High-resolution bent-crystal spectrometer for the ultrasoft x-ray region

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Abstract
A multichannel vacuum Bragg‐crystal spectrometer has been developed for high‐resolution measurements of the line emission from tokamak plasmas in the wavelength region between 4 and 25 Å. The spectrometer employs a bent crystal in Johann geometry and a microchannel‐plate‐intensified photodiode diode array. The instrument is capable of measuring high‐resolution spectra (λ/Δλ≊3000) with fast time resolution (4 ms per spectrum) and good spatial resolution (3 cm). The spectral bandwidth is Δλ/λ0=15% at λ0=8 Å. A simple tilt mechanism allows access to different wavelength intervals. In order to illustrate the utility of the new spectrometer, time‐ and space‐resolved measurements of the n=3–2 spectrum of selenium from the Princeton Large Torus tokamak plasmas are presented. The data are used to determine the plasma transport parameters and to infer the radial distribution of fluorinelike, neonlike, and sodiumlike ions of selenium in the plasma. The new ultrasoft x‐ray spectrometer has thus enabled us to demonstrate the utility of high‐resolution L‐shell spectroscopy of neonlike ions as a fusion diagnostic.