Freeman, R. R.; Gu, P.; Hatchett, S. P.; Hey, D.; Hill, J.; Key, M. H.; Izawa, Y.; King, J.; Kitagawa, Y.; Kodama, R.; Langdon, A. B.; Lasinski, B. F.; Lei, A.; MacKinnon, A. J.; Patel, P.; Stephens, R.; Tampo, M.; Tanaka, K. A.; Town, R.; Toyama, Y.; Tsutsumi, T.; Wilks, S. C.; Yabuuchi, T.; Zheng, J. Citation Physics of Plasmas.

A large number of hot electrons exceeding the Alfvén current can be produced when an ultraintense laser pulse irradiates a solid target. Self-excited extreme electrostatic and magnetic fields at the target rear could influence the electron trajectory. In order to investigate the influence, we measure the hot electrons when a plasma was created on the target rear surface in advance and observe an increase of the electron number by a factor of 2. This increase may be due to changes in the electrostatic potential formation process with the rear plasma. Using a one-dimensional particle-in-cell simulation, it is shown that the retardation in the electrostatic potential formation lengthens the gate time when electrons can escape from the target. The electron number escaping within the lengthened time window appears to be much smaller than the net produced number and is consistent with our estimation using the Alfvén limit.

We propose a foam cone-in-shell target design aiming at optimum hot electron production for the fast ignition. A thin low-density foam is proposed to cover the inner tip of a gold cone inserted in a fuel shell. An intense laser is then focused on the foam to generate hot electrons for the fast ignition. Element experiments demonstrate increased laser energy coupling efficiency into hot electrons without increasing the electron temperature and beam divergence with foam coated targets in comparison with solid targets. This may enhance the laser energy deposition in the compressed fuel plasma.