Compact multiple x-ray spectrometers for SPARC

The main design options and characteristics of SPARC’s x-ray spectrometers are presented.

SPARC’s first campaign mission of reaching Q > 1 requires a quick learning curve of operating the tokamak in the desired regime of temperature, density and radiation losses. This requires a means of keeping track of impurities in the plasma, especially with tungsten plasma facing components. A low-resolution survey X-ray spectrometer is designed for this task, complementing grating-based vacuum ultraviolet spectroscopy.

Measuring Q > 1 with a good level of confidence, and knowing the input power, can be done using neutron and kinetic measurements. A set of 2 different high-resolution X-ray spectrometers is designed to measure ion temperature via the Doppler-broadening of extrinsically seeded high-Z impurities. One will focus on Ne-like Xe (~2.720 AA) for accessing Ti in low-temperature plasma phases, the other will focus on He-like Kr (~0.944 AA) to access it in high-temperature plasmas phases.

A tokamak operating D-T shots comes with specific constraints, like a neutron-rich environment and the need to minimize risks of tritium exposure. It will be shown how these constraints drove some key aspects of the design of the spectrometers, which are grouped by pairs or triplets on a set of < 100 mm diameter and ~20m-long beamlines granting direct views through the plasma with minimal opening for neutrons. It will be explained how cost and risks are minimized by utilizing a novel compact layout, while meeting performance requirements for spectral resolution and signal levels.