Ultrafast Electron Beam Radiography of Self-Generated Magnetic Fields from High Intensity Laser-Solid Interactions

Using $\sim 30$ $fs$ electron bunches generated with laser wakefield acceleration (LWFA) as a probe, the femtosecond temporal evolution of a $\sim 4 \times 10^{19}$ $W/cm^2$ short laser pulse with solid targets has been studied experimentally. Magnetic fields of $\sim 100$ $megagauss$ were observed travelling outward from the interaction point of the laser with a $10$ $\mu m$ aluminum foil at nearly the speed of light under ideal laser conditions. With degraded contrast, a pre-plasma forms on the front surface, containing the front surface magnetic field to the hole-boring region. This proof-of-principle experiment demonstrates the utility of LWFA electrons as a diagnostic technique for magnetic fields with femtosecond timescale and/or in sufficiently dense plasma. These results are supported by OSIRIS particle-in-cell simulations.