Ion acceleration and D-D fusion neutron generation in relativistically transparent deuterated nanowire arrays

Abstract

Laser irradiation of deuterated polyethylene nanowire arrays at intensities of $5\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}2}$ was recently reported to accelerate deuterons to multi-MeV energies, resulting in microscale fusion. Here we show that irradiation of deuterated nanowire targets at intensities of $\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{21}\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}2}$ with high contrast $\ensuremath{\lambda}=400\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ laser pulses of 45 fs duration leads to different plasma dynamics in which the tip of the nanowires rapidly explodes to form an overdense plasma, but the onset of relativistic transparency allows the ultrashort laser pulse to penetrate deep into the nanowire array, enhancing particle acceleration. Experiments and particle-in-cell simulations show that ions are accelerated in the laser beam backward direction to energies up to 13 MeV by a target normal sheath acceleration (TNSA) field that develops in the front of the target, forming collimated beams characterized by full wave at half maximum divergence as low as 7.5\ifmmode^\circ\else\textdegree\fi{}. The simulations also show that deeper within the nanowire array ions are accelerated radially to MeV energies by an internal TNSA field normal to the nanowire surfaces. These radially accelerated deuterons collide with other D atoms, as do those directed toward the $\mathrm{C}{\mathrm{D}}_{2}$ substrate, leading to D-D fusion reactions. Irradiation with 8 J laser pulses is measured to generate up to $1.2\ifmmode\times\else\texttimes\fi{}{10}^{7}$ D-D fusion neutrons per shot. The simulations suggest the use of a thinner substrate would also accelerate ions originating from a small area in the back of the target in the laser forward direction.