Doug Homoelle

ND:GLASS LASER DESIGN FOR LASER ICF FISSION ENERGY (LIFE)

Abstract We have developed preliminary conceptual laser system designs for the Laser ICF (Inertial Confinement Fusion) Fission Energy (LIFE) application. Our approach leverages experience in high-energy Nd: glass laser technology developed for the National Ignition Facility (NIF)1, along with high-energy-class diode-pumped solid-state laser (HEC-DPSSL) technology developed for the DOE’s High Average Power Laser (HAPL) Program and embodied LLNL’s Mercury laser system.2 We present laser system designs suitable for both indirect-drive, hot spot ignition and indirect-drive, fast ignition targets. Main amplifiers for both systems use laser-diode-pumped Nd:glass slabs oriented at Brewster’s angle, as in NIF, but the slabs are much thinner to allow for cooling by high-velocity helium gas as in the Mercury laser system. We also describe a plan to mass-produce pump-diode lasers to bring diode costs down to the order of

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

0.01 per Watt of peak output power, as needed to make the LIFE application economically attractive.

The commissioning of the advanced radiographic capability laser system: experimental and modeling results at the main laser output

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 beams 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.

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

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory is the first of a kind megajoule-class laser with 192 beams capable of delivering over 1.8 MJ and 500TW of 351nm light [1], [2]. It has been commissioned and operated since 2009 to support a wide range of missions including the study of inertial confinement fusion, high energy density physics, material science, and laboratory astrophysics. In order to advance our understanding, and enable short-pulse multi-frame radiographic experiments of dense cores of cold material, the generation of very hard x-rays above 50 keV is necessary. X-rays with such characteristics can be efficiently generated with high intensity laser pulses above 1017 W/cm² [3]. The Advanced Radiographic Capability (ARC) [4] which is currently being commissioned on the NIF will provide eight, 1 ps to 50 ps, adjustable pulses with up to 1.7 kJ each to create x-ray point sources enabling dynamic, multi-frame x-ray backlighting. This paper will provide an overview of the ARC system and report on the laser performance tests conducted with a stretched-pulse up to the main laser output and their comparison with the results of our laser propagation codes.

High-energy (>70 keV) x-ray conversion efficiency measurement on the ARC laser 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.

Interferometric adaptive optics for high-power laser pointing and wavefront control and phasing

The Advanced Radiographic Capability (ARC) laser system at the National Ignition Facility (NIF) is designed to ultimately provide eight beamlets with a pulse duration adjustable from 1 to 30 ps, and energies up to 1.5 kJ per beamlet. Currently, four beamlets have been commissioned. In the first set of 6 commissioning target experiments, the individual beamlets were fired onto gold foil targets with energy up to 1 kJ per beamlet at 20–30 ps pulse length. The x-ray energy distribution and pulse duration were measured, yielding energy conversion efficiencies of 4–9 × 10−4 for x-rays with energies greater than 70 keV. With greater than 3 J of such x-rays, ARC provides a high-precision x-ray backlighting capability for upcoming inertial confinement fusion and high-energy-density physics experiments on NIF.

Measurement of the repeatability of the prompt flashlamp-induced wavefront aberration on beamlines at the National Ignition Facility

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 beams pointing and overall wavefront quality. One quad of the NIF beams, four beam pairs, will be utilized for these experiments and hydrodynamic and particle-in-cell simulations indicate that for the fast ignition experiments, these beams will be required to deliver 50% (4.0 kJ) of their total energy (7.96 kJ) within a 40-µm-diam spot at the end of a fast ignition cone target. This requirement implies a stringent pointing and overall phase conjugation error budget on the adaptive optics system used to correct these beam lines. The overall encircled energy requirement is more readily met by phasing of the beams in pairs but still requires high Strehl ratios and root-mean-square tip/tilt errors of approximately 1 µrad. To accomplish this task we have designed an interferometric adaptive optics system capable of beam pointing, high Strehl ratio, and beam phasing with a single pixilated microelectromechanical systems deformable mirror and interferometric wavefront sensor. We present the design of a testbed used to evaluate the performance of this wavefront sensor along with simulations of its expected performance level.

A Compact UV Timing Fiducial System for use with x-Ray Streak Cameras at NIF

We have undertaken a measurement campaign to determine the repeatability of the prompt flashlamp-induced wavefront aberration on beamlines at the National Ignition Facility (NIF) and determine the extent to which shot-to-shot variations in this aberration may degrade the performance of a proposed adaptive optics system for the short-pulse Advanced Radiographic Capability beamline on NIF. In this paper we will describe the unique NIF configuration that was required to make this measurement, present the results of the experiment, and discuss the implications of these results for the adaptive optics system design.

Interferometric adaptive optics for high-power laser pointing, wavefront control, and phasing

We present the design of a compact UV (263-nm) timing fiducial system for use with x-ray streak cameras at the National Ignition Facility (NIF). The design consists of remote fiber amplification of an infrared 1053-nm (1ω) seed, a free-space optical path that has two stages of frequency conversion from 1ω to the fourth harmonic (4ω), and fiber delivery of the 4ω signal via output fiber for use with an x-ray streak camera. This is all contained within an airbox that can reside in a vacuum. The 1ω seed and the pump light for the fiber amplifier is delivered to the airbox via optical fiber ( 100 meters) from a location in the NIF that is shielded from neutron radiation generated from imploding targets during system shots. When complete, the system will be able to provide timing fiducials to multiple x-ray streak cameras on the same system shot. We will present data that demonstrates end-to-end system performance.*

Interferometric adaptive optics for high-power laser beam correction in fast ignition experiments

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 beams pointing and overall wavefront quality. One quad of the NIF beams, 4 beam pairs, will be utilized for these experiments and hydrodynamic and particle-in-cell simulations indicate that for the fast ignition experiments, these beams will be required to deliver 50%(4.0 kJ) of their total energy(7.96 kJ) within a 40 μm diameter spot at the end of a fast ignition cone target. This requirement implies a stringent pointing and overall phase conjugation error budget on the adaptive optics system used to correct these beam lines. The overall encircled energy requirement is more readily met by phasing of the beams in pairs but still requires high Strehl ratios, Sr, and RMS tip/tilt errors of approximately one μrad. To accomplish this task we have designed an interferometric adaptive optics system capable of beam pointing, high Strehl ratio and beam phasing with a single pixilated MEMS deformable mirror and interferometric wave-front sensor. We present the design of a testbed used to evaluate the performance of this wave-front sensor below along with simulations of its expected performance level.