P. E. Young

Laser beam propagation and channel formation in underdense plasmas

Experimental results are presented which show the formation of density channels in a preformed plasma by a 100 ps laser beam focused with different f/number lenses. The density channels are diagnosed by an interferometer. The experiments are made with both line foci and circular foci up to intensities of 5×1016 W/cm2. The experimental channel size and density perturbation compare favorably with the predictions of two‐dimensional theoretical models. The limited axial extent of the channels is shown to be due to the onset of the filamentation instability.

Experimental study of filamentation in laser–plasma interactions

The filamentation instability can lead to regions of increased laser intensity when a spatially nonuniform laser beam interacts with a plasma. An experimental technique will be described that identifies the density perturbation produced by filaments. The growth of filaments has been investigated and, when the laser intensity is large enough, the transverse density profile of the filament can be measured. Evidence of filament growth influenced by plasma flow and density gradients is presented.

Backscattered light near the incident laser wavelength from 0.35 μm irradiated long scale length plasmas

Backscattered light at wavelengths near the incident laser wavelength of 0.35 μm irradiated thin targets has been spectrally and temporally resolved. Low‐Z targets show significant backscattered signals even after the foils burn through when the density gradient scale length is approximately 1000 times the laser wavelength. The results show the importance of considering seeding mechanisms in the plasma.

Laser coupling to reduced-scale hohlraum targets at the Early Light Program of the National Ignition Facility

A platform for analysis of material properties under extreme conditions, where a sample is bathed in radiation with a high temperature, is under development. Depositing maximum laser energy into a small, high-Z enclosure produces this hot environment. Such targets were recently included in an experimental campaign using the first four of the 192 beams of the National Ignition Facility [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)], under construction at the University of California Lawrence Livermore National Laboratory. These targets demonstrate good laser coupling, reaching a radiation temperature of 340 eV. In addition, there is a unique wavelength dependence of the Raman backscattered light that is consistent with Brillouin backscatter of Raman forward scatter [A. B. Langdon and D. E. Hinkel, Phys. Rev. Lett. 89, 015003 (2002)]. Finally, novel diagnostic capabilities indicate that 20% of the direct backscatter from these reduced-scale targets is in the polarization or...

High-speed horizontal-path atmospheric turbulence correction with a large-actuator-number microelectromechanical system spatial light modulator in an interferometric phase-conjugation engine

Results of atmospheric propagation for a high-speed, large-actuator-number adaptive optics system are presented. The system uses a microelectromechanical system- (MEMS-) based spatial light modulator correction device with 1024 actuators. Tests over a 1.35-km path achieved correction speeds in excess of 800 Hz and Strehl ratios close to 0.5. The wave-front sensor was based on a quadrature interferometer that directly measures phase. This technique does not require global wave-front reconstruction, making it relatively insensitive to scintillation and phase residues. The results demonstrate the potential of large-actuator-number MEMS-based spatial light modulators to replace conventional deformable mirrors.

Scattered light near the laser wavelength from Nova two‐color experiments

A laser‐produced plasma has been studied in which there is a peak plasma density between critical and quarter‐critical for a substantial amount of time (∼400 psec). The behavior of temporally and spectrally resolved light near the wavelength of the interaction laser λ0, is compared to the decay with time of the peak plasma density inferred from three‐halves harmonic emission and Raman scattered light. Evidence for seeding of stimulated Brillouin scattering by the two‐plasmon decay instability is presented.

Density and temperature profiles in strongly absorbing plasma with distributed absorption

A model is presented of the spatial structure of a laser‐heated, strongly absorbing, planar plasma. Inverse bremsstrahlung is very strong so that laser light absorption is not localized. Absorption is distributed over densities from well below critical to the critical density. It is shown that the spatial structure of the plasma is self‐consistent with laser energy deposition and heat transport so measurement of the plasma structure can be used as diagnostic of absorption and transport. Nonphysical discontinuities in density and temperature at the critical surface that are predicted by previous local absorption models for strongly flux‐limited heat transport are reduced. These jumps persist in the present model. It is shown that an improved flux‐limited heat transport model, strongly limited in the underdense plasma and weakly limited in the overdense plasma, results in continuous density and temperature profiles.

Experimental studies of stimulated Raman scattering in reactor‐size, laser‐produced plasmas

In this paper the results of experiments designed to provide information regarding the scaling of the stimulated Raman scattering (SRS) instability in long‐scale‐length, inertial‐ confinement‐fusion‐reactor‐size plasmas are described. Reactor‐scale plasma conditions were experimentally simulated by exploding thin CH foils with nine beams of the Nova laser [Rev. Sci. Instrum. 57, 2101 (1986)]. Using a one‐dimensional hydro model, the target and irradiation parameters were chosen to produce the largest‐possible scale length plasma consistent with the energy and pulse width capabilities of Nova. The SRS emissions, driven by both the nine plasma‐production beams and a tightly focused and delayed tenth beam, were recorded using an angularly resolved photodiode array and a temporally resolved optical spectrometer. These measurements verified the production of a reactor‐size plasma (L/λ ≊ 5000) and indicated that the absolute levels of SRS backscatter detected in the present experiments have not increased relati...

Open- and closed-loop aberration correction by use of a quadrature interferometric wave-front sensor

Experimental results are presented for an adaptive optics system based on a quadrature Twyman-Green interferometric wave-front sensor. The system uses a circularly polarized reference beam to form two interferograms with a pi/2 phase shift. The experiments conducted used Kolmogorov phase screens to simulate atmospheric phase distortions. Strehl ratio improvements by a factor of 8, to an absolute value of 0.45, are demonstrated.

Experimental observation of filamentation growth in laser‐produced plasmas

The filamentation instability has been observed and systematically studied in a low Z plasma under a variety of density gradients and electron temperatures. Growth lengths have been measured for several plasma conditions and as a function of the spatial size of the imposed intensity perturbation. The filamentation growth is compared to a model that assumes nonlocal electron energy transport and to ponderomotive theory.