Christopher D. Marshall

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.

Ultraviolet laser emission properties of Ce 3+ -doped LiSrAlF 6 and LiCaAlF 6

The UV laser properties of trivalent cerium-doped LiSrAlF6 (Ce:LiSAF) and LiCaAlF6 (Ce:LiCAF) are reported. The polarized absorption, emission, and excited-state absorption cross sections for both Ce:LiSAF and Ce:LiCAF were determined for the 266-nm pump and 290-nm laser emission wavelengths. The single-exponential emission lifetime was measured to be 28 ± 2 and 25 ± 2 ns for Ce:LiSAF and Ce:LiCAF, respectively. The gain was found to be highest when the probe was polarized parallel to the optical axis of the crystal, owing to the anisotropic nature of the excited-state absorption. The excited-state absorption was ascribed to a transition from the 5d orbital of Ce3+ to the conduction band of the host. Laser slope efficiencies as high as 29% and 21%, respectively, were measured for Ce:LiSAF and Ce:LiCAF when the 266-nm pulsed pump beam was polarized parallel to the optical or the c axis. These are the highest laser slope efficiencies yet observed to our knowledge for any known solid-state UV laser material. When the pump beam was polarized perpendicular to the optical axis of the crystals, the gain and the laser efficiencies dropped by as much as an order of magnitude. The magnitude of pump-induced solarization was found to be highly variable for different crystals and to have an insignificant effect on the laser performance when favorable crystals were used.

Induced optical absorption in gamma, neutron and ultraviolet irradiated fused quartz and silica

Abstract We have investigated the effects of ultraviolet (4.7 eV), gamma (∼ 1 MeV), and neutron (∼ 1 MeV) irradiations on the optical properties of SiO 2 glass samples. Samples from various sources were studied, including synthetic fused silicas and natural fused quartz. Several relationships among the different types of ionizing radiation were examined. For example, both UV light and gamma rays convert the germanium impurity to the B 1 absorption band in fused quartz samples. On the other hand, only neutrons are capable of inducing the oxygen deficient centers (ODCs) with the ∼krad-level doses employed here; the ODCs are produced by way of direct knock-on collisions. The ODCs generated by the neutrons can be converted into E′ centers afterwards with γ-rays. Transient grating and pump-probe optical experiments show that only a small fraction of the induced defect absorption remains permanently, while nearly all recover to the original condition after a UV pulse. Finally, thermal annealing experiments indicate that the radiation-induced defects can be annealed away at temperatures > 350°C. We compare the impacts of gamma, neutron, and ultraviolet radiation in terms of the mechanism by which defects are generated.

1.047-/spl mu/m Yb:Sr/sub 5/(PO/sub 4/)/sub 3/F energy storage optical amplifier

The pumping and gain properties of Yb/sup 3+/-doped Sr/sub 5/(PO/sub 4/)/sub 3/F (Yb:S-FAP) are reported. Using a tunable, free running 900-nm Cr:LiSAF oscillator as a pump source for a Yb:S-FAP rod, the saturation fluence for pumping was measured to be 2.2 J/cm/sup 2/ based on either the spatial, temporal, or energy transmission properties of the Yb:S-FAP rod. The emission peak of Yb:S-FAP (1047.5 nm in air) is shown to overlap with that of Nd:YLiF/sub 4/ (Nd:YLF) to within 0.1 nm, rendering Yb:S-FAP suitable as an effective power amplifier for Nd:YLF oscillators. The small signal gain, under varying pumping conditions, was measured with a cw Nd:YLF probe laser. These measurements implied emission cross sections of 6.0/spl times/10/sup -20/ and 1.5/spl times/10/sup -20/ cm/sup 2/ for /spl pi/ and /spl sigma/ polarized light. Respectively, which fall within the error limits of the previously reported values of 7.3/spl times/10/sup -20/ and 1.4/spl times/10/sup -20/ cm/sup 2/ for /spl pi/ and /spl sigma/ polarized light, obtained from purely spectroscopic techniques. The effects of radiation trapping on the emission lifetime have been quantified and have been shown to lead to emission lifetimes as long as 1.7 ms, for large optically dense crystals. This is substantially larger than the measured intrinsic lifetime of 1.10 ms. Yb:S-FAP crystal boules up to 25/spl times/25/spl times/175 mm in size, which were grown for the above experiments and were found to have acceptable loss characteristics ( >

Laser properties of a new average-power Nd-doped phosphate glass

The Nd-doped phosphate laser glass described herein can withstand 2.3 times greater thermal loading without fracture, compared to APG-1 (commercially available Average-Power Glass from Schott Glass Technologies). The enhanced thermal loading capability is established on the basis of the intrinsic thermomechanical properties (expansion, conduction, fracture toughness, and Youngs modulus), and by direct thermally induced fracture experiments using Ar-ion laser heating of the samples. This Nd-doped phosphate glass (referred to as APG-t) is found to be characterized by a 29% lower gain cross section and a 25% longer low-concentration emission lifetime. Other measurements pertaining to the concentration quenching, thermal lensing, and saturation of the extraction are also described in this article. It is note-worthy that APG-t offers increased bandwidth near the peak of the 1054 nm gain spectrum, suggesting that this material may have special utility as a means of generating and amplifying ultrashort pulses of light.

