Pulsed power generators create high-energy-density conditions by rapidly delivering an immense pulse of electrical current to a compact imploding load. Accurately measuring the shape and amplitude of this load current pulse is essential to understanding the behavior of all pulsed power experiments. At the Z Pulsed Power Facility, the closest-in load current measurements are provided by velocimetry techniques such as VISAR (velocity interferometer system for any reflector) and PDV (photonic Doppler velocimetry). Here, fiber-coupled interferometers measure the velocity history of an exploding metallic flyer plate that is embedded in the vertical walls of the current return can. The flyer plate is driven outward by the magnetic pressure from the load current such that magnetohydrodynamic modeling can be used to determine the load current waveform from the measured velocity history. In this paper, we present the first load current velocimetry measurements to be made from the horizontal top flyer plate that carries current radially inward from the return can to the load. These spatially resolved measurements, which span R = 5- 9 mm, are enabled by a transformative new velocimetry diagnostic-a line-imaging velocity interferometer called Z Line VISAR (ZLV)-whose optical performance overcomes the measurement challenges presented by the steep velocity gradients encountered on the top flyer plate. To validate ZLV's capabilities, a 14-MA, 100-ns experiment was conducted to losslessly couple current up the return can and radially inward across the top flyer plate. Comparisons between the ZLV data obtained from this experiment and two-dimensional magnetohydrodynamic simulations driven with the current measured on the return can indicate that the current delivery across the top flyer plate is indeed lossless to within the few-percent uncertainty of the ZLV data. Given that the current coupling is lossless, the experimental results are used to demonstrate that one-dimensional current unfold techniques can be applied to generate a radially resolved load current map from the ZLV velocity data. This analysis provides a template for how to use the ZLV diagnostic to determine the efficacy of current delivery in future experiments where losses may occur in close proximity to the load.

The Advanced Radiographic Capability (ARC) at the National Ignition Facility (NIF) is a petawatt-class, short-pulse laser system designed to provide x-ray backlighting of NIF targets. ARC uses four NIF beamlines to produce eight beamlets to create a sequence of eight images of an imploding fuel capsule using backlighting targets and diagnostic instrumentation. ARC employs a front end that produces two pulses, chirps the pulses out to 2 ns, and then injects the pulses into the two halves of each of four NIF beamlines. These pulses are amplified by NIF pre- and main amplifiers and transported to compressor vessels located in the NIF target area. The pulses are then compressed and pointed into the NIF target chamber where they impinge upon an array of backlighters. The interaction of the ARC laser pulses and the backlighting material produces bursts of high-energy x-rays that illuminate an imploding fuel capsule. The transmitted x-rays are imaged by diagnostic instrumentation to produce a sequence of radiograph images. A key component of the success of ARC is the automatic alignment system that accomplishes the precise alignment of the beamlets to avoid damaging equipment and to ensure that the beamlets are directed onto the tens-of-microns scale backlighters. In this paper, we describe the ARC automatic alignment system, with emphasis on control loops used to align the beampaths. We also provide a detailed discussion of the alignment image processing, because it plays a critical role in providing beam centering and pointing information for the control loops.

Four of the 192 beams of the National Ignition Facility (NIF) are currently being diverted into the Advanced Radiographic Capability (ARC) system to generate a sequence of short (1-50 picoseconds) 1053 nm laser pulses. When focused onto high Z wires in vacuum, these pulses create high energy x-ray pulses capable of penetrating the dense, imploding fusion fuel plasma during ignition scale experiments. The transmitted x-rays imaged with x-ray diagnostics can create movie radiographs that are expected to provide unprecedented insight into the implosion dynamics. The resulting images will serve as a diagnostic for tuning the experimental parameters towards successful fusion reactions. Beam delays introduced into the ARC pulses via independent, free-space optical trombones create the desired x-ray image sequence, or movie. However, these beam delays cause optical distortion of various alignment fiducials viewed by alignment sensors in the NIF and ARC beamlines. This work describes how the position of circular alignment fiducials is estimated in the presence of distortion.

