A. B. Langdon

Energetic proton generation in ultra-intense laser–solid interactions

An explanation for the energetic ions observed in the PetaWatt experiments is presented. In solid target experiments with focused intensities exceeding 1020 W/cm2, high-energy electron generation, hard bremsstrahlung, and energetic protons have been observed on the backside of the target. In this report, an attempt is made to explain the physical process present that will explain the presence of these energetic protons, as well as explain the number, energy, and angular spread of the protons observed in experiment. In particular, we hypothesize that hot electrons produced on the front of the target are sent through to the back off the target, where they ionize the hydrogen layer there. These ions are then accelerated by the hot electron cloud, to tens of MeV energies in distances of order tens of μm, whereupon they end up being detected in the radiographic and spectrographic detectors.

Effects of ion trapping on crossed-laser-beam stimulated Brillouin scattering

An analysis of the effects of ion trapping on ion acoustic waves excited by the stimulated Brillouin scattering of crossing intense laser beams is presented. Ion trapping alters the dispersion of ion acoustic waves by nonlinearly shifting the normal mode frequency and by reducing the ion Landau damping. This in turn can influence the energy transfer between two crossing laser beams in the presence of plasma flows such that stimulated Brillouin scattering (SBS) occurs. The same ion trapping physics can influence the saturation of SBS in other circumstances. A one-dimensional analytical model is presented along with reasonably successful comparisons of the theory to results from particle simulations and laboratory experiments. An analysis of the vulnerability of the National Ignition Facility Inertial Confinement Fusion point design [S. W. Haan et al., Fusion Sci. Technol. 41, 164 (2002)] is also presented.

Laser generated proton beam focusing and high temperature isochoric heating of solid matter

The results of laser-driven proton beam focusing and heating with a high energy (170J) short pulse are reported. Thin hemispherical aluminum shells are illuminated with the Gekko petawatt laser using 1μm light at intensities of ∼3×1018W∕cm2 and measured heating of thin Al slabs. The heating pattern is inferred by imaging visible and extreme-ultraviolet light Planckian emission from the rear surface. When Al slabs 100μm thick were placed at distances spanning the proton focus beam waist, the highest temperatures were produced at 0.94× the hemisphere radius beyond the equatorial plane. Isochoric heating temperatures reached 81eV in 15μm thick foils. The heating with a three-dimensional Monte Carlo model of proton transport with self-consistent heating and proton stopping in hot plasma was modeled.

Kinetic enhancement of Raman backscatter, and electron acoustic Thomson scatter

One-dimensional Eulerian Vlasov-Maxwell simulations are presented that show kinetic enhancement of stimulated Raman backscatter (SRBS) due to electron trapping in regimes of heavy linear Landau damping. The conventional Raman Langmuir wave is transformed into a set of beam acoustic modes [L. Yin et al., Phys. Rev. E 73, 025401 (2006)]. A low phase velocity electron acoustic wave (EAW) is seen developing from the self-consistent Raman physics. Backscatter of the pump laser off the EAW fluctuations is reported and referred to as electron acoustic Thomson scatter. This light is similar in wavelength to, although much lower in amplitude than, the reflected light between the pump and SRBS wavelengths observed in single-hot-spot experiments, and previously interpreted as stimulated electron acoustic scatter [D. S. Montgomery et al., Phys. Rev. Lett. 87, 155001 (2001)]. The EAW observed in our simulations is strongest well below the phase-matched frequency for electron acoustic scatter, and therefore the EAW is ...

Modeling the nonlinear saturation of stimulated Brillouin backscatter in laser heated plasmas

After showing that the stimulated Brillouin instability (SBS) is likely to be in a saturated regime under conditions of interest for inertial confinement fusion, two examples of reduced models of nonlinear effects that are included in a fluid model are described. Simulations using a nonlinear damping representing the saturation of the amplitude of acoustic waves in the fluid regime (i.e., weak Landau damping) are compared with experimental measurements done on CO2 plasmas. While good agreement is found between the model and a variety of independent experimental measurements, no simple explanation was found for the very low saturation level (well below the amplitude corresponding to the two-ion-decay instability) that has to be used. In the kinetic regime (i.e., large Landau damping), hybrid-particle-in-cell simulations show that nonlinear frequency shifts induced by trapping saturate SBS. A reduced steady-state model has been shown to be in correct agreement with time-integrated measurements done on Be pl...

