Evolution of laser-driven magnetic fields from proton tomography

Abstract

Self-generated magnetic fields are commonly produced in high-power laser-plasma interactions. These fields can inhibit plasma heat-flow which makes them important in inertial fusion and controlled laboratory astrophysics experiments. In this work, we characterize the time evolution of self-generated magnetic fields using multi-view proton tomography at two timings. Tomographic reconstructions of the magnetic field show a clear transition from fields located close to the target at early time to more extended coronal fields at later time. The tomographic inversion and mesh radiography also enable a direct measurement of the magnetic-flux evolution. Comparisons with extended-MHD simulations show only moderate agreement in field structure, but good agreement in magnetic flux. This suggests that the field generation model is largely correct under these conditions, while the magnetic transport model requires additional development to reproduce the observed field structure.

Figures (15)

• Figure 1: Experimental setup with different proton backlighter source positions (colored circles). The backlighter angle $\theta$ is defined relative to the target normal. The $t=0.7~$ns timing used 0° and 45° views, while the $t=1.4~$ns timing used 0°, 45°, 67 ° and 180° views.
• Figure 2: Experimental proton radiographs at $t=0.7~$ns (top row) and $t=1.4~$ns (bottom row) as the target is rotated about the $y$-axis in successive shots. All images show 15 MeV protons except (c) which is the x-ray reference image for 45$\degree$ and (i) which is a 3 MeV proton image. Darker regions received higher fluence. A clear shift is visible between the Biermann ring and the $x=0$ axis aligned with the laser spot in (b,f,g). (d) Path-integrated magnetic field profiles from the front and rear views at the two timings. Each dot is a beamlet deflection and the line is the azimuthally-averaged value.
• Figure 3: (a) Sample straight-line proton trajectories from the four proton sources (black dots) in 3D. (b,c) Trajectories projected onto cylindrical coordinates for $t=0.7~$ns and $t=1.4~$ns. The inversion domain is shown in by the black rectangle.
• Figure 4: Detector and foil coordinate systems for tilt angles of (a) $\theta=0\degree$ and (b) $\theta=45\degree$.
• Figure 5: (a,c) Number of constraints in each cell. (b,d) Number of independent constraints in each cell. The black contour line indicates 15 constraints.