Plasma Physics and Controlled Fusion

Particle-in-cell (PIC) methods have a long history in the study of laser-plasma interactions. Early electromagnetic codes used the Yee staggered grid for field variables combined with a leapfrog EM-field update and the Boris algorithm for particle pushing. The general properties of such schemes are well documented. Modern PIC codes tend to add to these high-order shape functions for particles, Poisson preserving field updates, collisions, ionisation, a hybrid scheme for solid density and high-field QED effects. In addition to these physics packages, the increase in computing power now allows simulations with real mass ratios, full 3D dynamics and multi-speckle interaction. This paper presents a review of the core algorithms used in current laser-plasma specific PIC codes. Also reported are estimates of self-heating rates, convergence of collisional routines and test of ionisation models which are not readily available elsewhere. Having reviewed the status of PIC algorithms we present a summary of recent applications of such codes in laser-plasma physics, concentrating on SRS, short-pulse laser-solid interactions, fast-electron transport, and QED effects.

An adequate modelling of the electromagnetic (EM) interaction of the plasma with the surrounding conductors is paramount for the correct reproduction of 3D plasma dynamics. Simulations of the latter provide in turn useful predictions regarding the plasma evolution, the related MHD modes leading to disruptions and the EM forces acting on the vacuum vessel's components when said disruptions occur. The latest modelling efforts with the 3D FEM non-linear JOREK code have been directed towards the eddy current coupling of a reduced magnetohydrodynamic (MHD) plasma model with thin and volumetric wall codes (STARWALL and CARIDDI). In this contribution, we present an eddy current coupling between the full MHD model of JOREK and the STARWALL code; this new coupling scheme describes the full three-dimensional interactions of the plasma with the vacuum region and external conductors, modeled by natural boundary conditions linking the magnetic vector potential A to the magnetic field B. The consistency of the new coupling scheme is validated via benchmarks for axisymmetric Vertical Displacement Events and multi-harmonics simulations of MHD modes.

The low frequency edge oscillation (LFEO) is a low frequency fluctuation in many plasma quantities in the pedestal region of the I-Mode confinement regime. It is observed on Alcator C-Mod between 10–30 kHz and on ASDEX Upgrade between 5–10 kHz. On both tokamaks it has been previously identified as a geodesic acoustic mode (GAM), however the recent discovery of the edge temperature ring oscillation (ETRO) in a similar frequency and spatial location as the LFEO in I-Modes on EAST has called this identification into question. In this paper we investigate the LFEO on C-Mod and AUG using a variety of different experimental techniques including spectral analysis, magnetic mode number analysis, localization, and direct measurement of the LFEO zonal structure and propagation using a mirror Langmuir probe. This investigation has reconfirmed the identification of the LFEO as a GAM and determined that it has several key differences from the ETRO.

CSD (Core, SOL and Divertor) models sampling a fusion device with three points (Core, SOL and Divertor) aim at understanding fusion plasmas in a wide range of parameter space. Conventional CSD models estimate interactions among plasmas, neutrals and impurities, which are incorporated as loss factors in basic equations in models, with the use of physical quantities of plasmas at three sampled points. Physical quantities are assumed to be uniform in each of the three regions. Neglecting plasma spatial distributions in a divertor prevents the models from accurately evaluating loss factors at low temperatures. This is because loss factors strongly depend on the location-dependent temperature when the temperature is low. Consequently, conventional CSD models have difficulties in reproduction of the detachment. This research increases sampling points for the calculation of loss factors in a divertor. Loss factors at each point are integrated in the volume, then effective loss factors (ELF) for the divertor are obtained. Conventional loss factors in transport equations are replaced with ELFs for an improved CSD model proposed in this study. As a result, the plasma detachment is successfully reproduced in the CSD model without changing the equations. A CSD model for qualitative analyses with low computational costs has been developed to study plasma detachment.

A dual-band frequency-modulated reflectometry is employed on the Helically Symmetric eXperiment (HSX) stellarator. This system equips a fast PIN switch to alternate the frequency source between two voltage-controlled oscillators, providing an operational frequency range from 14.5 GHz to 25.5 GHz. A monostatic antenna geometry and an ellipsoidal mirror are implemented in the vessel of HSX, where the polarization of the transmitted microwave can be switched between O- and X-mode to accommodate the magnetic field and plasma density. Although the original system was designed for density fluctuation measurement, significant efforts have been undertaken to improve the system performance and to realize the density profile inversion. Recent improvements of the system reported here include calibration of the dispersion in the transmission line, optimization of the waveform, and implementation of the Choi-Williams time–frequency distribution for spectral analysis. In this paper, we present the optimization of the reflectometry system along with first experimental measurements of the density profile and observations of fluctuations during gas puffing experiments.

