Suppression of Alfvén Modes on the National Spherical Torus Experiment Upgrade with Outboard Beam Injection

In this Letter we present data from experiments on the National Spherical Torus Experiment Upgrade, where it is shown for the first time that small amounts of high pitch-angle beam ions can strongly suppress the counterpropagating global Alfvén eigenmodes (GAE). GAE have been implicated in the redistribution of fast ions and modification of the electron power balance in previous experiments on NSTX. The ability to predict the stability of Alfvén modes, and developing methods to control them, is important for fusion reactors like the International Tokamak Experimental Reactor, which are heated by a large population of nonthermal, super-Alfvénic ions consisting of fusion generated $\alpha$’s and beam ions injected for current profile control. We present a qualitative interpretation of these observations using an analytic model of the Doppler-shifted ion-cyclotron resonance drive responsible for GAE instability which has an important dependence on $k_{\perp }{\rho }_L$. A quantitative analysis of this data with the hym stability code predicts both the frequencies and instability of the GAE prior to, and suppression of the GAE after the injection of high pitch-angle beam ions.

Figure 1

Sketch of neutral beam geometry. Original NSTX beams in green, labeled 1a, 1b, 1c; new beams for NSTX-U shown in red labeled 2a, 2b and 2c.

Figure 2

(a) Color-coded spectrogram showing counterpropagating GAE activity. Dominant modes are $n=-10$ (green) and $n=-11$ (blue). (b) Green curve is inboard beam power, red curve is the off-axis beam power (source 2c).

Figure 3

(a) Color-coded spectrogram showing GAE activity. Dominant modes are $n=-10$ (green), $n=-11$ (blue), $n=-12$ (cyan). (b) rms amplitude of GAE, pink bars indicate when the outboard source is on. (c) Injected beam power, green is the total inboard source power, red shows outboard beam pulses (source 2b).

Figure 4

transp fast ion distributions for before and after the outboard beam injection. The dashed blue line indicates fast ions with $k_{\infty }{\rho }_L=1.9$. Distribution functions are near the minimum of the continuum for the $n=-11$ mode. Contours are labeled in units of ${10}^7{cn}^{-3}{\mathrm{eV}}^{-1}{\mathrm{dA}}^{-1}$.

Figure 5

(a) Growth rates and (b) frequencies of unstable counter-GAEs from hym simulations for $t=0.44\mathrm{s}$. Blue line is Doppler-shift corrected frequencies, points—experimental values, (c) GAE burst with fit to exponential growth (red).