Acquiring information about helium ash production and transport is fundamental for future burning plasma devices, such as ITER, 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 chargeexchange recombination spectroscopy in conjunction with the JET neutral heating beam to measure the helium density at 20 radial locations across the JET plasma via the 4686Å He + line and an array of heated 1 mm quartz fibres. 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 a high quantum efficiency, a 10 MHz readout speed, and a time resolution of 5 ms.