Studies of magnetic fields at the laser entrance hole of scaled-down gas-filled hohlraums at OMEGA

In indirect drive inertial fusion designs, hohlraums are filled with a gas to slow the expansion of wall material into the path of the driving lasers. Previous work has shown that large magnetic fields generated at the laser entrance hole of NIF hohlraums could explain the observed anisotropies in fusion-produced D³He-proton spectra. The presented work shows the results of experiments at OMEGA in which proton radiography was used to map the morphology of magnetic fields in scaled-down gas-filled hohlraums. The independence of observed structures in radiographs across several hohlraum materials and imposed wall perturbation amplitudes suggests that the magnetic fields generated at the laser entrance hole are responsible for the observed proton deflection morphology. Extended MHD simulations (using FLASH) indicate the importance of magnetic fields at the laser-entrance-hole region in explaining the overall morphology of the proton radiographs. The magnitude of the FLASH-predicted magnetic field also matches the values extracted from reconstructions of the radiographs. The of electric fields to the deflections of protons, which can be constrained by co-registered T+³He-deuteron radiographs, is also discussed.