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

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.

The Concept of Spectral Accuracy for MS

“…What one actually obtains from any physical mass spectrometer with its finite resolving power, mass accuracy, and linear dynamic range is always a continuous response curve similar to the one shown in Figure B, which is a mathematical convolution between the discrete theoretical isotope distribution in Figure A and a continuous function called peak (or line) shape function . The complete profile of a peak shape function provides a numerical representation of the ion dispersion or aberration in the mass spectrometer, including spatial and velocity dispersion inside the ion source. − This is not unlike the line broadening observed in optical systems after laser light enters the entrance slit of a spectrophotometer …”

“…[32][33][34] This is not unlike the line broadening observed in optical systems after laser light enters the entrance slit of a spectrophotometer. 35 On a TOFMS instrument, the peak shape functionssometimes with a sharp rise followed by a slower declinesis simply the arrival time distribution of a population of isotopically pure ions. This reflects the statistical distribution introduced by the TOF analyzer, which includes the energy and velocity spread inside the ion source and the collisions with the residual gas throughout the flight path.…”