Curt W. Laumann

Harmonic conversion of large-aperture 1.05-μm laser beams forinertial-confinement fusion research

To provide high-energy, high-power beams at short wavelengths for inertial-confinement fusion experiments, we routinely convert the 1.05-microm output of the Nova, Nd:phosphate-glass, laser system to its second- or third-harmonic wavelength. We describe the design and performance of the 3 x 3 arrays of potassium dihydrogen phosphate crystal plates used for type-II-type-II phase-matched harmonic conversion of the Nova 0.74-m diameter beams. We also describe an alternate type-I-type-II phasematching configuration that improves third-harmonic conversion efficiency. These arrays provide conversion of a Nova beam of up to 75% to the second harmonic and of up to 70% to the third harmonic.

Power, energy, and temporal performance of the Nova laser facility with recent improvements to the amplifier system

High-powered glass-laser systems with multiple beams, frequency-conversion capabilities, and pulseshaping flexibility have made numerous contributions to the understanding of inertial confinement fusion and related laser-plasma interactions. The Nova laser at Lawrence Livermore National Laboratory is the largest such laser facility. We have made improvements to the Nova amplifier system that permit increased power and energy output. We summarize the nonlinear effects that now limit Novas performance and discuss power and energy produced at 1.05-, 0.53-, and 0.35-microm wavelengths, including the results with pulses temporally shaped to improve inertial confinement fusion target performance.

Temporal shaping of third-harmonic pulses on the Nova laser system

We demonstrate temporal shaping of 0.35-microm-wavelength pulses produced by a third-harmonic conversion of the output from the Nova Nd:phosphate glass-laser amplifier system for use in inertial confinement fusion experiments. We describe the computer models used to calculate the pulse shape that is required as the input to the amplifier system, the experimental apparatus used to produce these pulses, and the high-power 0.35-microm shaped pulses produced in recent experiments.

Beam control and diagnostic functions in the NIF transport spatial filter

Beam control and diagnostic systems are required to align the National Ignition Facility laser prior to a shot as well as to provide diagnostics on 192 beam lines at shot time. A design that allows each beams large spatial filter lenses to also serve as objective lenses for beam control and diagnostic sensor packages helps to accomplish the task at a reasonable cost. However, this approach also causes a high concentration of small optics near the pinhole plane of the transport spatial filter (TSF) at the output of each beam. This paper describes the optomechanical design in and near the central vacuum vessel of the TSF.

Modeling of large-aperture third-harmonic frequency conversion of high-power Nd:glass laser systems

To provide high-energy, high-power beams at short wavelengths for inertial-confinement-fusion experiments the authors rountinely converted the 1.053-micrometers output of the Nova, Nd:phosphate-glass, laser system to its third-harmonic wavelength. We describe performance and conversion efficiency modeling of the 3 X 3 arrays potassium-dihydrogen-phosphate crystal plates used for type II/type II phase-matched harmonic conversion of Nova 0.74-m diameter beams, and an alternate type I/type II phase-matching configuration that improves the third-harmonic conversion efficiency. These arrays provide energy conversion of up to 65% and intensity conversion to 70%.

A four-color beam smoothing irradiation system for laser-plasma interaction experiments at LLNL

A novel four-color beam smoothing scheme with a capability similar to that planned for the proposed National Ignition Facility has been deployed on the Nova laser, and has been successfully used for laser fusion experiments. Wavefront aberrations in high power laser systems produce nonuniformities in the energy distribution of the focal spot that can significantly degrade the coupling of energy into a fusion target, driving various plasma instabilities. The introduction of temporal and spatial incoherence over the face of the beam using techniques such as smoothing by spectral dispersion (SSD) can reduce these variations in the focal irradiance when averaged over a finite time interval. One of the limitations of beam smoothing techniques used to date with solid state laser systems has been the inability to efficiently frequency convert broadband pulses to the third harmonic (351 nm). To obtain high conversion efficiency, we developed a multiple frequency source that is spatially separated into four quadrants, each containing a different central frequency. Each quadrant is independently converted to the third harmonic in a four-segment Type I/Type II KDP crystal array with independent phase-matching for efficient frequency conversion. Up to 2.3 kJ of third harmonic light is generated in a 1 ns pulse, corresponding to up to 65% intrinsic conversion efficiency. SSD is implemented by adding limited frequency modulated bandwidth to each frequency component. This improves smoothing without significant impact on the frequency conversion process. The measured far field irradiance shows 25% rms intensity variation with four colors alone, and is calculated to reach this level within 3 ps. Smoothing by spectral dispersion is implemented during the spatial separation of the FM modulated beams to provide additional smoothing, reaching a 16% rms intensity variation level. Following activation the four-color system was successfully used to probe NIF-like plasmas, producing less than 1% SBS backscatter at greater than 2 multiplied by 1015 W/cm2. This paper discusses the detailed implementation and performance of the segmented four-color system on the Nova laser system.

High-gain, Nd-doped-glass Preamplifier For The National Ignition Facility (nif) Laser System

The large Nd-doped glass laser preamplifier system for the National Ignition Facility (NIF) is comprised of 192 separate beam lines that each produce about 20 kJ of 1.05 micron light. The architecture consists of a central, all-fiber master oscillator system, where the light is generated, shaped, modulated, and distributed to 192 beam lines. Next, 192 preamplifier modules amplifier the tailored pulses from 1 nJ up to 10 J, whereupon they are transported to the large amplifier chains where the laser energy is increased to the 20 kJ level. The preamplifier modules contain a diode-pumped regenerative amplifier (regen), two optical subsystems for spatial beam shaping and smoothing by spectral dispersion (SSD), and a larger four-pass amplifier. In this paper we describe the current design and performance of this high gain preamplifier.

Development of third-harmonic output beam diagnostics on NOVA

Precision multi-wavelength output beam diagnostics are currently under development for use in the Nova laser system. This diagnostic package will measure the energy, pulseshape, and near-field intensity distribution at wavelengths of 0.351 micrometers, 0.528 micrometers, and 1.05 micrometers.