Confinement of a Hot Electron Plasma in a Mirror Machine, DECA 1

Abstract

Plasma, produced by a hydrogenated titanium washer gun, was compressed between two pulsed magnetic mirrors (rise time : 150 μsec, decay time : 6 msec). For weak initial magnetic fields (below 150 G), and large compression ratios (above 100), the resulting plasma consists of very energetic electrons and of colder ions; it is confined during several milliseconds. Typically, with an initial field of 30 G antiparallel to a compression field of 10 kG, the compressed plasma has a diameter of 1.5 cm and a length equal to or less than 7 cm; the average hot electron density is (4 ± 2) × 1011 cm−3, with mean energy of 45 ± 10 keV; the density of the cold plasma, which might be present around the compressed plasma, is less than 1011 cm−3. Calculations show moreover that, in addition to the adiabatic heating, an important nonadiabatic acceleration occurs at the beginning of the compression. When the initial density is higher than 1013 cm−3, the energetic electrons do not appear. One observes, then, large radial leaks which might be caused by an instability. Such conditions correspond to a β which is higher than 1 to 3% for the compressed plasma.