Observation and Modeling of Optical Emission Patterns and Their Transitions in a Penning Discharge
2008
A Penning discharge tube has been used as the excitation source for optical
detection of gaseous species concentrations in a neutral gas. This type of diagnostic
has been primarily used in magnetic fusion energy experiments for the detection of minority
species in the effluent gas (e.g., for helium detection in a deuterium background).
Recent innovations (US Patent
no. 6351131, granted February 26, 2002) have allowed for extension of the operation range from <1 Pa to as high as 100 Pa and possibly beyond. This is done by dynamically varying the gauge magnetic
field and voltage to keep the optical signals nearly constant (or at least away from a nonlinear
dependence on the pressure). However, there are limitations to this approach, because the
Penning discharge can manifest itself in a number of modes, each exhibiting a different spatial
emission pattern. As a result, varying the discharge parameters can cause the gauge to
undergo transitions between these modes, disrupting any intended monotonic dependence
of the overall emission on the varied parameter and hence any predicable impact on the emission.
This paper discusses some of the modes observed experimentally using video imaging of
the discharge. It also presents a first successful application, a particle-in-cell (PIC) code, to
simulate these modes and a mode transition. The hope is that a good understanding of the
physics involved in the mode transitions may allow for methods of either avoiding or
suppressing such transitions. This would aid in broadening the use of this plasma-based sensor technology.
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