Observation and Modeling of Optical Emission Patterns and Their Transitions in a Penning Discharge

Abstract

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.

Laboratory-based validation of the baseline sensors of the ITER diagnostic residual gas analyzer

“…The design goal had been to have the pressure in the OGA region in the mid 10 −1 Pa when the pressure in the main analysis chamber is up to 10 −3 Pa, the operational limit for the quadrupole mass spectrometer. This would mean that for the range of likely ITER pumping duct pressures of 1-10 Pa, the analysis chamber would be in the 10 −4 to 10 −3 Pa, while the OGA sub-chamber would be in the mid 10 −2 to 10 −1 Pa, which is the range of optimal light emission for an OGA based on a commercial Penning gauge tube (some details on this Penning-OGA-specific pressure range can be found in [3,4]). This extraordinary range of neutral pressures involved in the ITER Divertor DRGA measurement system, which results from having to analyze a divertor expected to reach much higher pressures than in any currently operating tokamak, leads to substantially more differential pumping than previously applied, both to the mass spectroscopy (in the main analysis chamber) and to the OGA (at the IS port) than ever before for similar, plasma pulserelevant measurements (see, e.g.…”