Effects of preplasma scale length and critical surface dynamics on laser energy coupling to hot electrons

Summary form only given. Applications of intense laser-dense plasma interactions such as the fast ignition approach to fusion and laser-driven ion acceleration depend on the generation and transport of large currents of energetic electrons. The efficiency of coupling laser energy into fast electrons is a key issue for these applications and is sensitive to the scale length of the preformed plasma, the extent of the underdense plasma which the laser pulse propagates through and the dynamics of the critical density plasma surface. We report results from a recent experiment employing the VULCAN petawatt laser in which a controlled and well characterized preformed plasma is produced at the front surface of solid targets. The laser energy coupling to fast electrons is diagnosed by complementary measurement of the reflected laser light, K[L emission from a buried fluorescent layer and proton acceleration from the target rear surface. We find that two distinct regimes of laser energy coupling to fast electrons occur as the preplasma density scale length is increased. The extent of the underdense plasma is shown to be a critical factor, as this affects the laser beam propagation via inducing channeling and beam filamentation. An increase in laser energy absorption in the limit of large underdense plasma is attributed to `volume absorption' processes. The importance of the shape and dynamics of the critical density surface is also elucidated with the aid of experiment and hydrodynamic, ray-tracing and PIC simulations.