The Advance Radiographic Capability (ARC) at the National Ignition Facility (NIF) is a laser system designed to produce a sequence of short pulses used to backlight imploding fuel capsules. Laser pulses from a short-pulse oscillator are dispersed in wavelength into long, low-power pulses, injected in the NIF main laser for amplification, and then compressed into high-power pulses before being directed into the NIF target chamber. In the target chamber, the laser pulses hit targets which produce x-rays used to backlight imploding fuel capsules. Compression of the ARC laser pulses is accomplished with a set of precision-surveyed optical gratings mounted inside of vacuum vessels. The tilt of each grating is monitored by a measurement system consisting of a laser diode, camera and crosshair, all mounted in a pedestal outside of the vacuum vessel, and a mirror mounted on the back of a grating inside the vacuum vessel. The crosshair is mounted in front of the camera, and a diffraction pattern is formed when illuminated with the laser diode beam reflected from the mirror. This diffraction pattern contains information related to relative movements between the grating and the pedestal. Image analysis algorithms have been developed to determine the relative movements between the gratings and pedestal. In the paper we elaborate on features in the diffraction pattern, and describe the image analysis algorithms used to monitor grating tilt changes. Experimental results are provided which indicate the high degree of sensitivity provided by the tilt sensor and image analysis algorithms.

The Advanced Radiographic Capability (ARC) at the National Ignition Facility was developed to produce a sequence of short laser pulses that are used to backlight an imploding fuel capsule. This backlighting capability will enable the creation of a sequence of radiographs during capsule implosion and provide an unprecedented view into the dynamics of the implosion. A critical element of the ARC is the diagnostic instrumentation used to assess the quality of the pulses. Pulses are steered to the diagnostic package through a complex optical path that requires precision alignment. A central component of the alignment system is the image analysis algorithms, which are used to extract information from alignment imagery and provide feedback for the optical alignment control loops. Alignment imagery consists of complex patterns of light resulting from the diffraction of pilot beams around cross-hairs and other fiducials placed in the beam path. This paper describes the alignment imagery for two ARC automated alignment loops, and the image analysis algorithms used to extract information required for the operation of those loops.

Welcome to the Eleventh Annual C.A.S.I.S. Workshop, a yearly event at the Lawrence Livermore National Laboratory, presented by the Center for Advanced Signal & Image Sciences, or CASIS, and sponsored by the LLNL Engineering Directorate. Every November for the last 10 years we have convened a diverse set of engineering and scientific talent to share their work in signal processing, imaging, communications, controls, along with associated fields of mathematics, statistics, and computing sciences. This year is no exception, with sessions in Adaptive Optics, Applied Imaging, Scientific Data Mining, Electromagnetic Image and Signal Processing, Applied Signal Processing, National Ignition Facility (NIF) Imaging, and Nondestructive Characterization.

The NIF laser system will be capable of delivering 1.8MJ of 351nm energy in 192 beams. Diagnostics instruments must measure beam energy, power vs. time, wavefront quality, and beam intensity profile to characterize laser performance. Alignment and beam diagnostics are also used to set the laser up for the high power shots and to isolate problems when performance is less than expected. Alignment and beam diagnostics are multiplexed to keep the costs under control. At the front-end the beam is aligned and diagnosed in an input sensor package. The output 1053nm beam is sampled by collecting a 0.1% reflection from an output beam sampler and directing it to the output sensor package (OSP). The OSP also gets samples from final focus lens reflection and samples from the transport spatial filter pinhole plane. The output 351nm energy is measured by a calorimeter collecting the signal from an off-axis diffractive beam-sampler. Detailed information on the focused beam in the high-energy target focal plane region is gathered in the precision diagnostics. This paper describes the design of the alignment and diagnostics on the NIF laser system.

This System Design Requirement document establishes the performance, design, development, and test requirements for the NIP Laser System. The Laser System generates and delivers high-power optical pulses to the target chamber, and is composed of all optical puke creating and transport elements from Puke Generation through Final Optics as well as the special equipment that supports, energizes and controls them. The Laser System consists of the following WBS elements: 1.3 Laser System 1.4 Beam Transport System 1.6 Optical Components 1.7 Laser Control 1.8.7 Final Optics.

