in the low-shear high-β regime of interest to advanced tokamak operation scenarios.

Next, we turn to the related issue of transport in magnetic islands. Even in the 2D mono-helicity geometry, the effect of magnetic islands can be much more complex than a flattening (or lack thereof) of the pressure gradient, in particular exhibiting self-heating [3] and/or several distinct flattened regions separated by sharp gradients. On top of this, the 3D multi helicity geometry enables the complex issue of stochastic regions of the magnetic field. Indeed, while such regions look deceptively uniform in Poincare plots, they feature complex local variations: in addition to sticky regions around islands, particular manifolds called Lagrangian Coherent Structures (LCS) which have been touted as possible transport barriers. However, the complex cohabitation of turbulence and turbulence-driven magnetic islands raises several questions which we address in this work: is stochasticity dominated by the small-scale magnetic flutter, or by the coupling of large-scale islands? Are the large-scale generated LCS resilient to the presence of small-scale turbulence?

[1] N. Dubuit, et al, Physics of Plasmas 28, 022308 (2021)

[2] J. Frank, et. al., Physics of Plasmas 27 (2), 022119 (2020)

[3] D. Villa, et al., Journal of Plasma Physics, 88(6), 905880613 (2022)

ABSTRACT. The pursuit of inertial confinement fusion created advanced pulsed-power and laser systems for five decades, culminating in the world’s largest laser, the National Ignition Facility, demonstrating a target gain of more than 1 in 2022. In this journey, the laser systems, which deliver high-energy pulses in nanoseconds, have facilitated many other areas of high-energy-density science from studying materials at planetary conditions to simulating many aspects of astrophysics. The laser technology also advanced, including the development of chirped-pulsed amplification (CPA), to create picosecond pulses at petawatt levels of power. LLE is the home of two tens-of-kilojoule laser systems and one petawatt-class laser. The facilities support 2000 experiments per year for hundreds of users. In this presentation we will review the significant advances in fusion and high-energy-density science being conducted at LLE. This ranges from measuring fundamental plasma parameters like distribution functions to integrated experiments to study the role of magnetic fields in galaxy formation. In addition, the entire realm of short-pulse laser science, enabled by CPA, is expanding into advanced accelerator, THz radiation production, and intense x-ray sources. We will also discuss advances in short-pulse laser physics and a vision for 25-PW lasers to meet the scientific goals outlines in the recent international Multi-Petawatt Prioritization workshop.

Acknowledgements. This material is based upon work supported by the Department of Energy [National Nuclear Security Administration] University of Rochester “National Inertial Confinement Fusion Program” under Award Number DE-NA0004144.

ABSTRACT. Em/Prof. Robert (Bob) Leith Dewar, FAA, FAPS, FAIP (1944 - 2024) was a giant in the field of theoretical plasma physics, with important contributions in Magnetohydrodynamics (MHD) and in dynamical systems. These include MHD equilibrium and stability, MHD ballooning modes, Taylor relaxation and Hamiltonian maps. Bob worked closely with computer simulation and with experimentalists and has made important contributions to toroidal magnetic fusion research and to astrophysics. Over the last decade he had been instrumental in the development of a multiple region relaxed MHD model to describe general stellarator fields, and he was presently working on a generalisation of such models to systems that preserve magnetic helicity with a weak ideal Ohm’s law constraint. [1] Perhaps most importantly, he has left alegacy in both research and teaching, spanning 5 postdocs, 16 PhD, and many Masters and Honours students. Many of these now hold prominent positions in the field. Bob has also had a strong service contribution to the ICPP community, serving on the International Advisory Committee for decades, Chairing the 2002 ICPP meeting in Sydney, and was the architect of the ICPP statutes. In this proposed talk I will overview Bob’s major contributions to the field, and focus on his work to describe magnetic fields in toroidally asymmetric configurations with flux surfaces, islands and chaos.

[1] R. L. Dewar and Z. S. Qu, J. Plasma Phys., 88, 835880101 (2022)

ABSTRACT. Turbulence is often thought of as being constituted of a collection of interacting wavemodes, but at the outset of classical fluid dynamics, was seen to arise as a series of increasingly complex structures that saturate and became successively unstable. We show that gyrokinetic tokamak turbulence follows this paradigm. The case of interest is subcritical turbulence that arises when there is both a driving gradient and a sheared background flow; this is known do give rise to isolated radially-propagating structures, and the formation and destabilisation of these structures is the pathway to turbulence in this system. Not only can we observe this in initial value simulations, it is possible to directly determine these structures via a nonlinear root-finding technique. These relative periodic orbits (RPOs) are turbulence states that repeat after some time, modulo some symmetry operation like translation; direct numerical solution allows us to determine these states even when they are unstable and to examine the bifurcation pathway as simple translation transitions via Hopf bifurcations into more complex nonlinear oscillations and finally space-filling turbulence.

ABSTRACT. Extending work by Berk et al [1] on the case with a single interface, Bruno and Laurence [2] used KAM theory to make near axisymmetric magnetohydrostatic (MHS) fields with an arbitrary finite number of zones of constant pressure. The field is Beltrami in each zone and has a current sheet at each interface between zones, at which the pressure has a jump. Bob Dewar had the good idea, and the computationally expert team, to implement this strategy numerically [3]. The resulting stepped pressure equilibrium code (SPEC) is a hugely valuable tool in the search for suitable stellarator fields for confined nuclear fusion. In 2005, I began a discussion with Bob, in which I put the view that there could belimiting cases with infinitely many zones of constant pressure but for which the pressure is continuous, thereby eliminating the non-physicality of current sheets. The pressure gradient would be an L1 function supported on a fat Cantor set of flux surfaces. The same would hold for the current density. Such solutions would fit with Grad’s hunch that most nonaxisymmetric MHS fields are pathological [4]. Bob was keen on the idea, but we never reached a conclusion. Here, I will present what I can on the proposal.

[1] HL Berk, JP Freidberg, X Llobet, PJ Morrison, JA Tataronis, Phys. Fluids 29, 3281 (1986)

[2] OP Bruno, P Laurence, Commun. Pure Appl. Math. 49, 717 (1996)

[3] SR Hudson, RL Dewar, G Dennis, MJ Hole, M McGann, G von Nessi, S Lazerson, Phys.Plasmas 19, 112502 (2012)

[4] H Grad, Phys. Fluids 10, 137 (1967)

ABSTRACT. The basic elements (e.g., equivalent inductance and equivalent capacitance) of distributed-parameter and lumped-parameter characteristics will be described in a qualitative examination of multiple periodic nonlinear systems that are important as phenomenological models for demonstrating the salient features of the behavior of PNP that are encountered across a wide spectrum of discovery plasma science and beyond. This paper contains mostly theory but also contains many experimental examples from laboratory experiments.

- Basic, physical, and mathematical descriptions of PNP

- Prototype PNP systems

- Topological, analytical, variational, and perturbative methods

- Specific nonlinear characteristics of PNP

- Amplitude-frequency-phase relations and harmonic generation in PNP

- Experimental examples of PNP

- Coupled first-order systems

- Stabilization methods

Acknowledgments. Seminal contributions from H. Lashinsky (deceased) and collaborative contributions from R. Majeski, D. Hartley, K.D. Weltmann, P. Miller, T. Klinger,and N. Brenning are greatly appreciated.

ABSTRACT. Professor Robert Dewar was a great theoretical plasma physicist who made many important contributions to the development of research in magnetic fusion and astrophysics. The first part of this talk highlights one of his great achievements: his theory on ballooning representation of instabilities in toroidal magnetic configurations [1]. He presented an elegant formulation of ballooning representation to efficiently investigate pressure-driven MHD instabilities in tokamaks and nonaxisymmetric systems such as stellarators and heliotrons. His work had a significant impact not only on analyses of MHD modes but also on studies of microinstabilities and turbulence through gyrofluid and gyrokinetic theory and simulation, in which his ballooning formalism is used as a standard helpful tool. The second half of the talk will touch on Professor Dewar's interactions with Japanese researchers. Professor Dewar has often visited Japan, sometimes for long periods of time, and has conducted many joint research projects. This talk will forward some messages from Japanese collaborators. It is also remembered that he participated in the AAPPS-DPP2023 in Nagoya last November and played very active roles as the program committee member, the plenary session chair, and the invited speaker. After the conference, he also visited NIFS and gave a seminar on plasmarelaxation models, where he had a lively discussion and showed his energy. Therefore, we were even more shocked and saddened by the news of his sudden passing. We will never forget his great achievements and warm personality.

