Mark S. Pronko

The Nike KrF laser facility: Performance and initial target experiments

Krypton‐fluoride (KrF) lasers are of interest to laser fusion because they have both the large bandwidth capability (≳THz) desired for rapid beam smoothing and the short laser wavelength (1/4 μm) needed for good laser–target coupling. Nike is a recently completed 56‐beam KrF laser and target facility at the Naval Research Laboratory. Because of its bandwidth of 1 THz FWHM (full width at half‐maximum), Nike produces more uniform focal distributions than any other high‐energy ultraviolet laser. Nike was designed to study the hydrodynamic instability of ablatively accelerated planar targets. First results show that Nike has spatially uniform ablation pressures (Δp/p<2%). Targets have been accelerated for distances sufficient to study hydrodynamic instability while maintaining good planarity. In this review we present the performance of the Nike laser in producing uniform illumination, and its performance in correspondingly uniform acceleration of targets.

Production of high energy, uniform focal profiles with the Nike laser

Abstract Nike, a KrF laser facility at the Naval Research Laboratory, is designed to produce high intensity, ultra-uniform focal profiles for experiments relating to direct drive inertial confinement fusion. We present measurements of focal profiles through the next-to-last amplifier, a 20 × 20 cm 2 aperture electron beam pumped amplifier capable of producing more than 120 J of output in a 120 ns pulse. Using echelon free induced spatial incoherence beam smoothing this system has produced focal profiles with less than 2% tilt and curvature and less than 2% rms variation from a flat top distribution.

X‐ray emission from plasmas created by smoothed KrF laser irradiation

The x‐ray emission from plasmas created by the Naval Research Laboratory Nike KrF laser [Phys. Plasmas 3, 2098 (1996) ] was characterized using imaging and spectroscopic instruments. The laser wavelength was 1/4 μm, and the beams were smoothed by induced spatial incoherence (ISI). The targets were thin foils of CH, aluminum, titanium, and cobalt and were irradiated by laser energies in the range 100–1500 J. A multilayer mirror microscope operating at an energy of 95 eV recorded images of the plasma with a spatial resolution of 2 μm. The variation of the 95 eV emission across the 800 μm focal spot was 1.3% rms. Using a curved crystal imager operating in the 1–2 keV x‐ray region, the density, temperature, and opacity of aluminum plasmas were determined with a spatial resolution of 10 μm perpendicular to the target surface. The spectral line ratios indicated that the aluminum plasmas were relatively dense, cool, and optically thick near the target surface. The absolute radiation flux was determined at 95 eV ...

Efficient second harmonic conversion of broad-band high-peak-power Nd:glass laser radiation using large-aperture KDP crystals in quadrature

The authors have investigated the second-harmonic conversion efficiency of broadband Nd:glass laser light ( Delta nu /c >

X-ray spectroscopy of plasmas created by the Nike KrF laser

The x-ray emission from plasmas created by the Naval Research Laboratory Nike KrF laser was characterized using spectroscopic instruments. The targets were thin foils of aluminum and titanium and were irradiated by laser energies in the range 100–1500 J. Using a spherical-crystal imaging spectrometer operating in the 1–2 keV x-ray region, the density, temperature, and opacity of aluminum plasmas were determined with a spatial resolution of 10 μm in the direction perpendicular to the target surface. The spectral line ratios indicated that the aluminum plasmas were relatively dense, cool, and optically thick near the target surface.

Controlling output gain uniformity by spatial variation of the X-ray preionization in a large-aperture discharge-pumped KrF amplifier

A technique for improving the output uniformity in fast-discharge large-aperture lasers is described. Spatial variation of the X-ray preionization flux is used to compensate for the discharge-plasma skin depth and nonuniformity in the initial electric-field distribution. The technique was demonstrated using a 4.25/spl times/4.25 cm clear-aperture KrF laser that produces 2.7 J in a 15 ns FWHM pulse. >

Effects of random phase distortion and nonlinear optical processes on laser beam uniformity and spatial incoherence (ISI)

One of the key requirements for direct-drive laser fusion is a laser whose focal profile is sufficiently smooth and controllable to produce highly symmetric implosions. To achieve this, the NIKE laser will implement the echelon-free ISI technique, in which the desired focal profile is formed at the output of a spatially and temporally incoherent oscillator, then imaged through the laser amplifiers onto the target. Because the amplifiers are located in the quasi far- field of the oscillator, their large scalelength gain and phase nonuniformities will have little effect on the image as long as the light remains highly incoherent. However, small scalelength phase aberrations and nonlinear optical processes must be minimized to maintain good control over the image. After a brief description of the NIKE system, this paper reports on numerical simulations and measurements of profile distortion due to random amplitude and phase nonuniformities, nonlinear refraction, and self-seeded stimulated rotational Raman scattering, and describes the steps being taken to control these effects.

