Wave driven fast ion loss in the National Spherical Torus Experiment

Abstract

Spherical tokamaks have relatively low toroidal field which means that the fast-ion Larmor radius is relatively large ${\mathrm{(\rho }}_{\mathrm{fi}}{\text{>0.04\hspace{0.17em}a}}_p)$ and the fast ion velocity is much greater than the Alfvén speed ${\mathrm{(V}}_{\mathrm{fi}}{\text{>2\hspace{0.17em}V}}_{\mathrm{Alfvén}}\mathrm{).}$ This regime of large Larmor radius and low Alfvén speed is a regime in which fast ion driven instabilities are potentially virulent. It is therefore an important goal of the present proof-of-principle spherical tokamaks to evaluate the role of fast ion driven instabilities in fast ion confinement. This paper presents the first observations of fast ion losses in a spherical tokamak resulting from energetic particle driven modes. Two classes of instabilities are responsible for the losses. Multiple, simultaneously bursting modes in the toroidal Alfvén eigenmode frequency gap cause neutron drops of up to 15%. A bursting, chirping mode identified as precession and/or bounce resonance fishbone also causes significant neutron drops. Both modes are usually present when the losses are observed.