In our Petawatt laser experiments several hundred joules of 1 µm laser light in 0.5-5.0 ps pulses with intensities up to 3x10 20 Wcm -2 were incident on solid targets producing a strongly relativistic interaction. The energy content, spectra, and angular patterns of the photon, electron, and ion radiations were diagnosed in a number of ways, including several novel (to laser physics) nuclear activation techniques. From the beamed bremsstrahlung we infer that about 40-50% of the laser energy is converted to broadly beamed hot electrons. Their direction centroid varies from shot to shot, but the beam has a consistent width. Extraordinarily luminous ion beams almost precisely normal to the rear of various targets are seen -up to 3x10 13 protons with kT ion ~ several MeV representing ~6% of the laser energy.We observe ion energies up to at least 55 MeV. The ions appear to originate from the rear target surfaces.The edge of the ion beam is very sharp, and collimation increases with ion energy. At the highest energies, a narrow feature appears in the ion spectra, and the apparent size of the emitting spot is smaller than the full back surface area. Any ion emission from the front of the targets is much less than from the rear and is not sharply beamed. The hot electrons generate a Debye sheath with electrostatic fields of order MV per micron which apparently accelerate the ions.

International audienc

Electron transport within solid targets, irradiated by a high-intensity short-pulse laser, has been measured by imaging K(alpha) radiation from high- Z layers (Cu, Ti) buried in low- Z (CH, Al) foils. Although the laser spot is approximately 10 microm [full width at half maximum (FWHM)], the electron beam spreads to > or =70 microm FWHM within <20 microm of penetration into an Al target then, at depths >100 microm, diverges with a 40 degree spreading angle. Monte Carlo and analytic models are compared to our data. We find that a Monte Carlo model with a heuristic model for the electron injection gives a reasonable fit with our data.

We have performed experiments using Callisto, the Vulcan 100 TW and the Vulcan Petawatt high intensity lasers to understand the characteristics of high energy, Kα x-ray sources and to implement workable radiography solutions at 20-100 keV. Our measurements show that the Kα size from a simple foil target is larger than 60 µm, far larger than the experiment resolution requirement. The total Kα yield is independent of target thicknesses verifying that refluxing plays a major role in photon generation. Smaller radiating volumes emit brighter Kα radiation. 1-D radiography experiments using small-edge-on foils resolved 10 µm features with high contrast.We tested a variety of small volume 2-D point sources such as cones, wires, and embedded wires, measuring photon yields and comparing our measurements with predictions from hybrid-PIC LSP simulations. In addition to high-energy, high-resolution backlighters, future experiments will also need imaging detectors and diagnostic tools that are workable in the 20-100 keV energy range. An initial look at some of these detector issues is also presented.

In an experimental study of the physics of fast ignition the characteristics of the hot electron source at laser intensities up to 10 " Wcm"2and the heating produced at depth by hot electrons have been measured. Efficient generation of hot electrons but less than the anticipated heating have been observed.The concept of isochoric fast ignition originated by Tabak et al. ' is of importance through its potential to give higher inertially confined fusion (ICF) gain than isobaric central spark ignition used in the more developed indirect and direct drive schemes 'and thereby to reduce the driver efficiency required for inertial fusion energy (IFE). The physics is new and challenging involving strongly relativistic laser plasma interactions and transport of energy by MeV electrons where electrostatic potentials and self generated magnetic fields may strongly modify the transport 3. Experimental and theoretical studies aimed at assessing the feasibilityof fastignitionas a newrouteto ICF arenowbeing carried out at many laboratories world wide including the Lawrence Livennore National Laboratory (LLNL), where the Nova laser facility has been adapted to generate petawatt pulses using chirped puke ampliilcation (CPA)4 . Experimental FaciIityTwo beam lines at Nova have been adapted for CPA operation and for experiments reported here, generated typically 20J and 500J pulses respectively of duration in the range 0.4 to 20 ps (maximum power up to 1 PW). Focusing of the two beams respectively was with an off axis f73parabolic mirror of focal length 42 cm in a focal spot of 15 yrn diameter 1 and an axial f14parabola of focal length 170 cm in an asymmetrical spot of 40 pm x 20SThe focal spots had a speckle ,pattern sub structure with a broad power spec& of intensity. Work is in progress to correct the wavefront using a deformable mirror. T&b eam line produced a power weighted average intensity on target in 0.45 ps pulses estimated at 210'9 Wcm"2. The 500J, beam line produced 10 N Wcm'2 in 1 PW, 0.45 ps pulses. A thick glass plate debris shield protected the parabola in the 500J beam line for longer pulse operation down to 5 ps. Non linear effects precluded the me of the debris shield for lPW shots and here a plasma mirror was used to reverse the beam direction thus projecting ablated target debris away from the un-protected parabola. The off axis parabola was used with a thin debris shield for all pulse lengths in the 20 J experiments. Targets in these experiments were exposed to ASE and leakage prepulses before the main pulse. ASE in a typically 3 ns period before the pulse varied in experiments reported here from 210-5 to 2104 of the main pulse energy. The energy of leakage pulses ranged from 104 to 10'2of the main pulse, occurring 2 ns or 4 ns before the main pulse but could be made as low as 104 with precise adjustment of Pockels cell gates. The hot electron sourceA central theme of the experimental work has been the characterization of the hot electron source produced at a solid target. Electrons directed into the t...

