Stabilization of Alfvén Eigenmodes in DIII-D via Controlled Energetic Ion Density Ramp and Validation of Theory and Simulations

Fast-ion driven Alfvén waves with frequency close to the ion cyclotron frequency ($f=0.58f_{ci}$) excited by energetic ions from a neutral beam are stabilized via a controlled energetic ion density ramp for the first time in a fusion research plasma. The scaling of wave amplitude with injection rate is consistent with theory for single mode collisional saturation near marginal stability. The wave is identified as a shear-polarized global Alfvén eigenmode excited by Doppler-shifted cyclotron resonance with fast ions with sub-Alfvénic energetic ions, a first in fusion research plasmas.

Figure 1

(a) $|\delta b{|}^2$ spectrum on a logarithmic scale showing a highly coherent spectrum of high-frequency Alfvén eigenmodes at $f\sim 5550\mathrm{kHz}$, under a 60 dB power mask. Mode power $|\delta b{|}^2$ vs time (b) and beam injection rate vs time (c). Mode onset is $\sim 10\mathrm{ms}$ after turn-on of the 150RT beam, and the mode shuts off once beam injection rate crosses a threshold of $\sim 18.5\mathrm{A}$.

Figure 2

Mode power $|\delta b{|}^2$ versus beam injection rate $I_b$ shows that the mode is unstable for a threshold of $I_b\sim 18.5\mathrm{A}$. The theoretical scaling $|\delta b{|}^2\sim (1-I_{b,\text{threshold}}/I_b)$ is shown in red.

Figure 3

Fast-ion density as a function of pitch and energy, averaged over $\rho <0.3$, after mode onset at $t=2261\mathrm{ms}$. Resonance lines for $p=0$, 5, $n=-28$ are plotted.