Finite Element Modeling of the SPARC ICRF System*

Radiofrequency heating will raise the SPARC plasma temperature from an initial ~5 keV Ohmic plasma to the ~20 keV sufficient to produce a true burning plasma . As the only auxiliary heating system on SPARC, the ICRF system must reliably provide 25 MW of power through 12 4-strap dipole phased antennas. Finite element modeling has baselined antenna RF coupling, heating, and disruption loading. COMSOL Multiphysics allows the determination of coupled power and electric fields that can drive breakdown, which are themselves a function of coupling. The 8 straps of the antenna are fully 3D, 5 axis CNC machined facet to align with the plasma both poloidally and toroidally. A cold plasma tensor derived from TRANSP profiles is extended into the edge in a 2D equilibrium, and RF is applied to 16 ports  to drive a pair of 4 strap antennas to evaluate coupled power. Full 3D electric and magnetic RF fields can be evaluated for their B-field aligned and perpendicular components in a high fidelity 3D CAD model. Fields are compared with established empirical and modeled breakdown thresholds and combined with a tuning plan to establish the coupled power of 2.5MW/4 strap antenna, sufficient to meet the needs of SPARC.