[1] R.L. Dewar and A.H. Glasser, “Ballooning mode spectrum in general toroidal systems,”Phys. Fluids 26, 3038 (1983)

ABSTRACT. Tearing modes excited and driven by impurity radiation are investigated by using three-dimensional toroidal MHD code with the impurity module (CLT). Various types of impurities applied at the boundary under an equilibrium condition where the tearing mode is stable. It is observed that impurity radiation-induced plasma contraction results in a current gradient that excites the tearing mode at the resonance surface. Through a scan of the initial atomic number (Z) and impurity concentrations, it is found that impurities with different Z values exhibit similar behaviors in the radiation-driven tearing mode. The impurity radiation can drive tearing mode growth through temperature cooling near the resonance surface, and there exists a linear relationship between the temperature perturbation caused by impurity radiation and the linear growth rate of the tearing mode. Additionally, the impurity can promote the growth of magnetic islands through the cooling inside the magnetic island, and there exists a correlation between the initial parameters of impurity and the width of the saturated magnetic island.

ABSTRACT. Turbulent fluctuations play an important role in predicting the confinement of plasmas at the edge region as well as the core region. Namely, understanding the fluctuations in the outer core region connecting to the SOL/divertor region is important for predicting fuel supply, impurity pumping, divertor heat load control, and L-H transition of tokamak plasmas. These issues are particularly important for predicting the confinement of future fusion devices like JA-DEMO [1]. Although gyrokinetic simulation is an essential tool for studying these issues based on the first principles of magnetized plasmas, it is still challenging to apply it to the edge region due to high q values and complex magnetic surface geometries that are not present inthe inner core region. To address these issues, a core global gyrokinetic simulation code GKNET [2] has been extended to GKNET-FAC [3] by introducing a field-aligned coordinate system [4] with a shifted metric technique [5] and numerical MHD equilibria. The field-aligned coordinate can reduce computational costs for turbulence in the outer core region significantly. For calculating drift-wave instabilities with a high toroidal mode number in a JT-60SA plasma,GKNET-FAC requires only 1/94 smaller the number of grid points than that for GKNET.

In this study, we have extended GKNET-FAC to GKNET-X handling plasma turbulence in the edge region consisting of both the inside and outside of separatrix. Figures(a) and (b) show that the eigenfunction of the ion temperature gradient (ITG) mode calculatedby GKNET-X is in good agreement with that by GKNET-FAC for a toroidal mode number n= 60. In addition, we successfully calculate the ITG mode excited at the separatrix between the closed surface core region and the open surface SOL region as shown in Fig. (c). We will alsopresent some nonlinear simulation results by GKNET-X in the conference.

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Figure 1. Electrostatic potential  of the ITG mode in the core region calculated by (a)GKNET-FAC and by (b) GKNET-X. (c)  of the ITG mode at the separatrix by GKENT-X.

[1] K. Tobita, et al., Fusion Sci. Tech. 75, 372 (2019)

[2] K. Imadera, et al., Proc. 25th FEC, TH/P5-8 (2014)

[3] S. Okuda, et al., Plasma Fusion Res. 18, 2403040 (2023)

[4] M. A. Beer, et al., Phys. Plasmas 2, 7 (1995)

[5] B. Scott, Phys. Plasmas 8, 447 (2001)

ABSTRACT. In the spectroscopic measurement of fusion reactors, the reflected light from the plasma-facing surface has a significant effect on the measurement signal [1]. Therefore, to obtain accurate spectroscopic measurements, it is necessary to quantitatively assess the influence of the reflected light, which can be done by ray tracing calculations [2]. The accuracy of this assessment depends on the precision of the Bidirectional Reflectance Distribution Function (BRDF) of the reflecting surface used in the ray tracing calculations. Since the interaction between the plasma and the wall can change the surface condition, the BRDF of the plasma-facing surface will also change. Therefore, without the development of a method to measure this BRDF in situ, reliable spectroscopic measurements cannot be achieved, potentially leading to significant misunderstandings in spectroscopic diagnostics.

The aim of this study is to investigate the relationship between the surface geometry of plasma-facing materials and optical reflection models, and to clarify the possibility of in-situe stimation of the BRDF based on the surface topography. Among the plasma-facing components of ITER, the divertor surface will be made of tungsten, which may vary due to plasma-wall interactions, while the blanket surface will be made of beryllium, which has a large surface area and raises concerns about its substantial impact on optical diagnostics. Therefore, this study has conducted an analysis of BRDF for both tungsten and beryllium with various topographies.

Fitting was performed to optimize the free parameters in the optical reflection model. The topographic parameters were determined from the surface three-dimensional measured with a laser microscope. The relationship between the parameter (kd) defining the diffuse reflection in the optical reflection model (Phong model), and the topographic parameters showed that kd increased monotonically with increasing topographic parameters. In addition, topographic parameters using the slope of the surface geometry showed a higher coefficient of determination (R2). Furthermore, the relationship between the parameter (ks) defining the magnitude of the specular reflection in the Phong model and the topographic parameters didnot show as strong a correlation as kd. This is because the specular reflectance model does not accurately reproduce the measured BRDF, especially at the feet of the specular lobe where the brdf is often underestimated, suggesting the need to investigate more accurate specular reflectance models. However, the topographic parameters that account for surface slope showed a certain correlation with ks. Therefore, this indicates the potential to directly assess BRDF from measurements of surface geometry inside fusion reactors using tools such as laser interferometers, and to accurately assess the effect of reflected light in spectroscopic measurements.

[1] S. Kajita et. al., Plasma Phys. Control. Fusion, 55(8):085020, 2013.

[2] M. Carr et. al., Rev. Sci. Instrum., 90(4):043504, 2019.

ABSTRACT. It is expected that the heat flux reaching the divertor region of the future reactor such as DEMO will be several times higher than that of ITER. To sufficiently reduce this heat flux, a fully detached plasma must be maintained while preventing X-point MARFE. In large fusion devices, conducting fundamental research focused on detailed divertor plasma physics is challenging due to port limitations and machine time, and other factors. Therefore, it is efficient to use a small linear divertor simulator that can generate steady-state detached plasmas and can be flexibly connected to various measurement devices.

A common issue with the general linear divertor simulator is that the ion temperature of the generated plasma is as low as several eV, which is one order of magnitude lower than that of fusion divertor plasmas. The ion temperature of fusion divertor plasmas is several tens of eV, which is close to or higher than the electron temperature (~10 eV). Therefore, it is required to increase the ion temperature from several eV to about 20 eV to study reaction processes in detached plasmas.

In this study, to investigate the transition process of detached plasma in detail, high-densitysheet plasma (~10¹⁸-10¹⁹ m^-3) was generated using the linear divertor simulator TPD sheet-ICR [1,2]. Hydrogen or deuterium was used as the discharge gas. Additionally, ions in thesheet plasma were heated by ion cyclotron resonance heating (ICRH) using parallel-plate electrodes. We performed the Langumuir probe measurement of plasma parameters (electron density and temperature) of the detached plasma outflowing into the divergent magnetic field region. Futhermore, two-dimensional distribution measurements of Balmer series emission (Hα, Dα, Fulcher, Hγ and Dγ) were performed using a fast-framing camera equipped with an Arbaa prism. When both hydrogen and deuterium plasmas transition from the attached plasma state to the detached plasma state, the Fulcher band emission intensity decreases, while the Hγ and Dγ emission intensity increases. Further, it is evidenced that the increase in the ion temperatureby increasing the applied power of ICRH from 0 to 500 W (Ti: 2.4 to 6.5 eV) leads to rise in the electron temperature, resulting in a decrease in the emission from highly excited atoms such as Hγ and Dγ in the Balmer series.

[1] N.Okada et al, Fusion Engineering and Design,192,(2023)113596.

[2]A.Tonegawa et al ,Fusion Engineering and Design,203,(2024)114441.

