Whistler mode wave generation in a laboratory plasma
Thursday, 18 December 2014
The dynamic variability of Earth’s outer radiation, over a time scale of a day, is largely controlled by whistler mode chorus waves as demonstrated by satellite observations and numerical simulations. To better understand whistler mode wave generation in a controlled laboratory environment, we perform an experiment in the Large Plasma Device at UCLA. A broadband whistler-mode spectrum is generated by injecting a gyrating electron beam in a uniformly magnetized plasma. The mono-energetic electron beam is relaxed first in the parallel direction by rapidly growing electrostatic waves and collisions with background particles. After creating velocity spread in the parallel direction, the whistler mode wave grows due to the negative gradient of the velocity distribution function in the parallel direction (dF/dv//<0). We show that the unstable frequency range varies as either the beam tilt angle changes or the background electron density changes, which agrees well with the predictions of linear growth theory. The results reported are part of a long-term project to understand the physical process responsible for the generation of whistler-mode waves and their effects on energetic particles.