Electron Bernstein Wave Research on Overdense Plasmas in the National Spherical Torus Experiment
Authors: G. Taylor, T.S. Bigelow, J.B. Caughman, M.D. Carter, S. Diem, P.C. Efthimion, R.A. Ellis, N.M. Ershov, E. Fredd, R.W. Harvey, J. Hosea, F. Jaeger, B. LeBlanc, C.K. Phillips, J. Preinhaelter, A.K. Ram, D.A. Rasmussen, A.P. Smirnov, J. Urban, J.B. Wilgen, and J.R. Wilson
Date of PPPL Report: June 2006
Presented at: the Fourteenth Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Heating (EC14), 9 – 12 May 2006, Santorini, Greece.
Off-axis radiofrequency-driven current is expected to be critical for stabilizing and sustaining solenoid-free spherical torus (ST) plasmas when β > 20%. Electron Bernstein wave current drive (EBWCD) may be able to generate the required off-axis current in the overdense (ωpe >> ωce) ST plasma regime. Fokker-Planck and 3-D EBW ray-tracing models and EBW emission (EBE) measurements are being used to study EBW coupling, propagation, damping and EBWCD physics in NSTX. This research supports the proposed implementation of a multi-megawatt EBWCD system on NSTX that utilizes EBW coupling via obliquely launched, elliptically polarized, electromagnetic waves and efficient off-axis current generation via Ohkawa EBWCD. EBWCD modeling for plasmas with β of 20–40% and launch frequencies between 14 and 28 GHz predicts current drive efficiencies of 30–50 kA/MW and EBW-driven current densities that peak well off-axis on the outboard side of the NSTX plasma. The thermal EBE coupling efficiency has been measured to be 80±20% at 16–18 GHz, with the coupling efficiency and emission polarization being in good agreement with modeling. The NSTX EBE diagnostic has recently been upgraded to include two remotely steered, quad-ridged antennas, capable of detecting EBE at 8–18 and 18–40 GHz.