The Role of Far-Field RF Sheaths in SOL Losses of HHFW Power on NSTX, and Implications for Near-Field Studies of ICRF Antennae

Authors:  R. J. Perkins, J.C. Hosea, M.A. Jaworski, R.E. Bell, N. Bertelli, G. J. Kramer, L. Roquemore, G. Taylor, and J.R. Wilson

Abstract:  Radio-frequency   (RF)  rectification   is an  important sheath   phenomenon   for  wave  heating  of plasma  in fusion devices and  is proposed  to  be the  mechanism  responsible  for converting  high- harmonic  fast-wave  (HHFW)  power in the  National  Spherical  Torus  eXperiment  (NSTX)  into  a heat  flux to the divertor.  RF rectification  has two aspects:  current rectification  and voltage recti- fication,  and,  while the latter is emphasized  in many application, we demonstrate the importance of current rectification  in analysis  of the  NSTX  divertor  during  HHFW  heating.   When  rectified currents are  accounted  for in  first-principle  models  for the  heat  flux to  the  tiles,  we predict  a sizeable enhancement  for the  heat  flux in the  presence  of an RF  field:  for one case studied,  the predicted  heat flux increases from 0.103 MW/m2  to 0.209 MW/m2.  We also demonstrate how this rectification  scales with injected  HHFW  power by tracking  probe characteristics during  a HHFW power ramp;  the  rectified  current may be clamped  at  a certain  level.  This  work is important for minimizing SOL losses of HHFW  power in NSTX-U but  may also have implications  for near-field studies  of ICRF  antennae: ignoring rectified current may lead to underestimated heat  fluxes and overestimated rectified voltages.
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