EXPERIMENTAL STUDIES OF LINEAR BEAM-PLASMA INSTABILITIES IN A MAGNETIC FIELD

The interactions of an electron beam with a bounded, uniform, equilibrium plasma have been studied experimentally by varying several of the physical parameters that appear in the dispersion equation. This equation (which takes into account the geometry, the magnetic field, and collisions, but which neglects temperature), is solved for complex wavenumber roots in order to compare the wave variables w, kτ, and ki with their experimental counterparts. The only instability that has been observed with certainty is a convective one having a frequency f below both fpe and fce, all of which are in the neighborhood of 1 GHz. This frequency, and the associated wavelength and axial growth rate as well, are in reasonable agreement with the calculated values, if the absolute values of the effective plasma frequency as obtained from probe measurements are correct. Data are presented on the behavior of the wave properties with time, distance, beam voltage, frequency, and plasma and cyclotron frequencies; and the existence of several nonlinear effects is noted.