Fast Particles

Fast particles are those particles in a plasma with speeds that exceed the thermal speed. They are usually generated from plasma heating systems such as neutral beam injection (NBI) and radio-frequency waves: electron and ion cyclotron resonance heating (ECRH and ICRH) are two examples. A major source of energetic particles in a fusion device such as ITER is the alpha particles produced from the deuterium-tritium (D-T) fusion reactions. Understanding the effects of the fast particle population is therefore very important for ITER and fusion energy in general.

In addition to the effects on plasma performance, the plasma dynamics may be qualitatively altered by the presence of fast particles. Certain plasma waves can be driven unstable by energetic particles: the Toroidal Alfven Eigenmode (TAE) is one such example. In addition new modes of oscillation can also be generated, such as the energetic particle mode (EPM). We are interested in both classes of instability as they could lead to a premature loss of energetic particles before they are able to heat the plasma, possibly causing damage to the tokamak structure.

Further motivation to study fast particles comes with the observation that particular fast particle events, such as the experimentally witnessed frequency chirping regime, may provide particle-wave energy transfer mechanisms, which in turn could lead to improved confinement in tokamak plasmas. A final motivation for studying fast particle modes is that their properties can reveal information about the plasma and therefore serve as a valuable diagnostic tool.

The study of fast particles in York consists of both experimental observation and theory. Experimental data is analysed from the mega amp spherical tokamak (MAST). One aspect of our theory work compares experimentally observed fast particle phenomena with the Berk-Breizman model of the modified Vlasov equation. This involves large scale computational modelling, combined with careful interpretation of MAST data. There is qualitative agreement between the predictions of theory and the observations on MAST: our future work will aim to put these encouraging initial comparisons on a firmer basis.

A set of software tools to study fast particles is continually being developed at York. Data analysis techniques are important: there are over 3000 signals produced for each MAST discharge. Our particular expertise in York is the analysis of data from the OMAHA diagnostic: an array of approximately 30 coils, measuring magnetic field perturbations close to the plasma edge. To facilitate this data analysis, we have a dedicated room with two 6-screen displays. This will be upgraded to allow remote participation in MAST experiments from York in the future.