The Goal
Our goal was to explore the plasma wave-particle interaction utilizing
LIF spectroscopy, and to advance our understanding of wave-particle
interactions by applying LIF spectroscopy to the study of nonlinear ion
acoustic waves, ion acoustic shocks, and solitons.
We struck our first discharge during the summer of 2000 and examined it with
Langmuir probes. A Coherent model #699-21 scanning narrowband ring dye
laser system was already in place. We directed the laser beam into the
double plasma device, and the collection of the LIF signal with imaging
telescopes to yield a characterization of the argon plasma under diverse
steady-state operating conditions. This effort served to benchmark the
device, using for reference corresponding measurements performed previously
with intrusive instruments such as gridded electrostatic energy
analyzers. We would then proceed with experiments that explored
excitation and propagation of linear ion acoustic waves. We measured wave dispersion (w vs. k) using
LIF methods discussed above and
compared our results with predictions from linearized Vlasov
models. The linear wave-particle interaction was revealed by
measurements of ion Landau damping. This part of the plan was be executed in order to verify the applicability of this
diagnostic scheme to problems of conceptual simplicity appropriate for
uninitiated student assistants.
The Work
After a four year hiatus spent conducting space weather research at West
Virginia University and the University of Maryland, Professor Tim Good began,
in the Summer of 2000, to supervise some Gettysburg students' work with two
Gettysburg College physics students to make the Pickets Charged Plasma Device
(PCPD) operational.
Since the equipment had not been used for quite some time, the first task of
the summer for students Sean Lyman, '01, and Amy Kerr, '03, was to bring the
device back up to operating conditions. After initial tests of the
efficiency of the vacuum pumps, it was quickly discovered that the mechanical
pump needed repair, as it was only lowering the pressure of a small volume to
approximately 4 Torr. A replacement was
quickly found and installed; the mechanical pump alone can now lower the
pressure to approximately .05 Torr. With the
mechanical pump repaired, the oil diffusion pump's efficiency could also be
tested. Although their controllers were giving acceptable readings for
the pressure, they soon encountered another problem: their ion gauge proved
faulty, often turning itself off or not even lighting. They therefore
turned our attention to possible sources of leaks in the vacuum. A replacement
gauge soon proved more successful than the original gauge, yet they still
continued to check the joints and connections, taking care to clean and regrease all O-rings. They also checked for oil
contamination in the piping and changed the oil in our oil diffusion
pump. by the end of the summer of 2000, Amy
and Sean had succeeded in coaxing a faint purple glow from PCPD, as the first
argon plasmas were ignited in the device. Sean and Amy conquered some
vacuum and power supply problems and modified the plasma filament source,
while characterizing the plasma operating conditions using a Langmuir
probe.
The next cast of plasma researchers conducting a summer research campaign in
2001 included Holly Sheets, ’03, and Rob Vary, ‘03, thanks to stipend support
from the Delaware Space Grant Consortium. Holly and Rob learned to
excite ion acoustic waves in the double plasma of PCPD, and to diagnose the
ion velocity distribution via laser induced fluorescence spectroscopy.
The research was aimed at gaining a greater understanding of the plasma-wave
particle interaction employed in many particle acceleration and plasma
heating methods, as well as in wave generation methods.
Wave-particle and field-particle
interactions also play a key role in particle acceleration mechanisms
believed to be operating in the auroral acceleration region of the earth's
magnetosphere and more generally, in the domain of space plasmas.
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