Observation of polarization dependent raman scattering in a large scale plasma illuminated with multiple laser beams

Experiments in plasmas produced with 2mm diameter gas filled targets preheated with 10kJ of laser energy have shown that the stimulated Raman scattering (SRS) of a high intensity, 351nm, beam is affected by the presence of a second, counterpropagating, high intensity beam and that has its polarization aligned to the first when the plasma conditions are relevant to ignition by indirect drive. Separate experiments with the crossing beams polarization rotated to be normal to the first beams polarization show little change in the SRS backscatter when the second beam is added, consistent with the reduction in the SRS being caused by low frequency waves driven by the ponderomotive force produced by the beating of the two beams.

Reduction of stimulated scattering losses from hohlraum plasmas with laser beam smoothing

Laser beam smoothing by spectral dispersion and by polarization smoothing has been observed to significantly reduce the scattering losses by stimulated Brillouin and stimulated Raman scattering from inertial confinement fusion hohlraums. For these measurements, the laser beam smoothing and the high-Z hohlraum wall plasma parameters approach the conditions of future inertial confinement fusion experiments. The simultaneous application of the smoothing techniques has reduced the scattering losses by almost one order of magnitude down to the 1% level. The experimental scaling of the stimulated Brillouin reflectivity compares well to modeling assuming nonlinear damping on the ion acoustic waves in three-dimensional nonlinear wave simulations and calculated hohlraum plasma conditions from radiation-hydrodynamic modeling.

Saturation of power transfer between two copropagating laser beams by ion-wave scattering in a single-species plasma

Experiments show that power is transferred between two copropagating 351nm laser beams crossing in an Al plasma when the frequency of the driven ion wave is shifted by a Mach 1 flow. The resonant amplification of a low-intensity (⩽2.5×1014W∕cm2) beam intersected by a high-intensity (7.0×1014W∕cm2) pump beam is determined by comparing the transmitted beam power to that measured in experiments where the plasma flow direction is reversed and the ion wave is evidently detuned. The polarization of the amplified light is also observed to align to the pump polarization consistent with ion-wave scattering. The amplification is found to reduce with probe-beam intensity demonstrating a nonlinear saturation mechanism that is effective when the ion-wave damping is weak, which is modeled with a calculation including both the nonlinear ion-wave frequency shifts due to ion trapping and whole-beam pump depletion.

Stimulated Raman Scattering from Hot, Underdense Plasmas

Summary form only given. Stimulated Raman scattering (SRS) instability has been experimentally investigated in small-scale halfraums irradiated by intense laser pulses at 10 TW power. The driver energy was delivered by the OMEGA laser (LLE, Rochester) in 1 ns pulses, and deposited in a hot, under-dense Au plasma. The laser energy deposition shifts at later times outside the laser entrance hole (LEH) due to a rapid plasma fill of the hohlraum. Most of the SRS light is scattered from this region and detected with time and spectral resolution. The high temperature of the scattering volume is reflected in the SRS spectrum that extends beyond two times the laser wavelength, due to the Bohm-Gross shift. Information about the plasma parameters inferred from the SRS spectra will be discussed

Coupling of High Power Lasers to Small, Hot Targets at the National Ignition Facility

Summary form only given. An experimental campaign to study radiation drive in small-scale halfraums has been carried out using the first four beams of the National Ignition Facility (NIF) at the Lawerence Livermore National Laboratory (Livermore, CA). The targets fill with plasma so quickly that, late in time, most of the laser energy is deposited at the laser entrance hole. Experiments have shown the effect of laser beam conditioning, laser power, and target size on hohlraum performance. The experimental results on X-radiation drive, laser backscatter, hard X-rays, hard X-ray imaging, and X-ray burnthrough are discussed