BEAM SCATTERING AND HEATING AT THE FRONT OF AN ELECTRON-BEAM INJECTED INTO A PLASMA

A dense pulsed electron beam (t(r) less-than-or-equal-to 20 ns, V(b) = 200 V, i(b) less-than-or-equal-to 2 A, n(b)/n(e) less-than-or-equal-to 10%) is injected into a uniform magnetoplasma (n(e) = 1 - 4 x 10(10) cm-3, kT(e) less-than-or-equal-to 0.5 eV, B0 = 90 G) to study wave-particle interactions that occur at the front of the beam on short-time scales during which the ions are regarded as fixed and immobile. Above a threshold beam density (n(b)/n(e) > 5%), the beam front is observed using optical diagnostics to rapidly erode due to beam scattering and energetic tail production, and broaden into a warm front that propagates slower than its injected speed. Using radio-frequency (RF) antennas, the beam is observed to bunch due to the streaming beam-plasma instability and create a ballistic beam mode (f(B) = nf(pe)), which collapses into broadband electrostatic turbulence during the onset of beam scattering. A low-frequency electrostatic wave mode (f(pi) < f(LF) < f(pe)) is also detected that is coupled to the scattering process. These measurements show that rich beam-plasma phenomena occur in this electron time-scale regime, which are not adequately addressed by current plasma theory.