J. R. Murray

National Ignition Facility laser performance status

The National Ignition Facility (NIF) is the worlds largest laser system. It contains a 192 beam neodymium glass laser that is designed to deliver 1.8 MJ at 500 TW at 351 nm in order to achieve energy gain (ignition) in a deuterium-tritium nuclear fusion target. To meet this goal, laser design criteria include the ability to generate pulses of up to 1.8 MJ total energy, with peak power of 500 TW and temporal pulse shapes spanning 2 orders of magnitude at the third harmonic (351 nm or 3omega) of the laser wavelength. The focal-spot fluence distribution of these pulses is carefully controlled, through a combination of special optics in the 1omega (1053 nm) portion of the laser (continuous phase plates), smoothing by spectral dispersion, and the overlapping of multiple beams with orthogonal polarization (polarization smoothing). We report performance qualification tests of the first eight beams of the NIF laser. Measurements are reported at both 1omega and 3omega, both with and without focal-spot conditioning. When scaled to full 192 beam operation, these results demonstrate, to the best of our knowledge for the first time, that the NIF will meet its laser performance design criteria, and that the NIF can simultaneously meet the temporal pulse shaping, focal-spot conditioning, and peak power requirements for two candidate indirect drive ignition designs.

Raman pulse compression of excimer lasers for application to laser fusion

Application of efficient ultraviolet excimer lasers such as the 248 nm KrF laser to laser fusion requires that long laser pulses be efficiently converted to short pulses at high intensity. The backward Raman amplifier is shown to be a promising candidate for this application. Gain, saturation, and limits to amplifier performance are described. It is shown that pump beams of poor spatial quality may be converted to output beams of high spatial quality. Several common gaseous vibrational Raman scatterers are discussed, and it is shown that a simple KrF-pumped backward Raman amplifier using methane at atmospheric pressure will have a saturation fluence near 1 J/cm2and can produce an output five times as intense as the pump in a ten times shorter pulse with an efficiency of about 50 percent. Design tradeoffs and possible techniques for further improving the performance of such amplifiers are discussed.

Performance of a prototype for a large-aperture multipass Nd:glass laser for inertial confinement fusion

The Beamlet is a single-beam prototype of future multibeam megajoule-class Nd:glass laser drivers for inertial confinement fusion. It uses a multipass main amplifier, adaptive optics, and efficient, high-fluence frequency conversion to the third harmonic. The Beamlet amplifier contains Brewster-angle glass slabs with a clear aperture of 39 cm x 39 cm and a full-aperture plasma-electrode Pockels cell switch. It has been successfully tested over a range of pulse lengths from 1-10 ns up to energies at 1.053 mum of 5.8 kJ at 1 ns and 17.3 kJ at 10 ns. A 39-actuator deformable mirror corrects the beam quality to a Strehl ratio of as much as 0.4. The 1.053-mum output has been converted to the third harmonic at efficiencies as high as 80% and fluences as high as 8.7 J/cm(2) for 3-ns pulses.

ORION: Clearing near-Earth space debris using a 20-kW, 530-nm, Earth-based, repetitively pulsed laser

When a large piece of space debris forced a change of flight plan for arecent U.S. Space Shuttle mission, the concept that we are trashing space as well as Earth finally attained broad public awareness. Almost a million pieces of debris have been generated by 35 years of spaceflight, and now threaten long-term space missions. The most economical solution to this problem is to cause space debris items to reenter and burn up in the atmosphere. For safe handling of large objects, it is desired to do this on a precomputed trajectory. Due to the number, speed, and spacial distribution of the objects, a highly agile source of mechanical impulse, as well as a quantum leap in detection capability are required. For reasons we will discuss, we believe that the best means of accomplishing these goals is the system we propose here, which uses a ground-based laser system and active beam phase error correcting beam director to provide the impulse, together with a new, computer-intensive, very high-resolution optical detection system to locate objects as small as 1 cm at 500-km range. Illumination of the objects by the repetitively pulsed laser produces a laser-ablation jet that gives the impulse to de-orbit the object. A laser of just 20-kW average power and state-of-the-art detection capabilities could clear near-Earth space below 100-km altitude of all space debris larger than 1 cm but less massive than 100 kg in about 4 years, and all debris in the threatening 1–20-cm size range in about 2 years of continuous operation. The ORION laser would be sited near the Equator at a high altitude location (e.g., the Uhuru site on Kilimanjaro), minimizing turbulence correction, conversion by stimulated Raman scattering, and absorption of the 530-nm wavelength laser beam. ORION is a special case of Laser Impulse Space Propulsion (LISP), studied extensively by Los Alamos and others over the past 4 years.

Laser oscillation on the green bands of XeO and KrO

Laser oscillation has been observed on the green bands of the XeO and KrO excimers using direct electron beam excitation of high‐pressure Xe or Kr containing small concentrations of O2. These bands are transitions between molecular levels correlating to the 1S0 and 1D2 metastable levels of atomic oxygen plus ground‐state Xe or Kr. The XeO laser emission is at a number of wavelengths between 5300 and 5550 A, while the KrO emission is at a single wavelength near the free atom line at 5577 A. Laser pulse energies of 10 mJ and peak powers of 100 kW are seen for both excimers.

KrCl laser oscillation at 222 nm

Laser oscillation has been observed on the 2Σ+ 1/2‐2Σ+ 1/2 band of KrCl at 222 nm in an electron‐beam‐excited mixture of argon, krypton, and chlorine. The laser performance and spectral features of KrCl and KrF are compared.

Backward Raman gain measurements for KrF laser radiation scattered by CH4

Backward Raman small‐signal gain measurements for a KrF laser scattered by methane gas, including linewidth and pressure dependences, are presented. A 268‐nm probe beam is produced from a 249‐nm pump beam by superfluorescent emission in a methane cell, and is amplified by a counterpropagating KrF beam at 249 nm in a second methane cell. The results are consistent with theoretical predictions for the backward Raman amplifier.

Laser oscillation on the 292‐nm band system of Br2

Laser oscillation has been observed on numerous rotational transitions of the most intense bands of the Br2 emission system near 292 nm. The laser is electron‐beam excited in an Ar‐Br2 mixture. Quenching of the lower level by Br2 is inferred.

Photolytic pumping of the iodine laser by XeBr

The efficiency for producing fluorescence from the 220–340‐nm bands of XeBr has been measured to be 11±5% in an electron‐beam‐driven device, a value consistent with an assumed kinetic model. The photolytic pumping of the 1.315‐μm atomic iodine laser is demonstrated using this narrow‐band fluorescer.

Polarization smoothing in a convergent beam

A birefringent wedge in a collimated 351-nm beam provides polarization smoothing at the Omega laser facility and provided it for the Nova laser. At the National Ignition Facility, the best place to put such an optic is after the final focus lens. In a converging beam, a flat birefringent plate can closely mimic the polarization-smoothing action of a wedge. In this new design the flat plate is nearly a Z-cut crystal; for the wedges, the optical axis of the crystal lies far from the plate normal.