[IGPP Everyone] Fwd: Plasma Seminar Next Tuesday 11/10: Van Compernolle & An

[Emmanuel V. Masongsong]

 Dear all, 

We have a plasma seminar this Tuesday , November 10th at 11AM in Room 
4-330 of the Physics & Astronomy Building (PAB).  Coffee and cookies 
will be available in the room at 10:45.  Our speakers are Dr. Bart Van 
Compernolle and Mr. Xin An of UCLA.  They will each speak for 25 minutes 
on the following topics: 

"Laboratory studies of avalanches in magnetized plasmas" 
(Bart Van Compernolle) 

"Excitation of chirping whistler waves in a laboratory plasma" 
(Xin An) 

http://www.pa.ucla.edu/sites/default/files/plasma_VanCompernolle_An111015_.pdf 

Full abstracts below. 
~Seth 


Bart Van Compernolle (UCLA) 
"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) 
"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. 
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