Whistler spheromaks, instabilities and triggered emission experiments

Abstract

Whistler instabilities are observed in an EMHD spheromak in a large laboratory plasma. The spheromak slowly propagates along the ambient magnetic field, decays and converts its magnetic energy into electron kinetic energy. Anisotropies in the electron distribution function create magnetic oscillations below the electron cyclotron frequency. These are identified as whistler modes by measuring the frequency spectrum, wave field topology, polarization, helicity and propagation velocity. The instability is triggered by the transient formation of the spheromak but loses coherence in time. In order to investigate spatial and temporal growth a test whistler wave is propagated into the source region. The test wave does not grow but triggers a much larger instability amplitude. The triggered emission has a slightly different frequency from that of the test wave. The field topology of the triggered emission differs from that of the test wave. Space–time measurements in the source region show both convective wave amplification and an absolute instability in the current ring. These laboratory observations complement earlier studies of triggered whistler emissions in space plasmas.