R. K. Kirkwood

Symmetry tuning via controlled crossed-beam energy transfer on the National Ignition Facilitya)

The Hohlraum energetics experimental campaign started in the summer of 2009 on the National Ignition Facility (NIF) [E. I. Moses et al., Phys. Plasmas 16, 041006 (2009)]. These experiments showed good coupling of the laser energy into the targets [N. Meezan et al., Phys. Plasmas 17, 056304 (2010)]. They have also demonstrated controlled crossed-beam energy transfer between laser beams as an efficient and robust tool to tune the implosion symmetry of ignition capsules, as predicted by earlier calculations [P. Michel et al., Phys. Rev. Lett. 102, 025004 (2009)]. A new linear model calculating crossed-beam energy transfer between cones of beams on the NIF has been developed. The model has been applied to the subscale Hohlraum targets shot during the National Ignition Campaign in 2009. A good agreement can be found between the calculations and the experiments when the impaired propagation of the laser beams due to backscatter is accounted for.

Capsule implosion optimization during the indirect-drive National Ignition Campaign

Capsule performance optimization campaigns will be conducted at the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, 228 (2004)] to substantially increase the probability of ignition. The campaigns will experimentally correct for residual uncertainties in the implosion and hohlraum physics used in our radiation-hydrodynamic computational models using a variety of ignition capsule surrogates before proceeding to cryogenic-layered implosions and ignition experiments. The quantitative goals and technique options and down selections for the tuning campaigns are first explained. The computationally derived sensitivities to key laser and target parameters are compared to simple analytic models to gain further insight into the physics of the tuning techniques. The results of the validation of the tuning techniques at the OMEGA facility [J. M. Soures et al., Phys. Plasmas 3, 2108 (1996)] under scaled hohlraum and capsule conditions relevant to the ignition design are shown ...

Laser–plasma interactions in ignition‐scale hohlraum plasmas

Scattering of laser light by stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) is a concern for indirect drive inertial confinement fusion (ICF). The hohlraum designs for the National Ignition Facility (NIF) raise particular concerns due to the large scale and homogeneity of the plasmas within them. Experiments at Nova have studied laser–plasma interactions within large scale length plasmas that mimic many of the characteristics of the NIF hohlraum plasmas. Filamentation and scattering of laser light by SBS and SRS have been investigated as a function of beam smoothing and plasma conditions. Narrowly collimated SRS backscatter has been observed from low density, low‐Z, plasmas, which are representative of the plasma filling most of the NIF hohlraum. SBS backscatter is found to occur in the high‐Z plasma of gold ablated from the wall. Both SBS and SRS are observed to be at acceptable levels in experiments using smoothing by spectral dispersion (SSD).

Energy transfer between crossing laser beams

Induced scattering between crossing laser beams is considered, including the effects of long‐wavelength modulations in the plasma. This fundamental process can impact the choice of beam‐smoothing techniques for laser‐driven hohlraums. Study of this process is an ideal way to quantify stimulated scattering instabilities, since one can independently vary the intensity, polarization, and frequency separation of the crossing beams.

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.

Hohlraum energetics scaling to 520 TW on the National Ignition Facility

Indirect drive experiments have now been carried out with laser powers and energies up to 520 TW and 1.9 MJ. These experiments show that the energy coupling to the target is nearly constant at 84% ± 3% over a wide range of laser parameters from 350 to 520 TW and 1.2 to 1.9 MJ. Experiments at 520 TW with depleted uranium hohlraums achieve radiation temperatures of ∼330 ± 4 eV, enough to drive capsules 20 μm thicker than the ignition point design to velocities near the ignition goal of 370 km/s. A series of three symcap implosion experiments with nearly identical target, laser, and diagnostics configurations show the symmetry and drive are reproducible at the level of ±8.5% absolute and ±2% relative, respectively.

Capsule performance optimization in the National Ignition Campaign

A capsule performance optimization campaign will be conducted at the National Ignition Facility [G. H. Miller et al., Nucl. Fusion 44, 228 (2004)] to substantially increase the probability of ignition by laser-driven hohlraums [J. D. Lindl et al., Phys. Plasmas 11, 339 (2004)]. The campaign will experimentally correct for residual uncertainties in the implosion and hohlraum physics used in our radiation-hydrodynamic computational models before proceeding to cryogenic-layered implosions and ignition attempts. The required tuning techniques using a variety of ignition capsule surrogates have been demonstrated at the OMEGA facility under scaled hohlraum and capsule conditions relevant to the ignition design and shown to meet the required sensitivity and accuracy. In addition, a roll-up of all expected random and systematic uncertainties in setting the key ignition laser and target parameters due to residual measurement, calibration, cross-coupling, surrogacy, and scale-up errors has been derived that meets the required budget.

Amplification of an ultrashort pulse laser by stimulated Raman scattering of a 1ns pulse in a low density plasma

Experiments are described in which a 1mJ, 1ps, 1200nm seed laser beam is amplified by the interaction with an intersecting 350J, 1ns, 1054nm pump beam in a low density (1×1019∕cm3) plasma. The transmission of the seed beam is observed to be enhanced by ≳25× when the plasma is near the resonant density for stimulated Raman scattering, compared to measured transmissions at wavelengths just below the resonant value. The amplification is observed to increase rapidly with increases in both pump intensity and plasma density.

Development of a nanosecond-laser-pumped Raman amplifier for short laser pulses in plasma

Progress on developing a plasma amplifier/compressor based on stimulated Raman scattering of nanosecond laser pulses is reported. Generation of a millijoule seed pulse at a wavelength that is redshifted relative to the pump beam has been achieved using an external Raman gas cell. By interacting the shifted picosecond seed pulse and the nanosecond pump pulse in a gas jet plasma at a density of ∼1019 cm−3, the upper limit of the pump intensity to avoid angular spray of the amplified seed has been determined. The Raman amplification has been studied as a function of the pump and seed intensities. Although the heating of plasma by the nanosecond pump pulse results in strong Landau damping of the plasma wave, an amplified pulse with an energy of up to 14 mJ has been demonstrated, which is, to the best of our knowledge, the highest output energy so far by Raman amplification in a plasma. One-dimensional particle-in-cell simulations indicate that the saturation of amplification is consistent with onset of partic...

Observation of multiple mechanisms for stimulating ion waves in ignition scale plasmas

The laser and plasma conditions expected in ignition experiments using indirect drive inertial confinement have been studied experimentally. It has been shown that there are at least three ways in which ion waves can be stimulated in these plasmas and have significant effect on the energy balance and distribution in the target. First ion waves can be stimulated by a single laser beam by the process of Stimulated Brillouin Scattering (SBS) in which an ion acoustic and a scattered electromagnetic wave grow from noise. Second, in a plasma where more than one beam intersect, ion waves can Lie excited at the `beat` frequency and wave number of the intersecting beams,, causing the side scatter instability to be seeded, and substantial energy to be transferred between the beams [R. K. Kirkwood et. al. Phys. Rev. Lett. 76, 2065 (1996)]. And third, ion waves may be stimulated by the decay of electron plasma waves produced by Stimulated Raman Scattering (SRS), thereby inhibiting the SRS process [R. K. Kirkwood et. al. Phys. Rev. Lett. 77, 2706 (1996)].