Ignition requires precisely controlled, high convergence implosions to assemble a dense shell of deuterium-tritium (DT) fuel with ρR&gt;∼1 g/cm2 surrounding a 10 keV hot spot with ρR ∼ 0.3 g/cm2. A working definition of ignition has been a yield of ∼1 MJ. At this yield the α-particle energy deposited in the fuel would have been ∼200 kJ, which is already ∼10 × more than the kinetic energy of a typical implosion. The National Ignition Campaign includes low yield implosions with dudded fuel layers to study and optimize the hydrodynamic assembly of the fuel in a diagnostics rich environment. The fuel is a mixture of tritium-hydrogen-deuterium (THD) with a density equivalent to DT. The fraction of D can be adjusted to control the neutron yield. Yields of ∼1014−15 14 MeV (primary) neutrons are adequate to diagnose the hot spot as well as the dense fuel properties via down scattering of the primary neutrons. X-ray imaging diagnostics can function in this low yield environment providing additional information about the assembled fuel either by imaging the photons emitted by the hot central plasma, or by active probing of the dense shell by a separate high energy short pulse flash. The planned use of these targets and diagnostics to assess and optimize the assembly of the fuel and how this relates to the predicted performance of DT targets is described. It is found that a good predictor of DT target performance is the THD measurable parameter, Experimental Ignition Threshold Factor, ITFX ∼ Y × dsf 2.3, where Y is the measured neutron yield between 13 and 15 MeV, and dsf is the down scattered neutron fraction defined as the ratio of neutrons between 10 and 12 MeV and those between 13 and 15 MeV.

A hot, T e ~ 2-to 3-keV surface plasma was observed in the interaction of a 0.7-ps petawatt laser beam with solid copper-foil targets at intensities >10 20 W/cm 2 . Copper K-shell spectra were measured in the range of 8 to 9 keV using a single-photon-counting x-ray CCD camera. In addition to K α and K β inner-shell lines, the emission contained the Cu He α and Ly α lines, allowing the temperature to be inferred. These lines have not been observed previously with ultrafast laser pulses. For intensities less than 3 × 10 18 W/cm 2 , only the K α and K β inner-shell emissions are detected. Measurements of the absolute K α yield as a function of the laser intensity are in agreement with a model that includes refluxing and confinement of the suprathermal electrons in the target volume.2

The version of Fig. 4 that was inadvertently included in the original publication did not show the results of Monte Carlo modeling referred to in the caption and the text. The correct figure and its caption are shown below. This does not affect the conclusions of the article. FIG. 4. Integrated K ␣ fluorescence energy versus mass fraction of Cu fluor in Al/ Cu/ Al targets. The front Al layer varied from zero to 500 m, the Cu layer was 20-25 m. The back Al layer was 100 m for the four ϫ points, and 40 m for the ϩ, 10-20 m otherwise. The open symbols show the predictions from Monte Carlo modeling with an arbitrary relative normalization. The back Al layer in the model is either 16 m ͑square͒ or 100 m ͑triangle͒.