Remote Feedback Stabilization of the Ion-Sound Instability by a Modulated Source at the Electron-Cyclotron Resonance Frequency

Feedback stabilization results are presented for an ion-sound instability. Sensing of the instability was achieved by a photodiode outside the plasma, and the suppressor element was an electron-cyclotron resonance structure remote from the plasma. Radiation from this structure heats the electrons locally and feedback is achieved by amplitude modulation of the electron-cyclotron resonance at the instability frequency. A theory is developed which predicts the variation of the instability amplitude and frequency in terms of the change in electron temperature $\Delta T$, fractional modulation $f$, and phase change in the loop, $\varphi$. Comparison between theory and experiment shows good agreement.