4.4 Feedback Experiments on a High Energy Plasma Contained in the Phoenix II Simple Mirror Machine

Experiments using a single feedback loop, by which electrostatic signals from a plasma were amplified and fed back as potentials to the plasma boundary, have been carried out in the PHOENIX II mirror machine working as a simple mirror. With a flute unstable plasma the experiments show that, with feedback the density at which appreciable losses were observed increased by a factor of two. However, a residual instability was always present. Theoretical investigations, using a simple plasma model, show that the experimental plasma‐feedback system will always be unstable and, in particular, the density threshold of the instability is lower with feedback than that without; the lower plasma loss rate is thought to be due to the instability frequency being displaced away from the magnetic drift frequency of the ions. Measurements of the effect of the feedback system on a normally stable plasma showed that the system was unstable at frequencies close to those predicted by theory. It is concluded that a simple elec...