High-charge, long-pulse regime of relativistically transparent laser–microchannel interaction*

Relativistically induced transparency facilitates the efficient volumetric conversion of laser energy into hot electrons in structured targets. For short (⪅100-fs) pulses, ultra-relativistic intensity is predicted to also generate high-energy x rays with conversion efficiencies up to tens of percent. Ion motion has been previously considered detrimental to the electron acceleration and photon-production processes, which would constrain the choice of target materials and preclude interest in long pulses. Using particle-in-cell simulations and experiments on OMEGA EP, however, we demonstrate a long-pulse regime of laser–microchannel interaction featuring robust electron acceleration. In this regime, the relativisistically near-critical injection of electrons from the opaque channel wall leads to rapid laser absorption. The electron beam instability that would otherwise accompany this process is suppressed by truncating the channel after the laser is absorbed, resulting in a promising high-charge electron source for picosecond-duration, kilojoule-class laser facilities.