ABSTRACT. Auxiliary plasma heating and current drive (HCD) in the Spherical Tokamak for Energy Production (STEP) fusion reactor will be achieved by utilising a large number of high power microwave sources with a power of the order 1MW each. A combination of electron-cyclotron (EC) and electron Bernstein wave (EBW) HCD will be employed. In addition to the well-understood EC HCD method, the use of EBWs is considered due to their high efficiency current drive and ability to propagate in overdense plasmas. The proposed method to generate EBWs is by launching an O mode polarised microwave from the low field side at an optimal angle to the magnetic field. The O mode undergoes conversion to a slow X mode wave and finally to an EBW that can propagate into the overdense plasma interior. To obtain the most efficient power deposition and current drive, controllable access and power deposition of EBWs to all regions of the ST plasma is required. By employing GENRAY ray-tracing and CQL3D Fokker-Planck simulations, varying of the launch position and scaling the plasma parameters for a set of STEP Prototype Reactor (SPR) scenarios, we are able to optimise the propagation of the EBW and the locations of power deposition to achieve high levels of current drive. Analytical models have concurrently been developed through consideration of energy conservation and the fundamental resonance condition which give insight into the wave-plasma interaction. We also show how coupling of EBWs to Doppler-shifted electron cyclotron harmonics can impact the current drive through the excitation of oppositely directed currents.

ABSTRACT. It is crucial to understand physics of the edge region in fusion plasmas, since reducing heat flux at the divertor plate and reaching the thermo-mechanical engineering limit of the plasma facing materials are essential for the fusion energy development. Leveraging low construction and operation costs and simple geometry, linear devices have been utilized as a divertor simulator to investigate the edge plasma physics in closed magnetic field systems, including tokamaks [1–2]. This presentation will highlight the ongoing progress in developing a divertor plasma simulator using the magnetic mirror device at KAIST, KAIMIR [3]. The vacuum chamber of the device comprises three sub-chambers: source, center, and expander. We have modified the expander chamber as a divertor region of the tokamak to simulate the radiative divertor conditions and analyze their characteristics. To control the neutral pressureof the divertor chamber independently, we implemented a differential pumping system, allowing for a pressure difference of over 20 times between source and divertor chambers through the use of a skimmer structure. The gas injection valve, originally a piezoelectric valve (with a flow rate < 0.53 slm), has been replaced with a solenoid valve (with a flow rate ~ 17.8slm) to enhance neutral pressure control capability at the source chamber. We also developed additional diagnostics, such as probes and a mm-wave interferometer system to understand the physics relevant to the divertor region, complementing existing diagnostics (spectroscopy, diamagnetic loops, and probes) in the center chamber. Future plans involve the integration of an Electron Bernstein wave (EBW) heating system to regulate the temperature of the incoming plasma flow to the divertor region and mitigate plasma pressure degradation along the axis. Further details on the experimental setup and results will be discussed.

Acknowledgements. This work was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (Ministry of Science and ICT) (RS 2023 00212124, RS 2022 00155956).

[1] Ohno, N., Plasma Physics and Controlled Fusion, 59.3, (2017).

[2] Baldwin, M. J., et al., Nuclear Materials and Energy, 36, 1–10 (2023).

[3] Oh, D., et al. Journal of Plasma Physics, 90.2, 1–19 (2024).

ABSTRACT. In the late 1960s, the American physicists Eugene Parker and Ian Lerche studied a simplified model of the flow of the solar wind around Earth’s magnetic field, focusing on the region where the flow is parallel to the field lines (shown in red in the Figure 1). They discovered that, in the thin boundary layer between the plasma and the field, a secondary magnetic field arises, which is stronger than the primary field. Under certain circumstances, this effect causes the boundary layer to become unstable [1].

A decade later, Owen Storey and Laurent Cairó demonstrated that this effect, which they named “Parker’s Effect”, also occurs when a magnetic field encloses and confines a supersonically flowing plasma. However, in this scenario, it reinforces the confinement of the plasma which remains, in the bulk, field-free and current-free. They demonstrated analytically that a simple confinement system based on this effect would be intrinsically MHD-stable and have a global beta significantly higher than most magnetic confinement devices currently under investigation. They suggested that Parker’s Effect may have applications to fusion and outlined a toroidal device exploiting it [2]. To date, Parker’s Effect has not been observed experimentally or in simulation, mainly because it has not been sought.

Our research group has been reviewing these investigations and developing a full simulation model of a module of the device (figure 2), using an open-source software framework running on modern supercomputers. In this presentation, we will explain Parker’s discovery, the insights contributed by Storey and Cairó, and the main device they proposed. We will mention our team’s recent research efforts, share preliminary results and findings, and highlight potential avenues for collaboration.

[1] E.N. Parker. "Dynamical properties of the magnetosphere" In Physics of the Magnetosphere: Based upon the Proceedings of the Conference Held at Boston College June 19–28, 1967. Springer Netherlands (1968).

[2] L.R.O. Storey and L. Cairo. "Kinetic theory of the boundary layer between a flowing isotropic plasma and a magnetic field." Magnetospheric Boundary Layers 148 (1979)

ABSTRACT. and JET petition contributors+,*1Laboratory for Plasma Physics, Royal Military Academy, TEC Partner, Brussels, Belgium2Laboratorio Nacional de Fusion, CIEMAT, Madrid, Spain,*Petition to extend JET beyond 2023+writing as independent experts, not necessarily representing the views of our institutions

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We propose a research program for the continued operation of the JET facilities over the next 10-15 years, thereby making optimal use of JET’s unique capabilities (especially operation with Deuterium-Tritium mixtures) by integrating the ITER re-baselining decisions within JET. The primary objective is to transition JET to a phase with a tungsten wall and 10 MW of Electron Cyclotron Resonance Heating (ECRH) using 170GHz gyrotrons in addition to the existing additional heating systems, thereby aligning with the goals of both STEP and ITER.

Enhancing JET as proposed, would allow to increase the fusion performance for Baseline and Hybrid scenarios, and to test their compatibility with a Tungsten wall, aiming at steady conditionswith equal electron and ion temperatures. Synergies between Electron Cyclotron (EC), Ion Cyclotron (IC), Neutral Beam Injection (NBI), and alpha heating could be investigated in detail. Investigations in the ion temperature clamping effect seen on smaller machines with dominant electron heating (and methods to avoid it) would now become possible in a device with a size and under operational conditions as close as possible to ITER. The compatibility of Boronization with high performance plasmas in the presence of a high-Z wall can be investigated in detail. Control, mitigation and suppression techniques for Edge Localized Modes (ELMs) could be tested in more fusion-relevant situations, and the effect of controlled impurity seeding with Ar and Ne on plasma wall interaction and confinement could be explored. DT campaigns in the enhanced JET would also uniquely enable to develop advanced fusion diagnostics, including precise neutron measurements and γ-ray imaging to detect alpha-particles (from D+T or B⁺4He reactions),providing a unique opportunity to prepare necessary diagnostics for the rebaselined ITER. Preparing for ITER’s DT campaigns (presently planned to begin in 2039) implies training the next generation of scientists, technicians, and engineers within a large fusion facility. Extending JET's operation as proposed offers a unique opportunity to sustain and enhance the expertise of fusion researchers globally, thereby accelerating progress in the world-wide quest for fusion energy.

ABSTRACT. The generation of intense and coherent THz radiation has been a subject of intense research, with numerous methods developed to harness its potential. One of the promising techniques that has emerged is the generation of THz radiation using laser-induced plasma. This method utilizes the unique properties of ultrafast lasers to create plasma, which, in turn, emits THz radiation through a variety of mechanisms [1-5] . This study is significant for the generation of intense radiation fields having frequencies lying in the THz to near IR range.The novelty of the present mechanism is that the frequency of the generated radiation can be controlled by continuously varying the frequency difference between the two laser pulses. In order to portray the significance of the present mechanism laser frequency differences equal to twice and thrice the plasma frequency have been considered. However, arbitrary frequency differences can lead to a continuous range of tunable radiation. The present analytical mechanism of tunable radiation generation has been validated via simulation studies whichshow that the transverse orthogonal electric and magnetic fields generated in plasma are essentially radiation fields that can propagate in vacuum. The discrepancy in analytical and simulation results may be attributed to the approximation scheme used for the analytical case.