The NIKE KrF laser fusion facility

NIKE is a second generation high power KrF laser now under construction at the Naval Research Laboratory. The project is a collaborative effort between NRL and Los Alamos National Laboratory. NIKE is designed to deliver more than 2 kJ of energy to target in a 600-μm focal spot and a 4-ns pulse duration. Echelon free induced spatial incoherence (ISI) will be used to produce uniform target illumination. Flat targets will be ablatively accelerated to study both Rayleigh-Taylor and parametric instabilities. These results will have direct implications to direct-drive inertial confinement fusion for commercial energy applications. Reliable operation of a high power KrF laser is also an important goal of the NIKE laser, with the objective of 1000 target shots per year. This would be an important step in the development of the KrF laser as an ICF driver. NIKE is cheduled to begin target experiments in early 1994. If successful, these experiments will provide a technical basis to proceed with construction of an ignition facility.

Overview of the nike KrF laser program

Nike is a large angularly multiplexed Krypton-Fluoride (KrF) laser under development at the Naval Research Laboratory. It is designed to explore the technical and physics issues of direct drive laser fusion. When completed, Nike will deliver 2-3 kJ of 248 nm light in a 4 nsec pulse with intensities exceeding 2 x 10 {sup 14}W/cm{sup 2} onto a planar target. Spatially and temporally incoherent light will be used to reduce the ablation pressure nonuniformities to less than 2% in the target focal plane. The Nike laser consists of a commercial oscillator/amplifier front end, an array of gas discharge amplifiers, two electron beam pumped amplifiers (one with a 20x20 cm{sup 2} aperture, the other with a 60x60 cm{sup 2} aperture) and the optics required to relay, encode, and decode the beam. Approximately 90% of the system is operational and currently undergoing tests: the system is complete through the 20 cm amplifier, the 60 cm amplifier has completed all the necessary electron beam/pulsed power tests, and is currently being developed into a laser amplifier, and most of the optics have been installed. It is anticipated that Nike will be fully operational in the fall of 1994.

Short-range biological standoff detection system (SR-BSDS)

Fibertek is currently under contract to the US Army Soldier and Biological Chemical Command (SBCCOM) at Aberdeen Proving Ground, MD to develop a multi-wavelength lidar system. Under this effort, Fibertek will deliver a system that is capable of detecting the presence of biological aerosols. The SR-BSDS has successfully demonstrated the ability to detect and track a biological aerosol cloud while discriminating between biological and non-biological aerosols and hard targets. The SR-BSDS is an active standoff detection system with both ultraviolet (UV) and infrared (IR) capability. The UV wavelengths can provide near real time detection and ranging of a particulate cloud with demonstrated discrimination capability. Recent enhancements to the IR capability extended the cloud detection range and acquisition capability as well as providing an autonomous operation mode of operation. The SR-BSDS can be operated in one of two modes, manual or autonomous. In the manual mode the operator selects the desired scan field of view, resolution, wavelength, and degree of pulse coadding, then instructs the system to start scanning. The system will monitor its own performance and display information to the operator to indicate proper operation. The system will monitor cloud data and warn the operator when the sensor is aimed at an aerosol of interest. If a biological cloud of interest is found, an audible alarm will sound, and the operator can examine cloud imagery while the system continues to automatically monitor and track all clouds in the field-of-view. The scanning parameters can also be changed easily upon aerosol detection, if desired. In the autonomous mode, the operator selects the desired scan field of view. The system automatically scans for aerosol clouds with the IR beam. This is accomplished in a rapid, single pulse laser firing mode. Once a cloud with specified characteristics is acquired, the system automatically switches over to an UV beam for discrimination interrogation. System status, data and discrimination interrogation results will be transmitted over a wireless modem to a Command Post. All the above will continue to operate without additional operator intervention. The autonomous operation feature was recently demonstrated at Dugway Proving Ground in July of 1999. Field testing to date, both at Aberdeen Proving Ground, MD and Dugway Proving Ground, UT in 1998 and 1999 successfully demonstrated the systems detection, discrimination, scanning functions and autonomous operation. With the initial field testing and system demonstration testing successfully complete, emphasis is on several areas of enhancements in preparation for additional DPG testing and system delivery for field implementation in 2000.