JET returned to deuterium-tritium operations in 2023 (DTE3 campaign), approximately two years after DTE2. DTE3 was designed as an extension of JET's 2022-2023 deuterium campaigns, which focused on developing scenarios for ITER and DEMO, integrating in-depth physics understanding and control schemes. These scenarios were evaluated with mixed D-T fuel, using the only remaining tritium-capable tokamak until its closure in 2023. A core-edge-SOL integrated H-mode scenario was developed and tested in D-T, showing good confinement and partial divertor detachment with Ne-seeding. Stationary pulses with good performance, no tungsten accumulation, and even without ELMs were achieved in D-T. Plasmas with pedestals limited by peeling modes were studied with D, T-rich, and D-T fuel, revealing a positive correlation between pedestal electron pressure and pedestal electron density. The Quasi-Continuous Exhaust regime was successfully achieved with D-T fuel, with access criteria similar to those in D plasmas. A scenario with full detachment, the X-point radiator regime, was established in D-T, aided by the real-time control of the radiator's position. The crucial characterisation of tritium retention continued in DTE3, using gas balance measurements and the new LID-QMS diagnostic. Nuclear technology studies were advanced during the DTE3 campaign, addressing issues such as the activation of water in cooling loops and single event effects on electronics. Building on the previous D, T and DTE2 campaigns and the lessons learned from them, DTE3 extended our understanding of D-T plasmas, particularly in scenarios relevant to next-generation devices such as ITER and DEMO.

Ion cyclotron resonance heating (ICRH) has the potential of providing efficient ion heating of reactor grade fusion plasmas especially during the start-up phase. In order to assess such heating scenarios, ICRH modelling is required. However, the physics is complex and certain elements are not universally taken into account in ICRH modelling. In this paper we discuss the importance of including Doppler shift displacements of resonance points away from the cold resonance (i.e. where $\omega = n \Omega_{\textrm{c}}$) in Fokker–Planck calculations of the distribution function of resonating ions. In particular, the resonant interaction time and the wave electric field varies with the local Doppler shifted resonance positions. The importance of accounting for these variations in Fokker–Planck modelling is investigated. Furthermore, it is shown how these effects can be included in a simplified Fokker–Planck treatment that is sufficiently quick for integrated modelling frameworks of fusion plasmas. Because 2D effects in velocity space play a crucial role in determining Doppler shifts, we employ a model of the anisotropy of the non-thermal distribution function. Simulation results show that taking the Doppler effects into account in Fokker–Planck modelling can have a significant impact on the distribution functions of fast ions and important quantities, such as the collisional power transfer to the background plasma. This is especially important in cases where the poloidal variation of the left-hand component of the wave electric field is strong.

This article reviews applications of Bayesian inference and machine learning (ML) in nuclear fusion research. Current and next-generation nuclear fusion experiments require analysis and modelling efforts that integrate different models consistently and exploit information found across heterogeneous data sources in an efficient manner. Model-based Bayesian inference provides a framework well suited for the interpretation of observed data given physics and probabilistic assumptions, also for very complex systems, thanks to its rigorous and straightforward treatment of uncertainties and modelling hypothesis. On the other hand, ML, in particular neural networks and deep learning models, are based on black-box statistical models and allow the handling of large volumes of data and computation very efficiently. For this reason, approaches which make use of ML and Bayesian inference separately and also in conjunction are of particular interest for today's experiments and are the main topic of this review. This article also presents an approach where physics-based Bayesian inference and black-box ML play along, mitigating each other's drawbacks: the former is made more efficient, the latter more interpretable.

Simulating plasma turbulence presents significant computational challenges due to the complex interplay of multi-scale dynamics. In this work, we investigate the use of convolutional neural networks to improve the efficiency of plasma turbulence simulations, focusing on the Hasegawa–Wakatani model. The networks are trained to learn the closure terms in large eddy simulations, providing a computationally cheaper alternative to the high-resolution numerical solvers for capturing the effects of high-frequency components. This study is the first to successfully apply machine learning to predict plasma behavior for adiabatic coefficients beyond the training range for the Hasegawa–Wakatani equations. We generate ground truth simulations for three values of the adiabatic coefficient ($C\in\{0.2, 1.0, 5.0\}$), and train our models on one, or two. The evaluation is then performed on the remaining values. The models generalize well and accurately predict the particle flux up to a factor 5 outside the training range. Finally, we address a key challenge in machine-learning-accelerated plasma simulations—initialization—by starting simulations for previously unseen adiabatic coefficients C with states from existing simulations at other known C values. This approach removes the need for expensive direct numerical simulations for initialization while maintaining physical accuracy and a fast convergence rate. Overall, the results highlight the model's strong generalization capabilities and its potential for accelerating plasma turbulence simulations with more complex sets of parameters.

