J. Workman

High-resolution 17–75keV backlighters for high energy density experiments

We have developed 17 keV to 75 keV 1-dimensional and 2-dimensional high-resolution ( 10{sup 17} W/cm{sup 2}. We have achieved high resolution point projection 1-dimensional and 2-dimensional radiography using micro-foil and micro-wire targets attached to low-Z substrate materials. The micro-wire size was 10 {micro}m x 10 {micro}m x 300 {micro}m on a 300 {micro}m x 300 {micro}m x 5 {micro}m CH substrate. The radiography performance was demonstrated using the Titan laser at LLNL. We observed that the resolution is dominated by the micro-wire target size and there is very little degradation from the plasma plume, implying that the high energy x-ray photons are generated mostly within the micro-wire volume. We also observe that there are enough K{alpha} photons created with a 300 J, 1-{omega}, 40 ps pulse laser from these small volume targets, and that the signal-to-noise ratio is sufficiently high, for single shot radiography experiments. This unique technique will be used on future high energy density (HED) experiments at the new Omega-EP, ZR and NIF facilities.

X-RAY YIELD SCALING STUDIES PERFORMED ON THE OMEGA LASER

We have performed experiments with planar targets on the OMEGA laser facility at the University of Rochester. These experiments investigated the scaling of x-ray yield and conversion efficiency with the laser energy and focusing properties for several different target materials. The experiments were also designed to investigate the feasibility of high-energy backlighters under typical irradiance geometries. The scaling of Fe emission near 6.7 keV was investigated by varying laser irradiance from 1014 to 1016 W/cm2. In addition, the scaling of x-ray yield with emitted x-ray energy was studied at fixed laser irradiance near 1016 W/cm2 for Fe, Zn, and Ge. The time-integrated spectra as well as filtered x-ray film gave relative x-ray yields.

TRIDENT high-energy-density facility experimental capabilities and diagnostics

The newly upgraded TRIDENT high-energy-density (HED) facility provides high-energy short-pulse laser-matter interactions with powers in excess of 200 TW and energies greater than 120 J. In addition, TRIDENT retains two long-pulse (nanoseconds to microseconds) beams that are available for simultaneous use in either the same experiment or a separate one. The facilitys flexibility is enhanced by the presence of two separate target chambers with a third undergoing commissioning. This capability allows the experimental configuration to be optimized by choosing the chamber with the most advantageous geometry and features. The TRIDENT facility also provides a wide range of standard instruments including optical, x-ray, and particle diagnostics. In addition, one chamber has a 10 in. manipulator allowing OMEGA and National Ignition Facility (NIF) diagnostics to be prototyped and calibrated.

Scaling of x-ray K-shell sources from laser-solid interactions

The application of x-ray sources to imaging of dense objects is standard technique. The quality of the x-ray image depends both on the x-ray source wavelength and on the flux. To improve the flux of the x-ray source it is important to understand how the conversion efficiency scales with laser irradiance and target material. We present measurements of x-ray conversion efficiency in sold Cr, Fe, Ni Zn and Ge targets as s function of the laser irradiance using the OMEGA laser facility. The results show a steep decease in the conversion efficiency with increasing Z while the scaling of conversion efficiency with laser irradiance can show a peak. Values for x-ray yield are determined using time-integrated crystal spectrometer data.

Development of intense point x-ray sources for backlighting high energy density experiments (invited)

High-energy-density (HED) experiments are often diagnosed using x-ray backlighting. Recently, experiments have been designed and fielded that require x-ray backlighting having large fields of view and high x-ray energies. These types of experiments will be even more prevalent on the National Ignition Facility laser. Point backlighting offers the potential to obtain higher-energy x rays using less laser energy while giving a large, uniform field of view (millimeters). We present recent results from Los Alamos National Lab, Lawrence Livermore National Lab, and the University of Rochester’s Laboratory for laser energetics obtained on the OMEGA laser at the University of Rochester on the development of such bright sources. We include discussion of the challenges and successes to date.

