Nonlinear electron magnetohydrodynamics physics. V. Triggered whistler emissions

Laboratory experiments on whistler instabilities in the presence of small trigger waves have been performed. The instabilities arise from energizing electrons in magnetic null lines with time-varying magnetic fields. Such fields are created with loop antennas carrying large oscillating currents in the low-frequency whistler branch. X-type and O-type magnetic nulls are produced with electric fields along the toroidal separator. The magnetic field convects in the form of whistler spheromaks and whistler mirrors. Counterpropagating spheromaks merge and form field-reversed configurations (FRCs). Counterpropagating mirrors colliding with an FRC also energize electrons and produce high-frequency whistler emissions. The possibility that these emissions are triggered by incident waves from other null lines in the plasma has been investigated. A controlled experiment on triggered emissions where a test wave has been created with an independent antenna and propagated into the source region to investigate its amplification has also been performed. It is observed that the test wave does not grow but triggers a much larger instability in a spheromak. The enhanced emission has a different magnetic topology and a slightly different frequency from that of the test wave. Space-time measurements in the source region show both convective wave amplification occurs as well as an absolute instability in the current ring