Critical behavior near the instability threshold of an electron cyclotron maser formed in a mirror magnetic trap.

High-power electron cyclotron resonance discharge in a laboratory open trap is frequently accompanied by growth of electron cyclotron instabilities. We study a simple universal model that explains some of the stability conditions of such plasma observed experimentally, in particular, its crucial dependence on localization of the rf power deposition zone with respect to a minimum of B. An instability threshold seems to be sensitive to particular details of electron losses in a velocity space. Comparing theoretical predictions for the threshold condition with available measurements sheds light on a structure of fast-electron losses in ongoing experiments.