Joseph T. Salmon

Alignment and wavefront control systems of the National Ignition Facility

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. NIFs 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.

Experimental comparison of a Shack–Hartmann sensor and a phase-shifting interferometer for large-optics metrology applications

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 <20-nm rms systematic difference between the two measurements was due to diffraction artifacts that were present in the interferometer data but not in the Shack-Hartmann sensor data. The results suggest that Shack-Hartmann sensors could replace phase-shifting interferometers for many applications, with particular advantages for large-optic metrology.

National Ignition Facility alignment and wavefront control

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. NIFs 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.

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

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.

Beamlet pulse-generation and wavefront-control system

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.

Composite amplitude modulated phase only filter based detection and tracking of the back-reflection of KDP images

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 Beamlet laser system as a prototype for the National Ignition Facility (NIF)

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 (

Detection and Tracking of the Back-Reflection of KDP Images in the Presence or Absence of a Phase Mask

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.