Lead–lithium flows are key features in the design of tokamak breeding blanket concepts such as the dual-coolant lead–lithium (DCLL). Since they flow under magnetic fields, they are affected by magnetohydrodynamic (MHD) effects. The neutron flux originating in the tokamak plasma heats the breeding blanket channels in a non-uniform manner, inducing buoyancy forces in the liquid metal. Buoyancy may become a source of quasi-two-dimensional (Q2D) turbulence, and the appearance of eddies may affect the transport of heat and tritium across the blanket. Blankets characterized by high-speed liquid metal flows (such as DCLL) will need ceramic insulating walls to reduce the MHD-related pressure drop in the channels. In our simulations, we have used the Q2D model proposed by Sommeria and Moreau (SM82) which is especially suitable for modeling electrically insulating channel flows. On top of that, we have modelled buoyancy forces in the momentum equation using the Oberbeck–Boussinesq approximation. In this work, we include a validation of the implemented Q2D model in buoyancy-driven cases and identify a thermohydraulic configuration that promotes the generation of eddies and the accumulation of tritium. We also show the results provided by our post-processing tool based on the bi-dimensional fast Fourier transform for eddy detection and characterization. We complete our investigation by performing an initial assessment of how eddies can accumulate tritium in the breeding blanket. We conclude by discussing the relevance of preventing accumulation of tritium within the blanket and suggest a possible solution.

This computational study delves into the intricate interplay of alloying elements on the generation, recombination, and evolution of irradiation-induced defects. Molecular dynamics simulations were conducted for collision cascades at room temperature, spanning a range of primary knock-on atom energies from 1 to 10 keV. The investigation encompasses a series of model crystals, progressing from pure Ni to binary concentrated solid solution alloys (CSAs) such as NiFe₂₀, NiFe, NiCr₂₀, and NiFeCr₂₀ CSA. We observe that materials rich in Cr actively facilitate dislocation emissions and induce the nucleation of stacking fault tetrahedra in the proximity of nanovoids, due to Shockley partial interactions. This result is validated by molecular static simulations, which calculate the surface, vacancy, and defect formation energies. Among the various shapes considered, the spherical void proves to be the most stable, followed by the truncated octahedron and octahedron shapes. On the other hand, the tetrahedron cubic shape is identified as the most unstable, and stacking fault tetrahedra exhibit the highest formation energy. Notably, among the materials studied, NiCr₂₀ and NiFeCr₂₀ CSAs stood out as the sole alloys capable of manifesting this mechanism, mainly observed at high impact energies.

The gas electron multiplier camera has been used to investigate the electron dynamics in the presence of turbulence and/or a magnetic island in disruptive plasmas on the Experimental Advanced Superconducting Tokamak. Interactions between the micro turbulence and the macroscopic island structure are observed to be mediated by electrons experiencing multiscale instabilities, which are accelerated to generate non-thermal radiation peaks. After comparisons of two typical H-mode disruptive shots, a statistical database is constructed which clearly shows that the amplitude of non-thermal peaks is linearly dependent on the plasma poloidal beta before the thermal quench. Thus, the driving-energy for the electron energization is actually due to the plasma thermal pressure. Although not quantitatively compared with simulations, this paper provides multiple diagnostic data demonstrating electron dynamics under the complex multiscale instability fields. This advances the mechanism causing thermal quench of H-mode plasmas.

Millimeter-wave reflectometers are essential in magnetic fusion devices due to their capability to precisely measure plasma density profiles and shape. This study investigates the application of system-on-chip (SoC) technology in microwave imaging reflectometer, with a focus on designing and integrating receiver components. Key aspects of SoC chip development and packaging are discussed. The 90 nm Complementary Metal-Oxide-Semiconductor (CMOS) receiver circuit includes a radio frequency low-noise amplifier (LNA) with a balun, a double-balanced mixer, an intermediate frequency amplifier, and an local oscillator balun. The receiver chip has a compact footprint of 1.219 × 0.82 mm², containing 14 transistors and consuming 89 mW of power. Post-simulation results for the high-gain LNA demonstrate a gain range of 25–28 dB with a noise figure of 7.5 dB across a three-stage structure. The system achieves a measurable power sensitivity down to 0.25 nanowatts while maintaining a highly compact form factor. The SoC system shows strong potential for future fusion reactor applications, offering robustness and scalability under challenging operational conditions. Chip and module testing results further highlight the system's promise for high-performance, space-constrained environments.

