K. A. Gerber

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.

Measurements of laser-imprinted perturbations and Rayleigh–Taylor growth with the Nike KrF laser

Nike is a 56 beam Krypton Fluoride (KrF) laser system using Induced Spatial Incoherence (ISI) beam smoothing with a measured focal nonuniformity 〈ΔI/I〉 of 1% rms in a single beam [S. Obenschain et al., Phys. Plasmas 3, 1996 (2098)]. When 37 of these beams are overlapped on the target, we estimate that the beam nonuniformity is reduced by 37, to (ΔI/I)≅0.15% (excluding short-wavelength beam-to-beam interference). The extraordinary uniformity of the laser drive, along with a newly developed x-ray framing diagnostic, has provided a unique facility for the accurate measurements of Rayleigh–Taylor amplified laser-imprinted mass perturbations under conditions relevant to direct-drive laser fusion. Data from targets with smooth surfaces as well as those with impressed sine wave perturbations agree with our two-dimensional (2-D) radiation hydrodynamics code that includes the time-dependent ISI beam modulations. A 2-D simulation of a target with a 100 Au2009rms randomly rough surface finish driven by a completely unif...

Large area electron beam pumped krypton fluoride laser amplifier

Nike is a recently completed multi-kilojoule krypton fluoride (KrF) laser that has been built to study the physics of direct drive inertial confinement fusion. This paper describes in detail both the pulsed power and optical performance of the largest amplifier in the Nike laser, the 60 cm amplifier. This is a double pass, double sided, electron beam-pumped system that amplifies the laser beam from an input of 50 J to an output of up to 5 kJ. It has an optical aperture of 60 cm × 60 cm and a gain length of 200 cm. The two electron beams are 60 cm high × 200 cm wide, have a voltage of 640 kV, a current of 540 kA, and a flat top power pulse duration of 250 ns. A 2 kG magnetic field is used to guide the beams and prevent self-pinching. Each electron beam is produced by its own Marx/pulse forming line system. The amplifier has been fully integrated into the Nike system and is used on a daily basis for laser-target experiments.

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 ...

X-ray imaging of targets irradiated by the Nike KrF laser

Foil targets irradiated by the Naval Research Laboratory Nike KrF laser were imaged in the x-ray region with two-dimensional spatial resolution in the 2–10 μm range. The images revealed the smoothness of the emission from target and backlighter foils, the acceleration of the target foils, and the growth of Rayleigh–Taylor instabilities that were seeded by patterns on the irradiated sides of CH foils.

High-resolution x-ray imaging of planar foils irradiated by the Nike KrF laser

Thin plastic (CH) foils were irradiated by the Naval Research Laboratory Nike [Obenschain et al., Phys. Plasmas 3, 2098 (1996)] KrF laser and were imaged in the x-ray and extreme ultraviolet regions with two-dimensional spatial resolution in the 3–10 μm range. The CH foils were backlit by a silicon plasma. A spherically curved quartz crystal produced monochromatic images of the Si+12 resonance line radiation with energy 1865 eV that was transmitted by the CH foils. Instabilities that were seeded by linear ripple patterns on the irradiated sides of CH foils were observed. The ripple patterns had periods in the 31–125 μm range and amplitudes in the 0.25–5.0 μm range. The silicon backlighter emission was recorded by an x-ray spectrometer, and the 1865 eV resonance line emission was recorded by a fast x-ray diode. The multilayer mirror telescope recorded images of the C+3 1550 A emission (energy 8.0 eV) from the backside of the CH foils.

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.

X-Ray Bremsstrahlung Measurements of an Intense Relativistic Electron Beam Propagating in a Plasma

Information on the peak electron energy, angular spread, and current density of a relativistic electron beam propagating in a plasma column is obtained from measurements of two-absorber transmission ratios, anistropy, and intensity, respectively, of x-ray bremsstrahlung from thin target foils in the plasma. Measurements indicate a 10% loss in peak electron energy and a factor of two loss in peak beam intensity over the distance of the plasma column, and show a large angular spread in the beam.

The Nike electron-beam-pumped KrF laser amplifiers

Nike is a recently completed multikilojoule krypton-fluoride (KrF) laser that has been built to study the physics of direct-drive inertial confinement fusion. The two final amplifiers of the Nike laser are both electron-beam-pumped systems. This paper describes these two amplifiers, with an emphasis on the pulsed power. The smaller of the two has a 20/spl times/20 cm aperture, and produces an output laser beam energy in excess of 100 J. This 20 cm Amplifier uses a single 12 kJ Marx generator to inject two 300 kV, 75 kA, 140 ns flat-top electron beams into opposite sides of the laser cell. The larger amplifier in Nike has a 60/spl times/60 cm aperture, and amplifies the laser beam up to 5 W. This 60 cm amplifier has two independent electron beam systems. Each system has a 170 kT Marx generator that produces a 670 kV, 540 kA, 240 ns Bat-top electron beam. Both amplifiers are complete, fully integrated into the laser, meet the Nike system requirements, and are used routinely for laser-target experiments.