Time-domain modeling of RF sheath potentials for impurity production estimates in SPARC*

We discuss ongoing efforts to estimate impurity production from sputtering during ICRF heating on SPARC, arising from RF-rectified sheaths in the near field of the ICRF antennas.  Tech-X’s VSim time-domain plasma-and-sheath software model provides detailed RF sheath information, for use by UIUC’s plasma-material interaction codes [hPIC2/F-TriDYN/RustBCA/GITR], to provide local estimates of impurity yields.  This talk focuses on the first part of this process, i.e. the calculation of the ICRF antenna near-fields and sheath voltages.  Accurate CAD-derived 3D geometry of the vessel, ICRF antenna, and limiters is used, together with plasma profiles and confining magnetic fields based on previous SPARC operational studies in TRANSP.  Of primary concern are sheaths on the limiters immediately adjacent to the antenna, and on the Faraday shields.  Preliminary results indicate that SPARC’s high magnetic field and thin dense SOL yield sheath conditions that can result in sputtering when the RF antennas, as presently designed, are run at full power.  The nature of RF sheaths on the Faraday shields, in parallel alignment to the magnetic field, may also be more significant in this regime.