Interferometers based on phase measurement technique have been widely used to measure the plasma line integrated density (LID) in the magnetic fusion devices. Phase jump is an inherent drawback of phase measurement techniques. Once a phase jump occurs, phase errors should be compensated for by referring the complementary diagnostics. In a multi-chord interferometer used to measure plasma density profiles, phase errors can be compensated for using the phase relationship between adjacent receivers. To utilize the phase relationship between receivers, microwaves emitted from a single transmitter are spatially distributed to all receivers. The total phase changes are tracked by spatially accumulating the phase differences between adjacent receivers. Since temporal accumulation of the phase changes is not required, phase measurement failure due to phase jumps does not affect subsequent measurements. In multi-chord interferometers, phase loss due to beam refraction is also a serious problem because the beams are incident on the plasma at oblique angles. The beams are refracted due to the plasma density gradient. In a multi-chord interferometer with a single distributed transmitter, the phase loss problem is automatically solved because some of the microwaves always enter the receivers via curved beam paths, but interpretation of the measurements is problematic because the actual beam paths differ from the straight paths in vacuum. An algorithm is developed to find the actual curved beam paths by iterating four procedures: reconstructing density profile using tomography from the previously estimated LID, ray tracing beam path in the reconstructed density profile, estimating LID with the ray traced beam path information from phase measurements, and averaging the estimated LID with the previous LID. In this paper, the principles of single transmitter multi-chord interferometry are demonstrated by analyzing synthetically generated data and the algorithm of iterative tomography is described in detail.

In tokamak plasma merging experiments, high-power ion heating is achieved only when the current sheet thickness is compressed to ion gyroradius ρ_i. The magnetic reconnection converts 40%–50% of the reconnecting magnetic field energy (B_rec²/2μ₀) into the ion heating energy (W_i) with the scaling law of W_i proportional to B_rec². If the compressed current sheet thickness is larger than ρ_i the magnetic reconnection converts only 5%–10% of B_rec²/2μ₀ into W_i. The high-power ion heating can be understood from the experimental finding that the poloidal electric field due to electrostatic quadrupolar potential in the downstream region increases linearly with both B_rec and the guide field B_t.

In tokamak plasmas, non-linear interplay between transport and sources/sinks takes place for all transported quantities (current, heat, particles and momentum). Thanks to integrated modelling frameworks, we can iterate physics-based quasilinear turbulent transport models over multiple confinement times. Such modelling allows us to predict current, temperature, density and rotation profiles, and to disentangle the causality at play behind the modelled time evolution. An intense validation effort of such modelling against experimental measurements has been ongoing and has progressed our understanding. In dynamical phases, the so-called 'cold pulse' physics have been explained in the AUG tokamak, the isotope impact in plasma current ramp-up is understood in the JET tokamak, and the impact of the particle source (from neutral beam injection) on tungsten core accumulation has been clarified in JET and AUG. In stationary phases, the saturation of the ion temperature in electron-heated WEST plasmas has been clarified, and the energy content has been predicted with higher accuracy than empirical scaling laws with respect to the plasma current, magnetic field, plasma size and gas fueling, both in L and H modes on AUG. The validation of physics-based integrated modelling allows control optimisation in preparation for ITER operation as well as risk reduction for the design of future reactors. However, despite the reported progress, physics gaps remain on this path. For example, unlike today's devices, ITER-class devices will be opaque to neutrals and fuelled by pellets. In the absence of a physical understanding of the transport in the pedestal, extrapolation is uncertain. Moreover, in burning plasmas, the non-linear coupling between the central core profiles and the fusion power is very strong. The uncertainties in profile predictions due to unverified and unvalidated reduced transport models in such high-pressure plasmas lead to uncertain fusion power predictions. Solutions on how to address these challenges within integrated modelling will be proposed.

During its 40 years of operations, the Joint European Torus (JET) tokamak has consistently pushed the physics and engineering boundaries of fusion research, providing the scientific community with a unique testing ground for theories and innovative ideas. This paper covers a selection of remarkable contributions of JET to various fields of tokamak science, from transport and plasma heating studies to plasma-wall interaction and D-T experiments, and their impact on the fusion research progress.

Turbulence, a fascinating and intricate phenomenon, has captivated scientists over different domains, mainly for its complex cross-scale nature spanning a wide range of temporal and spatial scales. Despite significant advances in theories and observations in the last decades, some aspects of turbulence still remain unsolved, motivating new efforts to understand its underlying physical mechanisms and refine mathematical theories along with numerical models. This topical review explores recent findings from the Parker Solar Probe mission, providing a distinctive opportunity to characterize solar wind features at varying heliocentric distances. Analyzing the radial evolution of magnetic and velocity field fluctuations across the inertial range, a transition has been evidenced from local to global self-similarity as proximity to the Sun increases. This behavior has been reconciled with magnetohydrodynamic theory revising an old concept by emphasizing the evolving nature of the coupling between fields. This offers inspiration for novel modeling approaches to understand open challenges in interplanetary plasma physics as the heating and acceleration of the solar wind, as well as, its evolution within the inner Heliosphere.

