Speaker
Description
This talk will describe recent progress in modeling and diagnosing energetic particles in the SPARC tokamak, specifically DT-fusion alpha particles and runaway electrons (REs). First, alpha-driven MHD instabilities (such as Toroidal Alfven Eigenmodes) are simulated with several linear and nonlinear hybrid kinetic-MHD codes, including M3D-C1; these show good agreement in predicting a “most unstable” TAE with toroidal mode number n ~ 10. These sub-MHz range fluctuations are expected to be diagnosed via interferometry and arrays of Mirnov coils. Alpha transport - from TAEs, NTMs, and toroidal field ripple - is evaluated via particle following, and the resulting impact on plasma performance, confinement and losses to the first wall are assessed. For expected mode saturation amplitudes dB/B ~ $10^{-3}$, fusion power is predicted to only decrease by ~3%, while alpha power losses to the first wall could increase by a factor of ~2-4. Alpha knock-on reactions, leading to suprathermal fuel ions and high energy neutrons, are also modeled and could be measured via neutron spectrometry. Second, REs and MHD are solved self-consistently with M3D-C1; mitigation via massive gas injection and the passive RE mitigation coil (REMC) are simulated, including both thermal and current quenches, as well as the final RE termination event. REMC-induced stochasticity and resulting RE losses are found to be highly dependent on thermal conductivity and the magnetic geometry, with flux surfaces prone to rehealing near the magnetic axis; for SPARC, the REMC is predicted to be effective, although perhaps not fully preventive. RE transport and impacts on plasma-facing components are similarly evaluated via particle following, with analysis ongoing. REs are anticipated to be diagnosed via their synchrotron emission and hard x-ray bremsstrahlung, among other measurements.