Limitation on Prepulse Level for Cone-Guided Fast-Ignition Inertial Confinement Fusion
Physical Review Letters
The viability of fast-ignition (FI) inertial confinement fusion hinges on the efficient transfer of laser energy to the compressed fuel via multi-MeV electrons. Preformed plasma due to the laser prepulse strongly influences ultraintense laser plasma interactions and hot electron generation in the hollow cone of an FI target. We induced a prepulse and consequent preplasma in copper cone targets and measured the energy deposition zone of the main pulse by imaging the emitted K radiation.
... n of the radiation hydrodynamics of the preplasma and particle in cell modeling of the main pulse interaction agree well with the measured deposition zones and provide an insight into the energy deposition mechanism and electron distribution. It was demonstrated that a under these conditions a 100 mJ prepulse eliminates the forward going component of $2-4 MeV electrons. Cone-guided fast-ignition inertial confinement fusion (FI) depends on the efficient transfer of laser energy to a forward directed beam of $2 MeV electrons at the tip of a hollow cone embedded in the side of an inertialconfinement fusion fuel capsule . This scheme is particularly susceptible to laser prepulse [2,3] as the cone wall confines the expanding preformed plasma [4, 5] increasing both density scale lengths and laser beam filamentation  . The igniter laser pulse requirements for fast ignition depend on the conversion efficiency from laser energy to hot electrons , the electron energy spectrum , the electron transport efficiency to the ignition hot spot [9, 10] , and the electron energy deposition efficiency in the hot spot  . The required laser energy has been estimated at approximately 100 kJ in a 20 ps pulse [1, 11] . Since the ignition hot spot diameter is $40 m, the cone tip must be similar in diameter and the laser intensity $4 Â 10 20 W=cm 2 . Existing petawatt class laser systems deliver up to 1 kJ with typical energy contrast $1 Â 10 À5 and with nonlinear devices this ratio can be improved by a further order of magnitude  . Contrast due to amplified superfluorescence and spontaneous emission is independent of the final laser energy; hence, for an ignition pulse of 100 kJ the prepulse energy on target could range from 100 mJ to 1 J. Recent work by Baton et al.  has shown that some amount of prepulse can strongly affect coupling to cones; however, a detailed understanding of this limit has not been reported. In this Letter we report recent studies of laser interactions with hollow cone targets comparing simulations and experiments in conditions approaching full fast ignition (FI) using prepulse up to 100 mJ with main pulse irradiance $10 20 W cm À2 for picosecond durations. These parameters were accessible using the Titan laser at LLNL, which delivers ð150 AE 10Þ J in ð0:7 þ = À 0:2Þ ps at 1 m with $10% of the energy deposited above an intensity of $10 20 W cm À2 at best focus, as described in  . We compare coupling for two well-characterized prepulse conditions: (1) an intrinsic Titan laser prepulse with ð7:5 AE 3Þ mJ in 1.7 ns at 7:5 Â 10 10 W cm À2 and (2) ð100 AE 3Þ mJ, 3.0 ns prepulse at $10 12 W cm À2 . The larger prepulse was generated by injecting an auxiliary laser pulse into the short pulse amplifier chain prior to the main pulse. Targets were 1 mm long copper cones with 30 coangle, 25 m wall thickness, and 30 m internal tip diameter. Copper was chosen because copper K line emission is accessible to proven diagnostics. The energy distribution for the intrinsic prepulse, the auxiliary prepulse and the main pulse were measured by sampling beam leakage through the last turning mirror prior to the final focusing optic, at a plane equivalent to the focal plane on target  . Best focus for both the prepulse and the main pulse was set at the inside surface of the cone tip. The system was modeled in two parts: (i) The radiativehydro code HYDRA  was used to calculate the distribution of the preformed plasma created by laser ablation from the inside wall of the cone due to the prepulse; (ii) the plasma simulation code PSC  was used to perform a massively parallel particle in cell (PIC) simulation of the ultraintense short pulse laser interaction with the pre-PRL 104, 055002 (2010)