Experimental study of transport of relativistic electron beams in strong magnetic mirror field

Relativistic electron beams REB produced by ultra high intense laser pulses have generally a large divergence angle that results in degradation of energy coupling between the REB and a fuel core in the fast ignition scheme. Guiding and focusing of the REB by a strong external magnetic field was proposed to achieve high efficiency. We investigated REB transport through 50 $\mu$m or 250 $\mu$m thick plastic foils CuI doped under external magnetic fields, in magnetic mirror configurations of 1.2 or 4 mirror ratio. The experiment was carried out at the GEKKO XII and LFEX laser facility. Spatial pattern of the REB was measured by coherent transition radiation and/or Cu Ka x ray emission from the rear surface of the foil targets. Strong collimation of the REB by the external magnetic field was observed with 50 $\mu$m thick plastic targets, while the REB scattered in 250 $\mu$m thick targets. The experimental results are compared with computer simulations to understand the physical mechanisms of the REB transport in the external magnetic field.