[1] Mishra, D., Sharma, P., Singh, S., Kumar, B., & Jha, P. (2021). Generation of terahertz radiation by short laser pulses propagating in obliquely magnetized plasma. Plasma Research Express, 3(2), 025002

[2] Mishra, D., Singh, S., Kumar, B., & Jha, P. (2023). Terahertz to near-infrared radiation generation using two color counter and co-polarized laser pulses propagating in homogenous plasma. Physica Scripta, 11, 115612

[3] Albert F and Thomas A (2016) Applications of laser wakefield accelerator-based lightsources Plasma Phys. Control. Fusion 58 103001

[4] Frolov A 2018 Generation of terahertz waves under laser action on hot dense plasma Plasma Phys. Rep. 44 312

[5] Wang W, Gibbon P, Sheng Z and Li Y 2015 Tunable circularly polarized terahertz radiation from magnetized gas plasma Phys. Rev. Lett. 114 253901

ABSTRACT. Erosive plasmas sources creating dense, low-temperature plasma, in particular of the vacuum-arc type, are reliable and well-tested generators of metal plasma. They are widely used in academia and industry for the deposition of hard, protective and functional coatings. Moreover, they are used to modify surface properties of structural and decorative materials. However, the presence of microdroplets of the cathode material in the plasma prevents the use of erosive sources in applications requiring high-quality coatings, especially those demanding high uniformity and low roughness at the nanometer level. The use of modern filtering techniques to remove droplets, especially those of small sizes (< 1 μm), leads to a significant decrease in the deposition rate, which compromises one of the unique features of these sources, namely their high deposition rate. That is why alternative filtering concepts are of interest.

Here, we discuss an approach using energetic electrons injected into the arc plasma [1].They introduce additional energy into the plasma which can promote evaporation from the droplets which may eventually lead to complete destruction of droplets. By solving a self-consistent system of equations for the balance of energy, current to and from a droplet, and thedroplet mass, it is shown that favorable conditions for the evaporation of microdroplets may exist. It is shown that the main parameters influencing the droplet evaporation rate are the plasma density as well as the density and energy of the electron beam. It has been shown that droplets with a radius of 1 μm or less may completely evaporate during the time the droplet is in the system if the energy of the electron beam is 3 to 5 keV and the beam-to-plasma density ratio is greater than 0.01.

Acknowledgement. This work is supported partly by the grant DFG - Deutsche Forschungsgemeinschaft(DFG) under project number 525228371.

[1] Fisk A., Maslov V., Goncharov A. Device for the Elimination of Microdroplets from a Cathodic Arc Plasma Source U.S. patent application # 2014/0034484A1 (6 February 2014).

ABSTRACT. Plasma-activated water (PAW) has demonstrated potent antimicrobial efficacy, which is mainly attributed to the reactive oxygen and nitrogen species (RONS) [1]. Our study assessed bactericidal activity of PAW against multidrug-resistant Klebsiella pneumoniae in relation to plasma treatment time, volume of treated water and PAW application time on bacteria samples. The model organism we selected is an important no socomial pathogen, capable of persistence on various surfaces and difficult to eradicate due to resistance to harsh conditions and antimicrobials [2].

The in-house developed plasma source was 3-pin jet with needle electrodes inserted inside three separate glass tubes and positioned vertically above the water sample. Working gas was argon (2.5 slm), frequency of high-voltage power supply 340 kHz and applied power14 W. We have treated 150 ml of deionized water for 20 min (PAW1, pH=6.2) and 15 ml for10 min (PAW2, pH=6.5). Characterization of the PAW chemistry was performed by using colorimetric methods: PAW1 (H2O2=10 ppm, NO3-=34 ppm, NO2-=0 ppm) and PAW2 (H2O2=80 ppm, NO3-=97 ppm, NO2-=41 ppm).

An aliquot of 100 μL of the suspension (108 cells/ml, K. pneumoniae ATCC BAA-1705) was added to 900 μL of PAW in microtubes, homogenized, and maintained for 15 min and 60 min. Bactericidal effects of PAW were assessed by the standard plate count method and expressed in log CFU/mL. Inactivation activity of PAW against K. pneumoniae increased by reducing the water volume for PAW preparation and by extending the incubation time of the bacteria in PAW. PAW1 reduced K. pneumoniae by ≈1.1 log CFU after 60 min of application time, while no reduction was noted after 15 min exposure. PAW2 applied for 15min resulted in ≈1.6 log CFU reduction, while 60 min application achieved complete eradication of K. pneumoniae.

Acknowledgements. This research was supported by the Science Fund of the Republic of Serbia, 7739780,APPerTAin-BIOM.

[1]. F. Rezaei, et al., Materials (Basel) 12(17), 2751 (2019)

[2]. G. Wang, et al., Int. J Environ. Res. Public Health 17(17), 6278 (2020)

ABSTRACT. Radiation therapy has been used for the treatment of cancer disease for many decades. Normally, the dose rates in radiation therapy are of the order of tens to hundreds Gy/h. The effects of ultra-high dose rate irradiations (10³ ~ 10⁷ Gy/sec) on cancer cells are less explored. In the present study, several cell cultures, two colorectal cancer cells (DLD-1, HCT-116), onebreast cancer cell (MCF-7), one ovarian cell, and one non-cancerous colorectal cell (CCD-841-con) have been exposed to pulsed X-ray (~ ns duration time) with a high-dose-rate (~ 10⁷Gy/sec) keeping the total doses low (≤ 1 Gy) using a kilojoule plasma focus device, PF-2kJ.The obtained results are compared with the conventional X-ray irradiation source results. The DLD-1 cell line shows the low-dose hyper-radio-sensitivity (LDHRS) effect at lower doses for pulsed X-ray than the conventional X-ray irradiation. The HCT-116 cell line shows the HRS effect in the case of pulsed X-ray irradiation, which is reported to be absent in the case of conventional X-ray irradiation. The radio-resistive cell MCF-7 shows a larger number of cell death in the case of pulsed X-ray irradiation. The ovarian cancer cell shows a reduced proliferation capacity in the case of 2D cell culture irradiation and reduced vasculogenic-mimic structures in the case of 3D model irradiation using pulsed X-ray. The X-ray energy used for cell culture irradiation was in the range of 8-10 keV. To study further the irradiation effects of different energies of X-ray on cells, a simulation using the Geant4 tool kit was performed. It was found that low-energy X-ray irradiation induces larger toxic effects.

Acknowledgements. This work is supported by the ANID-FONDECYT Iniciación grant 11230594 and the ANID-FONDECYT-Regular grant 1240375.

ABSTRACT. Plasma-activated water (PAW) presents an effective strategy for microbial inactivation, which is linked to synergistic effects of reactive oxygen and nitrogen species (RONS) [1]. We evaluated the antimicrobial effect of PAW against a methicillin-resistant Staphylococcusaureus (MRSA) isolated from an odontogenic abscess. S. aureus strains, and in particular MRSA strains due to their resistance, have an important role in etiology of dental abscesses[2]. For the purpose of comparison, the reference MRSA strain routinely applied in disinfectant testing (ATCC 33591) was included in the study.

The plasma source for PAW preparation was 3-pin jet using argon as the working gas(2.5 slm), with frequency of high-voltage power supply 340 kHz and applied power 14 W. Needle electrodes were inserted inside three separate glass tubes and positioned vertically above the water sample. We have treated 15 ml of deionized water for 10 min (pH=6.5). The PAW chemistry was characterized by colorimetric methods (H2O2=80 ppm, NO3-=97 ppm,NO2-=41 ppm).

Bacterial suspensions containing 108 cells/ml were prepared in sterile saline, and analiquot of 100 μL per strain was added to 900 μL of PAW in microtubes. Following exposure to PAW for 15 min and 60 min, bactericidal effects were assessed by the plate count method and expressed in log CFU/mL. PAW treatment was ineffective against reference MRSA strainregardless of the exposure time. Only longer PAW treatment showed effectiveness in theclinical MRSA strain, resulting in log CFU reduction equivalent to a 81.67% reduction in the bacterial population. While these results show bactericidal capacity of PAW against MRSA, further optimization of the PAW generating parameters is required for achieving efficient eradication.

