High-harmonic fast magnetosonic wave coupling, propagation, and heating ina spherical torus plasma

A novel rotatable two-strap antenna has been installed in the current drive experiment upgrade (CDX-U) [T. Jones, Ph. D. thesis, Princeton University (1995)] in order to investigate high-harmonic fast wave coupling, propagati on, and electron heating as a function of strap angle and strap phasing in a spherical torus plasma. Radio-frequency-driven sheath effects are found t o fit antenna loading trends at very low power and become negligible above a few kilowatts. At sufficiently high power, the measured coupling efficien cy as a function of strap angle is found to agree favorably with cold plasm a wave theory. Far-forward microwave scattering from wave-induced density f luctuations in the plasma core tracks the predicted fast wave loading as th e antenna is rotated. Signs of electron heating during rf power injection h ave been observed in CDX-U with central Thomson scattering, impurity ion sp ectroscopy, and Langmuir probes. While these initial results appear promisi ng, damping of the fast wave on thermal ions at high ion-cyclotron-harmonic number may compete with electron damping at sufficiently high ion beta-pos sibly resulting in a significantly reduced current drive efficiency and pro duction of a fast ion population. Preliminary results from ray-tracing calc ulations which include these ion damping effects are presented. (C) 1999 Am erican Institute of Physics. [S1070-664X(99)97305-6].