Boundary Condition Effects in a Magnetic Mirror Compression Experiment

Abstract

The transition from unstable operation to quasi-stable plasma containment in a plasma-injected magnetic compression experiment has been found to be dependent upon the density and lifetime of plasma external to the confinement chamber. Transverse motion of the confined plasma is limited and the magnetic compression proceeds nearly adiabatically, provided the density of the external plasma is greater than ∼0.1% of the confined plasma. If the electrical connection to external conducting surfaces is lost after the magnetic compression is well under way (later than 50 μsec after beginning of compression for a magnetic field rise time of 120 μsec), the trapped plasma suddenly moves to the wall between the magnetic mirrors giving dramatic evidence of instability. In any case, during the magnetic compression the shifting external plasma-wall contact results in intermittent particle and/or energy losses from the hot plasma. Later than 100 μsec, the untrapped portion of the injected plasma which has been cooled, neutralized, and contaminated through its interaction with vacuum chamber walls diffuses into the containment chamber. Thus, at all times, there are loss processes in competition with the magnetic compression.