Acknowledgements. This research was supported by the Science Fund of the Republic of Serbia, 7739780,APPerTAin-BIOM.

[1] H.R. Lee, et al., Sci Rep. 12(1), 5968 (2022)

[2] E.S. Donkor, et al., Infect Dis. 13, 1 (2020)

ABSTRACT. Plasma polymers produced via atmospheric pressure plasma polymerization are an innovative alternative to polymers synthesized through wet chemistry. This solvent-free, eco-friendly technique creates advanced antibacterial polymer coatings suitable for food packaging. Despite advancements in plasma polymer manufacturing, less focus has been on the degradation of these coatings in contact with food. Proper identification of degradation products is crucial for assessing potential toxicity. This study uses an aerosol-assisted atmospheric pressure plasma system to deposit polyethylene glycol (PEG)-like coatings with1 wt% zinc oxide (ZnO) nanoparticles on a polymer substrate. Fourteen degradation products released into various food simulants were identified, with the highest releases linked to C₆H₁₄O₄ and C₁₀H₂₂O₅, which differ mainly in the number of ethylene oxide groups. This highlights the efficiency of the plasma polymerization approach. Increasing the plasma input power from 200 to 350 W produced nanocomposites with higher degrees of crosslinking and a greater presence of Z_nO nanoparticles (from 1.6 ± 0.3 to 5.9 ± 0.8 at. %), resulting in fewer degradation products being released. Toxicity evaluations, including Daphnia magna LC50 (48 hr) and oral rat LD50 tests, indicate that these substances are non-toxic, supporting the safe use of these plasma-polymerized coatings in antibacterial food packaging.

ABSTRACT. The glow discharge is a widely-used technique for improving the tribological properties of materials, including the production of thin films using ionic beams and the treatment of surfaces through the sputtering process [1]. In this study, we concentrate on numerical analysis of the glow discharge in a cylindrical quartz chamber filled with argon gas. The chamber has a length and radius of 42 cm and 5 cm, respectively, and it is bounded by two copper electrodes with fixed potentials of 0 V (Anode) and -350 V (Cathode). The simulation has been done with two different conditions for different size of electrodes. The behaviour of plasma parameters presented at low pressure of 0.05 torr. To investigate this time-independent discharge and explore phenomena like the self-sustaining generation of secondary electrons and the formation of fixed regions, we use the Plasma Module of COMSOL Multiphysics software for numerical modelling [2]. The results presented in this paper provide valuable insights for the design and development of new technologies in the field of surface treatment.

[1] Y.Q. Pacheco, Journal of Physics: Conf. Ser. 1386, 012122 (2019).

[2] COMSOL Plasma Module User’s Guide. 416(2019).

ABSTRACT. In this poster presentation, a fluid simulation study of overtaking soliton collisions is undertaken for a plasma consisting of cold ions and Boltzmann electrons. An asymptotic analysis approach is applied to construct single soliton initial conditions directly from the Sagdeev pseudopotential solution. This approach allows for the construction of large amplitude solitons with Mach numbers of up to 1.5, that is, solitons that propagate 50% faster than the acoustic speed. These solutions are used to simulate overtaking collisions of solitons. In the small-amplitude regime, collisions are shown to closely resemble two-soliton solutions obtained from reductive perturbation analysis. At larger amplitudes, results show elastic collisions despite the higher order nonlinear effects. The only effect of the higher order nonlinearities is the reduction (in magnitude) of the phase shift compared to that predicted from reductive perturbation analysis.

ABSTRACT. We develop analytical kinetic models of quasi-stationary multicomponent current sheets that remind magnetopause configurations and allow for an arbitrary particle energy distributions [1, 2]. Particle-in-cell numerical simulations of their evolution demonstrate that, despite the overall stability of these sheets, the Weibel-type magnetic turbulence can develop within them or be inhibited depending on the model parameters. Both cases are studied and compared for two variants of the particle energy distribution — Maxwellian and Kappa. The purpose of this work is to elucidate the structure and parameters of current sheets, in which the Weibel-type instability is possible despite the presence of a self-consistent magnetic field and inhomogeneity of the plasma. Another aim is to study the main features of a weak magnetic turbulence that exists inside such current sheets of a magnetopause type.

In agreement with analytical estimates, simulations show that our simplest models without countercurrents of particles are stable with respect to the aperiodic Weibel-type instability. On the contrary, sheets with countercurrents and a wide region of weak enough magnetic field between them can be unstable. Nevertheless, in our simulations even they are not destroyed by the Weibel-type instability, which saturates at a relatively low level and leads only to a weak deformation of the large-scale current structure.

The obtained results make it possible to identify the features of growth and nonlinear evolution of Weibel-type turbulence in the presence of a non-uniform self-consistent magnetic field and an inhomogeneous plasma with a non-trivial particle velocity anisotropy. The gradients of the magnetoactive plasma's local properties significantly influence the spectrum of growing magnetic perturbations and the character of its gradual evolution from short to long wavelengths in comparison with the case of a homogeneous plasma.

We show that for both Maxwellian and Kappa particle energy distributions, the global structure of the current sheet and the character of the Weibel-type instability in its inner part are similar. Hence, the possibility of long-term existence of small-scale quasi-magnetostatic turbulence in distributed multicomponent current sheets seems to be quite universal. We discuss the applicability of the constructed current sheet models and the features of their internal small-scale magnetic turbulence to the analysis of the phenomena in the vicinity of the magnetopauses of planets and late-type stars.

[1] Nechaev A. A., Kocharovsky V. V., Kocharovsky V. V., Garasev M. A., JETP Letters117, 214 (2023)

[2] Kocharovsky V. V., Kocharovsky Vl. V., Martyanov V. Yu., Nechaev A. A., AstronomyLetters 45, 551 (2019)

ABSTRACT. Based on the energy invariants for the Weibel instability in a collisionless nonrelativistic plasma, an analytical relation is obtained between the instantaneous values of the space-averaged energy density of the magnetic field, its dominant wavenumber, and the average plasma anisotropy parameter [1]. The relation is valid for arbitrary particle velocity distribution functions, including ones that vary with time at the nonlinear stage of instability. It is obtained under the assumption that the plasma is homogeneous along a certain axis, determined, for example, by an external magnetic field, and taking into account only the modes with wave vectors orthogonal to this axis. This limits the justification of the relation's correctness, but does not rule out its approximate validity for a wider class of real systems with a sufficiently narrow spectrum of the Weibel magnetic turbulence.

Universality of the analytical relation is verified for several typical examples with different particle energy distributions using two-dimensional particle-in-cell modelling. The simulations also reveales important features and a self-similar nature of the evolution of the spatial spectrum of the Weibel magnetic turbulence. In particular, the power-law profiles of the small- and large-wavelength slopes of this spectrum and the temporal power-law behavior of its maximum are demonstrated.

Using the obtained relation, we give an analytical estimate of the maximum magnetic field energy achievable during the Weibel instability. Namely, we show that its ratio to theinitial longitudinal energy of particles, even for a large initial plasma anisotropy parameter, cannot exceed the value approximately equal to 0.2. This estimate could be used to limit the potential mechanisms of the generation of magnetic fields observed in space and laboratory, especially since the performed simulations give reason to expect that the relation or its close analogues will be approximately valid for a class of quite diverse problems of quasi-two-dimensional Weibel turbulence under the conditions of a relatively narrow spatial spectrum and a weak inhomogeneity of the plasma number density.

We discuss possible applications of the newly found analytical relation to the analysis of explosive kinetic processes in laboratory and space plasmas leading to the small-scale magnetic turbulence due to self-consistent filamentation of the electric current.

