Initial Results from Relativistically Transparent Magnetic Filament Experiments on the OMEGA EP Laser

Relativistic transparency allows intense laser pulses to interact with overdense plasmas. With high enough intensity, the dense axial electron current driven by the ponderomotive force generates a magnetic filament with field strength of the order of the laser amplitude (>10⁵ T). This magnetic filament traps the electrons, enabling efficient acceleration and conversion of laser energy into MeV photons by electron oscillations. We present the results of initial experiments to study this phenomenon at moderate intensity (a₀ ~ 15 to 30) using the OMEGA EP laser. Experimental signatures of the magnetic filament phenomenon are observed in the recorded electron and photon spectra. The results are compared with 3-D particle-in-cell simulations to assess the dependence of electron acceleration on plasma density. We discuss the prospects for scaling this phenomenon to higher intensities for laser-driven studies of mega-Tesla fields in plasmas and high-efficiency secondary sources; above 6×10²¹ W/cm², laser conversion efficiency into MeV photons is predicted to exceed 10%. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856 and Department of Energy under Award Number DE-SC0020431.