Plasma response to the injection of an electron beam

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Abstract
The response of a plasma to the sudden injection of an electron beam was systematically studied using a Vlasov-Poisson solver in one-dimension. The temporal response of the plasma was dramatic and included the following processes: (1) The formation of ion bursts; (2) The creation of electrostatic electron shocks and holes, depending on the beam density; (3) A strong plasma heating and the consequent plasma expulsion from the simulation region; (4) The creation of density gradients due to the plasma expulsion; (5) The formation of a caviton through the pondermotive force of long wavelength fast oscillations and the consequent steepening of the density gradient; (6) The propagation of the steep density front and solitary pulse down the density gradient; and (7) The excitation of the Buneman mode and the subsequent formation of a virtual cathode and double layers. In the same simulation several types of double layers occur as the plasma evolves through various stages, including monotonic double layers, double layers with a dip on the low potential side, and double layers with a bump on the high potential side. When the electron current density is large, multiple double layers are common. On the other hand, when the electron current density decreases owing to disruptive plasma processes, a single strong double layer forms. Such double layers have localized negative potential dips at their low potential ends. Simulations carried out with several beam velocities indicate that the double layer strength approximately scales with the beam energy.