Effects of e-beam temperature on plasma heating

A series of experiments showed that the energy transfer from beam to plasma is greatest when the beam has the least perpendicular energy (the lowest temperature). The increase in perpendicular energy was due to beam scattering, which was caused by two effects: the anode foil, where the beam first entered the plasma, and by the beam-plasma interaction itself. The foil-scattering produced a predictable degradation of plasma heating, as measured by charge-exchange analysis, diamagnetic loops and 90° laser scattering. Heating levels were roughly one third as large with a 2 mil titanium anode foil as with a 1/2 mil foil. Scattering of beam electrons by the two-stream interaction in the plasma was very large, leading to trapping as much as 20% of the beam in the 2:1 magnetic mirror for at least two reflections. The interaction occurred during the first 20 to 30 cm of beam penetration, with little energy deposited in the plasma over the remaining 100 an of passage through the uniform plasma column.