[1] Nechaev A. A., Kuznetsov A. A., Kocharovsky Vl. V., J. Plasma Phys. 89, 175890601(2023)

ABSTRACT. We have been developing high-power ion heating of magnetic reconnection using two merging tokamak plasma with high guide field B_t ≫ B_p (poloidal field)[1, 2]. The heated ion energy by magnetic reconnection scales with the reconnecting magnetic field component energy (B_rec²/2μ₀) where B_rec~B_p. The B_rec²-scaling of ion heating energy by reconnection can be understood by the fact that in the reconnection downstream the ion energy is mainly in the form of outflow kinetic energy before ions are thermalized in further downstream. The ion outflow velocity is produced mainly by the large ExB drift velocity associated with largepoloidal electric field E_z, resulting from the formation of quadrupolar electrostatic potentialstructure in the downstream region and E_z depends linearly on B_tB_p as observed in the experiments. Hence, the outflow velocity scales with B_rec, and thus the ion heating energyscales with B_rec². High power ion heating with about 40-50% of B_rec²/2μ₀ converted into the heated ion energy can only be achieved when the reconnection current sheet thickness d iscompressed to thinner than the ion gyroradius r_i. The B_rec²-scaling of high power reconnectionion heating provides an efficient way to produce burning plasmas with T_i > 10keV by increasing B_rec to 0.6T (for n_e~1.5x10¹⁹m^-3) without using any auxiliary heating methods likeneutral beam injection (NBI). If d ≫ r_i, the magnetic reconnection converts only 5-10% of B_rec²/2μ₀ into ion energy. This operation is suitable for magnetic helicity injection (current drive) of tokamak with small heating loss.

[1] Y. Ono, et. al., Phys. Rev. Lett. 107, 185001, (2011).[2] Y. Ono, et. al., to be published in Nuclear Fusion.

ABSTRACT. Here we report ion and electron heating characteristics of magnetic reconnection under the influence of high guide field in ST40 and TS-6 plasma merging experiments using 96CH/320CH 2D ion Doppler tomography and Thomson scattering diagnostics. In addition to the extension of ion heating scaling ∆T_i ∝ B_rec² in keV range as demonstrated in MAST and ST40, our recent experiments explored the following 3 new findings using 2D ion Doppler tomography and Thomson scattering diagnostics: (1) formation of poloidally asymmetric global ion heating structure in TS-6 and highly localized electron heating around the X-point in ST40 via parallel electric field acceleration, (2) update of the heating scaling with ∆U_i ∝ B_rec² to 10kJ/m³ by including the contribution of electron density in collaboration withThomson scattering measurement in ST40 from 2023 (the increment of ion thermal energy ∆U_i in the downstream region is ~30% of the upstream magnetic energy of reconnecting field B_rec), and (3) exploration of further electron heating via magnetic reconnection under the influence of high guide field in the keV range in ST40.

The poloidally asymmetric ion heating structure depends on the polarity of toroidalfield B_t and the fine structure gets flipped when the guide field direction is reversed. Under the influence of high guide field, E×B drift is mainly driven by in-plane/poloidal electric field E_p from the quadruple potential structure, while parallel electric field E_// is mainlydriven by reconnection electric field E_rec (spontaneously formed toroidal electric field E_t around X-point) and higher T_i appears where plasma potential is positive, while high T_e mainly appears around the X-point. The portion of toroidal electric field E_t for parallel electric field E_// is higher for high guide field condition (B_t > 3B_rec) and the peaked electron heating structure around the X-point becomes clearer when higher guide field is applied. Under the influence of toroidal effect to have higher guide field in the inboard side of outflow direction (B_t ∝1/r), downstream heating also forms poloidally asymmetric structure, and more heating appears in the high field side. Perpendicular heat conduction in the outflow region is strongly suppressed by high guide field $kappa^i$_///$kappa^i$_⊥ ~ 2($omega_ci tau_ii$)² ≫1 and the field-aligned transport process leads to the formation of poloidally ring-like characteristic fine structure after merging.

%SVL fix equations

[1] H. Tanabe et al., Phys. Rev. Lett 115, 215004 (2015)

[2] H. Tanabe et al., Nucl. Fusion 57 056037 (2017)

[3] H. Tanabe et al., Nucl. Fusion 59, 086041 (2019)

[4] H. Tanabe et al., Nucl. Fusion 61, 106027 (2021)

[5] H. Tanabe et al., 29th IAEA Fusion Energy Conference (FEC 2023), 1695 (2023)

[6] Y. Ono, H. Tanabe and M. Inomoto, Nucl. Fusion, accepted for publication (2024)

ABSTRACT. The “Laser-hybrid Accelerator for Radiobiological Applications”, LhARA, is being developed toserve the Ion Therapy Research Facility (ITRF). ITRF/LhARA will be a novel, uniquely flexible facility dedicated to the study of the biological impact of proton and ion beams. LhARA will combine laser ion acceleration, plasma lens with conventional accelerator technologies, to accelerate ions to 10’s of MeV energies by using a high power laser focussed onto a thin target (target normal sheath acceleration). These ions, are captured and focussed by a non-neutral plasma lens known as a Gabor lens. The technologies can transform the clinical practice of proton and ion beam therapy (PBT) by creating a fully automated, highly flexible laser-driven system to:

- Deliver multi-ion PBT in completely new regimens at ultra-high dose rate in novel temporal-, spatial- and spectral fractionation schemes; and

- Make PBT widely available by integrating dose-deposition imaging with real-time treatment planning in an automatic, triggerable system.

The status of the ITRF/LhARA project will be described along with the collaboration’s vision for the development of a transformative proton- and ion-beam system.

ABSTRACT. The laser-plasma accelerator has advanced to the stage where control of beam parameters and stability are the main development challenges [1,2]. Achieving high quality electron beams in laser wakefield accelerators requires stable guiding of the driving laser pulse, which is challenging because of mode mismatching due to relativistic self-focusing. Here we show how an intense pre-pulse can be used to prepare the phase-space distribution of plasma electrons encountered by a trailing driver pulse so that it produces its own well-matched guiding channel that minimises wakefield evolution. We present a unique double-pulse scheme that creates astable plasma channel waveguide intrinsically aligned with the trailing driver pulse. The plasma momentum distribution and electrostatic field are prepared by an intense, short duration pre-pulse that precedes the arrival of a second, co-propagating pulse. The time delay between the two pulses is chosen to be slightly less than the plasma period and the moderately intense leading, or chaperone, pre-pulse excites a weakly-nonlinear plasma wave that is below the threshold for wavebreaking and self-injection. The pre-pulse does not directly produce aguiding structure for the trailing driver pulse, but rather its wake prepares the plasma by driving converging streams of electrons that are subsequently deflected by the ponderomotive force of the (trailing) driver pulse to produce a narrow, well-defined parabolic plasma channel. This acts as a self-aligned waveguide accompanying the driver over extended propagation lengths.The driver pulse initially undergoes relativistic self-focusing so that it matches the narrow density channel, which ensures high coupling efficiency. The chaperone pre-pulse stimulates guiding of the driver pulse, providing control over its evolution and enhancing the stability ofthe resulting wakefield [3].

Controlling high intensity driver pulses is an essential step in developing useful wakefield accelerators and compact radiation sources. Stable channels are necessary for developing next generation compact plasma undulators for compact synchrotron sources and free-electron lasers. Wide availability of ultra-compact accelerators and radiation sources could transform the way science is done.

[1] T. Tajima and J. M. Dawson, Phys. Rev. Lett. 43, 267 (1979).

[2] S. P. D. Mangles, et al. Nature 431, 535 (2004).

[3] D. N. Gupta, S. R. Yoffe, A. Jain, B. Ersfeld, and D. D. A. Jaroszynski, Scientific Reports 12, 20368 (2022).

ABSTRACT. Numerical methods for solving the relativistic motion of charged particles with a higher accuracy is an issue for scientific computing in various fields including plasma physics. The classic fourth-order Runge-Kutta method (RK4) has been used over many years for tracking charged particle motions, although RK4 does not satisfy any conservation law. However, the Boris method [1] has been used over a half century in particle-in-cell plasma simulations because of its property of the energy conservation during the gyro motion.

Recently, a new method for solving relativistic charged particle motions has been developed, which conserves the boosted Lorentz factor during the E-cross-B motion [2]. The new integrator has the second-order accuracy in time and is less accurate than RK4. Then, new integrator is extended to the fourth-order accuracy in time by combining RK4 [3]. However, it is not easy to implement the new fourth-order integrator into PIC codes, because the new method with RK4 adopted co-located time stepping for position and velocity vectors, which is not compatible with the charge conservation method.

