Backscatter Reduction Using Combined Spatial, Temporal, and Polarization Beam Smoothing in a Long-Scale-length Laser Plasma

Moody, J. D.; MacGowan, B. J.; Rothenberg, Joshua E.; Berger, R. L.; Divol, L.; Glenzer, S. H.; Kirkwood, R. K.; Williams, E. A.; Young, P. E.

doi:10.1103/physrevlett.86.2810

Cited by 79 publications

(35 citation statements)

References 34 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…3(b) are the power cone fraction for two experiments. We employ smoothed beams [18,[38][39][40][41][42][43] with polarization rotation [44,45], smoothing by spectral dispersion with a laser bandwidth of 45 or 60 GHz and a 17 GHz frequency oscillator. In addition, continuous phase plates are employed that give elliptical vacuum spot sizes [46,47] At these energies and powers, the hohlraum absorbs 80 − 90 % of the incident energy with the dominating loss mechanism being due to Stimulated Raman Scattering (SRS) [48,49] on the inner cones of beams.…”

Section: Laser and Hohlraum Drivementioning

confidence: 99%

Cryogenic thermonuclear fuel implosions on the National Ignition Facility

et al. 2012

Self Cite

View full text Add to dashboard Cite

The first inertial confinement fusion implosion experiments with equimolar deuterium-tritium thermonuclear fuel have been performed on the National Ignition Facility. These experiments use 0.17 mg of fuel with the potential for ignition and significant fusion yield conditions. The thermonuclear fuel has been fielded as a cryogenic layer on the inside of a spherical plastic capsule that is mounted in the center of a cylindrical gold hohlraum. Heating the hohlraum with 192 laser beams for a total laser energy of 1.6 mega joules produces a soft x-ray field with 300 eV temperature. The ablation pressure produced by the radiation field compresses the initially 2.2-mm diameter capsule by a factor of 30 to a spherical dense fuel shell that surrounds a central hot-spot plasma of 50 µm diameter. While an extensive set of x-ray and neutron diagnostics has been applied to characterize hot spot formation from the x-ray emission and 14.1 MeV deuterium-tritium primary fusion neutrons, thermonuclear fuel assembly is studied by measuring the down-scattered neutrons with energies in the range of 10 to 12 MeV. X-ray and neutron imaging of the compressed core and fuel indicate a fuel thickness of (14 ± 3) µm, which combined with magnetic recoil spectrometer measurements of the fuel areal density of (1 ± 0.09) g cm −2 result in fuel densities approaching 600 g cm −3 . The fuel surrounds a hot-spot plasma with average ion temperatures of (3.5 ± 0.1) keV that is measured with neutron time of flight spectra. Absolute neutron yields of (7.5 ± 0.1) × 10 14 have been recorded from the magnetic recoil spectrometer and nuclear activation diagnostics while gamma ray measurements provide the duration of nuclear activity of (170 ± 30) ps. These indirect-drive implosions result in the highest areal densities and neutron yields achieved on laser facilities to date. This achievement is the result of the first hohlraum and capsule tuning experiments where the stagnation pressures have been systematically increased by more than a factor of 10 by fielding low-entropy implosions through the control of radiation symmetry, small hot electron production, and proper shock timing. The stagnation pressure is above 100 Gbar resulting in high Lawson confinement parameters of P τ 10 atm s. Comparisons with radiation-hydrodynamic simulations indicate that the pressure is within a factor of three required for reaching ignition and high yield. This will be the focus of future higher-velocity implosions that will employ additional optimizations of hohlraum, capsule and laser pulse shape conditions.