This paper reviews current understanding of key physics elements that control the H-mode pedestal structure, which exists at the boundary of magnetically confined plasmas. The structure of interest is the width, height and gradient of temperature, density and pressure profiles in the pedestal. Emphasis is placed on understanding obtained from combined experimental, theoretical and simulation work and on results observed on multiple machines. Pedestal profiles are determined by the self-consistent interaction of sources, transport and magnetohydrodynamic limits. The heat source is primarily from heat deposited in the core and flowing to the pedestal. This source is computed from modeling of experimental data and is generally well understood. Neutrals at the periphery of the plasma provide the dominant particle source in current machines. This source has a complex spatial structure, is very difficult to measure and is poorly understood. For typical H-mode operation, the achievable pedestal pressure is limited by repetitive, transient magnetohydrodynamic instabilities. First principles models of peeling–ballooning modes are generally able to explain the observed limits. In some regimes, instability occurs below the predicted limits and these remain unexplained. Several mechanisms have been identified as plausible sources of heat transport. These include neoclassical processes for ion heat transport and several turbulent processes, driven by the steep pedestal gradients, as sources of electron and ion heat transport. Reduced models have successfully predicted the pedestal or density at the pedestal top. Firming up understanding of heat and particle transport remains a primary challenge for developing more complete predictive pedestal models.

This article reviews applications of Bayesian inference and machine learning (ML) in nuclear fusion research. Current and next-generation nuclear fusion experiments require analysis and modelling efforts that integrate different models consistently and exploit information found across heterogeneous data sources in an efficient manner. Model-based Bayesian inference provides a framework well suited for the interpretation of observed data given physics and probabilistic assumptions, also for very complex systems, thanks to its rigorous and straightforward treatment of uncertainties and modelling hypothesis. On the other hand, ML, in particular neural networks and deep learning models, are based on black-box statistical models and allow the handling of large volumes of data and computation very efficiently. For this reason, approaches which make use of ML and Bayesian inference separately and also in conjunction are of particular interest for today's experiments and are the main topic of this review. This article also presents an approach where physics-based Bayesian inference and black-box ML play along, mitigating each other's drawbacks: the former is made more efficient, the latter more interpretable.

A new self-consistent 1D scrape-off layer model is recently developed in BOUT++ framework, named SD1D, which includes equations of various particle species (e.g. main plasma, neutrals and impurities) and couples open databases like ADAS and AMJUEL. It is able to fast and effectively simulate the divertor detachment experiments. In this work, a typical detachment experiment (shot #6270) on HL-3 with neon seeding is simulated using the SD1D code. It is found that the target electron temperature and the target ion saturation current in simulations are consistent with experimental results measured by Langmuir probes on the target plate. The variation of Dα radiation intensity in the divertor is qualitatively similar to the measured Dα signal. Following the experimental validations, different upstream densities are set in the simulations for studying impurity distribution in different plasma density conditions. It is found that increasing upstream density can be helpful for control of the neon radiation front (closer to the target).
This work also compares two detachment regimes in simulations. Based on the same initial experimental parameters (shot #6270) on HL-3, a scan of upstream density and a scan of neon seeding rate are carried out respectively. It is found that the role of atomic and molecular processes is different in the two detachment regimes. The current density roll-over is ascribed to a drop in the divertor ion source, and the variation of D_α radiation intensity via different excitation channels is associated to the relevant collisional reaction sources.


A bright X-ray backlighter with a broad spectrum in the multi-keV range is essential for diagnostics of opacity and plasma states in high-energy-density physics. This paper experimentally investigates the X-ray conversion efficiency and spectra across the 1.6-4.0 keV range for three types of material targets irradiated by lasers: Au films, Au foams, and WBi mixture films. The multi-keV X-ray conversion efficiency of WBi mixture targets is approximately 34% and 17% higher than that of the Au film and Au foam targets respectively, and the spectral range of the X-ray backlight produced by the WBi mixture target is also extended. Consequently, a combination of Au, W, and Bi is proposed to generate a bright and broadband X-ray source that covers the entire 2.0 keV - 3.8 keV range, which will have extensive applications in X-ray absorption spectroscopy.

A one-dimensional mixing model, incorporating the effects of laser ablation and initial perturbations, is developed to study the influence of ablative Rayleigh-Taylor instability on compression dynamics. The length of the mixing region is determined with the buoyancy-drag model[arXiv:2411.12392v2 (2024)]. The mixing effect on laser ablation is mainly described with an additional heat source which depends on turbulent kinetic energy and initial perturbation level through a free multiplier. The model is integrated into a one-dimensional radiation hydrodynamics code and validated against two-dimensional planar simulations. The relative errors of the model for quantifying compression remain below 10%. The further application of our model to spherical implosion simulations reveals that the model can give reasonable predictions of implosion degradation due to mixing, such as lowered shell compression, reduced stagnation pressure, and decreased areal density, etc. It is found that the time interval between the convergence of the main shock and stagnation may offer an estimate of mixing level in single-shot experiments.

