NONUNIFORM DENSITY DISTRIBUTION MODEL IN AN INVESTIGATION OF NONNEUTRAL ELECTRON PLASMA COLUMNS SUPPORTED BY A PENNING DISCHARGE (CONFINEMENT, ELECTRONICS).

摘要

Study of the electron plasma in the plasma lens is a continuation of Lefevre and Connolly's work.('1) The lens tube with tapered elec- trodes was designed so that the ends of the electrodes are all located on the same magnetic flux tube with diameter of 2.6 cm at the center. The electron plasma is confined axially by electrode potentials and radially by a nonuniform magnetic field. The nonuniform magnetic field is necessary to achieve an almost pure electron plasma. The nonuniform magnetic fields cause the plasma density distribution to be nonuniform not only radially, but also axially. The electrode potentials impose a boundary condition to the plasma and dictate the plasma shape. For a stable plasma mode, the plasma surface follows a flux tube in some range of the z axis and gradually shrinks towards the end. Plasma modes whose surfaces almost follow flux tubes have been found. Configuration instability is discussed for the plasma whose surface expands outward from the flux tube.;Electron plasma density is limited by the plasma radius and its temperature. The plasma density limit is derived based on Poisson's equation with one assumption about the plasma temperature. At.;any given plasma radius n(,0)/B('2) is a constant. This relation has been observed experimentally.;('1)H. W. Lefevre et al., Nucl. Instr. Meth. 218, 39 (1983). B10/11, 707 (1985). Also see R. C. Connolly, Ph.D. thesis University of Oregon, (1985).;Positive ions are dynamically concentrated near the plasma axis. An estimate of the order of magnitude of the dynamic ion concentra- tion is given based on the experimental data and the nonuniform model. The estimated dynamic ion concentration near the axis is about 1-3% of the electron density there at a gas pressure of about 9 x 10('-8) Torr. The ion concentration will vary with plasma mode and gas pressure. The plasma temperature has a combined effect: an increase in the plasma temperature may cause an increase in the rate of ion production due to an increase of the ionization coeffi- cient; but it also increases the ion loss rate and the pumping rate of the plasma.