Magnetohydrodynamic simulation of two capillary discharge channels

Abstract

Two plasma channels, corresponding to a slow capillary discharge and a fast capillary discharge plasma, are investigated numerically. A long-lifetime parabolic plasma channel is formed in the slow capillary discharge channel. Its radial density profile is determined by the energy balance between Ohmic heating and thermal conduction, together with force balance. The profile and the on-axis density value vary only weakly with the peak value of the slow discharge current (300–600 A). A rather dynamic plasma channel is formed under the compression by a magnetic piston in the fast capillary discharge channel. In a narrow time window of the order of nanoseconds a suitable hollow relatively low density channel is reached, followed by a shock-dominated rebound phase. The on-axis density minimum features of these two channels make both of them suitable guiding channels for the laser wakefield accelerators. In particular, a low-density channel ( $n_{\rm e}\sim 5\times 10^{16}~{\rm cm}^{-3}$ ) formed in the fast capillary discharge is in favor of controlled beam injection, but requires precise timing, while the slow capillary discharge is in essence steady. We discuss possible strategies for using them in a segmented approach.