To enhance the power absorption of ion cyclotron resonance heating(ICRH) and lower hybrid wave current
driven(LHCD) heating for high-parameter tokamak, it is essential to increase the plasma density near edge and here
we employ helicon wave for such a purpose. This work simulates the penetration and power deposition of helicon
wave excited by half turn helical antenna in plasma with sharp density and temperature gradient. An electromagnetic
wave solver code ( EMS ) is used which is based on Maxwell's equations and cold plasma dielectric tensor. With
profiles which are uniform in the core and decay exponentially, simulating the radial configuration of fusion plasma
in high-parameter tokamaks, we scan decay coefficients and antenna lengths with different frequencies. Two damping
mechanisms are taken into account and vary in different radial locations. The results show that the wave propagates
inward and downstream or bidirectionally, depending on the antenna length. Most of the absorbed power concentrates
on the surface where the damping collision frequency changes abruptly, which plays a role of reflection mirror. With
smaller decay coefficient, the wave trajectory is clear, while blurred with larger decay coefficient. In addition, with
the increasing of wave frequency, the power absorbed in plasma increases and clear standing wave structures form in
scrape-off layer (SOL) at specific frequencies. It shows a trend that the absorbed power peak raises first and then drops
with the antenna lengths increasing. This work presents the propagation features of helicon wave in simulated tokamak
SOL environment and provides a basis for further helicon discharge experiments in SOL. These findings are important
to the ongoing helicon discharge experiments on a tokamak and the efficient power coupling of ICRH and LHCD for
future advanced tokamaks. Overall, the length, frequency, and shape of antenna is crucial for helicon current drive.

The propagation of high-frequency (HF) waves through inhomogeneous magnetized plasmas, to investigate energy absorption mechanisms, mode conversion, and the resulting stimulated electromagnetic emissions (SEEs) is studied, aiming to illuminate the underlying physical mechanisms. A Fully electromagnetic (EM) particle-in-cell (PIC) method is employed to simulate the interaction between a high-power left-hand circularly polarized HF wave and a magnetized plasma with a linearly increasing density gradient. Two conditions are considered: magnetic field aligned with the HF wave propagation direction and perpendicular to the HF wave propagation direction. The dynamics of HF wave-plasma interactions, the nature of mode conversion, excited stimulated modes, and the conditions that enhance or inhibit SEEs are studied. Parallel propagation to the magnetic field induces small amplitude plasma modes near the left-hand cutoff frequency. In perpendicular propagation into the magnetic field, the incident circularly polarized HF wave decomposes into two distinct polarization modes: the ordinary mode (O-mode) and the extraordinary mode (X-mode). The O-mode behaves as a linearly polarized EM wave and reflects at the ordinary wave cutoff frequency which leads to excitation of plasma modes. However, the X-mode propagating partly transverse and partly longitudinal through the plasma stops propagating at the right-hand cutoff frequency. At this point, it is absorbed and converted into strong electromagnetic and electrostatic modes, resulting in a substantial energy loss. The results indicate that considering an elliptically polarized HF wave instead of a circular one can lead to more efficient heating of plasmas. The generated linear and nonlinear plasma modes are investigated using Fast Fourier Transform (FFT) analysis. The suppression of SEEs at integer multiples of electron gyroharmonic frequencies observed in experiments is also investigated. It is shown that the resonance of the transmitted HF wave at the upper-hybrid frequency causes this suppression. These results have important implications for plasma diagnostics and heating experiments.

This computational study delves into the intricate interplay of alloying elements on the generation, recombination, and evolution of irradiation-induced defects. Molecular dynamics simulations were conducted for collision cascades at room temperature, spanning a range of primary knock-on atom energies from 1 to 10 keV. The investigation encompasses a series of model crystals, progressing from pure Ni to binary concentrated solid solution alloys (CSAs) such as NiFe₂₀, NiFe, NiCr₂₀, and NiFeCr₂₀ CSA. We observe that materials rich in Cr actively facilitate dislocation emissions and induce the nucleation of stacking fault tetrahedra in the proximity of nanovoids, due to Shockley partial interactions. This result is validated by molecular static simulations, which calculate the surface, vacancy, and defect formation energies. Among the various shapes considered, the spherical void proves to be the most stable, followed by the truncated octahedron and octahedron shapes. On the other hand, the tetrahedron cubic shape is identified as the most unstable, and stacking fault tetrahedra exhibit the highest formation energy. Notably, among the materials studied, NiCr₂₀ and NiFeCr₂₀ CSAs stood out as the sole alloys capable of manifesting this mechanism, mainly observed at high impact energies.

