Bhavin S. Patel

This paper presents gradient-driven global electromagnetic gyrokinetic simulations for a conceptual burning flat-top operating point of STEP (Tholerus et al 2024 Nucl. Fusion 64 106030), STEP-EC-HD, and investigates how non-local effects influence the nonlinear saturation and transport of the electromagnetic turbulence at finite β . Local gyrokinetic simulations have shown that including δ B ∥ is essential for the dominant hybrid kinetic ballooning modes, or hKBMs, to be unstable in STEP (Kennedy et al 2024 Nucl. Fusion 64 086049). Using the long-wavelength δ B ∥ solver (Sheffield et al 2024 Plasma Phys. Control. Fusion 67 015028) implemented in GENE (Jenko et al 2000 Phys. Plasmas 7 1904–10), this work demonstrates that the linear mode spectrum can be accurately captured in global geometry, which results in good agreement with an ensemble of local flux-tube simulations. The global framework reproduces the hKBMs identified in Kennedy et al (2023 Nucl. Fusion 63 126061), while microtearing modes remain challenging to resolve due to their shorter radial scales. Nonlinear simulations reveal clear evidence of an electromagnetic transition to states with extremely large heat fluxes, consistent with local simulations and with the predicted loss of zonal-flow regulation for this proposed operating point (Kennedy et al 2026 Nucl. Fusion ). These findings establish the capability of global gyrokinetics to capture finite- β dynamics in STEP-like plasmas and motivate future work to identify the conditions governing this transition.

Beam emission spectroscopy (BES) is used to diagnose ion-scale ( k ⊥ ρ i < 1 ) plasma turbulence in mega ampere spherical tokamak MAST Upgrade (MAST-U) by analysing the Doppler shifted D α light emitted from the on-axis neutral deuterium heating beam. These are the first results of ion-scale turbulence measured in MAST-U, and the first results from the upgraded BES diagnostic system which has an 8 × 8 array of avalanche photo-diodes in the R − z plane, increased from 4 poloidal channels in the old BES system on MAST, spaced ≈ 2 cm apart at up to 4 MHz sampling frequency. A series of three repeatable L-mode discharges are analysed, with the BES sampling different locations in each shot, to obtain measurements over almost the entire low-field side minor radius, 0.15 ⩽ Ψ N ⩽ 1.1 , giving radial profiles of key statistical quantities. The 2D coverage of the BES diagnostic ( R × z ≈ 13 × 15 cm) is exploited to give simultaneous measurements of radial and poloidal correlation lengths and wavenumbers, as well as tilt angles of turbulent eddies. Compared with the system’s predecessor, the increased poloidal coverage allows for significant improvements in the inference of poloidal quantities (correlation lengths and wavenumbers), as well as improved turbulence decorrelation time and poloidal velocity inferences. Local maxima are seen mid-radius ( R ≈ 1.3 m), which corresponds closely with a local maximum in the plasma rotation velocity, and a flattening of the electron density gradient. The apparent z -velocity from the BES is used in tandem with other plasma profiles (i.e. electron temperature and density, ion temperature, plasma rotation velocity, and magnetic field components) to infer the E × B velocity profile. These are compared with incomplete E × B velocity estimates from the charge exchange diagnostic. Some features of the velocity profiles are present in both inferences, giving good qualitative agreement, but the absolute values do not agree. As there is no poloidal velocity measurement from charge exchange in MAST-U, we assume it is small in our inference of the radial electric field, E r . For MAST plasmas, however, it has been shown that the poloidal flow contribution to E r can be large, and we suggest this may be the cause of the quantitative difference in our profiles.

Stationary safety factor q profiles with q min ≈ 1 and without sawtooth activity have been measured for the first time in MAST Upgrade (MAST-U) plasmas, indicative of the magnetic flux pumping mechanism. This anomalous current re-distribution phenomenon is observed in MAST-U plasmas with 2 / 1 tearing magneto-hydrodynamic (MHD) instabilities. Simulations using the ideal MHD code MISHKA indicate that q profiles, measured using the motional Stark effect (MSE) diagnostic, low magnetic shear are also ideally unstable to n = 1 , m = 1 MHD modes. Database analysis of MAST-U pulses demonstrates overlap in β N ≈ 2 in pulses which exhibit magnetic flux pumping and sawtooth activity. The flux pumping regime on MAST-U is observed within the operational space of q 95 > 6 and β p ≈ 0.6, when combined with both on and off-axis neutral beam injection (NBI) heating.

