ABSTRACT. Y. Peysson1, A. Jardin2), Gerenton3), G. Verdoolaege4), H. Wu4,5), M.Chernyshova6), A. Wojenski7), J. Colnel1), D. Guibert1), T. Czarski6), K. Malinowski6), P.Linczuk7), D. Colette8), G. Kasprowicz7), K.T. Poźniak7), M. Walsh8), and the WEST team*

1 CEA, IRFM F-13108, Saint Paul Lez Durance, France

2 Institute of Nuclear Physics Polish Academy of Sciences (IFJ PAN), PL-31-342, Krakow,Poland.

4Department of Applied Physics, Ghent University, Ghent, 9000, Belgium.

5Southwestern Institute of Physics, CNNC, Chengdu, 610041, People’s Republic of China.

6 Institute of Plasma Physics and Laser Microfusion, Hery 23, 01-497 Warsaw, Poland.

7 Warsaw University of Technology, Institute of Electronic Systems,Nowowiejska 15/19, 00-665 Warsaw, Poland.

8ITER Organization, Route de Vinon sur Verdon, CS 90 046 - 13067 Saint Paul Lez DuranceCedex, France

* http://west.cea.fr/WESTteam

%SVL edit author list

In modern tokamaks like ITER—International Thermonuclear Experimental Reactor, or WEST – Tunsgten (W) Environment in Steady-State Tokamak in Cadarache, the choice of W instead of traditional carbon as the main plasma facing material, to minimize tritium retention in the walls, has raised the essential issue of heavy impurity radiation. Monitoring and control of W concentration below 0.01% in the plasma core will be indeed necessary to avoid significant cooling of the plasma by impurity radiation, in particular in the soft X-ray(SXR) energy range of 0.1–20 keV.

In fusion devices, SXR plasma emissivity contains rich information not only about impurity transport, but also about the magnetohydrodynamic activity, magnetic equilibrium, plasma density and temperature. Since the radial impurity transport can be impacted by their poloidal distribution, SXR tomographic tools are valuable to infer the 2D impurity distribution and select adequate mitigation strategies.

In this context, this paper describes different efficient numerical tools, including artificial intelligence, neural network and Bayesian techniques for tungsten and/or electron temperature profiles evaluation in WEST plasmas based on experimental data from X-Ray measurements [1] and modelling tools [2], including synthetic diagnostics.

[1] D. Mazon et al, JINST 17 C01073 (2022)

[2] Y. Peysson et al., Nucl. Fusion 63 12604 (2023)

ABSTRACT. representing the APEX Collaboration including V. C. Bayer ^1,2, E. Buglione-Ceresa², A. Card², J. R. Danielson³, A. Deller^1,3, P. Gil¹, C. P. Hugenschmidt², P. Huslage¹, J. von der Linden¹, D. Mendonça², S. Nißl¹, D. Orona⁴, H. Saitoh⁵, D. Schmeling⁶, E. von Schoenberg⁷, L. Schweikhard⁸, M. Singer^1,8, J. Smoniewski¹, P. Steinbrunner^1,8, M. R. Stoneking⁹, C. M. Surko³, A. Zettl⁸

¹Max Planck Institute for Plasma Physics, Garching & Greifswald, Germany

²Technische Universität München, Garching, Germany

³University of California San Diego, La Jolla, CA, U.S.A.

⁴Massachusetts Institute of Technology, Cambridge, MA, U.S.A.

⁵University of Tokyo, Kashiwa, Japan

⁶Columbia University, New York City, NY, U.S.A.

⁷Concordia University, Montréal, Quebec, Canada

⁸University of Greifswald, Greifswald, Germany

⁹Lawrence University, Appleton, WI, U.S.A.

