High spectral resolution, photon counting detector for Doppler temperature measurements in magnetically confined plasmas

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Abstract
This paper describes the design, construction, and performance of a single-stage microchannel-plate image intensifier used as a photon counting detector over the wavelength range from 1150 to ∼2000 Å. The intensifer incorporates three high strip current (∼300 μA) microchannel plates, constructed with 12-μ-diam channels and 15-μ center–center spacing, in a ‘‘Z’’ configuration. The use of high strip current MCPs requires gating the power supply to protect the plates from thermal runaway of the strip current. The output pulses are proximity focused onto a P-46 phosphor screen, which is fiber-optically coupled to a linear, self-scanning photodiode array. Maximum frame rates for the photodiode array are ∼ 125 000 frames/s, with maximum count rates of ∼25 000 photoevents/s. The detector was placed at the focal plane of a 1-m focal length Ebert–Fastie spectrometer and the performance characteristics of the spectrometer-detector system were evaluated using a hollow cathode Pt lamp. The linewidths measured during this evaluation demonstrate that the spatial resolution of the detector is better than 50 μ. The spectrometer-detector system was then used to determine ion temperatures from Doppler broadened impurity lines emitted from plasmas of the Alcator C tokamak. This detector demonstrated more than an order of magnitude increase in sensitivity compared to a photon-counting photomultiplier tube with a vibrating mirror previously used for these measurements with the same spectrometer. This permitted a determination of the central ion temperature of the Alcator C tokamak using the ‘‘forbidden’’ line of Fe xii at 1354.1 Å which was not detected with the previous system.