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

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 A. 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