Residual gas analysis hardware and software data acquisition system at NSTX

The National Spherical Torus Experiment (NSTX) is the first step in an investigation of the physics principles of low-aspect-ratio spherical tori (ST) designed to study non-inductive start-up, current sustainability, current profile control, confinement, transport, pressure limits, stability and disruption resilience, as well as unique scrape-off layer (SOL) and divertor characteristics. NSTX started plasma operations in February 1999. During the first extended period of experiments, plasma discharge reproducibility and performance were strongly affected by impurity control and wall conditions. During this time, residual gas analyzer (RGA) data has been used during non-operating periods and between discharges to measure contributions to the vessel base-pressure (2-3E/sup -8/ Torr) that resulted from impurity gases evolving from internal surfaces. The RGA is a quadrupole mass spectrometer (QMS) with a Faraday Cup and electron multiplier. Recently, a second RGA system was installed to monitor fast changes in impurity gas production during and immediately after plasma discharges, which are indicative of changing conditions. In order to achieve the required response time, scan rates were increased by limiting the range of masses scanned. A data acquisition board was used to drive the new RGA system in concurrence with the NSTX shot cycle. Specialized interactive software to put acquired data into MDSplus, control data acquisition and assist in data analysis was written in Visual Basic and IDL.