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Using 120-fs, 800-nm Ti:sapphire laser pulses, ionic yields of singly and doubly charged magnesium ions were measured as a function of intensity and laser field ellipticity. A clear ''nonsequential'' enhancement of the doubly charged ion is observed for circularly polarized light in addition to linearly polarized light. Over the entire intensity range the double-ionization yield is considerably higher for linear polarization than it is for circular polarization.

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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. Simulation 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 [1]. 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 [6].The igniter laser pulse requirements for fast ignition depend on the conversion efficiency from laser energy to hot electrons [7], the electron energy spectrum [8], the electron transport efficiency to the ignition hot spot [9,10], and the electron energy deposition efficiency in the hot spot [10]. 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 [12]. 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. [5] 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 [13].We compare coupling for two well-characterized prepulse conditions: (1) an intrinsic Titan laser prepulse with ð7:5 AE 3Þ mJ in 1.7 n...

Using a 110 fs, 800 nm Ti:sapphire laser, we have collected angle-resolved, highresolution (< 25 meV) photoelectron kinetic energy spectra of xenon for multiple intensities in the range 10 13 -10 14 W cm −2 . For various ATI peaks, we present angular distributions, which show complex and varied structures not previously reported or predicted. For each intensity, abrupt variations in the structures of the angular distributions coincide energetically with the onset of the plateau region in the photoelectron spectra.