x-ray crystal spectrometer is a diagnostic that disperses a plasma's X-ray line emission by Bragg reflection from a bent crystal, reading ion temperature from the width of the lines and toroidal rotation from their shift.
The instrument is 📝spectroscopy carried into a band where ruled gratings fail. X-ray wavelengths are comparable to the spacing between planes of atoms in a crystal, so a crystal reflects a given wavelength only at a specific angle and becomes the dispersing element; bending it onto a curved surface focuses the reflection and lets one instrument image many sightlines at once, so tomographic inversion returns profiles rather than a single chord average. The emitters are not the fuel — hydrogen isotopes radiate no lines at core temperatures — but highly charged mid-Z impurity ions, intrinsic or deliberately injected, whose helium-like and hydrogen-like line complexes fall in the X-ray range and are atomically well characterized. Doppler broadening of those lines gives 📝ion temperature; the Doppler shift gives 📝plasma rotation.
The distinction against charge-exchange recombination spectroscopy, the other standard route to core temperature and rotation profiles, is that the crystal spectrometer is passive: charge exchange requires a neutral beam fired into the core, while the crystal reads light the 📝plasma already emits. That has been the workhorse arrangement for decades. On 📝Alcator C-Mod the HIREX spectrometers imaged helium-like and hydrogen-like argon, and the profiles they measured underpin 📝John Rice's work on intrinsic rotation — spin-up with no applied torque — including the 2007 inter-machine comparison across tokamaks. 📝JET has fielded a high-resolution Johann bent-crystal spectrometer since the late 1980s, and 📝ITER plans both a core imaging system and a survey spectrometer.
📝SPARC is designed with three arrays: two high-resolution bent-crystal systems imaging helium-like krypton and neon-like xenon for temperature and toroidal velocity, and a survey system monitoring neon-like 📝tungsten alongside chromium, iron, cobalt, nickel and copper emission. The 2024 performance study by Perks, Vezinet, Rice and 📝Matt Reinke, Performance predictions of the SPARC x-ray crystal spectrometers, is a synthetic-diagnostic prediction: line intensities computed from atomic and collisional-radiative codes, ray-traced into modeled detector images. No SPARC spectrometer has yet viewed a discharge.