C. Marcus1, T.M. Biewer1, C.C. Klepper1, H.D. Mandliya1, R. Mellor2, B.R. Quinlan1, K.J. Witek1
1Oak Ridge National Laboratory, Oak Ridge, TN
2Hiden Analytical, Warrington, United Kingdom
A diagnostic residual gas analyzer (DRGA) system is being designed for ITER to analyze neutral gases from tokamak fusion reactions. The measurement regions of interest include both low-amu hydrogen and helium species (masses 1 through 6) as well as neon (primarily mass 20), which is injected to radiate plasma energy in the divertor to prevent detrimental damage to the plasma facing components (PFCs) armoring the confinement vessel wall. From a mass spectroscopy perspective, the conditions needed for successfully obtaining stable, accurate ion current measurements for helium-4, when present as a minor analyte, can be complex. First, the resolving power, when using a residual gas analyzer (RGA), must be sufficient to deconvolute ion current intensities separated by less than one mass unit; as relevant when deuterium molecules are present (Δm ~0.025 amu). Also, the analyzed gas concentrations, sampled from the fusion processes, must be free of bias caused by the latent presence of these same gases (i.e., residual effects from system outgassing and/or vacuum backstreaming) to enable a high sensitivity measurement of the ion current signal.
Recently, research was performed in support of the US-ITER project at the Oak Ridge National Laboratory to validate the resolving power of a specialized quadrupole mass spectrometer (QMS) system. The mass filter can operate within the Mathieu Second Stability Zone (via variable input settings to the RF circuitry) for greater mass peak resolution. The QMS tests analyzed various concentrations of gas leak mixtures, comprising helium-4, deuterium, and/or neon. Using both a relative sensitivity factor and an ion current signal ratio, known as “RS” and “R”-values, respectively, the concentration for helium-4 was determined and compared to known, analyzed mixtures. Then, the measurement accuracy and validation acceptance basis of the QMS were determined.
This presentation will encompass this QMS testing and subsequent data analyses. In addition, the structure and function of a remote data acquisition control system will be described. The architecture performs as a “virtual instrument” to transmit command signals as well as receive the scanned ion mass currents for compilation measurements. These functions enable the processing of relevant data, the generation of graphical plots, and performing the gas species calculations for the parameters of interest within the ITER diagnostic mission.