Scaling laws for energetic ions from the commissioning of the new Los Alamos National Laboratory 200 TW Trident laser

The recent Los Alamos National Laboratory Trident laser enhanced from 30 to 200 TW in power allows more than 100 J to be delivered on target in 500 fs with a spot size smaller than 12 microm at full width at half maximum. 15 microm flat-foil targets have been observed to produce proton beams in excess of 50 MeV at an intensity of only approximately 4x10(19) W/cm(2) with efficiencies approaching 5%. The Trident laser beam characteristics are presented along with the data compared to published scaling laws for proton acceleration.

Characterization and cross calibration of Agfa D4, D7, and D8 and Kodak SR45 x-ray films against direct exposure film at 4.0–5.5keV

Kodak direct exposure film (DEF) [B. L. Henke et al., J. Opt. Soc. Am. B 3, 1540 (1986)] has been the standard for moderate energy (1–10keV) x-ray diagnostic applications among the high-energy-density and inertial confinement fusion research communities. However, market forces have prompted Kodak to discontinue production of DEF, leaving these specialized communities searching for a replacement. We have conducted cross-calibration experiments and film characterizations on five possible substitutes for Kodak DEF. The film types studied were Kodak’s Biomax MR (BMR) and SR45 along with Agfa’s D8, D7, and D4sc. None of the films tested matched the speed of DEF. BMR and D8 were closest but D8 exhibited lower noise, with superior resolution and dynamic range. Agfa D7, Agfa D4sc, and Kodak SR45 were significantly less sensitive than BMR and D8, however, the improvements they yielded in resolution and dynamic range warrant their use if experimental constraints allow.

Phase-contrast imaging using ultrafast x-rays in laser-shocked materials.

High-energy x-rays, >10 keV, can be efficiently produced from ultrafast laser target interactions with many applications to dense target materials in inertial confinement fusion and high-energy density physics. These same x-rays can also be applied to measurements of low-density materials inside high-density Hohlraum environments. In the experiments presented, high-energy x-ray images of laser-shocked polystyrene are produced through phase contrast imaging. The plastic targets are nominally transparent to traditional x-ray absorption but show detailed features in regions of high density gradients due to refractive effects often called phase contrast imaging. The 200 TW Trident laser is used both to produce the x-ray source and to shock the polystyrene target. X-rays at 17 keV produced from 2 ps, 100 J laser interactions with a 12 μm molybdenum wire are used to produce a small source size, required for optimizing refractive effects. Shocks are driven in the 1 mm thick polystyrene target using 2 ns, 250 J, 532 nm laser drive with phase plates. X-ray images of shocks compare well to one-dimensional hydro calculations.

High-energy, high-resolution x-ray imaging on the Trident short-pulse laser facility.

With the completion of the Trident laser facility upgrade, 200 TW high-energy laser pulses are now capable of producing x-ray pulses with energies in the range of 15-40 keV, which will be used for high-spatial resolution radiography. A diagnostic suite is being developed on the laser system to investigate and characterize the x-ray emission from high-Z targets. This includes charge coupled device based single-photon counters, imaging plates, a high-energy electronic imager, spectral diagnostics, and optical and x-ray spot size diagnostics. We describe recent x-ray results from a commissioning campaign as well as describe the development and design of a high-energy spectrometer. X-ray radiographs taken at 22 keV with a spatial resolution of 25 mum are a first demonstration on this facility of high-energy, high-spatial resolution capability.

Uniform large-area x-ray imaging at 9 keV using a backlit pinhole

The development and application of point backlighting at high x-ray energies is an essential step in diagnosing radiation-driven experiments. The point-backlighting technique provides uniform backlighter irradiance over a large field of view. This technique circumvents the large laser energy required for area backlighters at energies of 9 keV and above. We present the results of a Zn 9 keV point-backlighter source using the technique of pinhole aperturing to define the source size and hence the resolution. Details of the design and application of this technique to an undriven gold-walled hohlraum are described.