Theoretical astrophysics and plasma physics at RPC

Abstract:

We present evidence of tearing mode seeding by nonlinear three-wave coupling of Alfvén eigenmodes in a tokamak plasma. Here, a m,n = 2,1 magnetic island appears to be seeded in a MAST-U discharge by coupling of (Formula presented) (Formula presented) and (Formula presented) (Formula presented) Alfvén eigenmodes, which is demonstrated by increased squared normalised bicoherence calculated from magnetic probes preceding tearing mode onset. This observation offers a new explanation for tearing modes that would be otherwise addressed as spontaneous, and indicates three-wave coupling can seed 2,1 magnetic islands in plasmas without pre-existing (Formula presented) (Formula presented) tearing modes. Further, this mechanism has the potential to be more prevalent in burning plasmas expected in future fusion devices.

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:

The adiabatic equation of state upper P proportional to n Superscript upper Gamma $P\propto n^{\Gamma }$ $P \propto n^{\varGamma }$ describes the pressure evolution of highly collisional, isotropic plasmas in terms of their density, providing a possible closure of the fluid moment hierarchy in the absence of heat fluxes and dissipation. An analogous closure exists for collisionless, magnetised plasmas, whose pressure tensor is anisotropic with respect to the magnetic field, and the closure is therefore double adiabatic, prescribing the evolution of the parallel and perpendicular pressures in terms of the magnetic-field strength and density. Here, we present a general first-principles formalism to derive adiabatic laws using symmetries and phase-space conservation properties of the distribution function. With this theory we recover the adiabatic equation of state upper P proportional to n Superscript upper Gamma $P\propto n^{\Gamma }$ $P \propto n^{\varGamma }$ for isotropic plasmas and the double-adiabatic equations of state for collisionless, magnetised plasmas. We extend the latter to the relativistic regime, finding that their exact functional form depends on the pressure anisotropy and is not a simple power law. Our double-adiabatic equations of state describe simple geometries, like magnetic mirrors or compressed homogeneous plasmas, as well as complex high-energy astrophysical processes, such as the evolution of plasmoid structures formed during magnetic reconnection.

Abstract:

Abstract Gyrokinetic simulations of tokamak turbulence indicate that fluctuation levels increase abruptly and dramatically when the plasma beta exceeds a certain critical value. This increase in fluctuation levels coincides with a transition from a state dominated by zonal flow to one in which turbulent eddies form radially-elongated 'streamers'. Here we derive from gyrokinetics a minimal fluid model for electromagnetic ion-temperature-gradient (ITG) turbulence that captures the key features of this transition. Due to the relative simplicity of the model, we are able to conduct a detailed numerical study of the interplay between the turbulence and the zonal flow across a broad range of values of the plasma beta and the ITG. We find that the transition occurs when the Reynolds stress, which tends to strengthen zonal flows, is overwhelmed by the Maxwell and diamagnetic stresses, which tend to weaken them. Power-law scalings of the stress ratios with plasma beta and ITG are obtained, indicating a possible means by which the location of the transition could be predicted with minimal computational cost.