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.

Analytic stability boundaries for compressional and global Alfvén eigenmodes driven by fast ions. I. Interaction via ordinary and anomalous cyclotron resonances

“…A mixture of high frequency compressional (CAE) and global (GAE) Alfvén eigenmodes were commonly observed on the spherical tokamaks NSTX(-U) [1][2][3][4][5][6][7][8][9][10][11] and MAST. [12][13][14] These modes may propagate either in the same direction as the plasma current and neutral beam injection (co-propagating) or opposing it (cntrpropagating).…”

Analytic stability boundaries for compressional and global Alfvén eigenmodes driven by fast ions. II. Interaction via Landau resonance

“…High frequency co-propagating compressional Alfvén eigenmodes (CAE) have been observed in the spherical tokamaks NSTX(-U) [1][2][3] and MAST. [4][5][6] These instabilities are more easily excited on these devices than conventional tokamaks due to their lower magnetic fields and large neutral beam power, which together generate a substantial population of super-Alfvénic fast ions (v 0 /v A = 2 − 6).…”

First observation and interpretation of spontaneous collective radiation from fusion-born ions in a stellarator plasma

“…There are some notable apparent parallels between recent ICE measurements from NSTX-U and the observed link between the LHD bursting ICE locale and the "Tongue" that we report here. Submillisecond bursts of ICE driven by marginally super-Alfvénic NBI deuterons in NSTX-U were observed [75] to be spatially colocated with internal transport barriers that are well within (by tens of centimetres) the plasma. The redistribution of the initially helically-trapped ions that are implicated in burst kinetic MHD [16,17,19] or abrupt tongue deformation [21,22] could generate a distinct, transient, highly non-Maxwellian distribution in velocity space, perhaps involving the expelled ions and freshly trapped ions.…”

“…of[75] display striking measurements of the correlation of the gently time-varying radial location of the ICE emission with the location where contours of electron density are concentrated, together with the steep ion temperature gradient. The experimental results from the present paper thus share several intriguing physical features with the results reported in Ref [75]…”