Physics basis for the ICRF system of the SPARC tokamak

Ion cyclotron range of frequencies (ICRF) heating will be the sole auxiliary heating method on SPARC for both full field (Bt0 ~ 12 T) D-T operation and reduced field (Bt0 ~ 8 T) D-D operation. Using the fast wave at ~120 MHz, good wave penetration and strong single pass absorption is expected for D-T(3He), D(3He), D(H) and 4He(H) heating scenarios. The dependences of wave absorption on k||, 3He concentration, resonance location, antenna poloidal location and losses on alpha particles and ash have been studied. The antenna loading has been assessed by comparing to the Alcator C-Mod antennas. Antenna spectrum k|| ~ 15-18 m-1 is shown to be good for both core absorption and edge coupling. For the control of impurity sources, the antenna straps are rotated ~10o to be perpendicular to the B field and the straps can run with different power levels in order to optimize the antenna spectrum and to minimize the image current on the antenna frame. Combining the physics constraints with the SPARC port design, maintenance requirement and contingency against antenna failure during D-T operation, we plan to mount on the inner wall of the vacuum vessel a total of 12 4-strap antennas in 6 ports while keeping 3-strap antennas that are insertable and removable on port plugs as the backup option.