Observation and modeling of optical emission patterns and their transitions in a Penning discharge

A Penning discharge tube has been used as the excitation source for opticaldetection of gaseous species concentrations in a neutral gas. This type of diagnostichas been primarily used in magnetic fusion energy experiments for the detection of minorityspecies in the effluent gas (e.g., for helium detection in a deuterium background).Recent innovations (US Patentno. 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 magneticfield and voltage to keep the optical signals nearly constant (or at least away from a nonlineardependence on the pressure). However, there are limitations to this approach, because thePenning discharge can manifest itself in a number of modes, each exhibiting a different spatialemission pattern. As a result, varying the discharge parameters can cause the gauge toundergo transitions between these modes, disrupting any intended monotonic dependenceof 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 ofthe discharge. It also presents a first successful application, a particle-in-cell (PIC) code, tosimulate these modes and a mode transition. The hope is that a good understanding of thephysics involved in the mode transitions may allow for methods of either avoiding orsuppressing such transitions. This would aid in broadening the use of this plasma-based sensor technology.