Reflectometer sensing of rf waves in front of the high harmonic fast wave antenna on NSTX

Abstract

The ability to measure rf driven waves in the edge of the plasma can help to elucidate the role that surface waves and parametric decay instabilities (PDIs) play in rf power losses on NSTX. A microwave reflectometer has recently been modified to monitor rf plasma waves in the scrape-off layer in front of the 30MHz high harmonic fast wave antenna array on NSTX. In rf heated plasmas, the plasma-reflected microwave signal exhibits 30MHz sidebands, due primarily to the modulation of the cutoff layer by the electrostatic component of the heating wave. Similarly, electrostatic parametric decay waves (when present) are detected at frequencies below the heating frequency, near 28, 26,…MHz, separated from the heating frequency by harmonics of the local ion cyclotron frequency of about 2MHz. In addition, a corresponding frequency matched set of decay waves is also detected near the ion cyclotron harmonics at 2, 4,…MHz. The rf plasma-wave sensing capability is useful for determination of the PDI power threshold as a function of antenna array phasing (including toroidal wavelength), outer gap spacing, and various plasma parameters such as the magnetic field and the plasma current.

Spectral effects on fast wave core heating and current drive

“…This density is above the critical density for propagation consistent with the conclusion that high edge densities lead to increased edge power losses and corresponding degradation of the core heating efficiency. With additional edge conditioning obtained with lithium injection, a further reduction in the edge density at -30˚ phasing to 0.5 × 10 18 m -3 was observed with the edge reflectometer [20], resulting in the first significant heating in D plasmas at this phasing, as indicated by the open red circle at -30˚ in figure 6. However, the rf pulse length in the discharges with Li conditioning were only about 67 msec in duration, compared to the 230 msec duration for the scan of deuterium discharges shown in figure 6.…”

Transition to ELM-free improved H-mode by lithium deposition on NSTX graphite divertor surfaces

“…Displayed in Fig. 5 are two edge density profiles measured with an X-mode reflectometer in 129024 during ELM activity and in 129041 with no ELMs [22]. Distinct changes in shape and magnitude of the SOL density indicate strong pumping due to evaporated lithium.…”

Using X-mode L, R and O-mode reflectometry cutoffs to measure scrape-off-layer density profiles for upgraded ORNL reflectometer on NSTX-U

“…The reflectometer electronics, combined with a separate 100 MHz digitizer, was then upgraded in 2005-2006 to also detect sub-30 MHz RF waves and parametric decay instabilities in order to help understand lower than expected heating and current drive efficiencies. 6 It is desired to maintain and improve the measurement capabilities of this reflectometer for NSTX-U operation. 7 NSTX-U will operate at toroidal magnetic fields up to 1 T, plasma currents of 2 MA, and outer boundary radius of 1.574 m. Since 6-27 GHz X-mode R-cutoff was originally used for NSTX toroidal magnetic fields up to 0.55 T and plasma currents of 1 MA, it was expected that ∼10-40 GHz X-mode R-cutoff would be necessary to maintain the capability to measure SOL densities at the increased magnetic fields.…”