Study of self-generated magnetic field at the front of a strong shock in helium*

We report for the first time a measurement of the magnetic field at the front of a Mach 6 shock propagating in a low-density helium gas system. In the experiments, strong shock waves were generated using two long pulse beams of 1 kJ total energy in 0.5 ns square pulse from OMEGA EP laser system. The generated shock was propagating under a quasi-planar geometry and diagnosed by broadband proton radiography. TNSA protons are produced by a 10 ps, 400 J short pulse interacting with a copper foil. Shock conditions such as temperature and density are constrained by X-ray spectrometry measurements to be ~ 140 eV and ~ 1.5 mg/cc respectively. The existence of the magnetic field is confirmed by point projection proton radiography from different angles. Calculations of the field strength ~ 5 to 7 T and thickness ~ 120 μm are also achieved by quantitative analysis of the proton radiography data. The measured self-generated magnetic field was further confirmed using the hybrid particle-in-cell code.