The problem of avoiding saturation of the coil currents is critical in large tokamaks with superconducting coils like ITER. Indeed, if the current limits are reached, a loss of control of the plasma may lead to a major disruption. Therefore, a current limit avoidance (CLA) system is essential to operate safely. This paper provides the first experimental evidence that the online solution of a constrained quadratic optimization problem can offer a valid methodology to implement a CLA. Experiments are carried out on the tokamak à configuration variable at the Swiss Plasma Center, showing the effectiveness of the proposed approach and its suitability for real-time application in view of future reactors such as ITER.

A new self-consistent 1D scrape-off layer model is recently developed in BOUT++ framework, named SD1D, which includes equations of various particle species (e.g. main plasma, neutrals and impurities) and couples open databases like ADAS and AMJUEL. It is able to fast and effectively simulate the divertor detachment experiments. In this work, a typical detachment experiment (shot #6270) on HL-3 with neon seeding is simulated using the SD1D code. It is found that the target electron temperature and the target ion saturation current in simulations are consistent with experimental results measured by Langmuir probes on the target plate. The variation of Dα radiation intensity in the divertor is qualitatively similar to the measured Dα signal. Following the experimental validations, different upstream densities are set in the simulations for studying impurity distribution in different plasma density conditions. It is found that increasing upstream density can be helpful for control of the neon radiation front (closer to the target).
This work also compares two detachment regimes in simulations. Based on the same initial experimental parameters (shot #6270) on HL-3, a scan of upstream density and a scan of neon seeding rate are carried out respectively. It is found that the role of atomic and molecular processes is different in the two detachment regimes. The current density roll-over is ascribed to a drop in the divertor ion source, and the variation of D_α radiation intensity via different excitation channels is associated to the relevant collisional reaction sources.


This study presents analysis of gyrokinetic simulations on the National Spherical Torus Experiment (NSTX) to investigate the effects of electromagnetic fields on plasma turbulence and transport. The simulations, performed with varying levels of fidelity using the gyrokinetic CGYRO code, include electrostatic (ES), single-field electromagnetic (EM1), and two-field electromagnetic (EM2) models. A detailed comparison across the simulation database reveals that electromagnetic effects increase both predicted growth rates and quasilinear fluxes, with EM2 simulations producing stronger turbulence than ES and EM1 cases. Quasilinear modeling using QLGYRO demonstrates that while the perturbed parallel magnetic field $(\delta B_\parallel$) does not drastically affect the total flux at experimental gradients, it leads to a shift in the dominant instability, altering mode structures from microtearing to kinetic ballooning modes (KBMs). The proximity of the plasma profiles to the KBM threshold is explored, with the experimental conditions being near the onset of KBM-driven transport. The KBM, with its large growth rates, is identified as a potential driver of electron temperature flattening, as it can rapidly transport heat across flux surfaces. Performing stability analysis shows core-localized unstable a low-$n_\mathrm{tor}$ mode that could contribute to the flattening at the early times of the discharge. TGYRO predictive modeling, incorporating both TGLF and QLGYRO, indicates that the inclusion of $\delta B_\parallel$ significantly improves the accuracy of temperature profile predictions in NSTX high-beta plasmas, although challenges remain in modeling the sharp flux discontinuities caused by KBM-driven instabilities.

With the increased urgency to design fusion pilot plants, fast optimization of electron cyclotron current drive (ECCD) launchers is paramount. Traditionally, this is done by coarsely sampling the 4D parameter space of possible launch conditions consisting of (1) the launch location (constrained to lie along the reactor vessel), (2) the launch frequency, (3) the toroidal launch angle, and (4) the poloidal launch angle. For each initial condition, a ray-tracing simulation is performed to evaluate the ECCD efficiency. Unfortunately, this approach often requires a large number of simulations (sometimes millions in extreme cases) to build up a dataset that adequately covers the plasma volume, which must then be repeated every time the design point changes. Here we adopt a different approach. Rather than launching rays from the plasma periphery and hoping for the best, we instead directly reconstruct the optimal ray for driving current at a given flux surface using a reduced physics model coupled with a commercial ray-tracing code. Repeating this throughout the plasma volume requires only hundreds of simulations, constituting a significant speedup. The new method is validated on two separate example tokamak profiles, and is shown to reliably drive localized current at the specified flux surface with the same optimal efficiency as obtained from the traditional approach.

A bright X-ray backlighter with a broad spectrum in the multi-keV range is essential for diagnostics of opacity and plasma states in high-energy-density physics. This paper experimentally investigates the X-ray conversion efficiency and spectra across the 1.6-4.0 keV range for three types of material targets irradiated by lasers: Au films, Au foams, and WBi mixture films. The multi-keV X-ray conversion efficiency of WBi mixture targets is approximately 34% and 17% higher than that of the Au film and Au foam targets respectively, and the spectral range of the X-ray backlight produced by the WBi mixture target is also extended. Consequently, a combination of Au, W, and Bi is proposed to generate a bright and broadband X-ray source that covers the entire 2.0 keV - 3.8 keV range, which will have extensive applications in X-ray absorption spectroscopy.