The creation and study of confined, long-lived, electron-positron plasma in the laboratory is the grand challenge of the APEX (A Positron-Electron eXperiment) Collaboration. Conducting experiments with this unique hybrid of matter and antimatter willenable comparisons to fundamental plasma physics predictions for this uncommonly symmetric system. Ideally, these can in turn contribute to our understanding of positrons and/or pair plasmas in astrophysics --- or even the early universe. As part of striving towards this goal, we are employing, validating, combining, and advancing diverse, state-of-the-art science and technology [1]. This talk will give an overview of recent highlights --- including novel techniques in the areas of non-neutral plasmas [2], positron beams, and gamma diagnostics [3] --- as well as the progress on our two, complementary, table top-sized, pair-plasma traps: a levitated dipole and an optimized stellarator, both based on small, non-insulated, HTS (high-temperature superconducting) coils. Finally, it will summarize our roadmap for the next few years, when we will put all of these elements together.

Acknowledgements. We gratefully acknowledge funding from the Helmholtz Association; the Deutsche Forschungsgemeinschaft (DFG); the Alexander von Humboldt Foundation; the UC San Diego Foundation; the Deutscher Akademischer Austauschdienst (DAAD) RISE program; the United States Department of Energy; the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme; the Japan Society for the Promotion of Science (JSPS); and the National Institute for Fusion Science (NIFS).

[1] Stoneking, M. et al. Journal of Plasma Physics 86, 155860601 (2020).

[2] M. Singer, et al., Journal of Plasma Physics 89, 935890501 (2023).

[3] A. Deller, et al. Phys. Rev. E 110, L023201 (2024).

ABSTRACT. for the JET L2H Team*, JET contributors**Laboratorio Nacional de Fusion, CIEMAT, Madrid, Spain;* See the author list of E.R. Solano et al, Nucl. Fusion 63 112011 (2023)**See the author list of “Overview of T and D-T results in JET with ITER-like wall”by C.F. Maggi et al. 2023 Nucl. Fusion 63 110201

%SVL fix authors

The transition between L and H-mode has fascinated plasma physicists since its discovery [1]. It is a clear phase transition between plasma confinement regimes, which takes place when the plasma is sufficiently heated. Here we present and discuss results from recent dedicated L-H transition experiments at JET [2]. Uniquely, we studied the power threshold (PLH) in plasmas composed of pure Tritium, to be compared with Hydrogen and Deuterium plasmas, D+T mixtures, and H+T mixtures [3]. We show that critical pressure profiles are required for the L-H transition to occur, and such critical profiles are independent of plasma content, but the power threshold itself depends strongly on isotopic content. PLH is in fact determined by the plasma transport characteristics in L-mode. We also show that an analysis of Doppler reflectometer measurements of the edge perpendicular velocity in D and He plasmas and observe that there are no critical radial electric field value or critical v_ExB rotation before the L-H transition [4]. Instead, there appears to a v_ExB profile that is characteristic of the L-mode. The diamagnetic velocity, proportional to ∇p, is a better indicator of proximity to the L-H transition. Together, these results enable us to challenge the widely accepted model of the L-H transition being associated to the stabilisation of electrostatic turbulence by sufficient v_ExB shear. An alternate model of the L-H transition is briefly discussed, based on a magnetisation transition of the plasma.

[1] F.F. Wagner et al., Phys. Rev. Lett. 49, 1408 (1982)

[2] E.R. Solano et al, Nuclear Fusion 63 (11), 112011, 2023

[3] G. Birkenmeier et al, PPCF 65 (5), 054001, 2023

[4] C. Silva et al, NuclearFusion 61 (12), 126006, 2021

ABSTRACT. T. Maurel–Oujia1, B. Kadoch2, S. Benkadda3 and K. Schneider11Aix-Marseille Université, CNRS, I2M, UMR 7373, 13453 Marseille, France2Aix-Marseille Université, CNRS, IUSTI, UMR 7343, 13453 Marseille, France3Aix-Marseille Université, CNRS, PIIM, UMR 7345, 13397 Marseille, France

%SVL fix authors

Confinement quality in fusion plasma is significantly influenced by the presence of heavy impurities, which can lead to radiative heat loss and reduced confinement. This study explores the clustering of heavy impurity, i.e., Tungsten in edge plasma of tokamaks. The two-dimensional Hasagawa-Wakatani model of drift-wave turbulence is used as a paradigm to describe edge tokamak turbulence. To this end high-resolution direct numerical simulations of the Hasegawa-Wakatani equations are carried out with millions of charged inertial particles. We use Stokes number to quantify the inertia of impurity particles. It is found that particle inertia will cause spatial intermittency in particle distribution and the formation of large-scale structures, i.e., the clustering of impurity particles. The degrees of clustering are influenced by Stokes number [1]. To quantify these observations, we apply a modified Voronoi tessellation method, which assigns specific volumes to impurity particles [2]. By determining time changes of these volumes, the impurity velocity divergence can be calculated, where negative values indicate cluster formation while positive values correspond to cluster destruction. Thus the clustering dynamics can be assessed. Additionally, Lagrangian statistics are used to provide further insights into the dynamics of heavy impurity behavior inthe edge plasma.

