Theoretical astrophysics and plasma physics at RPC

Abstract:

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, Te,ped is limited with gas fuelling to ∼350 eV to maintain regular ELMs; higher Te,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.

Abstract:

Negative ion based neutral beam injection (NBI) systems will be used for heating and current drive at the ITER tokamak and require stringent beam quality. Including a beamlet core divergence of less than 7 mrad and high uniformity across individual beamlets (better than 90%), at extracted current densities of 329 A/m 2 for hydrogen and 286 A/m 2 for deuterium. The BATMAN Upgrade (BUG) test facility, a 1/8 scale ITER source, is used to investigate and optimize these beam properties. Beamlet divergence and uniformity at BUG test facility are measured using a retractable Carbon Fiber Composite (CFC) calorimeter. However, under standard operation (with all 70 apertures open), strong beamlet overlap prevents the extraction of single-beamlet divergences from the CFC data. To overcome this limitation, a checkerboard masking configuration was implemented. This masking, combined with a multi-Gaussian model allows to obtain beamlet properties, including size, shape, and intensity along the whole extraction area. The experimental methodology and data analysis techniques are presented in detail. Their capabilities are showcased by presenting measurements performed at BUG under different configurations of the magnetic filter field (drift-up and drift-down conditions) and for both hydrogen and deuterium operation.

Abstract:

Multiscale temperature fluctuations are abundant in the intracluster medium (ICM) outside of galaxy cluster cores (∼100 kpc). Their origin is often attributed to turbulent stirring by subhalos or accreting baryons crossing the virial radius. However, their apparent resistance to mixing and thermal conduction in a collisional medium has not been explained. We propose a new mechanism by which steady-state temperature fluctuations can form and persist outside the cluster core. Local thermal instability, or Field instability, is used to explain filamentary condensates in cluster cores but is usually dismissed outside them because thermal conduction should suppress instability. In weakly collisional or collisionless plasmas, however, thermal conduction can be anomalously suppressed by heat-flux-driven plasma instabilities triggered in the presence of a local magnetic field, leading to two effects: (i) condensates form in a new parameter regime that overlaps with conditions outside the core, and (ii) condensates reach a steady state as in the hydrodynamic limit. This extends the regime of instability-driven fluctuations to over ≳50% (depending on hot plasma temperature) of the cluster. We use one-dimensional hydrodynamic simulations of condensates to test our analytical ideas.

Abstract:

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 q2βe, where q is the safety factor and βe is the ratio of electron thermal pressure to magnetic pressure, and relate this to a limit on the poloidal beta βpol. Crucially, this critical value lies below any relevant linear stability limit in the (q,βe) 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 βe, 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- βe devices.