Search

We have achieved the NIF design goals for power and energy by delivering 1.86 MJ of ultra-violet energy in a wide dynamic range (>300:1), 22.5-ns shaped ignition pulse with a peak power of 520 TW.

Quenching-independent fluorescence by atomic hydrogen has been measured with photoionization controlled-loss spectroscopy (PICLS) in stoichiometric and fuel-rich premixed These H2/O2/N2 flames at a pressure of 20 Torr. These measurements are compared with conventional fluorescence measurements in the same flames. When matched in the postflame zone, the two sets of measurements diverge in the preheat zone between the burner surface and the peak of the fluorescence profiles. This divergence, caused by changes in the local quenching rate coefficient, shows the utility of PICLS for determining the kinetics of atomic hydrogen in the preheat zone.

We performed a direct side-by-side comparison of a Shack-Hartmann wave-front sensor and a phase-shifting interferometer for the purpose of characterizing large optics. An expansion telescope of our own design allowed us to measure the surface figure of a 400-mm-square mirror with both instruments simultaneously. The Shack-Hartmann sensor produced data that closely matched the interferometer data over spatial scales appropriate for the lenslet spacing, and much of the

An algorithm for determining the position of the KDP back-reflection image was developed. It was compared to a centroid-based algorithm. While the algorithm based on centroiding exhibited a radial standard deviation of 9 pixels, the newly proposed algorithm based on classical matched filtering (CMF) and a Gaussian fit to correlation peak provided a radial standard deviation of less than 1 pixel. The speed of the peak detection was improved from an average of 5.5 seconds for Gaussian fit to 0.022 seconds by using a polynomial fit. The performance was enhanced even further by utilizing a composite amplitude modulated phase only filter; producing a radial standard deviation of 0.27 pixels. The proposed technique was evaluated on 900+ images with varying degrees of noise and image amplitude as well as real National Ignition Facility (NIF) images.

The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is a stadium-sized facility containing a 192-beam, 1.8-Megajoule, 500-Terawatt, ultraviolet laser system. High-energy-density and inertial confinement fusion physics experiments require the ability to precisely align and focus pulses with single beam energy up to 20KJ in a few nanoseconds onto mm-sized targets. NIF's alignment control system now regularly provides automatic alignment of the four commissioned beams prior to every NIF shot in approximately 45min., and speed improvements are being implemented. NIF utilizes adaptive optics for wavefront control, which significantly improves the ability to tightly focus each laser beam onto a target. Multiple sources of both static and dynamic aberration are corrected. This presentation provides an overview of the NIF Automatic Alignment and Wavefront Control Systems including the accuracy and target spot size performance achieved.

The Beamlet pulse-generation system (or {open_quotes}front end{close_quotes}) refers to the laser hardware that generates the spatially and temporally shaped pulse that is injected into the main laser cavity. All large ICF lasers have pulse-generation systems that typically consist of a narrow-band oscillator, elector-optic modulators for temporal and bandwidth shaping, and one or more preamplifiers. Temporal shaping is used to provide the desired laser output pulse shape and also to compensate for gain saturation effects in the large-aperture amplifiers. Bandwidth is applied to fulfill specific target irradiation requirements and to avoid stimulated Brillouin scattering (SBS) in large-aperture laser components. Usually the sharp edge of the beam`s spatial intensity profile is apodized before injection in the main amplifier beam line. This prevents large-amplitude ripples on the intensity profile. Here the authors briefly review the front-end design and discuss improvements to the oscillator and modulator systems. Their main focus, however, is to describe Beamlet`s novel beam-shaping and wavefront-control systems that have recently been fully activated and tested.

Summary form only given. The National Ignition Facility is designed to ignite inertial-confinement fusion (ICF) targets using 1.8 MJ of ultraviolet (351 nm) laser light generated by frequency tripling the output of 192 neodymium glass laser beams. The Beamlet laser system is a full scale scientific prototype of one of the 192 NIF beamlines. Because the estimated cost of the NIF facility is substantial ($1.1 billion) it is imperative that the performance be cost optimized. This implies operation as close as possible to power and energy extraction limits imposed by fundamental physical constraints. Control of beam quality in the NIF and the Beamlet prototype is enhanced through the use of a deformable mirror. Beamlet employs a sophisticated suite of laser diagnostic systems to measure beam quality.

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.

The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is a stadium-sized facility containing a 192-beam Nd glass laser. Its 1.053-µm output is frequency converted to produce 1.8-MJ, 500-TW pulses in the ultraviolet. Refer to the companion overview articles in this issue for more information. High-energy-density and inertial confinement fusion physics experiments require the ability to precisely align and focus pulses with single-beam energy up to 20 KJ and durations of a few nanoseconds onto millimeter-sized targets. NIF's alignment control system now regularly provides automatic alignment of the four commissioned beams prior to every NIF shot in approximately 45 min, and speed improvements are being implemented. NIF utilizes adaptive optics for wavefront control, which significantly improves the ability to tightly focus each laser beam onto a target. Multiple sources of both static and dynamic aberration are corrected. This article provides an overview of the NIF automatic alignment and wavefront control systems, and provides data to show that the facility is expected to meet its primary requirements to position beams on the target with an accuracy of 50-µm rms over the 192 beams and to focus the pulses into a 600-µm spot.

Calibration of laser-saturated fluorescence measurements using Rayleigh scattering is presented as an alternative to absorption. This new procedure is advantageous when measuring radical species at concentrations well below the corresponding detection limit for absorption. The calibration accounts for nonuniform laser irradiation by extracting the local fluorescence emission along the laser axis and works equally well for both saturated and near-saturated center-line conditions. The predicted error due to misfocusing of the collection optics is nearly negligible when the measured fluorescence is within 10% of its peak value. Number densities obtained using this method are within 15% of those obtained from absorption measurements.

An experimental protocol, using photoionization controlled-loss spectroscopy (PICLS), has been developed for obtaining absolute number densities of atomic hydrogen from laser-induced fluorescence measurements in flames. Two laser beams are employed, the first to excite hydrogen atoms from the ground state to the second excited state via two-photon absorption and the second to strongly photoionize the excited atoms. The resulting fluorescence measurements are independent of quenching. A model is presented that assures the viability of PICLS as long as the photoionization rate is greater than or equal to the quenching rate. The model is verified in fuel-lean, stoichiometric, and fuel-rich flat premixed H(2)/O(2)/N(2) flames at pressures of 20 and 72 Torr. Over this range in pressure, the ratio of number densities obtained from PICLS to those calculated from partial equilibrium is constant to within 20%. Most of the error arises from the sensitivity of the partial equilibrium calculat ions to small uncertainties in both the fuel-oxidizer ratio and the measured OH concentration. Because of the quenching-independent nature of PICLS, quantitative fluorescence measurements can be made by calibrating at a single favorable flame condition.