Electrostatic structure of a magnetized laser-produced plasma

Measurements of the structure of the electrostatic fields produced by the expansion of a laser-produced plasma into a background magnetized plasma are presented. The three-dimensional measurements of the electrostatic field are made using an emissive probe that measures the time-varying plasma potential on two orthogonal planes, one across and one containing the background magnetic field. The inductive electric field is also calculated from probe measurements of the time-varying magnetic fields. Deviations from local charge neutrality at the level of ${10}^{-4}$ generate a radial electrostatic field with peak strength an order of magnitude larger than the corresponding inductive field. The electrostatic energy density near full expansion is over an order of magnitude larger than that of the induced azimuthal electric field. These measurements show that electrostatic fields must be included in theoretical and computational models of collisionless coupling in magnetized point explosions of laser-produced plasmas and their relation to similar phenomena such as magnetospheric chemical releases.

Figure 1

(a) False color visible light emission with carbon target drawn in white and (b) cartoon rendering of the experimental arrangement. Blue planes depict planes in which data were acquired with $\delta x=\delta z=\delta y=2$ mm resolution. The origin is taken to be the laser focal point. The laser enters $\sim {30}^{\circ }$ from the $z$ axis in the $xz$ plane, on the side away from the diagnostic planes. The transparent ellipsoid denotes the approximate size of the LPP at the time of peak diamagnetism: ${\tau }_D=240\mathrm{ns}$.

Figure 2

Vector plot of electrostatic potential gradients in a plane at $z=2$ cm (see Fig. 1) for two times (a) $t=160\mathrm{ns}$ and (b) $t={\tau }_D=240\mathrm{ns}$.

Figure 3

Vector plot of electrostatic electric field in a plane at $y=0$ (see Fig. 1) for times (a) $t=120\mathrm{ns}$ and (b) $t={\tau }_D=240\mathrm{ns}$, with $B_0=-750G\widehat{z}$.

Figure 4

(a) An $xy$ plane of the calculated charge density relative to $e{n}_0$ ($n_0$ is the background density) at $z=2$ cm. (b) Integrated charge density (normalized) showing a net positive charge of the cavity.

Figure 5

Comparison of the mean energy density in the electrostatic field and the induced azimuthal electric field (scaled by a factor of 10) as a function of time in the plane $z=2$ cm.