Impurity analysis using a space resolved transmission grating based imaging spectrometer on NSTX

Figure 1: Principle of operation of TGIS. The two slits define a narrow fan beam which is diffracted by the free standing transmission grating. The aperture of the grating determines the spatial resolution of the instrument. Measurement of the impurity content and transport is critical for fusion experiments. The plasma spectroscopy group at the Johns Hopkins University is developing an impurity diagnostic package consisting of two com-plimentary diagnostics-(a) Multi-energy filtered SXR arrays (ME-SXR) measure the plasma emission with high space and time resolution but with coarse energy discrimination[1], (b) A transmission grating imaging spectrometer (TGIS) measures the EUV impurity line emission with lower time resolution but higher energy resolution[2]. The space resolved TGIS spectra determine the impurity fractions needed for modeling the ME-SXR data and also for standalone impurity monitoring. This paper briefly describes the application of the TGIS for diagnosing the impurity fractions on NSTX. The basic layout of the TGIS is shown in Fig. 1. The detailed design of the TGIS has been published earlier[2]. The device uses a CsI coated MCP image intensifier as a 2-D detector. The wavelength coverage is 30-700 Å and the spectral resolution around 10 Å. The time resolution of the instrument is limited by a slow image acquisition rate of 3 frames/s. The TGIS has a tangential midplane view covering the region from r/a ∼ 1 to the magnetic axis with spatial resolution around a/15. The field of view of TGIS includes the region where the heating beams enter the plasma, thus enabling measurement of both fully stripped low-Z impurities through their CX excited emission and higher-Z impurities through their electron excited emission.