Energetic particles in spherical tokamak plasmas

Energetic particles in spherical tokamak plasmas

Energetic particles in spherical tokamak plasmas 150 150 UKAEA Opendata
CCFE-PR(16)62

Energetic particles in spherical tokamak plasmas

Spherical tokamaks (STs) typically have lower magnetic fields than conventional tokamaks, but similar mass densities. Suprathermal ions with relatively modest energies, in particular beam-injected ions, consequently have speeds close to or exceeding the Alfven velocity, and can therefore excite a range of Alfvenic instabilities. STs heated with neutral beams, such as the Mega Amp Spherical Tokamak (MAST) and the National Spherical Torus Experiment (NSTX), have thus provided an opportunity to study toroidal Alfven eigenmodes (TAEs), together with higher frequency global Alfven eigenmodes (GAEs) and compressional Alfven eigenmodes (CAEs), which could affect beam current drive and channel fast ion energy into bulk ions in future devices. In NSTX GAEs were correlated with a degradation of core electron energy confinement. In MAST pulses with reduced magnetic field, CAEs were excited across a wide range of frequencies, extending to the ion cyclotron range, but were suppressed when hydrogen was introduced to the deuterium plasma, apparently due to mode conversion at ion-ion hybrid resonances. At lower frequencies fishbone instabilities caused fast particle redistribution in some MAST and NSTX pulses, but this could be avoided by moving the neutral beam line away from the magnetic axis or by operating the plasma at either high density or elevated safety factor. Fast ion redistribution has been observed during GAE avalanches on NSTX, while in both NSTX and MAST fast ions were transported by saturated kink modes with toroidal mode number n = 1, sawtooth crashes, resonant magnetic perturbations and TAEs. High energy charged fusion products are unconfined in present-day STs, but have been shown in MAST to provide a useful diagnostic of beam ion behaviour, supplementing the information provided by neutron detectors. In MAST electrons were accelerated to highly suprathermal energies as a result of edge localised modes, while in both MAST and NSTX ions were accelerated due to internal reconnection events. Ion acceleration has also been observed during merging-compression start-up. Finite Larmor radius effects often cannot be neglected in the theoretical description of fast ions in STs due to the relatively low magnetic field. Also the restriction of many kinetic codes to modes in the sub-cyclotron range means that they cannot be used to model the highest frequency CAEs observed in MAST and NSTX. The resulting severe demands placed on the modelling of energetic particles in existing STs will apply also to future ST reactors, since fusion alpha-particles in such devices will have orbits similar to those of beam ions in MAST and NSTX.

Collection:
Journals
Journal:
Plasma Physics and Controlled Fusion
Publisher:
IOP
Published date:
21/03/2017