Fast Advection of Magnetic Fields by Hot Electrons

Experiments where a laser-generated proton beam is used to probe the megagauss strength self-generated magnetic fields from a nanosecond laser interaction with an aluminum target are presented. At intensities of ${10}^{15}\mathrm{W}{\mathrm{cm}}^{-2}$ and under conditions of significant fast electron production and strong heat fluxes, the electron mean-free-path is long compared with the temperature gradient scale length and hence nonlocal transport is important for the dynamics of the magnetic field in the plasma. The hot electron flux transports self-generated magnetic fields away from the focal region through the Nernst effect [A. Nishiguchi et al., Phys. Rev. Lett. 53, 262 (1984)] at significantly higher velocities than the fluid velocity. Two-dimensional implicit Vlasov-Fokker-Planck modeling shows that the Nernst effect allows advection and self-generation transports magnetic fields at significantly faster than the ion fluid velocity, $v_N/c_s\approx 10$.

Figure 1

Proton radiographs using the proton energy indicated on the images to look at a null shot and times after $t_0$ of 50, 100, 500, and 800 ps. Top panels: raw images. Middle panels: enhanced images. Bottom panels: magnetic field maps. The mesh is too disrupted for measurement in the green regions.

Figure 2

Left panel: A plot of the product of the magnetic field and the spatial extent ($\mathbf{B}·\mathbf{L}$) against distance from the focal spot for times after $t_0$ of 50, 100, 500, and 800 ps. Right panel: Electron density maps of plasma plumes from interferometry at times of $t_0+100\mathrm{ps}$ and $t_0+400\mathrm{ps}$.

Figure 3

The 2D simulation plane is perpendicular to the target surface, with the heating occurring at one end of the box so that the magnetic field expansion can be observed. (a) and (b) are the initial $n_e$ profiles.

Figure 4

The simulation results showing (a) and (b) the magnetic field profile at 25 and 50 ps, respectively. (c) Nernst velocity, ${\mathbf{v}}_{\mathbf{N}}$, at 25 ps, calculated directly from the distribution function.