Imaging transmission grating spectrometer for magnetic fusion experiments

Abstract

The Johns Hopkins Plasma Spectroscopy Group is developing a transmission grating based imaging spectrometer for the ultrasoft x-ray [(USXR), 10–300 Å] range. The spectrometer will be integrated into an impurity diagnostic package for magnetic fusion experiments, which provides time and space resolved information about radiation losses, Zeff profiles, and particle transport. The spectrometer has a simple layout, consisting of collimating and space resolving slits, a transmission grating, and a two-dimensional imaging USXR detector. We tested two types of detectors, a CsI coated multichannel plate and a phosphor P45 coated fiber optic plate, both with intensified charge-coupled-device image readout. The performance of the 5000 1/mm, 3:1 bar to open area ratio transmission grating has been evaluated in the laboratory using Kα lines from a Manson source and the emission from a Penning discharge. A prototype spectrometer equiped with the first type detector and optimized for 6 Å spectral resolution has been tested successfully on the CDX-U tokamak at the Princeton Plasma Physics Laboratory. A spectrometer using the second detector version has been developed for the NSTX spherical torus at Princeton. Spatially resolved spectra have been recorded with 25–250 ms time integration with both spectrometers. In both experiments, spectra are dominated by low-Z impurities, C, N, and O.