show abstract

Section: Laser and Hohlraum Drivementioning

confidence: 99%

Cryogenic thermonuclear fuel implosions on the National Ignition Facility

et al. 2012

Self Cite

View full text Add to dashboard Cite

show abstract

“…8,9 At the National Ignition Facility, the laser beams are smoothed spatially through the use of random phase plates, 10 which break up the beam into a collection of laser speckles that in vacuum have width $1:2Fk 0 and length 2pF 2 k 0 , where k 0 is the wavelength of the laser light (351 nm) and F is the focal parameter. Because of diffraction, beam overlap, and the use of polarization smoothing, 11 the effective speckle size in gas-filled hohlraums is somewhat smaller than its vacuum values (F $ 6-8 vs. F ¼ 20 in vacuum).…”

Section: Introductionmentioning

confidence: 99%

Multi-dimensional dynamics of stimulated Brillouin scattering in a laser speckle: Ion acoustic wave bowing, breakup, and laser-seeded two-ion-wave decay

et al. 2016

View full text Add to dashboard Cite

Two- and three-dimensional particle-in-cell simulations of stimulated Brillouin scattering (SBS) in laser speckle geometry have been analyzed to evaluate the relative importance of competing nonlinear processes in the evolution and saturation of SBS. It is found that ion-trapping-induced wavefront bowing and breakup of ion acoustic waves (IAW) and the associated side-loss of trapped ions dominate electron-trapping-induced IAW wavefront bowing and breakup, as well as the two-ion-wave decay instability over a range of ZTe/Ti conditions and incident laser intensities. In the simulations, the latter instability does not govern the nonlinear saturation of SBS; however, evidence of two-ion-wave decay is seen, appearing as a modulation of the ion acoustic wavefronts. This modulation is periodic in the laser polarization plane, anti-symmetric across the speckle axis, and of a wavenumber matching that of the incident laser pulse. A simple analytic model is provided for how spatial “imprinting” from a high frequency inhomogeneity (in this case, the density modulation from the laser) in an unstable system with continuum eigenmodes can selectively amplify modes with wavenumbers that match that of the inhomogeneity.

show abstract

“…For example, introducing a thin layer of borated gold on the inside of the hohlraum wall increases the electron temperature and thus enhances the Landau damping of the EPW and reduces SRS [2]. On the laser side, multiple manipulation * ibarth@princeton.edu techniques of the incident laser are used to mitigate PB including smoothing methods as spatial smoothing [10], spectral dispersion [11], polarization smoothing [12,13], and a combination of these three smoothing methods [8,9]. Interestingly, perpendicular polarizations can also mitigate cross beam energy transfer, and thereby affect symmetry [14].…”

Section: Introductionmentioning

confidence: 99%

Reducing parametric backscattering by polarization rotation

Barth

Fisch

2016

Physics of Plasmas

View full text Add to dashboard Cite

When a laser passes through underdense plasmas, Raman and Brillouin Backscattering can reflect a substantial portion of the incident laser energy. This is a major loss mechanism, for example, in employing lasers in inertial confinement fusion. However, by slow rotation of the incident linear polarization, the overall reflectivity can be reduced significantly. Particle in cell simulations show that, for parameters similar to those of indirect drive fusion experiments, polarization rotation reduces the reflectivity by a factor of 5. A general, fluid-model based, analytical estimation for the reflectivity reduction agrees with simulations. However, in identifying the source of the backscatter reduction, it is difficult to disentangle the rotating polarization from the frequency separation based approach used to engineer the beam's polarization. Although the backscatter reduction arises similarly to other approaches that employ frequency separation, in the case here, the intensity remains constant in time.

show abstract

Backscatter Reduction Using Combined Spatial, Temporal, and Polarization Beam Smoothing in a Long-Scale-length Laser Plasma

Cited by 79 publications

References 34 publications

Cryogenic thermonuclear fuel implosions on the National Ignition Facility

Cryogenic thermonuclear fuel implosions on the National Ignition Facility

Multi-dimensional dynamics of stimulated Brillouin scattering in a laser speckle: Ion acoustic wave bowing, breakup, and laser-seeded two-ion-wave decay

Reducing parametric backscattering by polarization rotation

Contact Info

Product

Resources

About