ECR breakdown of a low-pressure gas in a mirror confinement system with a longitudinal microwave power injection

The effect of the gas pressure and location of the ECR region (at a constant level of the microwave power) on the initiation of the ECR discharge in a mirror device is studied. The microwave (MW) radiation is injected in the longitudinal direction. Good agreement is demonstrated between experimental results and theoretical estimates for the breakdown threshold in a low-pressure gas; these estimates take into account losses of electrons in their resonant interaction with the MW pumping. It is found that a dielectric window positioned in the magnetic mirror region and intended for the injection of the MW radiation into the vacuum chamber greatly affects the discharge.