Emission in the ion cyclotron range of frequencies (ICE) on NSTX and NSTX-U

Abstract

We report here on observations of magnetic fluctuations in the ion-cyclotron frequency range on NSTX and NSTX-U. In many respects, the fluctuations appear similar to the ion cyclotron emission (ICE) seen in conventional tokamaks. However, a significant difference between previous observations of ICE and the ICE on NSTX is that the frequency of ICE in conventional tokamaks is typically near the ion cyclotron frequency of the energetic fast ions at the plasma edge. In NSTX and NSTX-U, the magnetic fluctuation frequency corresponds to the ion cyclotron frequency deeper in the plasma, near the location of an internal transport barrier. As on conventional tokamaks, higher harmonics of the deuterium cyclotron frequency, as high as the seventh, are seen with the strongest signal sometimes from higher harmonics. The emission usually appears as an irregular sequence of short bursts typically ≤100 μs in duration although nearly continuous emission for several ms has also been seen under some conditions. Measurements of the emission with a toroidal array of fast probes show that the emission is a long wavelength, spatially coherent mode. The emission frequency does not follow an Alfvénic scaling with density, as seen for compressional Alfvén eigenmodes, but does show a linear scaling with local magnetic field strength. The measured emission shows a compressional polarization consistent with a compressional Alfvén wave. No correlation between the neutron rate and the ICE amplitude is seen. Three-wave coupling between the instability responsible for the ICE and lower frequency modes has also been observed.