Upgrades to x-ray microcalorimeter fusion diagnostic to improve calibration, spectral bandwidth selection, and count rate adjustment

This paper presents progress made toward the overarching goal to adapt single-photon-counting microcalorimeters to magnetic fusion energy research and demonstrate the value of such measurements for fusion. Microcalorimeter spectrometers combine the best characteristics of x-ray instrumentation currently available on fusion devices: high spectral resolution similar to an x-ray crystal spectrometer and broad spectral coverage sufficient to measure impurity species from Be to W. As a proof-of-principle experiment, a NASA-built x-ray microcalorimeter spectrometer has been installed on the Madison Symmetric Torus (MST) at the Wisconsin Plasma Physics Laboratory. For fusion applications, microcalorimeters will provide multi-species impurity ion data and will also contribute to the measurements of impurity ion temperature and flow velocity, Zeff, and electron density. Here, improvements to the microcalorimeter setup at MST and spectra acquired with the upgraded system are presented. Four motor-controlled wheels, loaded with x-ray fluorescence targets for continuous gain-drift tracking and broad energy-scale calibration, filters, and apertures, were added to the diagnostic's beamline. A stronger 55Fe source was added as an additional x-ray source for calibration. The 55Fe source, targets, filters, and apertures can be moved into and out of the optical path. Several configurations are now accessible for shot-to-shot adjustments to the count rate and waveband, enabling the measurement of x-ray photons emitted by a variety of impurity ions in the majority deuterium plasma, including aluminum, carbon, and argon.