[1] Z. Lin, T. Maurel-Oujia, B. Kadoch, P. Krah, N. Saura, S. Benkadda and K. Schneider.Synthesizing impurity clustering in the edge plasma of tokamaks using neural networks. Physics of Plasmas, 31, 032505, 2024.

[2] Z. Lin, T. Maurel-Oujia, B. Kadoch, S. Benkadda and K. Schneider. Tessellation-basedAnalysis of Impurity Clustering in the Edge Plasma of Tokamaks. Preprint, 04/2024

ABSTRACT. Tungsten (W) has been selected as the divertor and first wall material for the International Thermonuclear Experimental Reactor (ITER) project [1, 2]. The tungsten divertor technology of ITER is tested on several devices, including the W Environment in Steady-state Tokamak (WEST) located at Cadarache. Despite the high melting point and low erosion rate of W, the interactions between the edge plasma and the plasma-facing components are an important sourceof tungsten impurities in the plasma. The accumulation of heavy impurities like tungsten in the plasma core of fusion devices poses a major risk to the core performance through strong radiative cooling. Therefore, reliable estimates of impurity concentrations in fusion devices are of vital importance. The soft X-ray (SXR) diagnostic system at WEST records the line-integratedSXR emissivity from impurities as well as the bulk plasma [2]. The two-dimensional SXR emissivity profile in a poloidal cross section can be reconstructed through Bayesian inference witha nonstationary Gaussian process prior, resulting in a closed-form expression for the posterior distribution [3]. Combined with density and temperature measurements, the tungsten concentration profile can be directly inferred from the reconstructed emissivity profile. However, such sequential analysis does not take into account the various sources of uncertainty from the individual diagnostic systems. Instead, we use Bayesian probability theory to jointly estimate the density, temperature and tungsten concentration profiles from their joint posterior distribution. This approach is called integrated data analysis (IDA) in the fusion community [4]. The IDA approach enables reliable uncertainty estimates without explicit error propagation analysis and allows exploiting the diagnostics’ interdependencies. The high-dimensional joint posterior distribution of the quantities of interest can be explored by a Markov chain Monte Carlo (MCMC) sampler. Tests on synthetic data suggest large uncertainty on the inferred tungsten concentra-tion near the plasma edge, due to the weak SXR signal from low ionization states of tungsten at low electron temperature. Therefore, to improve the edge inference we also investigate the possibility of including additional diagnostics, like bolometry. First results using synthetic datashow that this is a promising approach.

[1] R.A. Pitts, et al., Nucl. Mater. Energy 20, 100696 (2019)

[2] P. Barabaschi, proceedings of the 29th FEC, IAEA-CN-316-2354, London (2023)

[3] D. Mazon et al., J. Instrum. 11, C08006 (2016)

[4] T. Wang et al., Rev. of Sci. Instrum. 89, 10F103 (2018)

[5] R. Fischer et al., Fusion Sci. Technol. 58, 675-684 (2010)

ABSTRACT. Since the end of 2021, the Laser Mégajoule (LMJ) at CEA in France can be operated ina symmetric configuration with 10 laser chains [80 beams distributed into 2 upper hemisphere cones (polar angle 33° and 49°) and 2 lower hemisphere cones (147° and 131°)] giving up to ~270 kJ delivered at 351 nm. LMJ presently offers more than 14 plasma diagnostics operating in the visible-UV wavelengths ranges and in multiple hard and soft x-ray ranges as well as particles diagnostics (neutrons, electrons and ions). Notably, Laser Plasma Interaction (LPI) is characterized on LMJ with (i) backscattering measurements (time-resolved spectra end energy) on one inner quadruplet and (ii) near backscattering [1] covering polar angles from 20° to 65°  over ~1/5 of the azimuth of the LMJ target chamber.

