P. Lake

Hot Dense Capsule-Implosion Cores Produced by Z -Pinch Dynamic Hohlraum Radiation

Hot dense capsule implosions driven by Z-pinch x rays have been measured using a approximately 220 eV dynamic Hohlraum to implode 1.7-2.1 mm diameter gas-filled CH capsules. The capsules absorbed up to approximately 20 kJ of x rays. Argon tracer atom spectra were used to measure the T(e) approximately 1 keV electron temperature and the n(e) approximately 1-4 x 10(23) cm(-3) electron density. Spectra from multiple directions provide core symmetry estimates. Computer simulations agree well with the peak emission values of T(e), n(e), and symmetry, indicating reasonable understanding of the Hohlraum and implosion physics.

Dynamic hohlraum driven inertial fusion capsules

A dynamic hohlraum is formed when an imploding annular cylindrical Z-pinch driven plasma collides with an internal low density convertor. This collision generates an inward traveling shock wave that emits x rays, which are trapped by the optically thick Z-pinch plasma and can be used to drive an inertial fusion capsule embedded in the convertor. This scheme has the potential to efficiently drive high yield capsules due to the close coupling between the intense radiation generation and the capsule. In prior dynamic hohlraum experiments [J. E. Bailey et al., Phys. Rev Lett. 89, 095004 (2002)] the convertor shock wave has been imaged with gated x-ray pinhole cameras. The shock emission was observed to be very circular and to be quite narrow in the radial direction. This implies that there is minimal Rayleigh–Taylor imprinting on the shock wave. Thus, the dominant source of radiation asymmetry is not random and in principle could be significantly decreased by proper design. Due to the closed geometry of the d...

Radiation science using Z-pinch x rays

Present-day Z-pinch experiments generate 200 TW peak power, 5–10 ns duration x-ray bursts that provide new possibilities to advance radiation science. The experiments support both the underlying atomic and plasma physics, as well as inertial confinement fusion and astrophysics applications. A typical configuration consists of a sample located 1–10 cm away from the pinch, where it is heated to 10–100 eV temperatures by the pinch radiation. The spectrally-resolved sample-plasma absorption is measured by aiming x-ray spectrographs through the sample at the pinch. The pinch plasma thus both heats the sample and serves as a backlighter. Opacity measurements with this source are promising because of the large sample size, the relatively long radiation duration, and the possibility to measure opacities at temperatures above 100 eV. Initial opacity experiments are under way with CH-tamped NaBr foil samples. The Na serves as a thermometer and absorption spectra are recorded to determine the opacity of Br with a pa...

Neon Photoionization Experiments Driven By Z-Pinch Radiation

Abstract Present-day Z-pinch experiments generate ∼2×10 21 erg / s peak power, ∼6 ns full-width at half-maximum X-ray bursts that provide new possibilities to study radiation-heated matter. This source is being used to investigate the production of plasmas in which photoionization dominates collisional ionization. Spectroscopic measurements of such plasmas can serve to benchmark atomic physics models of the photoionized plasmas. Beyond intrinsic interest in the atomic physics, these models will be applied to the interpretation of data from the new generation of satellite X-ray spectrographs that will promote the understanding of accretion-powered objects such as X-ray binaries and active galactic nuclei. Moreover, this information is needed for X-ray laser research. Our experiments use a 1-cm-scale neon gas cell to expose 10 18 atoms / cm 3 to an X-ray flux of ∼5×10 18 erg / cm 2 / s . Thin mylar ( 1.5 μm ) windows confine the gas and allow the radiation to flow into the cell. The ionization is monitored with absorption spectra recorded with crystal spectrometers, using the pinch as a backlight source. In initial experiments we acquired an absorption spectrum from Li- and He-like Ne, confirming the ability to produce a highly ionized neon plasma.

Dynamic hohlraum radiation hydrodynamics

Z-pinch dynamic hohlraums are a promising indirect-drive inertial confinement fusion approach. Comparison of multiple experimental methods with integrated Z-pinch∕hohlraum∕capsule computer simulations builds understanding of the hohlraum interior conditions. Time-resolved x-ray images determine the motion of the radiating shock that heats the hohlraum as it propagates toward the hohlraum axis. The images also measure the radius of radiation-driven capsules as they implode. Dynamic hohlraum LASNEX [G. Zimmerman and W. Kruer, Comments Plasma Phys. Control. Fusion 2, 85 (1975)] simulations are found to overpredict the shock velocity by ∼20–40%, but simulated capsule implosion trajectories agree reasonably well with the data. Measurements of the capsule implosion core conditions using time- and space-resolved Ar tracer x-ray spectroscopy and the fusion neutron yield provide additional tests of the integrated hohlraum-implosion system understanding. The neutron yield in the highest performing CH capsule implos...

