|The HELIX helicon source was constructed to generate the high-density plasmas needed for the Large Experiment on Instabilities and Anisotropies (LEIA). Shown below is HELIX (on the right) and LEIA (on the left).|
|Helicon sources themselves are surprisingly rich sources of
plasma phenomena. In our group we have explored a variety of helicon
source phenomena. Perhaps the most surprising observation was the
discovery of hot ions, already anisotropically heated, in the HELIX
source. The ion temperatures were measured by laser-induced
fluorescence (LIF). Because we had expected relatively cold ions, we
had also embarked on a series of external ion heating experiments.
During those experiments, we successfully heated argon ions in HELIX
plasmas to over 2 eV by launching electrostatic ion cyclotron waves
into the helicon source plasma. That electrostatic ion cyclotron waves
were responsible for the observed ion heating was proven by comparing
the measured, perturbed ion distribution function with theoretical
predictions. The source of ion heating in helicon plasma sources
without external heating systems was still unknown and we conducted a
series of experiments to determine what plasma conditions and antenna
geometries lead to the largest plasma densities and ion temperatures.
Those experiments indicated a clear correlation between density
production and the on-axis lower hybrid frequency in HELIX and a clear
correlation between the edge lower hybrid frequency and ion heating in
HELIX. Additional experiments confirmed the relationship between ion
heating and the lower hybrid resonance for the slow or
"Trivelpiece-Gold" wave at the edge of HELIX. Those experiments
provided convincing evidence of slow wave propagation in helicon
sources (a matter of considerable debate in the helicon plasma source
community) and a clear explanation for ion heating in helicon sources.
Combined with theoretical predictions for the optimal conditions for
slow wave resonances at the lower hybrid frequency, the experimental
measurements of ion heating have lead to a significant improvement in
our understanding of helicon source physics.
During our studies of ion heating in helicon sources, we also noticed that additional waves were being excited parametrically in the helicon source. That helicon sources spontaneously excite a broad spectrum of low and medium frequency waves was a new observation and led to a careful investigation of the conditions under which parametrically driven waves were most strongly excited. Our work in ion heating and parametrically driven instabilities attracted the interest of a theory group in the Ukraine and we have recently completed a theoretical study of anisotropic ion heating and parametrically driven ion-sound turbulence in helicon plasmas.
As a result research into helicon source physics, our group has become known around the world as a leader in helicon source development. Currently we are completing experimental studies involving microwave scattering from slow waves in helicon sources, the formation of double layers in helicon sources, and the effects of multiple ion species on plasma sheaths.
Shown below are the CHEWIE helicon source (left) and additional views of HELIX plasmas