An overview of experimental ICRF research on NSTX-U

The National Spherical Torus eXperiment Upgrade (NSTX-U) will begin operation in 2015. The same twelve-strap 30 MHz fast-wave antenna from NSTX will be available on NSTX-U with up to 6 MW of source power; however the higher magnetic-field strength of NSTX-U will put the waves at the fourth to sixth harmonic of the ion cyclotron frequency. This regime, the "high harmonic fast wave" (HHFW) regime, is intermediate to conventional ICRF heating (minority heating, second-harmonic heating) and fast waves in the lower hybrid frequency range (helicon current drive), which makes for good comparison of fast-wave physics across a broad range of machines. Additional grounding points have been added to the HHFW antenna to improve the high-voltage standoff. Diagnostic upgrades include an infrared camera to monitor the heat flux to the antenna, high-frequency Langmuir divertor probes with electronics suitable for detection of 30 MHz waves, a wide-angle IR camera for edge loss studies, and a reflectometer suited for SOL density profile measurements. Planned ICRF experiments will first focus on characterization of SOL losses of HHFW power and later to study interactions between beam ions and fast waves and well as solenoid-free start-up. Significant losses of HHFW power were sometimes observed along SOL field lines in NSTX, leading to bright and hot spirals on both upper and lower divertors. The diagnostic upgrades described above will allow for a quantitative characterization of these losses and help determine optimum conditions for coupling HHFW power into H-mode plasmas. Later experiments will focus on the interaction of beam ions with fast waves, both as a power-loss mechanism and as a potential tool to influence fast-ion modes, as well as solenoid-free start-up. Other more technical issues to be addressed on NSTX-U are the compatibility of the HHFW antenna with the new neutral beam (and higher level of NBI power), high-voltage standoff of the antenna, and performance of the double-feed antenna in boronized and lithiated conditions. Additionally, work on a test stand will elucidate the role of induced currents in the antenna sidewalls on the launched wave spectrum and on outgassing. (author)