Measurements for fusion power to demonstrate Q fus > 1 on SPARC (invited)

The SPARC tokamak’s primary goal in the first operational campaign is achieving net fusion energy gain (Qfus> 1), targeting a fusion power (Pfus) of 8–30 MW in deuterium–tritium (DT) plasma. For an optimally accurate and widely accepted demonstration of breakeven, the SPARC team is preparing for three independent methodologies to measure Pfus. The first method relies on neutron flux monitors and a neutron activation system, which together offer high time-resolution data using ionization chambers and dosimetry foils for absolute yield measurements. These are calibrated via an in situ DT neutron generator and detailed neutronics models. The second method employs collimated neutron diagnostics. A core-viewing magnetic-proton recoil spectrometer analyzes the neutron energy spectrum, while a poloidal profiler uses scintillator and diamond detectors to derive the DT neutron emissivity profile. Together, these diagnostics allow for a precise reconstruction of Pfus from core emissions. The third method calculates Pfus by volume-integrating the fusion reactivity using data from an array of kinetic profile diagnostics. Key inputs include temperature profiles from x-ray spectroscopy and electron–cyclotron emission measurements, plasma/electron density from Thomson-scattering, interferometry and edge-reflectometry, and fuel ion and impurity levels (Zeff) from optical and vacuum-UV spectroscopy. Neutral gas diagnostics and optical spectroscopy provide the critical fuel ion concentrations, which are also needed for corrections for the non-DT neutron contributions. By integrating these complementary techniques, SPARC ensures a robust and verified demonstration of Qfus > 1 in its first campaign.