Time-Resolved Fuel Density Profiles of the Stagnation Phase of Indirect-Drive Inertial Confinement Implosions.

The implosion efficiency in inertial confinement fusion depends on the degree of stagnated fuel compression, density uniformity, sphericity, and minimum residual kinetic energy achieved. Compton scattering-mediated 50-200 keV x-ray radiographs of indirect-drive cryogenic implosions at the National Ignition Facility capture the dynamic evolution of the fuel as it goes through peak compression, revealing low-mode 3D nonuniformities and thicker fuel with lower peak density than simulated. By differencing two radiographs taken at different times during the same implosion, we also measure the residual kinetic energy not transferred to the hot spot and quantify its impact on the implosion performance.

Production of relativistic electrons at subrelativistic laser intensities.

Relativistic electron temperatures were measured from kilojoule, subrelativistic laser-plasma interactions. Experiments show an order of magnitude higher temperatures than expected from a ponderomotive scaling, where temperatures of up to 2.2 MeV were generated using an intensity of 1×10^{18}W/cm^{2}. Two-dimensional particle-in-cell simulations suggest that electrons gain superponderomotive energies by stochastic acceleration as they sample a large area of rapidly changing laser phase. We demonstrate that such high temperatures are possible from subrelativistic intensities by using lasers with long pulse durations and large spatial scales.

X-ray streak camera cathode development and timing accuracy of the 4ω ultraviolet fiducial system at the National Ignition Facility.

The convergent ablator experiments at the National Ignition Facility (NIF) are designed to measure the peak velocity and remaining ablator mass of an indirectly driven imploding capsule. Such a measurement can be performed using an x-ray source to backlight the capsule and an x-ray streak camera to record the capsule as it implodes. The ultimate goal of this experiment is to achieve an accuracy of 2% in the velocity measurement, which translates to a ±2 ps temporal accuracy over any 300 ps interval for the streak camera. In order to achieve this, a 4ω (263 nm) temporal fiducial system has been implemented for the x-ray streak camera at NIF. Aluminum, titanium, gold, and silver photocathode materials have been tested. Aluminum showed the highest relative quantum efficiency, with five times more peak signal counts per fiducial pulse when compared to Gold. The fiducial pulse data were analyzed to determine the centroiding statistical accuracy for incident laser pulse energies of 1 and 10 nJ, showing an accuracy of ±1.6 ps and ±0.7 ps, respectively.

High performance imaging streak camera for the National Ignition Facility.

An x-ray streak camera platform has been characterized and implemented for use at the National Ignition Facility. The camera has been modified to meet the experiment requirements of the National Ignition Campaign and to perform reliably in conditions that produce high electromagnetic interference. A train of temporal ultra-violet timing markers has been added to the diagnostic in order to calibrate the temporal axis of the instrument and the detector efficiency of the streak camera was improved by using a CsI photocathode. The performance of the streak camera has been characterized and is summarized in this paper. The detector efficiency and cathode measurements are also presented.

Phasing beams with different dispersions and application to the petawatt-class beamline at the National Ignition Facility.

In order to achieve the highest intensities possible with the short-pulse Advanced Radiographic Capability beamline at the National Ignition Facility (NIF), it will be necessary to phase the individual ARC apertures. This is made especially challenging because the design of ARC results in two laser beams with different dispersions sharing the same NIF aperture. The extent to which two beams with different dispersions can be phased with each other has been an open question. This paper presents results of an analysis showing that the different dispersion values that will be encountered by the shared-aperture beams will not preclude the phasing of the two beams. We also highlight a situation in which dispersion mismatch will prevent good phasing between apertures, and discuss the limits to which higher-order dispersion values may differ before the beams begin to dephase.

Phasing rectangular apertures.

Several techniques have been developed to phase apertures in the context of astronomical telescopes with segmented mirrors. Phasing multiple apertures, however, is important in a wide range of optical applications. The application of primary interest in this paper is the phasing of multiple short pulse laser beams for fast ignition fusion experiments. In this paper analytic expressions are derived for parameters such as the far-field distribution, a line-integrated form of the far-field distribution that could be fit to measured data, enclosed energy or energy-in-a-bucket and center-of-mass that can then be used to phase two rectangular apertures. Experimental data is taken with a MEMS device to simulate the two apertures and comparisons are made between the analytic parameters and those derived from the measurements. Two methods, fitting the measured far-field distribution to the theoretical distribution and measuring the ensquared energy in the far-field, produced overall phase variance between the 100 measurements of less than 0.005 rad(2) or an RMS displacement of less than 12 nm.

Interferometric adaptive optics testbed for laser pointing, wave-front control and phasing.

Implementing the capability to perform fast ignition experiments, as well as, radiography experiments on the National Ignition Facility (NIF) places stringent requirements on the control of each of the beam's pointing, intra-beam phasing and overall wave-front quality. In this article experimental results are presented which were taken on an interferometric adaptive optics testbed that was designed and built to test the capabilities of such a system to control phasing, pointing and higher order beam aberrations. These measurements included quantification of the reduction in Strehl ratio incurred when using the MEMS device to correct for pointing errors in the system. The interferometric adaptive optics system achieved a Strehl ratio of 0.83 when correcting for a piston, tip/tilt error between two adjacent rectangular apertures, the geometry expected for the National ignition Facility. The interferometric adaptive optics system also achieved a Strehl ratio of 0.66 when used to correct for a phase plate aberration of similar magnitude as expected from simulations of the ARC beam line. All of these corrections included measuring both the upstream and downstream aberrations in the testbed and applying the sum of these two measurements in open-loop to the MEMS deformable mirror.

Conical harmonic generation in isotropic materials.

A novel class of nonlinear optical processes is described in which radiation at the nth harmonic is generated through the use of a (2n+1)-order nonlinearity. Utilizing an odd-order nonlinearity, this process allows for the generation and amplification of both odd- and even-order harmonics in isotropic materials. Additionally, this process can always be phase matched in normal-dispersion materials without the use of birefringence. Experimental results are presented in which conical third-harmonic emission is generated from a sapphire sample.

Pulse contrast enhancement of high-energy pulses by use of a gas-filled hollow waveguide.

Using nonlinear ellipse rotation in a gas-filled hollow waveguide, we have increased the pulse contrast of a microjoule femtosecond laser pulse by several orders of magnitude. This scheme offers a number of advantages over competing techniques, including a high degree of tunability that allows for a broad range of input pulse parameters, higher throughput, greater stability, and an output pulse with high spatial quality that is compressible to a quarter of the original temporal width.

Nonlinear propagation dynamics of an ultrashort pulse in a hollow waveguide.

We present a theoretical investigation of the self-focusing dynamics of femtosecond pulses in a hollow waveguide. We show that transverse effects play an important role in these dynamics, even for pulses that are significantly below the critical power for self-focusing in free space, and that excitation of higher-order modes of the waveguide results in the spreading of the pulse in time. Inclusion of self-steepening and space-time focusing in our model is necessary for properly capturing the pulse dynamics.

Infrared photosensitivity in silica glasses exposed to femtosecond laser pulses.

We investigate the use of infrared femtosecond laser pulses to induce highly localized refractive-index changes in fused-silica glasses. We characterize the magnitude of the change as a function of exposure and measure index changes as large as 3x10(-3) and 5x10(-3) in pure fused silica and boron-doped silica, respectively. The potential of this technique for writing three-dimensional photonic structures in bulk glasses is demonstrated by the fabrication of a Y coupler within a sample of pure fused silica.