In this paper, the two new relativistic integrators are reviewed. Then, a new leap-frog integrator with fourth-order accuracy in time is developed, which adopts staggered timestepping for position and velocity vectors.

[1] J. P. Boris, Proc. 4th Conf. Num. Sim. Plasmas, 120310 (1970)

[2] T. Umeda, J. Comput. Phys. 472, 111694 (2023)

[3] T. Umeda and R. Ozaki, Earth Planets Space 75, 157 (2023)

ABSTRACT. This study provides a new numerical method for relaxation of the Courant condition and correction of numerical errors in the Finite-Difference Time-Domain (FDTD) method with the time-development equations using higher-degree difference terms. The FDTD method [1] is a numerical method for solving the time development of electromagnetic fieldsby approximating Maxwell's equations in both space and time with the finite difference of the second-order accuracy, which is widely used in plasma kinetic simulations. A staggered grid system is adopted in the spatial differences, in which Gauss’s law is always satisfied. The FDTD method has a disadvantage that numerical oscillations occur due to the error between the numerical phase velocity and the theoretical phase velocity. The FDTD (2,4) method [2, 3],which uses the fourth-order spatial difference, is proposed for reduction of the numerical errors. However, the Courant condition becomes more restricted by using higher-order finite differences in space and a larger number of dimensions.

Recently, a numerical method has been developed by adding third-degree differenceterms to the time-development equations of FDTD(2,4) [4]. Although the new method relaxes the Courant condition, there exist large numerical errors with large Courant numbers. In the present study, a new explicit and non-dissipative FDTD method is proposed with two types of the higher-degree difference operators for relaxation of the Courant condition andreduction of numerical errors. First, the one-dimensional third- and fifth-degree difference terms are added to the time-development equations of FDTD(2,6) [5]. Second, the third-degree difference terms including Laplacian are added to those of FDTD(2,4) [6]. The results of the test simulations show that numerical oscillations are not reduced so much with the one-dimensional difference operator, whereas the Laplacian operator suppresses an anisotropy in the waveforms and reduces the numerical oscillations. Furthermore, numerical instability is suppressed with large Courant numbers up to 1, which reduces the computational time ofplasma kinetic simulations significantly.

[1] K. S. Yee, IEEE Transactions on Antennas and Propagation 14, 302-307 (1966)

[2] J. Fang, Ph.D. dissertation, Department of Electrical Engineering, University of California, Berkeley, CA (1989)

[3] P. G. Petropoulos, IEEE Transactions on Antennas and Propagation, 42, 859-862 (1994)

[4] H. Sekido and T. Umeda, IEEE Transactions on Antennas and Propagation, 71, 1630-1639 (2023)

[5] H. Sekido and T. Umeda, Progress In Electromagnetics Research M, 123, 83-93 (2024)

[6] H. Sekido and T. Umeda, Earth, Planets and Space, 76, 5 (2024)

ABSTRACT. Electromagnetic micro instabilities are likely to limit performance in future advanced steady state tokamak plasmas and are expected to dominate transport in high β next generation spherical tokamaks (STs) such as STEP [1]. While gyrokinetic (GK) simulations have thus far proven to be a very accurate tool in modelling turbulent transport in predominantly electrostatic regimes, obtaining saturated nonlinear simulations in higher β plasmas with unstable kinetic ballooning modes (KBMs) and microtearing modes (MTMs) has proven computationally challenging (see e.g. [2]). Recent simulations of STEP-relevant equilibria that retain only MTMs and exclude KBMs (by neglecting compressional perturbations) saturate cleanly at very modest electron heat flux. However, local GK simulations find that including δB_∥ in such plasmas can unleash a hybrid KBM-like (hKBM) instability which drives very large heat fluxes (orders of magnitude greater than the available heating power) in the absence of strong equilibrium shear flows [3, 4]. These simulations underscore that understanding and mitigating hKBM-induced turbulence will be essential for the development of consistent flat-top operating regimes for future high performance ST devices.

In this talk we will present recent advances in our understanding of electromagnetic turbulence and discuss hKBM-driven turbulence in high β STs, with a particular focus on avoiding the high-transport state. We will present: (i) linear and nonlinear local simulations of hKBM turbulence, exploring the sensitivity of the turbulence and its associated transport to local parameters as well as different saturation mechanisms; (ii) a quasi-linear inspired reduced transport model for the hKBM turbulence; (iii) first flux-driven simulations for a high β ST that support the existence of a transport steady state in STEP with a fusion power comparable to that in the burning flat top of the conceptual design; and (iv) the first global nonlinear electromagnetic simulations of STEP to include δB_∥. These first-of-their-kind global simulations support the conclusions drawn from local GK simulations by: confirming that high-transport states in some equilibrium conditions are a robust prediction of GK and not simply an artefact of the local approximation; and tentatively supporting, in simulations that are currently running, the existence of a transport steady state in the vicinity of the conceptual STEP flat top. We will also discuss the most pressing priorities to be addressed in future work.

[1] STEP - Spherical Tokamak for Energy Production, https://step.ukaea.uk/

[2] S. M. Kaye et. al., Plasma Phys. Control. Fusion 63, 123001 (2021)

[3] D. Kennedy, at. al., Nuclear Fusion 63(12), 126061 (2023)

[4] M. Giacomin, et. al. Plasma Phys. Control. Fusion 66, 055010 (2024)

ABSTRACT. Spherical tokamak (ST) is a type of tokamak with low aspect ratio (A) that is the ratio of major radius (R) to minor radius (a) of torus and is intensively studied as a candidate for a cost-effective fusion power plant. Taking advantage of better MHD stability, higher bootstrap current fraction, and favorable confinement trend with magnetic field than traditional tokamaks (TTs), several designs of ST based fusion power plant have been proposed. But, the intrinsic natures of STs give rise to issues that need to be resolved. In particular, since the magnetic flux available for inductive plasma current generation is less than in TTs, it is important to develop methods to start and maintain the plasma current and to increase the toroidal magnetic field,BT. Recently, a small- sized ST, ST-40 developed in a private company (Tokamak Energy Ltd.) in UK achieved 10keV of ion temperature for several tens ms at B_T=2.2T. The machine size of ST-40 (R=0.4m, a=0.25m) is significantly smaller than TTs that obtained more than 10 keV that is need to cause a DT nuclear fusion reaction. This proves a high potential of STs.

QUEST is a medium sized spherical tokamak with R=0.64m, a=0.4m, and B_T<0.25T at R=0.64m. The QUEST aims at effective plasma current start up and stable maintenance of plasma discharge. To resolve the ST specific issues mentioned above, electron cyclotron current drive (ECCD) and coaxial helicity injection (CHI) are introduced on QUEST. An efficient ECCD could be achieved owing to energetic electrons (EEs), but the excess fraction of EEs may limit the application of toroidal electric field to drive plasma current to avoid a significant damage on the vacuum vessel due to local heat load caused by EEs. The methods we developed are to control the number of EEs with the assistance of control of injection refractive index to the magnetic field, N_// of electron cyclotron wave (ECW) and/or to apply negative toroidal electric field using center solenoid. The combination of both methods could drive the bulk electron temperature up to 1keV. Another non-inductive start-up tool, the Transient coaxial helicity injection (T-CHI) has been succesfully used in NSTX and a new electrode configuration is applied on QUEST. The electrode has been installed on lower divertor plates and electrically insulated by ceramic plates, and a plasma discharge between the electrode and the vacuum vessel is generated to supply magnetic helicity. The discharge develops in a force-free manner and magnetic reconnection drives a production of closed flux surface. The plasma induced by CHI is high density up to 1020 m^-3, but the temperature is still 10 eV. For long pulse operations on QUEST, it is impeded frequently due to particle imbalance. To resolve the issue, QUEST equipped a temperature controllable plasma facing wall (PFW) called hot wall. With help of the hot wall, 6 h discharges were obtained. In particular, the plasma duration of 40 min at 673K could be extended to more than 3 h through the temperature control of the hot wall.

Augmentation of B_T up to 0.5T and a CW gyrotorn of 28GHz will be prepared as future plans of QUEST. Long pulse operations with higher plasma parameters will be expected.

