Direct Observations of Particle Dynamics in Magnetized Collisionless Shock Precursors in Laser-Produced Plasmas

We present the first laboratory observations of time-resolved electron and ion velocity distributions in magnetized collisionless shock precursors. Thomson scattering of a probe laser beam was used to observe the interaction of a laser-driven, supersonic piston plasma expanding through an ambient plasma in an external magnetic field. From the Thomson-scattered spectra we measure time-resolved profiles of electron density, temperature, and ion flow speed, as well as spatially resolved magnetic fields from proton radiography. We observe direct evidence of the coupling between piston and ambient plasmas, including the acceleration of ambient ions driven by magnetic and pressure gradient electric fields, and deformation of the piston ion flow, key steps in the formation of magnetized collisionless shocks. Even before a shock has fully formed, we observe strong density compressions and electron heating associated with the pileup of piston ions. The results demonstrate that laboratory experiments can probe particle velocity distributions relevant to collisionless shocks, and can complement, and in some cases overcome, the limitations of similar measurements undertaken by spacecraft missions.

Figure 1

(a) Experimental setup. A background magnetic field primarily directed along $\widehat{y}$ is preimposed using current-carrying copper wires. A laser ablates a CH target to create an ambient plasma. Two drive beams then generate a CH piston plasma that expands through the ambient plasma to drive a shock. Temperature, density, and velocity are diagnosed in the $\widehat{x}$ direction using Thomson scattering with a $2\omega$ probe beam. Twenty beams (not shown) compress a DHe3 backlighter capsule to generate monoenergetic protons that probe the magnetic field structure in the $x\text{−}y$ plane. (b) Top-down schematic view of the setup and Thomson scattering geometry.

Figure 2

IAW spectra of piston-ambient interactions under three experimental conditions: (a) magnetized ambient plasma, (b) unmagnetized ambient plasma, and (c) no ambient plasma. Data in (a) and (c) were taken at $x=3\mathrm{mm}$ (TCC), while in (b) were taken at $x=4\mathrm{mm}$. The wavelength shift $\mathrm{\Delta }\lambda$ is relative to the probe beam, and the marks at the bottom of (a) are timing fiducials. (d) Simulated ion velocity space in conditions similar to (a), with velocity relative to the piston speed and time relative to the upstream gyrofrequency. Regions of interest are labeled with Roman numerals and discussed in the text.

Figure 3

(a) Streaked Thomson-scattered spectrum of the EPW feature taken at TCC, corresponding to Fig. 2. (b) Two example profiles at time $t_0+3.85\mathrm{ns}$ (green) and $t_0+4.15\mathrm{ns}$ (red), along with best fits (black). (c) Proton radiography image taken at time $t_0+3.75\mathrm{ns}$ using 14.7 MeV protons. (d) Proton intensity (red squares) taken from the red region in (c), normalized to the mean intensity, and the associated reconstructed path-integrated magnetic field $\int B_{y}dz$ (black). Also shown is the normalized proton intensity (green dashed) forwarded-modeled from a 2D synthetic magnetic field $B_y(x,z)$, which has the dashed blue profile at $z=0$. The model uncertainties are shown as shaded regions.

Figure 4

Thomson scattering results. Measured (a) piston and ambient ion flow speed $v_x$, (b) electron density $n_e$, and (c) electron temperature $T_{ex}$ for a piston plasma expanding through a magnetized (black squares) and unmagnetized (red circles) ambient plasma. (d) Electron temperature (black), density (green), magnetic field (magenta), and piston (red) and ambient (blue) ion flow speed for the magnetized case. The magnetic field profile was constructed from the model in Fig. 3. Error bars are shown as shaded regions. The magnetized plasma were probed at $x=3\mathrm{mm}$ (TCC), while the unmagnetized plasma was probed at $x=4\mathrm{mm}$. The unmagnetized data have been shifted forward in time by 1.5 ns for ease of comparison.