A high throughput spectrometer system for helium ash detection on JET

Acquiring information about helium ash production and transport is fundamental for future burning plasma devices, such as International Thermonuclear Experimental Reactor, since the helium ash must be continuously removed from the plasma to prevent the dilution of the deuterium-tritium (DT) fuel. This diagnostic for future JET DT operation uses charge-exchange recombination spectroscopy (CXRS) in conjunction with the JET neutral heating beam to measure the helium density at 20 radial locations across the JET plasma via the 4686 A He+ line and an array of heated 1 mm quartz fibers. The CXRS diagnostic utilizes a high throughput short focal length spectrometer with f/1.8 input optics, two entrance slits, a holographic transmission grating, and refractive optics. The detector is a thinned back-illuminated charge coupled device that has high quantum efficiency, a 10 MHz readout speed, and a time resolution of 5 ms