Use of optical gas analysis to determine helium detection limits for the ITER DRGA prototype

The Diagnostic Residual Gas Analyzer (DRGA) is an instrumentation set that will be integrated into the ITER fuel cycle system to analyze the gas composition dynamically in the ITER tokamak. The present prototype DRGA uses a dual pumping system, where primary and secondary Turbomolecular Pumps (TMPs) are placed in series, to create vacuum conditions (high and low) in separate analysis chambers. The former is needed for testing a rad-hardened quadrupole mass spectrometer (QMS); the latter, for plasma cells used in optical gas analysis (OGA). The plasma cell produces a glow discharge for the spectral analysis of similar-mass gases such as helium-4 and deuterium, which are difficult to resolve with a QMS.



In this work, several tests were performed. First, pressure profiling was done to quantify the effects of the secondary TMP rotational speed on the pressures in both, upstream chambers; with the goal to optimize the pressure for the plasma cells. Second, to understand the correlation of gas species concentration to emission line intensity, varying ratios of helium in hydrogen gas were introduced from a gas mixing station. Lastly, these measurements were used to estimate the percent detectability limit of helium in the two chambers as a function of the system configuration with varying TMP rotational speed. Results from these tests and analysis will be summarized in this presentation.