Field dynamics and filament growth following high-intensity laser-solid interactions

The electromagnetic fields generated following the interaction of a 3x1019 W cm-2 infrared pulse with a gold wire have been investigated using the well-documented proton radiography diagnostic. The pulse is observed to drive a transient current along the length of the wire, the spread velocity of which is confidently measured via a novel experimental arrangement to be ~c. Within a temporal window of 20 ps, the current rises to its peak magnitude ~104 A before decaying to below measurable levels. Both PIC simulation results and simple theoretical reasoning are used to demonstrate that the ultrafast target charging and transient current observed are a direct product of the permanent escape of a small fraction of the laser-accelerated hot electron population to vacuum. Additional shots were performed at a similar intensity using target materials of varying atomic number. The development of fine-scale filamentary structures normal to the wire surface was observed. Notably, (i) the growth times of these structures and (ii) the Z-dependence of their spatial wavelength were found to be consistent with those of the electrothermal instability proposed by Haines. The findings of this research are of relevance to fast ignition and a number of schemes for the optimisation of a laser-driven proton beam to its minimal bandwidth and divergence angle.