Comparison of Signal Resolution from Two Plasma Sources under Various Gas Sampling Conditions for Optical Emission Spectroscopy

The role of the ITER diagnostic residual gas analyzer (DRGA), is to measure the composition of the exhaust gases leaving the tokamak within a one-second response time. The current DRGA design comprises an instrumented, high-vacuum chamber, including a plasma cell-activated optical emission spectroscopy arrangement, referred to as an optical gas analyzer (OGA). It is used to measure both the hydrogen and helium isotopic content as well as other gas species. In the determination of which plasma cell and OGA configuration is optimal, various gas leak mixtures were produced and analyzed in a laboratory setting to determine the measurements with the highest signal resolution. These mixtures, comprising various concentrations of hydrogen (H2), deuterium (D2), and helium (He), were selected because they are relevant to the gas species expected in the ITER tokamak. The mixtures were leaked into a test chamber at DRGA-relevant, steady-state sampling pressures. Initially, the OGA analyses were done using the Optix System in its commercial configuration: plasma cell (inverted magnetron geometry) and integrated spectrometer. In additional tests, the emission from the plasma cell was coupled to an IsoPlane™ spectrometer via an optical fiber. This latter configuration was also tested concurrently using the traditional OGA, a Penning-geometry cold cathode gauge, coupled to the same spectrometer. The measurements were carried out in steady state and over a range of gas sampling pressures. A preliminary assessment of the Optix™ plasma cell for the ITER OGA application will be presented.