A. V. Deniz

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.

Comparison between measured and calculated nonuniformities of Nike laser beams smoothed by induced spatial incoherence

We have studied the spatial fluence nonuniformities in single beams from the Nike KrF laser smoothed by the Induced Spatial Incoherence (ISI) technique. This study compares time-integrated CCD camera measurements with a numerical model of ISI speckle used in the NRL FAST2D hydrocode, and with analytic calculations that examine the ensemble-average behavior of that model. The results verify the model and calculations, showing good agreement of both the spatial mode spectra and the total nonuniformity versus pulsewidth and bandwidth.

Improved measurements of noise and resolution of x-ray framing cameras at 1–2 keV

We describe x-ray framing cameras that were developed for experiments on the Nike laser system at the Naval Research Laboratory. These cameras have internal impedance matching to allow use of 50Ω pulsing systems from 100 ps to several nanoseconds, or for long pulse/dc operation. The cameras have been optimized for spatial resolution and low light scatter using aluminized phosphors. One of these cameras has been coupled to a charge coupled device camera and the total system was tested for resolution and image noise using both electrons and x rays. The spatial resolution was found to be well fitted to a Gaussian modulation transfer function with a value of 0.5 at 10 lp/mm. A detailed analysis of the noise in the images was performed for electrons and x rays. The noise in the images is determined by a detective quantum efficiency of approximately 10% for 1–2 keV x-ray photons.

The determination of absolutely calibrated spectra from laser produced plasmas using a transmission grating spectrometer at the Nike laser facility

A transmission grating spectrometer has been developed that is capable of recording time-resolved spectra from laser-irradiated target foils in the wavelength range from 10 to 150 A. Since the primary goal is to quantify the radiation flux from the resulting plasma, a detailed investigation has been conducted to evaluate the estimation of the absolute radiation intensity. Time-integrated spectra were observed on photographic film using a 5000 l/mm grating and clearly show the superposition of four diffraction orders. The deconvolution of the observed spectrum is based on a quantitative description of the spectrometer’s performance using the calibrated transmission efficiencies of the grating’s diffraction orders. This deconvolution procedure can provide an estimate of the absolute spectral intensity with a residual error of ∼20%. Results from a high spectral resolution grazing-incidence spectrometer have been used in the development of this method. A streak camera (not absolutely calibrated) was also used with the transmission grating spectrometer to record the time-resolved spectra.

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.

A comparison of glass fluorescers used to measure a pulsed ultraviolet image at F/2

We built and characterized an instrument to measure the output profile of a pulsed KrF laser. The laser beam is focused onto a ∼1-mm-thick fluorescer, and the resulting fluorescence is imaged by a charge coupled device (CCD) camera. We tested 21 materials for efficiency, linearity, and fidelity. The best material is Schott BG-26 color filter glass. It has acceptable linearity for a fluence of 50 mJ/cm2, and a spatial resolution of ∼50 μm with a laser convergence angle of 30° (F/2), when the fluorescence is imaged with F/3 optics. We present a list of the materials tested and comparisons of profiles measured by using the fluorescer and by direct recording with a CCD camera.

Rear Surface Light Emission Measurements from Laser-produced Shock Waves in Clear and Al-coated Polystyrene Targets

Abstract The Nike KrF laser, with its very uniform focal distributions, has been used at intensities near 10 14 W/cm 2 to launch shock waves in polystyrene targets. The rear surface visible light emission differed between clear polystyrene (CH) targets and targets with a thin (125 nm) Al coating on the rear side. The uncoated CH targets showed a relatively slowly rising emission followed by a sudden fall when the shock emerges, while the Al-coated targets showed a rapid rise in emission when the shock emerges followed by a slower fall, allowing an unambiguous determination of the time the shock arrived at the rear surface. A half-aluminized target allowed us to observe this difference in a single shot. The brightness temperature of both the aluminized targets and the non-aluminized targets was slightly below but close to rear surface temperature predictions of a hydrodynamic code. A discussion of preheat effects is given.

Nike direct-drive ICF program

The Nike program coordinates experimental and theoretical efforts to evaluate and verify the physics base needed to proceed with direct-drive inertial confinement fusion. The program emphasizes the use of KrF laser drivers because of their demonstrated ultra-smooth target illumination and high absorption efficiency but is also applicable to short- wavelength glass laser drivers. Currently, the Nike program is concentration on the problem of increasing the gain of direct-drive target designs while simultaneously minimizing the growth of hydrodynamic instabilities. Advanced target design have been identified which achieve low isentropic compression while reducing RT instability through the use of tuned soft x-ray or shock preheating of the target ablator. Experiments measure the growth and saturation of the ablative RT instability and its modification as a result of tuning of the ablator isentrope and variations in laser imprinting. The important fundamental processes are also investigated in individual experiments to measure the emission and transport of x-ray radiation, the equations-of- state of deuterium, foams, and other relevant materials, as well as the acceleration of cryogenic filled foams.

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 2×1014 W/cm2 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. These parameters are predicted to be those required for a high gain ICF pellet. The NIKE system consists of a commercial oscillator/amplifier front end, an array of gas discharge amplifiers, two electron beam pumped amplifiers (one with a 20×20 cm2 aperture, the other with a 60×60 cm2 aperture) and the optics required to relay, encode, and decode the beam. Approximately two‐thirds of the system is operational and currently undergoing tests. The output of the smaller e‐beam system, the 20 cm Amplifier, exceeds both the unifo...

Noise and sensitivity of x-ray framing cameras at Nike (abstract)

X-ray framing cameras are the most widely used tool for radiographing density distributions in laser and Z-pinch driven experiments. The x-ray framing cameras that were developed specifically for experiments on the Nike laser system are described. One of these cameras has been coupled to a CCD camera and was tested for resolution and image noise using both electrons and x rays. The largest source of noise in the images was found to be due to low quantum detection efficiency of x-ray photons.