Abstract Guiding-centre (GC) and full-orbit (FO) simulations of the beam-injected fast ion distribution and the corresponding neutron emissivity have been carried out for magnetohydrodynamics-quiescent NSTX plasmas, using a combination of ASCOT5 and DRESS, to assess the suitability of the GC approximation for fast ions in NSTX. It was found that GC and FO simulations predicted substantially different steady-state distributions in both position and velocity space and different neutron emissivity profiles, leading to a 15% reduction in the predicted global neutron rate for FO relative to GC. These changes accompany a higher magnetic moment in FO, and correspond to a change in particle orbits from co-passing to trapped and stagnation orbits. ASCOT5 was also benchmarked against TRANSP/NUBEAM with input loaded entirely from TRANSP/NUBEAM output files, with agreement found between the GC simulations when finite Larmor radius (FLR) corrections were omitted. ASCOT5 FO and TRANSP/NUBEAM with FLR produced fast ion distributions which differed in localised regions, but predicted global neutron rates which agree within 3%.

Abstract Operation with beam heating is prone to central accumulation of high-Z impurities in present-day tokamaks due to a dominant neoclassical pinch. Fusion reactors are expected to have a different core transport regime for high-Z impurities, where turbulent transport completely dominates in both diffusion and convection, causing flat impurity profiles. In this work, we present integrated simulations of plasmas that approach this regime of less dominant neoclassical and stronger turbulent high-Z impurity transport in three tokamaks: ASDEX Upgrade, JET and Alcator C-Mod. The modelling is first validated against a suite of diagnostics measuring both the main plasma and the impurity profiles. The high-Z impurity densities are found to be flat in both experiments and simulations. The impurity transport coefficients as calculated by theory-based turbulent and neoclassical transport models are then analyzed, showing comparable or even dominant turbulent components instead of the more typical dominant neoclassical high-Z impurity convection. The implications for a reactor are discussed.

Abstract Gyrotrons having a high output-frequency of more than 200 GHz will be necessary for future fusion devices, such as the DEMO reactor. At 170 GHz, gyrotrons for ITER achieved a high power of 1 MW with a high electrical efficiency of approximately 50% (oscillation efficiency of approximately 30%) in a long pulse operation. At frequencies higher than 200 GHz, no gyrotrons have achieved ITER-like performance, which this study attempted to demonstrate using a prototype multi-frequency gyrotron for 170 GHz, 203 GHz, and 236 GHz. At 203 GHz, an output power of 0.87 MW was achieved at the gyrotron window with an oscillation efficiency of 25% and an electrical efficiency of 41% in a long pulse operation of 100 s. Although this performance is lower than that of the ITER gyrotrons at 170 GHz, this study represents the first demonstration that ITER-like performance is achievable at frequencies above 200 GHz. In addition, short 1-ms pulse tests at 236 GHz achieved an output power of 1 MW.

Abstract Self-emission X-ray imaging is a key diagnostic in inertial confinement fusion (ICF), yet the recorded images are heavily degraded by shot noise arising from photon-counting statistics and scintillator blur, as well as impulsive noise induced by neutron interactions with silicon-based detectors. The complexity of these noise sources, together with the limited availability of experimental data, poses significant challenges for both physical interpretation and data-driven denoising.
To address this problem, we propose a hybrid physics-informed and generative framework that enables realistic data synthesis and robust denoising of self-emission X-ray images from ICF experiments. Hot-spot signals are modeled using Legendre polynomial expansions, while shot and impulsive noise are synthesized from empirically measured distributions. A generative enhancement module further reduces the domain gap between synthetic and experimental data, resulting in more realistic training inputs and improved denoising performance.
Self-emission X-ray images from experiments conducted on SG-180kJ laser facility demonstrate the effectiveness of the proposed approach. The synthesized data achieve PSNR values approaching 50,dB, and the denoising results improve the SNR of real experimental images from approximately $-30$,dB to $+35$,dB.

Abstract The goal of the ST40 programme is to explore the physics of high-field spherical tokamaks (STs), to validate empirical and theoretical models and, hence, to build confidence in predictions required to support the design of future generations of STs. ST40 is a compact high-field ST that has achieved the following parameters: R0 = 0.4–0.55 m, Ip = 0.20–0.85 MA, Bt(R = 0.4 m) = 0.7–2.1 T, κ ≤ 1.9, and A = 1.6–1.9. Highlights of recent experimental results include (i) H-mode and confinement studies at Bt ≤ 2.1 T, (ii) observation of bifurcation of the scrape-off-layer power fall-off width, λq, into a ‘wide’ branch that follows existing H-mode scalings and a ‘narrow’ branch that exhibits λq values that are up to 10 times lower than the predictions of established scalings, (iii) development of high-performance scenarios with plasma current, Ip, up to 0.85 MA, (iv) development of highly non-inductive scenarios with high βp , and (v) the first ST40 experiments utilising the newly commissioned impurity powder dropper. The work on all these topics has been supported by a number of advancements in ST40 hardware and software, from plasma control to data analysis and interpretation. At the end of 2025, ST40 embarked on a major upgrade to further expand its capabilities by introducing, among other improvements, all-metal plasma-facing components, 1MW of electron cyclotron heating, a pellet injector, and a pair of lithium evaporators for wall conditioning.

Abstract We present the first gyrokinetic simulations of multiscale turbulence in a stellarator, using the magnetic geometry of Wendelstein 7-X (W7-X) and experimentally relevant parameters. A broad range of scenarios is explored, including regimes where electron-temperature-gradient (ETG) turbulence coexists with varying levels of ion-temperature-gradient (ITG) turbulence, as well as cases involving microtearing modes (MTMs) relevant to high- β and reactor-like conditions. Notably, while ETG turbulence does not form radial streamers as in tokamaks, it can still drive significant transport and interact with ion-scale turbulence. In electrostatic ITG-dominated regimes, electron-scale fluctuations erode zonal flows, enhancing ion-scale transport, while ion-scale turbulence suppresses ETG activity. In contrast, under electromagnetic MTM conditions, the isotropic nature of ETG turbulence limits its suppressive effect, allowing MTMs to persist. These findings underscore the critical role of cross-scale effects for accurate transport predictions in W7-X and future stellarators.

Abstract Recent experiments reported a correlation between power law core temperature spectra and D α emission, suggesting that heat avalanches penetrate the SOL. This paper derives a threshold criterion for avalanche penetration using a reduced model. Avalanches with ( ∇ T ~ ) rms > ∇ T ~ crit at the separatrix are predicted to penetrate, and so broaden the SOL and heat load distribution. ∇ T ~ crit is ∼ 1 / τ ∥ , where τ ∥ is the parallel heat flow time through the SOL. Penetration occurs when avalanches are strong enough to steepen sufficiently to shock at the separatrix. A positive correlation is found between the nonlinear drive for steepening and the penetration depth. In particular, penetration depth exceeds that of the heuristic drift limit when shocks form. Implications for numerical and physical experiments are also discussed.