In the indirect-drive configuration to ICF, the laser energy is converted into a bath of x-rays inside a high-Z enclosure (a hohlraum) that further implode the capsule (containing thefuel), its symmetry of irradiation being adjusted through the energy balance between inner and outer cones of beams. A specific “rugby-ball shaped” hohlraum design (in contrast to the regular cylinder shape) is well suited to the LMJ energy partition with its ½-½ power balance. However, this particular prolate spheroid hohlraum shape introduces challenges in the control of the inner beams propagation. Namely, the significant expansion of the gold holhraum wall into the path of the inners can give rise to significant Stimulated Brillouin Scattering (SBS). The latter involves the coupling of the incident laser with ion acoustic waves thus amplifying scattered electromagnetic waves. Large amounts of SBS may be expected for inner beams propagating in the high-Z mm scale gold bubble plasma.

In the direct-drive configuration to ICF, the laser beams irradiate the capsule in a spherical geometry giving rise to an expanding exponential density profile plasma. Thecoupling of the laser beams with such a plasma at the relevant scale (laser intensity, plasma temperature, density scalelength) had been studied with a planar geometry. The unique angular coverage of the LMJ LPI diagnostics recently allowed to investigate the Raman scattering mechanisms responsible for the hot electrons generation in direct-drive ICF experiments thanks to this planar target platform. We will report on the first indirect-drive experiments performed on LMJ in rugby ballshaped gas-filled hohlraums with laser pulses duration up to 9 ns. We will illustrate the challenges of getting a round implosion while mastering the LPI mechanisms. The configuration and preliminary results of the planar direct-drive experiments will also bepresented.

[1] V. Trauchessec et al., Rev. Sci. Instrum. 93, 103519 (2022).

ABSTRACT. The talk aims at the classical problem of the decay of a strong plasma discontinuity and a similar problem of the injection of a plasma with hot electrons into a vacuum or a rarefied cold plasma with a magnetic field in the absence of significant particle collisions. The main goal is to elucidate the transient phenomena of the formation of an anisotropic electron velocity distribution leading to quasi-magnetostatic turbulence associated with Weibel-type instabilities. We focus mainly on the formation and decay of current filaments and sheets and review the theoretical and numerical results on this account obtained previously for space and laser plasmas.

We describe and analyze in detail the original results of particle-in-cell modeling of a collisionless expansion of an anisotropic plasma cloud with hot electrons from a flat surface into a background magnetoactive plasma in different settings entailing [1](i) a hot-electron spot of a circular or cylindrical form within an initial-value problem or a finite-time injection of electrons from the surface,(ii) an external magnetic field with three orthogonal orientations: perpendicular to the surface or along it, directed either across or parallel to a long axis of the hot-electron spot,and (iii) inhomogeneous layers of cold background plasma of different spatial scales and densities.

Bearing in mind typical laser-plasma experiments, we outline the development of the principal structures of currents and highly inhomogeneous magnetic fields linked with distinct forms of the anisotropic electron velocity distribution and sophisticated dynamics of the instability process for diverse sets of attributes (i)–(iii). Strong magnetic fields generatedat the main transition stage are turbulent in nature, can reach, in typical laser experiments, mega-Gauss and higher values, and modify interaction between charged particles notably. In particular, according to the analytical estimates, numerical simulations and laser ablation experiments, the magnetic fields of self-consistent currents of cold and hot electrons quickly lead to the spatial separation of particle counter flows and suppress the beam instability of plasma (Langmuir) waves. This situation is qualitatively different from the well-studied decay of a weak plasma discontinuity, where the electrons obey a Boltzmann distribution and the formation of magnetic turbulence is replaced with the generation of ion-acoustic solitons. We discuss applications of the obtained results to the analysis of laboratory and space plasma problems involving an explosive development of the small-scale magnetic turbulence due to the filamentation of electric currents.

This work has been supported by the RSF, project No. 24-12-00457.

