The SPARC diagnostics papers are the seventeen companion publications that appeared in Review of Scientific Instruments through 2024, plus a 2022 antecedent, specifying instrument by instrument how 📝SPARC will be measured. They are the layer beneath 📝SPARC Early Campaign Diagnostics, the invited overview that promises them. Not one of these instruments has measured a plasma, because SPARC has not run: they are design studies, synthetic-diagnostic predictions, and bench tests of detectors against radioactive sources. They are arranged here by what each instrument is for — counting neutrons, reading x-rays, feeding the control room, and watching the machine rather than the plasma.
Counting the neutrons
Neutrons are the primary route to fusion power, and the largest group of papers is about measuring them. 📝Prasoon Raj's overview of the neutron diagnostic systems sets the requirement — 10 percent uncertainty on fusion power across more than eight orders of magnitude, up to 5×10¹⁹ neutrons per second — and divides the job across four subsystems: roughly fifteen flux monitors, two independent foil 📝activation analysis systems, a spectrometric radial neutron camera, and a high-resolution 📝magnetic proton recoil spectrometer. Four papers build out those pieces. Neutronics simulations size the flux monitors in OpenMC benchmarked against a CAD-based MCNP6 model, finding that boron-10 ionization chambers in borated housings respond overwhelmingly to neutrons below 100 keV while uranium-238 fission chambers see roughly 60 percent direct neutrons. The MPR spectrometer is covered by an ion optical design reaching a modeled resolution better than one percent and a hodoscope study that settled on EJ276D scintillator rods 0.7 cm by 13 cm. Two more concern the 📝neutron spectrometer detectors themselves: deuterated xylene outperformed its protiated counterpart against an AmBe source and D-T generators, and diamond spectrometer electronics for the neutron camera achieved 20–25 nanosecond signal development. That camera's concept comes from the 2022 paper in which 📝Università di Milano-Bicocca and CNR-ISTP carried forward the diamond spectrometers that resolved 14 MeV neutrons to about 1 percent FWHM in 📝JET's DTE2 campaign at yields up to 4.7×10¹⁸ per second. Those JET figures are measurements. Everything downstream of them is design.
Reading x-rays, and finding runaways
SPARC's x-ray diagnostics overview describes three systems with three different jobs: in-vessel 📝soft X-ray imaging for early-campaign plasma position, 📝MHD activity and impurity content; ex-vessel hard x-ray scintillators to catch 📝runaway electrons; and Bragg 📝x-ray crystal spectrometer arrays sited outside the tokamak hall for 📝ion temperature, rotation and impurity emission. Each has its own paper. The crystal spectrometers' performance predictions are a 📝synthetic diagnostic throughout — line intensities from atomic and collisional-radiative codes, ray-traced into modeled detector images and inverted back to profiles. The diamond soft x-ray camera exists because the silicon diode arrays used on most tokamaks are damaged by neutrons, so single-crystal CVD diamonds go into the upper and lower port plugs instead. The hard x-ray monitor prototype is a LaBr₃ scintillator on a photomultiplier, characterized against γ-ray sources, intended to detect 📝bremsstrahlung above roughly 100 keV during startup and tell the control system to terminate the discharge. 📝Alex Tinguely's synthetic synchrotron study completes the runaway picture: at 12.2 📝tesla the 📝synchrotron radiation from a runaway beam peaks in the visible and infrared, so matched clockwise and counterclockwise wide-angle views can see it in either current direction.
Feeding the control room
Four instruments exist to keep the discharge under control in real time. Edge scanning 📝reflectometry sweeps 18 to 90 GHz to return the electron density profile from the far 📝scrape-off layer to the 📝pedestal top, spanning roughly 4×10¹⁸ to 4×10²⁰ per cubic meter at 12 tesla. A two-color heterodyne laser 📝interferometry system provides density feedback from day one, its in-vessel optics designed to survive a major 📝disruption. Multichannel vacuum-ultraviolet @spectroscopy covers 10–2000 Å on five sightlines — core plus four 📝divertor regions — for impurity control. 📝Matt Reinke co-authored the bolometry design, which puts 248 📝bolometer lines of sight into pinhole cameras at 20 locations, fourteen for two-dimensional equilibrium radiated power and six for three-dimensional radiated energy during disruptions.
Watching the machine, not the plasma
Two systems measure hardware. The thermal diagnostic uses thermocouples and 📝fiber Bragg gratings to keep in-vessel structures inside their design limits, with two spring-loaded thermocouples per divertor target tile acting as calorimeters; everything must survive a 350 °C bake, and because the vessel becomes activated, a failed sensor cannot be replaced. The neutral gas diagnostic system fields pressure sensors and gas analyzers for wall conditioning, 📝helium ash removal and plasma control, and states its second purpose plainly: de-risking the design of 📝ARC.
We published the design of every one of these instruments before any of them saw a plasma. That is deliberate: it means SPARC's first campaign can contradict us instrument by instrument, in detail, rather than in general.