Abstract: Understanding MHD instabilities and their impact on energetic alpha particle transport in SPARC will provide insights for future fusion pilot plants. As a first step, comprehensive MHD simulations are performed for the primary reference discharge (PRD)[1] to study low-n instabilities in the SPARC tokamak, using the M3D-C1 code. A resistivity scan identifies a dominant internal kink mode at the q=1 flux surface with a toroidal mode number n=1. A plasma β scan (by varying the temperature) shows that the mode’s linear growth rate is greatly reduced with a low-pressure profile (low β). This pressure effect is also verified by solving a simplified 1D eigenvalue problem. In 3D nonlinear simulations, the PRD case, with on-axis T0=20 keV, undergoes a very strong profile crash, which shows a magnetic reconnection and a hollowed pressure profile. Increasing the on-axis safety factor q0 toward 1 can reduce the initial sawtooth crash, which suggests the actual crash may occur in SPARC before q0 drops far below unity. Exploring more combinations of q profile and pressure profile for the SPARC tokamak is needed. Based on MHD results, the kinetic-MHD code, M3D-C1-k[2], is used to include alpha particles for transport studies in the presence of MHD instabilities. An initial result shows a reduction of the linear growth of the n=1 mode, when including a fraction of kinetic alpha particles. In future, a full-orbit model will also be implemented for high energy particles, such as runaway electrons, alpha particles, etc.
[1]. Rodriguez-Fernandez, P., et al. 2022. Nuclear Fusion, 62(4), p.042003.
[2]. Liu, C., et al. 2022. Computer Physics Communications, 275, p.108313.