[1] Kocharovsky V. V., Nechaev A. A., Garasev M. A., Reviews of Modern Plasma Physics8, 17 (2024)

ABSTRACT. A. Kinsella %SVL as author

Turbulence involves the transport of energy from large scales to the small scales at which it is dissipated. The properties of the structures which dissipate the magnetohydrodynamic (MHD) turbulence in star forming regions are important in determining the conditions within which stars and planets form. In MHD turbulence a significant fraction of energy is dissipated in current sheets which are formed as magnetic field lines are pressed together. In planet-forming discs these current sheets influence the chemistry and even the ionisation fraction, and thus the dynamics, of the plasma thereby potentially influencing star and planet formation.

Dissipation of energy in current sheets has been studied in the context of compressible and incompressible non-ideal MHD incorporating the effects of both Ohmic and ambipolar resistivity [1, 2]. However, these studies have not incorporated the Hall effect which is thought to be important during gravitational collapse (in molecular clouds) and certain parts of planet-forming discs.

In this work we report, for the first time, on the impact of the Hall effect on dissipative structures in MHD turbulence. We use the multi-fluid MHD code HYDRA [3, 4] to simulate driven, multi-fluid MHD turbulence and show that the size distribution of current sheets is changed, resulting in energy being dissipated equally across current sheets of all sizes, in contrast to previous results without the Hall effect and similar to previously published results for ideal MHD [5].

[1] G. Momferratos, P. Lesaffre, E. Falgarone and G. Pineau des Forêts, MNRAS 443, 86(2014)

[2] J. Ross, H.N. Latter, MNRAS 477, 3329 (2018)

[3] S. O’Sullivan and T.P. Downes, MNRAS 366, 1329 (2006)

[4] S. O’Sullivan and T.P. Downes, MNRAS 376, 1648 (2007)

[5] V. Zhdankin, S. Boldyrev, J.C. Perez and S.M. Tobias, ApJ 795, 127

ABSTRACT. Black hole accretion disks are assumed to be turbulent as they are highly unstable to magnetic instabilities driven by shear flow, resulting in angular momentum transport and an effective turbulent viscosity. The axisymmetric, weak-field magnetorotational instability (MRI) and its derivation through local WKB approximations has been the topic of many studies [1]. In contrast, its non-axisymmetric counterparts have received relatively little attention as they require the full power of magnetohydrodynamic (MHD) spectroscopy for a general description of the eigenspectrum of waves and instabilities [2]. This is essential for global cylindrical disk models, where the influence singularities from the overlapping MHD continua complicates the analysis. Recently, rigorous MHD spectroscopy identified a new type of ultra-localised, non-axisymmetric instability in global cylindrical disks with super-Alfvénic flow [3]. These super-Alfvénic rotational instabilities (SARIs) make up 2D unstable regions in the complex eigenfrequency plane with (near-eigen)modes that corotate at the local Doppler velocity and are radially localised between Alfvénic resonances, utterly insensitive to inner/outer radial disk boundaries.

Here, we independently confirm the existence of these unprecedented modes using the novel spectral MHD code Legolas (https://legolas.science/), reproducing and extendingour earlier study with detailed eigenspectra and eigenfunctions [4]. We also compare growth rates of the discrete SARIs and MRI in a variety of disk equilibria, highlighting the impact of field strength and orientation, as well as compressibility in suprathermal fields. We show that non-axisymmetric modes can significantly extend instability regimes at high mode numbers, with instability determined by the Alfvén frequency and maximal growth rates comparable to the MRI. Furthermore, we explicitly show the existence of modes that are highly localised in all directions, with possible applications to global and shearing box simulations. A visualisation in the time and space domain of MRI/SARI modes highlights their differences and similarities and gives further indication that the onset instability in accretion disks could very well be governedby localised non-axisymmetric SARI modes.

[1] S.A. Balbus & J.F. Hawley, ApJ 376, 214 (1991)

[2] R. Keppens, F. Casse & J.P. Goedbloed, ApJL 569, L121 (2002)

[3] H. Goedbloed & R. Keppens, ApJS 259, 65 (2022)

[4] N. Brughmans, R. Keppens & H. Goedbloed, ApJ (accepted for publication) (2024)