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:

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 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.

Abstract:

Abstract Magnetic reconnection jets in Earth's magnetotail are regions of electron heating and acceleration. They are often associated with dipolarization fronts (DFs), which are characterized by sharp increases in the northward magnetic field component . Electrons can be accelerated at DFs mainly through the betatron and Fermi mechanisms. We perform a statistical study of betatron and Fermi electron rate of energy change at Earthward‐propagating DFs observed by the Magnetospheric Multiscale (MMS) spacecraft. Our results show that betatron acceleration is dominant ahead of DFs, where suprathermal electron fluxes perpendicular to the magnetic field are detected. Up to ion inertial lengths inside the outflow, Fermi acceleration acts on the low‐density electron population, potentially driving them to high energies (∼100 ). Spatially in the magnetotail, betatron acceleration is statistically larger near the most probable X‐line location, while Fermi‐dominated events are observed closer to the Earth.

Abstract:

Ground-based gravitational-wave (GW) observatories have detected approximately 200 binary black hole (BH) mergers. The astrophysical origin of these events is debated, with evidence suggesting that at least a subset originated from dynamic environments characterized by frequent close encounters. Accretion disks in active galactic nuclei (AGNs) are of particular interest, as certain observed features could be more readily produced within such environments. In this paper, we investigate the expected properties of mergers in these environments, and their dependence on various parameters, using 1D N-body simulations combined with a comprehensive semianalytical model. In our fiducial model, the distributions of masses (m1 and m2) and mass ratios (q ≡ m2/m1 ≤ 1) are similar to those observed. However, they depend strongly on the lifetime and density of the AGN disk and on the number and accretion efficiency of BHs, with higher masses predicted as these quantities increase. The most massive mergers, such as GW231123, can be produced either by efficient gas accretion or by hierarchical mergers among ≥3 generations of BHs. The observed negative correlation between q and the average effective spin (χeff), along with the positive correlation between χeff and the chirp mass (Mchirp), can be explained by a combination of efficient gas accretion, which promotes spin alignment, and hierarchical mergers, which produce high-∣χeff∣ and low-q binaries. Hierarchical mergers can also explain the negative correlation between q and the dispersion of χeff, as well as the positive correlation between ∣χeff∣and Mchirp. We present a comprehensive study on how the expected distribution of each of these quantities depends on model parameters and assumptions, which will aid the interpretation of observed GW population properties.

Abstract:

Abstract We study the influence of fuzzy-dark-matter cores on the orbits of stars at the Galactic centre. This dark matter candidate condenses into dense, solitonic cores, and, if a super-massive black hole is present at the centre of such a core, its central part forms a ‘gravitational atom’. Here, we calculate the atom’s contribution to the gravitational potential felt by a Galactic-centre star, for a general state of the atom. We study the angular-momentum dynamics this potential induces, and show that it is similar to vector resonant relaxation. Its influence is found to be potentially sufficiently strong that such a dynamical component should be accounted for in Galactic-centre modelling. For the Milky Way, the atom is expected to have some spherical asymmetry, and we use this to derive a stability condition for the disc of young, massive stars at the Galactic centre—if the atom’s mass is too large, then the disc would be destroyed. Thus, the existence of this disc constrains the mass of the particles comprising the solitonic core. We study an example model of the core, where all of the rotation of the core’s inner region is assumed to come from an l = 1 state, and its amplitude is determined by the halo’s spin parameter; such a core is found to be in tension with the stability of the clockwise stellar disc for 4.2 × 10−20 eV ≤ ma ≤ 5.4 × 10−20 eV at 2σ. Other core models could vary the constrained values of ma. These constraints will tighten significantly with future, improved data.