HHFW Heating and Current Drive Studies of NSTX H-Mode Plasmas
Authors: G. Taylor, P.T. Bonoli, D.L. Green, R.W. Harvey, J.C. Hosea, E.F. Jaeger, B.P. LeBlanc, R. Maingi, C.K. Phillips, P.M. Ryan, E.J. Valeo, J.R. Wilson, J.C. Wright, and the NSTX Team
Abstract:
30 MHz high-harmonic fast wave (HHFW) heating and current drive are being
developed to assist fully non-inductive plasma current (Ip ) ramp-up in NSTX. The initial
approach to achieving this goal has been to heat Ip = 300 kA inductive plasmas with current
drive antenna phasing in order to generate an HHFW H-mode with significant bootstrap and RFdriven
current. Recent experiments, using only 1.4 MW of RF power (PRF ), achieved a noninductive
current fraction, fNI ~ 0.65. Improved antenna conditioning resulted in the generation
of Ip = 650 kA HHFW H-mode plasmas, with fNI ~ 0.35, when PRF ≥ 2.5 MW. These plasmas
have little or no edge localized mode (ELM) activity during HHFW heating, a substantial
increase in stored energy and a sustained central electron temperature of 5-6 keV. Another focus
of NSTX HHFW research is to heat an H-mode generated by 90 keV neutral beam injection
(NBI). Improved HHFW coupling to NBI-generated H-modes has resulted in a broad increase in
electron temperature profile when HHFW heating is applied. Analysis of a closely matched pair
of NBI and HHFW+NBI H-mode plasmas revealed that about half of the antenna power is
deposited inside the last closed flux surface (LCFS). Of the power damped inside the LCFS
about two-thirds is absorbed directly by electrons and one-third accelerates fast-ions that are
mostly promptly lost from the plasma. At longer toroidal launch wavelengths, HHFW+NBI
H-mode plasmas can have an RF power flow to the divertor outside the LCFS that significantly
reduces RF power deposition to the core. ELMs can also reduce RF power deposition to the core
and increase power deposition to the edge. Recent full wave modeling of NSTX HHFW+NBI
H-mode plasmas, with the model extended to the vessel wall, predicts a coaxial standing mode
between the LCFS and the wall that can have large amplitudes at longer launch wavelengths.
These simulation results qualitatively agree with HHFW+NBI H-mode data that show
decreasing core RF heating efficiency and increasing RF power flow to the lower divertor at
longer launch wavelengths.
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Submitted to: AIP Proceedings of the 19th Topical Conference on Radio Frequency Power in Plasmas. Presented at: 19th Topical Conference on Radio Frequency Power in Plasmas (June 1-3, 2011)
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Download PPPL-4634 (pdf 3.32 MB 8 pp)
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