Nanosecond Intense Relativistic Electron Beam Propagation in Gas and Preformed Plasma

An experimental and numerical study on the propagation of a typical flash radiography electron beam in helium gas and a preformed plasma is presented, in the context of a plasma lens applicability for such beams. The experiments consist of measuring the beam profile at the exit of an interaction chamber as a function of time, for various gas pressure and plasma densities. The plasma is produced before the beam’s arrival in the gas at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 1 \times 10^{-3} \,\mathrm {mbar}$</tex-math> </inline-formula> by a seeded, pulsed inductive discharge. The plasma density reaches a peak value of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$(1.2~\pm~0.4) \times 10^{12} \,\mathrm {cm}^{-3}$</tex-math> </inline-formula> then decreases exponentially over several tens of microseconds. This allows probing the propagation in different plasma densities by varying the delay between the plasma discharge and the electron beam. The influence of the preformed plasma on the beam propagation is experimentally demonstrated, as well as an evolution of the propagation at the tens of nanoseconds time scale. Simulations are carried out in the regime where the plasma density (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2 \times 10^{10} \,\mathrm {cm}^{-3}$</tex-math> </inline-formula> or less) is lower than the beam density, using a full kinetic description with a particle-in-cell (PIC)/Monte-Carlo code. The simulations reproduce the main features of the observations, and show that after the effect of the fast plasma electron response (at the nanosecond time scale), the evolution of the propagation at the tens of nanoseconds time scale is mainly due to the plasma ion redistribution driven by the beam space charge.