Prof Michael Barnes

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:

Strongly driven ion-scale turbulence in tokamak plasmas is shown to be regulated by a new propagating zonal flow mode, the toroidal secondary mode, which is nonlinearly supported by the turbulence. The mode grows and propagates due to the combined effects of zonal flow shearing and advection by the magnetic drift. Above a threshold in the turbulence level, small-scale toroidal secondary modes become unstable and shear apart turbulent eddies, forcing the turbulence level to remain near the threshold. This threshold condition is used to derive scaling laws for the turbulent heat flux, fluctuation spectra, and zonal flow amplitude, which are validated in nonlinear gyrokinetic simulations and explain previous experimental observations.

Abstract:

The ITER Neutral Beam Injection (NBI) system is based on negative ions, produced in an RF-driven plasma source. The ITER NBI lines must deliver a current density of 230 A/m2 of negative hydrogen ions, accelerated to 870 keV, or a current density of 200 A/m2 of negative deuterium ions accelerated to 1 MeV. NBI systems, based on negative ions, are compromised by a process known as stripping losses, in which negative ions are neutralized in the grid system before achieving full energy. For a source filling pressure of p fill = 0.3 Pa, 29% of the extracted H -(D -) ions are predicted to be lost by stripping in the ITER full-scale NBIs system (7 grid acceleration system). To compensate for these stripping losses, a larger amount of negative ions has to be extracted from the source (329 A/m2 in hydrogen and 286 A/m2 in deuterium). The ELISE test facility is based on a 1/2-size ITER source. It extracts H -(D -) ions using a 3-grid acceleration system, with a maximum extraction voltage of 10 kV and acceleration voltage of 50 kV is achieved. In a 3-grid acceleration system, 10% of stripping losses is predicted for both isotopes. This contribution focuses on experimental measurements of stripping losses at ELISE. Experimentally, stripping losses are monitored using Beam Emission Spectroscopy (BES), which analyzes the Doppler-shifted spectrum of the Balmer Hα (Dα ). To not underestimate the number of stripping losses the full area between the unshifted Peak background (H 2 dissociation and excitation) and the Doppler Peak (fully-accelerated beam particles excitation) needs to be considered. However, the influence of BES background and signal-to-noise ratio (SNR) can affect the calculation of stripping losses, mainly for hydrogen measurements at low filling pressures (< 0.4 Pa). To accurately predict the value of the stripping losses, only signals with high-enought SNR should be used. When this effect is considered, no differences between hydrogen and deuterium are found in terms of stripping losses. For a filling pressure of 0.3 Pa, a stripping fraction of 6.0±0.8% was found for hydrogen and 6.2±0.7% for deuterium. A systematic comparison of the stripping losses between hydrogen and deuterium under various experimental conditions is presented.