Experimental fast-ion transport studies on the Mega-Amp Spherical Tokamak

Abstract

Nuclear fusion holds the promise of a sustainable means of electrical power generation. The technical challenge posed by controlled nuclear fusion however is formidable. One key aspect of research into magnetically-confined fusion plasmas is the study of the behaviour of fast ions. Produced by auxiliary heating systems including neutral beam injection (NBI) and ion cyclotron resonance heating, as well as by the fusion reactions themselves, the energies of these particles range from tens of keV to several MeV. Four fast-ion diagnostics on a medium-sized experimental fusion research device, namely the Mega-Amp Spherical Tokamak (MAST), have been used to investigate the transport of NBI-generated fast ions under the influence of various magnetohydrodynamic (MHD) instabilities. These include frequency-chirping fast-ion-driven modes known as toroidal Alfvn eigenmodes (TAE) and fishbones, as well as saturated internal kink modes and sawtooth reconnection events. The frequencies of these modes in the plasma frame ranged from 0 - 150 kHz in MAST. The effects of these modes on the fast ions have been investigated with the use of a fission chamber, a collimated neutron detector, a fast-ion deuterium alpha spectrometer and a charged fusion product detector. Data from each diagnostic are examined and compared for consistency in the presence of each type of instability. Fast-ion transport modelling is coupled with synthetic diagnostics to model the observed signals based on calculated fast-ion distributions. The data reveal a significant enhancement of fast-ion transport due to each of the MHD modes. It is found that the transport of fast ions in the presence of TAE and fishbones, averaged over the chirping mode cycle, is well-represented by assuming anomalous radial diffusion of these ions. A simple model for fast-ion mixing during sawtooth reconnection is found to reproduce partially the observations accompanying such events. The effects of the saturated internal kink mode cannot be modelled in such a simple way, and partial consistency with the measurements requires the use of tailored transport coefficients. Shortcomings and possible enhancements of the diagnostic capability are discussed in the light of these results.