Recent Results on the Beam-Plasma-Discharge

The beam-plasma-discharge (BPD) studied at M.I.T. uses an electron beam launched from a cathode operating under near vacuum conditions (space charge limited). The gun is located at the one peak of a magnetic mirror (2 ≲ R ≲ 6) and the beam collector is at the other peak. When gas is fed into the system continuously the minimum pressure for establishing a discharge is about 10-4 Torr. Under these conditions, the maximum beam power input is limited by an ''axial breakdown'' - plasma flows into the gun so that the cathode current increases by factors of 2 -10; this loads down the beam power supply and drastically reduces the power input to the beam. The axial breakdown can be essentially eliminated by introducing a metered amount of gas into the beam collector by a fast acting Marshall valve. In this mode of operation the initial ionization takes place within the beam collector where the pressure is very high and the plasma rapidly flows along the B lines into the region between the mirrors. The ambient gas pressure around the plasma can be kept below 10-6Torr during the beam pulse. The resulting plasma exhibits very long stable decays, up to 2 sec, characteristic of the energetic electrons. Data describing some of the interesting aspects of the pulsed gas BPD is presented. Electron densities of 1013/cm3 are achieved, and energy densities of 1010/cm3 (in units of nxV/cm3) are common. From our studies of the plasma decay, the energetic electron density (several keV and upwards) is about 1010/cm3. Although the electron beam is about 2.5 cm in diameter in the centre of the mirror, the energetic plasma occupies a diameter in excess of 15-20 cm. The dependence of plasma kinetic energy on electron beam power and on magnetic field strength is discussed. In general, there is a monotonie (but less than linear) increase of W⊥ with beam power. For equal beam power levels, also increases with magnetic field strength (constant R). However, the dependence upon B is clouded by the fact that the perveance of the magnetion injection gun decreases with increasing field strength. (author)