This work investigates the nonlinear transition to large heat fluxes observed in local gyrokinetic (GK) simulations of electromagnetic (EM) turbulence in STEP (Meyer 2024 Phil. Trans. R. Soc. 382 20230406). Using the stress-balance framework of (Zhang et al 2026 J. Plasma Phys. ) we confirm that the onset of extreme transport correlates with a critical value of q 2 β e , where q is the safety factor and β e is the ratio of electron thermal pressure to magnetic pressure, and relate this to a limit on the poloidal beta β pol . Crucially, this critical value lies below any relevant linear stability limit in the ( q , β e ) space (e.g. the onset of ideal or kinetic ballooning modes). Using an extensive set of nonlinear GK simulations, we demonstrate that the transition to large fluxes in STEP is governed by a balance between the electrostatic and magnetic-flutter stresses. We argue, and also show numerically, that larger-major-radius tokamaks reach the EM non-zonal regime at lower β e , making this MHD-controlled saturation limit more accessible in reactor-scale devices than in small spherical tokamaks. We also demonstrate that access to a second-stable regime enables re-saturation at larger values of β ′ . We further show that the ideal ballooning mode (IBM) threshold serves as a useful proxy for delineating this second-stable region and also tracks the onset of large fluxes. These results provide a predictive framework for identifying no-go zone predictions from local GKs and offer new insight into the EM saturation physics relevant to STEP and other high- β e devices.

The ability to capture the isotope mass scaling of core confinement seen in experiment is validated with JETTO-TGLF for low power JET-ILW discharges across H, D and T. The cases analysed include Ohmic discharges spanning the linear and saturated Ohmic confinement regimes as well as a trio of L-modes. The TGLF saturation rules SAT1-SAT3 are seen to predict a similar isotope scaling across both the ITG- and TEM-dominated discharges simulated, despite for the latter case the inclusion of the TEM branch of SAT3. The models demonstrate good agreement with experiment for the scaling between D and T plasmas, however a discrepancy is observed for H in the ITG-dominated discharges of higher density, as well as a systematic overprediction of the confinement time on the order of 20% in most cases. A retuned version of the SAT3 model, which was fit to better recreate fluxes close to the transport threshold, is seen to improve the magnitude of confinement predictions across all shots owing to an increased transport stiffness. This retuning was not seen to influence the confinement isotope scaling however, and possible transport mechanisms responsible for the continued discrepancy of higher density Ohmic and L-mode discharges in H are discussed.

The Korea Superconducting Tokamak Advanced Research (KSTAR) device has recently entered a new phase of operation following the installation of a tungsten mono-block divertor, providing a unique platform to investigate high-performance, long-pulse plasma scenarios for future reactors. The 2023–2024 experimental campaigns demonstrated significant progress in commissioning the tungsten divertor, developing plasma operation scenarios, and achieving real-time control. Comparative studies revealed increased core radiation losses and associated performance degradation relative to the carbon divertor environment. Alleviation strategies, such as optimized neutral beam injection timing, boron powder seeding, and impurity control through gas puffing, were shown to reduce tungsten accumulation and extend high-performance phases, including record H-mode operation exceeding 100 s. International collaboration enabled the first demonstration of high poloidal beta scenarios with an internal transport barrier on KSTAR. Advances in control included upgraded real-time resonant magnetic perturbation schemes and disruption forecasting via the DECAF system. Furthermore, divertor detachment control using a new surrogate-model-based approach with real-time radiation imaging demonstrated active impurity and heat flux management. Supported by enhanced heating, current drive, and diagnostic systems, these achievements offer valuable insights into tungsten impurity behavior, transport physics, and control strategies for future reactors including ITER. Planned upgrades toward a full-tungsten wall and ITER-aligned real-time plasma control will further establish KSTAR as a leading experimental platform for developing operational scenarios required to achieve steady-state, burning plasma conditions.

