Electron energy distributions measured during electron beam/plasma interactions

In the large vacuum facility at the NASA-Johnson Space Center an electron beam was projected approx.20 m parallel to B from a gun with variable accelerating potential (1.0 to 2.5kV) to an aluminum target. The ionospheric neutral pressure and field were approximated. Beam electron energy distributions were measured diretcly using an electrostatic deflection analyzer and indirectly with a detector that responded to the X-rays produced by electron impact on the target. At low currents the distribution is sharply peaked at the acceleration potential. At high currents a beam plasma discharge occurs and electrons are redistributed in energy so that the former energy peak broadens to 10--15 percent FWHM with a strongly enhanced low energy tail. At the 10% of maximum point the energy spectrum ranges from <1/2 to approx.1.2 times the gun energy. The effect is qualitative the same at all pitch angles and locations sampled.