Paul A. Renard

Effects of nonlinear propagation on laser focusing properties

In a high-power laser system both the nonlinear growth of small-scale spatial irregularities on a pulse (beam breakup) and the ac- cumulation of self-induced phase-front distortion on the spatial enve- lope of the pulse (wholebeam self-focusing) alter the focusing proper- ties of the system output. We present experimental results which show simultaneously and in detail the influence of these two effects on the spatial and temporal profiles of the focused output of a simple system. In an extreme case the focused on-axis intensity is observed to be re- dud by beam breakup to 20 percent of the value obtained in the absence of breakup. Diffraction code calculations of whole-beam self- focusing, which in these experiments tends to enhance the focused on- axis intensity, are found to agree closely with the measured results and demonstrate that whole-beam self-focusing effects depend strongly on the plane being observed.

Valkyrie, A CO 2 laser system for laser fusion experiments

The Livermore COz laser system, Valkyrie, was constructed to provide an experimental facility capable of delivering 50 J in a diffraction limited beam at 10.6 pm in 1 nsec. This device has met all of its design goals and is in operation as a target irradiation facility. On a typical day, Valkyrie provides 40 J 10 X in 1 nsec at a repetition rate limited by the time required to replace targets. In order to design Valkyrie a CO, amplifier modeling computer program predicted the output of individual amplifier modules. This information was fed into a laser system simulation routine in order to determine amplifier staging. Once specified and manufactured, Valkyries amplifiers were subjected to an exhaustive series of tests. Small signal gain as a function of gas mixture, E/n, pump pulse duration, and spatial position was measured. Valkyries oscillator employs a specially designed UV preionized TFA gain medium (Lumonics 142A), a Ge Brewsters angle acoustooptic mode-locker, and an NaCl etalon as an output mirror. This oscillator generates a burst of 1 nsec duration pulses with peak pulse energies in excess of 50 mJ distributed reproducibly among -six rotational lines centered on the P20 transition of the 10.4 pm hand. A single pulse is selected by a switch-out assembly consisting of a laser triggered spark gap and CdTe Pockels cells. The pulse is then directed through a second Lumonics 142A which serves as a preamplifier and into a 3 x Keplerian beam expander with a spatial filter at its focus. Expended and collimated, the pulse propagates through a nine centimeter aperture UV-preionized TEA amplifier (Lumonics 602A) followed by two eleven centimeter aperture cold-cathode E-beam sustained amplifiers operated at two atmospheres. Emerging in the target area, the pulse contains 50 J in 1 nsec with an approximately Gaussian spatial intensity distribution. Two NaCl beamsplitters which are wedged and anti-reflection

Application of adaptive optics for controlling the NIF laser performance and spot size

The National Ignition Facility (NIF) laser will use a 192- beam multi-pass architecture capable of delivering several MJ of UV energy in temporal phase formats varying from sub- ns square to 20 ns precisely-defined high-contrast shapes. Each beam wavefront will be subjected to effects of optics inhomogeneities, figuring errors, mounting distortions, prompt and slow thermal effects from flashlamps, driven and passive air-path turbulence, and gravity-driven deformations. A 39-actuator intra-cavity deformable mirror, controlled by data from a 77-lenslet Hartman sensor will be used to correct these wavefront aberrations and thus to assure that stringent farfield spot requirements are met. We have developed numerical models for the expected distortions, the operation of the adaptive optics systems, and the anticipated effects on beam propagation, component damage, frequency conversion, and target-plane energy distribution. These models have been extensively validated against data from LLNLs Beamlet, and Amplab lasers. We review the expected beam wavefront aberrations and their potential for adverse effects on the laser performance, describe our model of the corrective system operation, and display our predictions for corrected-beam operation of the NIF laser.

Optimized NIF laser system based on ICF target requirements

The design of the National Ignition Facility (NIF) is the result of optimization studies that maximized laser performance and reliability within a restricted cost budget. We modeled the laser using a suite of tools that included a 1D propagation code, a frequency conversion code, a 2D ray trace code for calculating the gain profile, thermo- mechanical codes for calculating the pump-induced distortions in the slabs, a database giving estimates of optics bulk/finish quality, and costing models of the laser/building. By exploiting parallel processing, we were able to consider approximately 750 possible designs per hour using a cluster of 28 workstations. For our optimization studies, we used a temporally shaped (ICF indirect drive) pulse producing at least 2.2 MJ and 600 TW in a 600 micron diameter hole at the target entrance plane. We varied as many as 20 design variables (e.g., slab counts, slab thickness, Nd concentration, amplifier pulse length) and applied as many as 40 constants (e.g., flashlamp voltage and fluence damage/filamentation at various points in the chain). We did not vary the number of beamlets (fixed at 192 or the aperture (fixed at 40 cm). We used three different optimization approaches: a variable metric algorithm, an exhaustive grid search of more than 50,000 candidate designs, and a parabolic interpolation scheme. All three approaches gave similar results. Moreover, a graphical analysis of the parameter scan data (analogous to sorting and pruning designs using a spreadsheet) has allowed us to understand why the optimizers eliminated alternate designs. The most inexpensive main-switch-boot slab configuration meeting the mission requirements and satisfying all constraints was 9-5-3. The cost of this configuration is approximately

One-micron focal spot and the power spectrum of the noise field at the entrance to the UV generation and transport subsystem

DOL10M less than the 9-5-5 conceptual design. However, the NIF Project has chosen a slightly more expensive 11-0-7 configuration for continued Title I engineering because of its similarity to the Beamlet 11-0-5 design and a lower B-integral.

Focal spot characterization

We discuss the partitioning of effects on the fundamental focal irradiance distribution from whole beam self-focusing, and PSDs of the optical components in the laser chain. Beamlet and NIF simulation results are discussed.

Characterization of the one-micron beam profile at the interface between the one-micron laser and the UV generation and transport subsystem

We discuss the partitioning of effects on the fundamental focal irradiance distribution from whole beam self-focusing, and PSDs of the optical components in the laser chain, and the resulting third harmonic focal distribution. Beamlet and NIF simulation results are discussed.

The NIF's power and energy ratings for flat-in-time pulses

The near field irradiance parameters at the interface between the one micron laser, the UV generation, and transport subsystem will be discussed. The test results obtained from the Beamlet and Nova lasers used to validate the mathematical models will be presented.