|Ph.D Student||Ilan Be'ery|
|Subject||Active Feedback Stabilization of Magnetically Confined|
|Department||Department of Physics||Supervisors||Professor Emeritus Ron Amiram|
|Full Professor Amnon Fruchtman|
|Dr. Amnon Fisher|
|Full Thesis text|
Axi-symmetric mirror machines are the simplest magnetic confinement devices, and as such could be attractive for fusion machines, ions separation, and more. However, mirror machines suffer from particle and energy loss due to the violent flute instability. This work demonstrates for the first time the stabilization of fast growing, slowly rotating flute instability by multi-input, multi-output active feedback system. Step response of the plasma to the actuators’ input and MHD-spectroscopy experiments indicates a non-linear temporal response with delay of 10-20 ms. This response it explained by the gradual charging of plasma virtual electrodes through the sheath-limited current of the feedback electrodes. A simplified model predicts that the feedback-plasma system should be unstable on short time scales, but stable on longer times. Indeed, the power spectra of feedback-stabilized plasma shows the reduction of power by feedback at low frequencies, but the increase of power at higher frequencies. Using local, proportional feedback algorithm, an almost complete suppression of m=1 mode and significant simultaneous suppression of m=1,2 modes are demonstrated. In a cold and dense plasma, the suppression of the flute instability does not increases the plasma density, which is governed by particle loss through the mirrors. In lower density plasma, heated by ion-cyclotron RF, the life-time of the plasma is dominated by the flute instability and consequently the feedback increases the plasma density by up to 40%. Analysis of the required feedback power and its scaling to fusion-scale plasma traps indicates that the demonstrated temporal response should be suffice and the feedback power should be only a few percent of the heating power. A future research is intended at feedback-stabilize hotter plasma with sensors and actuators which will be applicable to keV-scale hydrogen plasma.