Mixed tungsten and cobalt oxides-based thin films were fabricated using plasma-enhanced chemical vapor deposition (PECVD) with CpCo(CO)₂ and W(CO)₆ as precursors, providing a reliable method for producing materials with potential applications in various fields. The influence of varying the partial pressure ratio of the precursors (feed gas ratio) during the co-deposition process, while maintaining a constant total partial pressure, on the growth rate, surface morphology, and atomic composition was systematically investigated using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results indicate that W(CO)₆ exhibits a lower polymerization ability than CpCo(CO)₂. The growth rate of the mixed CoO/WO₃ thin films corresponds to the combined contributions of each precursor under the applied plasma deposition parameters. Cauliflower-like globules in the films were attributed to the surface morphology of the calcined kanthal steel substrate. The atomic ratio of tungsten to cobalt in the films was slightly lower than in the precursor mixture, suggesting the preferential incorporation of cobalt into the film. Analysis of the [W]/[Co+W] and [C]/[Co+W] atomic ratios relative to the precursor feed gas ratio revealed the non-additive nature of the co-deposition process, highlighting the complex interactions between gas-phase species during the plasma deposition. These findings enable the fabrication of thin films with a predictable chemical structure in the plasma co-deposition process by adjusting the partial pressures of the precursors.

The gyrokinetic particle-in-cell code PICLS is a full-f finite element tool to simulate turbulence in open and closed field lines.
During the previous year, the capability of PICLS was extended to encompass electromagnetic effects. Successful tests using the method of manufactured solutions were conducted on the freshly added Ampère's-law-solver, and shear Alfvén waves were simulated to verify the new electromagnetic time step. 
However, as a code based on the p_||-formulation of the gyrokinetic equations, PICLS is affected by the Ampère-cancellation problem. In order to bring higher-beta simulations within reach of our computational capacity, we implemented the mixed-variable formulation with pullback-scheme in a similar fashion to, e.g., EUTERPE, ORB5, or XGC. Here, we present the successful verification of the different electromagnetic formulations of PICLS by simulating shear-Alfvén waves in a test setup designed to minimize kinetic effects.

A systematic study of the power threshold for L–H transition has been conducted in TCV. Only results from NB heated plasmas are reported. The dependencies of $\mathrm{\mathit{P}_{LH}}$ on plasma current, plasma density and main ion species have been investigated. For deuterium plasmas and at low $\mathrm{\mathit{q}_{95}}$, the L–H power threshold dependence on density is well described by the International Tokamak Physics Activities scaling law. Nevertheless at larger $\mathrm{\mathit{q}_{95}}$ values (reduced plasma current at constant toroidal magnetic field), the threshold deviates significantly from the scaling law and $\mathrm{\mathit{P}_{LH}}$ increases with $\mathrm{\mathit{q}_{95}}$. As a consequence, a strong correlation of power threshold with transport is reported suggesting that L–H transition is more favorable for L-mode plasmas with already good confinement.

Tearing modes (TM) are a significant challenge for the operation of large tokamak devices as they cause confinement degradation, can lead to a disruption and are part of the disruption process itself. Detecting and characterising tearing modes is basis for both, improved understanding of their evolution as well as adequate countermeasures. The most harmful tearing mode has the toroidal mode number n = 1, which, due to toroidal coupling, usually consists of different helicities, referred to as poloidal harmonics. Determining the poloidal harmonics requires modelling the magnetic measurements with different challenges for rotating and locked modes. Rotating modes produce frequency-dependent shielding currents in conducting structures that modify the perturbation field. Locked modes require pick-up coils measuring mainly the radial perturbation field component $B_\textrm{r}$, of which there are far fewer than Mirnov coils in ASDEX Upgrade. The agreement between $B_\textrm{r}$ coils and Mirnov coils in a model description can be validated in the low frequency range, where all coils observe the modes and shielding currents are important. We employ a three-dimensional finite element method model to calculate the expected magnetic measurements of the perturbation field produced by an n = 1 tearing mode with a single helicity and frequency. The TMs are represented by helical perturbation currents at the corresponding resonant surfaces while the rest of the plasma is treated as vacuum. We show that, besides the vacuum vessel and the passive stabilisation loop, it is crucial to include other in-vessel components of ASDEX Upgrade. The poloidal composition of an n = 1 TM is determined by the linear superposition of modelled TMs with single helicities that best matches the measurements by a poloidal array of Mirnov coils. The resulting amplitudes and phases of the poloidal harmonics give simulated measurements that are found to be in agreement with their measured values for all coil types.