ABSTRACT. The pursuit of high-charge, high-density electron bunches driven by multi-petawatt lasers is essential for applications such as neutron generation, bright synchrotron radiation, and laboratory studies of electron-positron plasmas. Experiments have proven that at the hundred-terawatt scale, the OMEGA-EP facility can achieve electron energies of several hundred MeV and total charge over one hundred nanocoulombs via direct laser acceleration [1]. We push the acceleration mechanism towards multi-PW laser facilities to obtain electrons with energies surpassing GeV level and keeping the important property of high total charge. Our analytical scaling shows the dependence of an electron energy on laser intensity, and plasma density, and we provide a pathway to optimize this acceleration process for both existing and future facilities [2]. By precisely guiding a laser pulse over an appropriate distance, we can produce a wide Maxwellian spectrum of electrons with energies reaching several GeV and laser-to-electron conversion efficiencies in the tens of percent.

This presentation the nonlinear trade-off between the laser intensity and laser spot size will be explained. Understanding this balance provides insight into maximizing the transfer of laser energy to electrons, thus achieving the highest possible electron energies for a given laser power. We will compare our analytical scaling with Quasi-3D particle-in-cell simulations using the OSIRIS framework. Furthermore, we will discuss the theoretical aspects of plasma interactions under varying density conditions [3], crucial for understanding interactions in the preplasma of thin foils for ion acceleration. This analysis is necessary for experiments where the density profile of a gas jet varies along the laser propagation. In the end, we will highlight the potential of these electron bunches to emit high-brightness radiation at energies >100 MeV.

[1] A. E. Hussein, et. al., New J. Phys. 23, 023031 (2021)

[2] R. Babjak, et. al., Phys. Rev. Lett. 132, 125001 (2024)

[3] R. Babjak, et al., arXiv:2406.10702

ABSTRACT. The number of particles in an electron beam from laser wakefield acceleration is determined at the moment of trapping of background electrons. The longitudinal and transverse wave-breaking initiates the electron trapping. After some time, the trapping stops because of the repulsive force by the trapped particles. From many simulations and experiments, it has been well known that trapping of the background electrons begins much below the longitudinal wave-breaking limit [1,2]. This is related with transverse motion of the electrons. As an ultra-intense laser pulse propagates through a plasma, it pushes out the background plasma electrons and leaves behind a periodically-repeated bubble-like region. Inside the bubble, the electron density is very low, while the electron density at the rim of the bubble is very high. Highly energetic electrons make their trajectories along the rim of the bubble. Though many of such electrons turn around the rim and leave the bubble, some of those electrons are trapped in the transverse direction when their kinetic energies are lower than the depth of the potential well of the bubble [3].

The idea suggested in this paper is to use the double-gas system for electron injection. Based on the ionization of higher atomic number gas, enhanced injection has been estimated through particle-in-cell (PIC) simulations. The laser ionizes a lower atomic number gas to facilitate the laser wakefield via ponderomotive push of the electrons. However, the electron injection isweak in this case. The inclusion of higher atomic number gas (such as nitrogen) can enhance the electron injection via gas ionization. The advantage of this technique is to obtain the control of the beam charge in the laser-plasma accelerators, while keeping other parameters unmodified. Though the required gas ionization should be controlled via various parameters.A systematic bunch optimization is estimated in this work for future course of compact accelerator development.

[1] W. P. Leemans, B. Nagler, A. J. Gonsalves, Cs. Toth, K. Nakamura, C. G. R. Geddes, E.Esarey, C. B. Schroeder, and S. M. Hooker, Nature 2, 696 (2006).[

2] J. Faure, C. Rechatin, A. Norlin, A. Lifschitz, Y. Glinec, and V. Malka, Nature, 444, 737(2006).

[3] A. Pak, K. A. Marsh, S. F. Martins,3 W. Lu, W. B. Mori, and C. Joshi, Phys. Rev. Lett. 104,025003 (2010).

ABSTRACT. Wave propagation in relativistic plasmas is a subject of interest in many astrophysical and space systems, where electromagnetic fields may accelerate particles up to relativistic velocities, which in turn modifies the physics of wave propagation. Besides, kinetic effects further modify the dispersion properties of waves and their nonlinear interactions with the plasma particles. Thus, it is of interest to study wave propagation, and its nonlinear decays, considering both relativistic and kinetic effects.

However, the traditional approach of equilibrium statistics, where kinetic effects are described by Maxwellian velocity distributions, is not satisfactory in several environments where long range correlations, or memory effects, may lead the distributions to deviate from the Maxwellian case. Here, the proposal to describe plasma distribution functions in terms of nonextensive distribution functions, either of the Tsallis (where deviation from the Maxwellian case is given by a parameter q) [3] or the kappa type (where deviations are parametrized by a κ factor), [4, 5] allows to extend the traditional formalisms, to study wave linear and nonlinear propagation for systems out of thermodynamical equilibrium.

Following these ideas, in this work, parametric decays of an electromagnetic wave in an electron-positron plasma are studied. Kinetic effects are considered by means of the collisionless Vlasov equation, which is coupled to Maxwell equations. Relativistic effects on the particle motion are also taken into account. [6]

In the weakly relativistic case, although some of the instabilities involve strongly damped, electroacoustic pseudomodes, instabilites found using fluid theory are present as well in the kinetic regime. [1, 2] We study in detail the dependence on κ of the dispersion relation, the growth rate, and the phase velocity of the waves. As expected, results reduce to the Boltzmann-Gibbs statistics for κ → ∞ (q → 1).

Acknowledegements. This project has been financially funded by FONDECyT, grant number 1242013 (VM).

[1] V. Muñoz and L. Gomberoff, Kinetic effects on the parametric decays of circularly polarized electromagnetic waves in an electron-positron plasma, Phys. Plasmas 9, 2534–2540,2002.

[2] V. Muñoz, Kinetic effects on the parametric decays of circularly polarized electromagneticwaves in a relativistic pair plasma, Phys. Plasmas 11, 3497–3501, 2004.

[3] C. Tsallis, Possible generalization of Boltzmann-Gibbs statistics, J. Stat. Phys. 52, 479–487, 1988.

[4] J. A. S. Lima, R. Silva, Jr., and J. Santos, Plasma oscillations and nonextensive statistics, Phys. Rev. E 61, 3260–3263, 2000.

[5] G. Livadiotis, Thermodynamic origin of kappa distributions, Europhys. Lett. 122, 50001,2018.

[6] V. Muñoz, A nonextensive statistics approach for Langmuir waves in relativistic plasmas, Nonlinear Proc. Geophys. 13, 237–241, 2006.

ABSTRACT. The Visibility Graph, a technique for mapping time series into complex networks, is employed to research underlying physical mechanisms in collisionless, turbulent plasmas. We analyze four distinct time series of magnetic field fluctuations obtained from Particle in Cell (PIC) simulations, initialized varying the $\kappa$ parameter of its particle velocity distributions to explore departures from thermodynamic equilibrium. All studied cases exhibit a power law behavior in the degree distribution of the nodes. The critical exponent of this distribution unveils information about network properties, including particle correlations and heterogeneity. We compute the $\gamma$ exponent for the degree distribution of the scale-free network and observe its evolution according to $\kappa$, peaking at $\kappa = 3$. This trend suggests that long-range correlations are more prominent in plasmas far from thermal equilibrium, while short-range correlations dominate in thermal plasmas following a Maxwellian distribution. These findings align with previous non-collisional plasma studies.Additionally, we investigate the $\mu$ and $\nu$ exponents associated with the slopes of power spectra of the magnetic fluctuations, obtaining insights into the energy dissipation and temporal persistence of the time series.  Our findings reveal that low-frequency fluctuations exhibit the sharpest energy dissipation in thermal equilibrium environments, while high-frequency fluctuations dominate in systems described by velocity distributions with small $\kappa$. When comparing the correlation between these exponents and $\gamma$ as a function of $\kappa$, we find a direct correlation for the exponent $\nu$ associated with high-frequencies, and an anticorrelation for the low-frequencies exponent $\mu$.This finding underscores the connection between long- and short-range correlations and the Debye sphere of the plasma, revealing that the $\gamma$ metric of the Visibility Graph is only able to see the smaller scales of a time series.

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