Time-Dependent Response of the ITER Diagnostic Residual Gas Analyzer

Analysis of the ITER diagnostic residual gas analyzer (DRGA) vacuum system is critical for the design and accuracy of the gas composition measurement which is used for the control of plasma heating and fuel-cycle processing. The DRGA is a diagnostic system capable of resolving hydrogen and helium isotopes as well as heavier impurity gases. The DRGA is located approximately 15 m away from the divertor and is intended to measure the composition of gases in the ITER sub-divertor region. The DRGA for the divertor analysis relies on an evacuated sampling pipe to transport the exhaust gases to the analysis station where spectrometers measure the exhaust gas composition. At the sampling pipe entrance, an in-line orifice creates molecular flow conditions in the entire length of the sampling pipe. For plasma heating control and fuel-cycle processing, it is necessary to analyze the time-dependent response resulting from changes in gas composition from injected gas and/or fueling pellets. Molflow+, which is a test particle Monte Carlo (TPMC) simulation code, is used to simulate the time-dependent response of the DRGA from injections of hydrogen gas. An innovative way to model an in-line turbomolecular pump is presented in this work. The results from a simulation code based on the physical geometry of an in-lab test article. The results from the simulation code and physical testing of the in-lab test article are compared and show good agreement. The simulation is then extended to an ITER like geometry and compared to previous work. The results of the time dependent response are shown from the sampling pipe orifice to the RGA and a plasma cell which is used for optical gas analysis (OGA) which are two distinct locations in the DRGA system, and the latter of these locations has not been simulated previously. This study provides important design validation of the ITER DRGA time-dependent response as it relates to fuel-cycle and plasma heating control.