Diode-pumped ytterbium-doped Sr/sub 5/(PO/sub 4/)/sub 3/F laser performance

The performance of the first diode-pumped Yb/sup 3+/-doped Sr/sub 5/(PO/sub 4/)/sub 3/F (Yb:S-FAP) solid-state laser is discussed. An InGaAs diode array has been fabricated that has suitable specifications for pumping a 3/spl times/3/spl times/30 mm Yb:S-FAP rod. The saturation fluence for diode pumping was deduced to be 5.5 J/cm/sup 2/ for the particular 2.8 kW peak power diode array utilized in our studies. This is 2.5/spl times/ higher than the intrinsic 2.2 J/cm/sup 2/ saturation fluence as is attributed to the 6.5 nm bandwidth of our diode pump array. The small signal gain is consistent with the previously measured emission cross section of 6.0/spl times/10/sup -20/ cm/sup 2/, obtained from a narrowband-laser pumped gain experiment. Up to 1.7 J/cm/sup 3/ of stored energy density was achieved in a 6/spl times/6/spl times/44 mm Yb:S-FAP amplifier rod. In a free running configuration, diode-pumped slope efficiencies up to 43% (laser output energy/absorbed pump energy) were observed with output energies up to /spl sim/0.5 J per 1 ms pulse. When the rod was mounted in a copper block for cooling, 13 W of average power was produced with power supply limited operation at 70 Hz with 500 /spl mu/s pulses.

Diode arrays, crystals, and thermal management for solid-state lasers

We summarize our efforts in the development of solid-state lasers, including the laser diode arrays, pump light delivery, approaches to thermal management, and novel gain media. Our interests are in developing unique solid-state lasers, including those operating at higher powers, offering less common wavelengths, and having other specialized features. In this paper, we discuss high-power Tm:YAG and Yb:YAG lasers. The gas cooled slab laser concept using Yb:S-FAP, and side-pumped Er:YAG and Cr:ZnSe lasers. We address the optical and thermal physics of these systems and also mention several additional gain media that have the potential of offering unique performance characteristics: Ce:LiSAF, APG-2 laser glass, Dy:LaCl/sub 3/, and Yb:BCBF.

Ce : LiSrAlF6 laser performance with antisolarant pump beam

Abstract We have explored the impact of 266 nm pump-induced solarization on the 290 nm laser performance of Ce: LiSrAlF 6 crystals. Among the issues considered are the incorporation of codopants (e.g. Na + , Mg 2+ , Zn 2+ ), and the use of an additional 532 nm beam to rapidly destroy the interfering color centers. The solarization mechanism has been unraveled and found to involve two-photon creation of color centers (via the 4f→ 5d→ conduction band pathway of Ce 3+ ), followed by the one-photon bleaching of the color centers. Ce : LiSrAlF 6 (Ce : LiSAF) laser slope efficiencies as high as 47% can be achieved with the simultaneous introduction of the 266 nm pump and 532 nm bleaching beams; 33% with the 266 nm beam only.

Three-Dimensional Hydrodynamic Experiments on the National Ignition Facility

The production of supersonic jets of material via the interaction of a strong shock wave with a spatially localized density perturbation is a common feature of inertial confinement fusion and astrophysics. The behavior of two-dimensional (2D) supersonic jets has previously been investigated in detail [J. M. Foster et. al, Phys. Plasmas 9, 2251 (2002)]. In three-dimensions (3D), however, there are new aspects to the behavior of supersonic jets in compressible media. In this paper, the commissioning activities on the National Ignition Facility (NIF) [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] to enable hydrodynamic experiments will be presented as well as the results from the first series of hydrodynamic experiments. In these experiments, two of the first four beams of NIF are used to drive a 40 Mbar shock wave into millimeter scale aluminum targets backed by 100 mg/cc carbon aerogel foam. The remaining beams are delayed in time and are used to provide a point-projection x-ray backlighter source for diagnosing the three-dimensional structure of the jet evolution resulting from a variety of 2D and 3D features. Comparisons between data and simulations using several codes will be presented.

National Ignition Facility commissioning and performance

The National Ignition Facility at LLNL recently commissioned the first set of four beam lines into the target chamber. This effort, called NIF Early Light, demonstrated the entire laser system architecture from master oscillator through the laser amplifiers and final optics to target and initial X-ray diagnostics. This paper describes the major installation and commissioning steps for one of NIFs 48 beam quads. Using a dedicated single beam line Precision Diagnostic System, performance was explored over the entire power versus energy space up to 6.4 TW/beam for sub-nanosecond pulses and 25 kJ/beam for 23 ns pulses at 1w. NEL also demonstrated frequency converted Nd:Glass laser energies from a single beamline of 11.3 kJ at 2w and 10.4 kJ at 3w.