C. Marcus1, Robert Mellor2
1Oak Ridge National Laboratory, Oak Ridge, TN
2Hiden Analytical, Warrington, United Kingdom
The Diagnostic Residual Gas Analyzer (DRGA) is being designed for ITER to analyze neutral species (gases) formed during the fusion reaction. One measurement range of interest comprises low-amu species (1 to 6), which will exist during the tokamak pulse events. Therein, the gas species will comprise trace amounts of helium isotopes within a large concentration of deuterium (D2). The challenge in successfully obtaining stable, accurate measurements of Helium-4 (4He) as the minor analyte is two-fold. First, the resolving power for a mass spectroscopy (MS) method must be sufficient to deconvolute ion current intensities, separated by less than one mass unit (∆m ~0.025 amu, relative to D2). Second, the gas concentration from the fusion processes must be free of bias caused by the latent presence (i.e., artifacts from system outgassing and/or vacuum backstreaming) of these gases to enable an accurate measurement of the real-time signal. This latter criterion was addressed in a prior technical paper, which discusses an effective solution to eliminate backstreaming using a conductance-limiting orifice plate.
To achieve the MS goal, research and testing is underway in the US-ITER Project, conducted at the Oak Ridge National Laboratory, to validate the resolution capabilities (including scanning stability and repeatability) of a specialized, quadrupole mass spectrometer (QMS) system prototype. It is unique in both the remote system configuration needed to survive the intense radiation environment (i.e., ~140-meter cabling offset for signal processing between instrument and controller) and the optional higher frequency RF power circuit (to operate the mass filter within Mathieu’s second stability zone, which produces better peak resolution). This will ensure the system meets ITER objectives for both design and mass resolution capabilities; specifically, where the region of interest (ROI) encompasses 4He and D2, the resolving power must detect the 4He at the lowest concentration of three percent.
This presentation will encompass unique details of the QMS system hardware, which are enhanced versus commercially available mass spectrometers using similar ion optics but operating in the first stability zone and integrated (on-board) electronics. Also, content herein will detail a methodology used to evaluate the satisfactory performance of the prototype system. This is accomplished by comparing calculated concentrations and partial pressures for 4He and D2 versus a comparison to a resident cold cathode (ionization gauge) signal, corrected for the analyzed species, as well as known values in the mixed-gas leaks being analyzed during the QMS evaluation process.