Beam scattering and heating at the front of an electron beam injected into a plasma

A dense pulsed electron beam (tr≤20 ns, Vb=200 V, ib≤2 A, nb/ne≤10%) is injected into a uniform magnetoplasma (ne=1-4x1010 cm-3, kTe≤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 (nb/ne>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 (fB=nfpe), which collapses into broadband electrostatic turbulence during the onset of beam scattering. A low-frequency electrostatic wave mode (fpi<fLF<fpe) 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