Laser electron acceleration using relativistic transparency injection

Accelerating charged particles using intense lasers has been actively pursued over the past few decades. Such laser-based compact accelerators have several potential applications including fast ignition, high-energy physics and radiography. Among the several schemes of laser-plasma based electron acceleration, the vacuum laser acceleration (VLA) features great simplicity where electrons are directly accelerated by the intense laser field. However, a grand challenge of VLA lies in how to efficiently load free electrons into the fast-oscillating laser field such that the injected electrons remain within given half cycles of the laser wave and see a unipolar field for continuous acceleration. This requirement necessitates the injected electrons to be pre-accelerated close to the speed of laser light before they are captured and accelerated by the laser field. Recently, we have experimentally demonstrated electron acceleration up to 20 MeV by driving a thin solid plasma in the transparency regime. Numerical simulations and theoretical analyses suggest a new electron injection scheme for VLA based on the relativistic transparency effect, where an initial opaque plasma becomes transparent to the driving laser due to relativistic electron mass increase. When a high-contrast intense laser drives a thin solid foil, the electrons from the dense plasma are first accelerated to near-light speed by the standing laser wave formed in front of the opaque foil and they are subsequently injected into the transmitted laser field as the dense plasma becomes relativistically transparent. This work not only solves an outstanding problem in VLA by demonstrating a viable injection method, but also provides insight into the electron acceleration in relativistically transparent plasmas, which acts as the primary driver for laser-foil based ion accelerators, x-ray sources and relativistic optics.