Measurement of the global structure of interchange modes driven by energetic electrons trapped in a magnetic dipole

Abstract

Measurements of the radial, azimuthal, and field-aligned mode structures of interchange instabilities excited by energetic electrons confined by a magnetic dipole are presented. The mode structures are determined using a correlation analysis of movable high-impedance floating potential probes located at various positions within the plasma. The hot electron population, produced by electron cyclotron resonance heating, becomes unstable to hot electron interchange (HEI) instabilities which saturate nonlinearly with a complex and time-varying frequency spectrum. Although the mode frequencies vary dramatically, it is found that the mode structures do not evolve significantly in time, being determined by the azimuthal mode numbers. These measurements are compared to a self-consistent nonlinear particle simulation of the HEI mode in dipole geometry. Upon appropriate adjustment of the boundary conditions, the simulation reproduces the measured radial and azimuthal structures at large amplitudes.