"Laboratory studies of avalanches in magnetized plasmas," by Bart Van Compernolle (UCLA) and "Excitation of chirping whistler waves in a laboratory plasma," by Xin An (UCLA)

Tuesday, November 10, 2015 - 11:00am to 12:00pm
Plasma Seminars

Plasma Seminar

Physics & Astronomy Building (PAB) Room 4-330
Tuesday, November 10, 2015

Joint Speakers: Bart Van Compernolle (UCLA) and Xin An (UCLA)

Bart Van Compernolle (UCLA) 
Talk Title: "Laboratory studies of avalanches in magnetized plasmas"
Results of a basic heat transport experiment involving an off-axis heat source are presented. A ring-shaped electron beam source injects low energy electrons (below ionization energy) along a strong magnetic field into a preexisting, large and cold plasma resulting in a long, hollow, cylindrical region of elevated electron pressure embedded in a colder plasma, and far from the machine walls. It is demonstrated that this heating configuration provides an ideal environment to study avalanche phenomena under controlled conditions. The avalanches are identified as sudden rearrangements of the pressure profile following the growth of fluctuations from ambient noise. The intermittent collapses of the plasma pressure profile are associated with unstable drift-Alfven waves and exhibit radial, poloidal and axial dynamics. After each collapse the plasma enters a quiescent phase in which the pressure profile slowly recovers and steepens until a threshold is exceeded, and the process repeats. At higher heating powers the system transitions from the avalanche regime into a regime dominated by sustained drift-Alfven wave activity. The pressure profile then transitions to a near steady-state in which anomalous transport balances the external pressure source. 

Xin An (UCLA)
Talk Title: "Excitation of chirping whistler waves in a laboratory plasma"
Whistler mode chorus emissions with a characteristic frequency chirp largely control the dynamic variability of the Earth's outer radiation belt. They are responsible for the acceleration of outer radiation belt electrons to relativistic energies and also for the scattering loss of these electrons into the atmosphere. Here, we report on the first laboratory experiment where whistler waves exhibiting fast frequency chirping have been artificially produced using a gyrating beam of energetic electrons injected into a cold plasma. It is shown that there is an optimal beam density for frequency chirps, which indicates the existence of optimum wave amplitude for the generation of chirps. Also, frequency chirps only occur for a very narrow range of ratio of fpe/fce similar to that observed in space. Strong magnetic field gradient, which prohibits the formation of phase space electron hole, disrupts frequency chirps as expected. Broadband whistler waves similar to magnetospheric hiss are also observed at relatively high plasma density. Their mode structures are identified by the phase-correlation technique. It is demonstrated that broadband whistlers are excited through Landau resonance, cyclotron resonance and anomalous cyclotron resonance. Wave growth rate and wave normal angle given by linear theory are consistent with experimental results in general. The results have implications for the generation process of whistler waves in the Earth's inner magnetosphere.

PAB 4-330