The research programme performed on the Mega Amp Spherical Tokamak (MAST) Upgrade experiment has made significant advances in developing the physics understanding of low aspect ratio tokamaks in support of the operation of ITER and design of fusion powerplants. High performance plasma scenarios have been developed to facilitate a broad programme of experiments, in which confinement is constrained by the presence of m / n = 2/1 modes that cause substantial losses of fast ions. The onset of these modes coincides with the q = 2 surface residing in a local minimum in the toroidal current density profile. The maximum electron temperature at the pedestal top, T _e,ped is limited with gas fuelling to ∼350 eV to maintain regular ELMs; higher T _e,ped results in a transition to a non-stationary ELM-free regime. The operational space of spherical tokamaks has been expanded into small and ELM-free regimes. Strong shaping of the last closed flux surface can induce a transition from large to small ELMs, and ELM suppression with resonant magnetic perturbations has been observed for the first time in a low aspect ratio tokamak. Negative triangularity shaping has induced a transition from ELMy H-mode to a high-performance L-mode regime for the first time in a low aspect ratio tokamak. In studies of fast ion confinement, losses of fast particles due to Global Alfvén Eigenmodes have been identified. Interactions between fast ions generated by off-axis neutral beam injection and thermal neutrals can result in significant losses of fast ions. Experiments with on- and off-axis neutral beam injection exhibit a flux pumping mechanism, where the central safety factor is held to ∼1 in the absence of sawteeth. In studies of pedestal physics, it has been found that elevated main chamber neutral pressures result in an increase in the electron density and reduction in the temperature at the pedestal top. Advances in understanding plasma exhaust include the integration of a high-performance plasma core with detached outer divertors in the X -point target configuration. A newly commissioned lower divertor cryopump reduces the lower divertor neutral pressure by up to 50%, with minimal effect on the main chamber or upper divertor. New measurements and SOLPS-ITER simulations emphasise the importance of plasma–neutral interactions on divertor detachment in the conditions accessible in experiments. Real-time control of the ionisation front location in both divertor chambers independently has been demonstrated in double null experiments, enabled by the tightly baffled divertor chambers.

Strong E × B plasma flow shear is beneficial for reducing turbulent transport. However, traditional methods of driving flow shear do not scale well to large devices such as future fusion power plants. In this paper, we use a large number of nonlinear gyrokinetic simulations to study a novel approach to increase flow shear: decreasing the momentum diffusivity to make the plasma ‘easier to push’. We first use an idealized circular geometry and find that one can obtain low momentum diffusivity (LMD) at tight aspect ratio, low safety factor, high magnetic shear and low temperature gradient. This is the so-called LMD regime. To drive intrinsic momentum flux, we then tilt the flux surface, making it up–down asymmetric. In the LMD regime, this intrinsic momentum flux drives strong flow shear that can significantly reduce the heat flux and increase the critical temperature gradient. We also consider the actual experimental geometry of the Mega Ampere Spherical Tokamak (MAST) to illustrate that this strategy can be practical and create experimentally significant flow shear. Lastly, a preliminary prediction for the SMall Aspect Ratio Tokamak (SMART) is made.

The National Spherical Torus Experiment Upgrade (NSTX-U) is preparing to resume operation, representing a crucial step toward realizing compact, cost-effective fusion pilot plants (FPPs). In advance of this, extensive modeling and data analysis have been conducted to advance the physics basis for low-aspect-ratio, high-performance plasma regimes, focusing on three core objectives: confinement and stability, power and particle handling, and steady-state operation. Significant progress has been made in understanding the electron temperature flattening in high-β plasmas, which is shown to be driven by a complex interplay of MHD instabilities (e.g., non-resonant infernal modes), fast-ion-driven Alfvén eigenmodes, and electron and ion-scale micro-instabilities, particularly Kinetic Ballooning Modes (KBMs), whose destabilization is strongly dependent on parallel magnetic field fluctuations (δB _‖ ). Furthermore, a new Gyrokinetic Critical Pedestal model was developed, accurately predicting pedestal structure by identifying KBMs as the primary stability limit, offering a critical constraint for future high-confinement scenarios. To address the challenge of high heat flux, novel liquid lithium plasma-facing components were modeled. The analysis confirmed that lithium vapor shielding is a self-regulating mechanism for heat mitigation, while also emphasizing that strong main ion parallel flow is essential to minimize core lithium contamination. Finally, progress toward steady-state operation was anchored by developing the required physics basis and control tools. This includes predictive modeling for Reversed Magnetic Shear sustainment, demonstrating that magnetic island-induced bootstrap current reduction is negligible in STs, and advancing real-time control and disruption avoidance capabilities. The development of high-speed surrogate models (e.g., MMMNet) provides computationally efficient tools vital for non-inductive scenario optimization and integrated, low-disruptivity operations planned for NSTX-U.

Based on the analysis of a large number of high-fidelity nonlinear gyrokinetic simulations, we propose a novel strategy to improve confinement in spherical tokamak plasmas by combining up-down asymmetric flux surface shaping with the Low Momentum Diffusivity (LMD) regime. We show that the intrinsic momentum flux driven by up-down asymmetry creates strong flow shear in the LMD regime that can significantly reduce energy transport, increasing the critical gradient by up to 25%. In contrast to traditional methods for generating flow shear, such as neutral beam injection, this approach requires no external momentum source and is expected to scale well to large fusion devices. The experimental applicability of this strategy in spherical tokamaks is addressed via simulations by considering actual equilibria from Mega Ampere Spherical Tokamak and a preliminary equilibrium from SMART.