The authors measure the absolute wavefront of Beamlet output pulses and actively correct for slowly varying aberrations present when the power amplifiers are not fired. Provision is also made for active precompensation of pump-induced amplifier aberrations.

Light propagating through a large laser system is subject to numerous perturbations which can cause deviations from the desired path. Closed loop alignment controls are typically located at key points to compensate for such errors. We have tested a decetralized approach to the alignment control of such a laser system. The decentralized control system has a modular structure in which the control command for each subsystem is a function of only two errors, the error detected by that subsystem's own sensor and the error detected by the sensor in the preceding subsystem. Therefore, although the controller uses local information, the whole alignment system can operate in a coordinated fashion. On the other hand, any design change of a subsystem will cause redesign of only one additional controller, the one for the subsystem immediately downstream. The rest of the system remains unchanged, as we verified in the laboratory by disabling one control loop to simulate a change in the subsystem. The control accuracy of the whole system remained the same after the controller downstream of the disabled loop was modified. This modular approach increases design flexibility and facilitates future expansion of the system.

A dye master-oscillator-power-amplifier chain pumped by copper lasers produced 1200 W output at 50% conversion efficiency and >10 kHz pulse repetition frequency. 2 refs., 3 figs.

A deformable mirror has been designed and built to correct thermally induced aberrations in a medium average power solid-state laser (MPSSL) system. The mirror is capable of correcting typically induced errors of up to two waves in the narrow dimension and up to six waves in the long dimension. It has a clear aperture of 12 mm x 130 mm and serves as one element in the laser cavity. Piezoelectric translators are attached to the mirror through a unique flexure mechanism. This simple mechanical design has proven adequate for controlling low-spatial-frequency aberrations. Although designed specifically for the MPSSL, modifications of this design have application to other optical systems. The mirror design and performance as well as the methodology and techniques used in this development are discussed.

A rectangular deformable mirror is described which meets the low spatial frequency and low bandwidth requirements associated with thermal aberrations in a solid state zig-zag amplifier. We incorporate design features which simplify the design of the mirror itself and reduce the complexity of the associated control system. 4 refs., 7 figs.

The Nova laser, in operation since December 1984, is capable of irradiating targets with light at 1.05 µm, 0.53 µm, and 0.35 µm. Correct alignment of these harmonic beams uses a system called a target plane imager (TPI). It is a large microscope (four meters long, weighing one thousand kilograms) that relays images from the target chamber center to a video optics module located on the outside of the chamber. Several modes of operation are possible including: near-field viewing and far-field viewing at three magnifications and three wavelengths. In addition, the entire instrument can be scanned in X,Y,Z to examine various planes near chamber center. Performance of this system and its computer controls will be described.

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Requirements for diagnostic-data acquisition and processing include rapid turnaround, effective interfacing with the alignment and power-conditioning controls, preservation of data under failure conditions, options for local or control-room modes of operation, and portability of the hardware required for off-line troubleshooting and calibration. The major elements of a digital system which meets these requirements are shown. A further potential difficulty, operating in the presence of prompt radiation from the target, has been avoided by locating all laser diagnostic components outside of the target room.

The alignment and laser diagnostic systems guide laser pulses through the separate amplifier chains to the target, measure their temporal, spatial and energy characteristics, and ensure simultaneous arrival at the target to within 5 picoseconds. Alignment tasks accomplished prior to each target shot involve automatic or remote-manual adjustments of approximately 2000 stepper motors and other actuators for the full 20 beam, 3 wavelength system. The primary detectors for alignment functions are CCD cameras with both digital and standard video output. Diagnostic data handling and processing is accomplished digitally, and both the alignment and diagnostic systems are integrated into the facility-wide digital control network.

The Shiva laser system is part of a new 20 terawatt laser facility at Lawrence Livermore Laboratory.. The system contains more than $5,000,000 worth of optics. This paper discusses the various optical components, typical-component quantities and specification, and the problem of laser damage to components.