Time- and space-resolved elliptical crystal spectrometers for high energy density physics research

X-ray spectrometers used in high energy density plasma experiments must provide high time, space, and spectral resolution while overcoming the difficulties imposed by x-ray background, debris, and mechanical shocks. At the Z facility these problems are addressed using a suite of elliptical crystal spectrometers. The elliptical geometry isolates the detector from the line of sight with a slit placed at the elliptical focus, while the sensitivity enables locating the crystal 2–4 m from the plasma source. Space and time resolution are obtained by using an array of slits to project one dimensional plasma images onto the crystal and recording the spectrally dispersed images with a gated microchannel plate detector.

High performance capsule implosions driven by the Z-pinch dynamic hohlraum

The Z-pinch dynamic hohlraum (ZPDH) is a high-power x-ray source that has been used in a variety of high energy-density experiments including inertial confinement fusion (ICF) studies. The system consists of a tungsten wire-array Z pinch that implodes onto a low-density CH2 foam converter launching a radiating shock that heats the hohlraum to radiation temperatures >200 eV. Through time-gated pinhole camera measurements, the mean shock speed is measured from 28 experiments to be 326 ± 4 µm ns−1 with a shot-to-shot standard deviation of 7%. Broad-band x-ray measurements indicate that the shot-to-shot reproducibility in the power emission and pulse-shape of the source shock is 40 kJ of x-ray energy, within a factor of 4 of the energy believed sufficient for ICF ignition. The capsule types imploded by the ZPDH have evolved over four years culminating in a design that produces record indirect-drive DD thermonuclear neutron yields of up to 3.5E11.

Wavelength-dependent measurements of optical-fiber transit time, material dispersion, and attenuation

A new, to our knowledge, method for measuring the wavelength dependence of the transit time, material dispersion, and attenuation of an optical fiber is described. We inject light from a 4-ns rise-time pulsed broadband flash lamp into fibers of various lengths and record the transmitted signals with a time-resolved spectrograph. Segments of data spanning a range of approximately 3000 A are recorded from a single flash-lamp pulse. Comparison of data acquired with short and long fibers enables the determination of the transit time and the material dispersion as functions of wavelength dependence for the entire recorded spectrum simultaneously. The wavelength-dependent attenuation is also determined from the signal intensities. The method is demonstrated with experiments using a step-index 200-mum-diameter SiO(2) fiber. The results agree with the transit time determined from the bulk glass refractive index to within ?0.035% for the visible (4000-7200-A) spectrum and 0.12% for the UV (2650-4000-A) spectrum and with the attenuation specified by the fiber manufacturer to within ?10%.

Diagnostic methods for time-resolved optical spectroscopy of shocked liquid deuterium

Sandia National Laboratories Z facility generates shocks in liquid deuterium with pressures up to 100 GPa. Temperature measurements using spectroscopy of the shocked D2 self-emission can help discriminate between different deuterium equation of state models. Time-resolved spectra are recorded using four diagnostic systems, each composed of a fiber optic probe that transmits light from the shocked D2 to a remote streaked spectrometer. Calibration of the entire system in the streaked mode is performed using a xenon arc lamp. The absolute xenon arc lamp spectrum is determined by comparison with National Institute of Standards and Technology (NIST) standards. Data analysis is performed by measuring the wavelength-dependent efficiency for each system and applying this to determine the shocked D2 self-emission spectrum. Temperature is deduced from either the wavelength dependence of the spectral radiance, ignoring the absolute intensity, or from both the wavelength dependence and the absolute intensity. The sho...

High-accuracy time- and space-resolved Stark shift measurements (invited)

Stark-shift measurements using emission spectroscopy are a powerful tool for advancing understanding in many plasma physics experiments. We use simultaneous two-dimensional space- and time-resolved spectra to study the electric field evolution in the 20 TW Particle Beam Fusion Accelerator II ion diode acceleration gap. Fiber optic arrays transport light from the gap to remote streaked spectrographs operated in a multiplexed mode that enables recording time-resolved spectra from eight spatial locations on a single instrument. Design optimization and characterization measurements of the multiplexed spectrograph properties include the astigmatism, resolution, dispersion, and sensitivity. A semiautomated line-fitting procedure determines the Stark shift and the related uncertainties. Fields up to 10 MV/cm are measured with an accuracy ±2%–4%. Detailed tests of the procedure confirm that the uncertainty in the wavelength-shift error bars is less than ±20%. Development of an active spectroscopy probe technique ...