Abstract
High harmonic fast wave heating and current drive CD are being developed on the National
Spherical Torus Experiment M. Ono et al., Nucl. Fusion 41, 1435 2001 for supporting startup
and sustainment of the spherical torus plasma. Considerable enhancement of the core heating
efficiency from 44% to 65% has been obtained for CD phasing of the antenna strap-to-strap
=−90°, k=−8 m−1 by increasing the magnetic field from 4.5 to 5.5 kG. This increase in
efficiency is strongly correlated to moving the location of the onset density for perpendicular fast
wave propagation nonsetBk
2 / away from the antenna face and wall, and hence reducing the
propagating surface wave fields. Radio frequency RF waves propagating close to the wall at lower
B and k can enhance power losses from both the parametric decay instability PDI and wave
dissipation in sheaths and structures around the machine. The improved efficiency found here is
attributed to a reduction in the latter, as PDI losses are little changed at the higher magnetic field.
Under these conditions of higher coupling efficiency, initial measurements of localized CD effects
have been made and compared with advanced RF code simulations.