Thomas C. Sangster

First results from cryogenic target implosions on OMEGA

Initial results from direct-drive spherical cryogenic target implosions on the 60-beam OMEGA laser system [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1997)] are presented. These experiments are part of the scientific base leading to direct-drive ignition implosions planned for the National Ignition Facility (NIF) [W. J. Hogan, E. I. Moses, B. E. Warner et al., Nucl. Fusion 41, 567 (2001)]. Polymer shells (1-mm diam with walls <3 μm) are filled with up to 1000 atm of D2 to provide 100-μm-thick ice layers. The ice layers are smoothed by IR heating with 3.16-μm laser light and are characterized using shadowgraphy. The targets are imploded by a 1-ns square pulse with up to ∼24 kJ of 351-nm laser light at a beam-to-beam rms energy balance of <3% and full-beam smoothing. Results shown include neutron yield, secondary neutron and proton yields, the time of peak neutron emission, and both time-integrated and time-resolved x-ray images of the imploding core. The experimental values are...

Absolute calibration method for laser megajoule neutron yield measurement by activation diagnostics.

Guohong Li, Adina Luican, and Eva Y. Andrei Department of Physics & Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA We demonstrate a simple capacitive based method to quickly and efficiently locate micron size conductive samples on insulating substrates in a scanning tunneling microscope (STM). By using edge recognition the method is designed to locate and identify small features when the STM tip is far above the surface allowing for crash-free search and navigation. The method can be implemented in any STM environment even at low temperatures and in strong magnetic field, with minimal or no hardware modifications.The laser megajoule (LMJ) and the National Ignition Facility (NIF) plan to demonstrate thermonuclear ignition using inertial confinement fusion (ICF). The neutron yield is one of the most important parameters to characterize ICF experiment performance. For decades, the activation diagnostic was chosen as a reference at ICF facilities and is now planned to be the first nuclear diagnostic on LMJ, measuring both 2.45 MeV and 14.1 MeV neutron yields. Challenges for the activation diagnostic development are absolute calibration, accuracy, range requirement, and harsh environment. At this time, copper and zirconium material are identified for 14.1 MeV neutron yield measurement and indium material for 2.45 MeV neutrons. A series of calibrations were performed at Commissariat à lEnergie Atomique (CEA) on a Van de Graff facility to determine activation diagnostics efficiencies and to compare them with results from calculations. The CEA copper activation diagnostic was tested on the OMEGA facility during DT implosion. Experiments showed that CEA and Laboratory for Laser Energetics (LLE) diagnostics agree to better than 1% on the neutron yield measurement, with an independent calibration for each system. Also, experimental sensitivities are in good agreement with simulations and allow us to scale activation diagnostics for the LMJ measurement range.

Radial Structure of Shell Modulations Near Peak Compression of Spherical Implosions

The structure of shell modulations is measured at peak compression of directly driven spherical implosions using absorption of titanium-doped layers placed at various distances of 1, 5, 7, and 9 μm from the inner surface of 20 μm thick plastic CH shells filled with 18 atm of D3He gas. The modulations are measured using the ratios of monochromatic core images taken inside and outside of the titanium 1s–2p absorption spectral region. Peak-compression, time-integrated areal-density modulations are higher at the inner shell surface, which is unstable during the deceleration phase of an implosion with a modulation level of 59±14%. The perturbations are lower in the central part of the shell, having a modulation level of 18±5%. The outer surface of the shell, which is unstable during the acceleration phase of an implosion, has a modulation level of 52±20%.

Observations of modulated shock waves in solid targets driven by spatially modulated laser beams

The growth of surface perturbations due to nonuniformities in the drive laser is an important subject in laser–matter interactions. We present results of experiments using drive lasers with known, single-mode modulations to produce nonuniform shocks that propagate into planar plastic (CH) targets. An optical probe beam is used to measure the arrival of these modulated shocks at various surfaces in the target. Experiments at moderate laser intensities (≲1013u2009W/cm2) exhibit behavior predicted by hydrocodes and simple scaling laws. This technique will be used to observe various dynamic effects in laser-produced plasmas and shock-wave propagation.

Alignment effects on a neutron imaging system using coded apertures.

A high resolution neutron imaging system is being developed and tested on the OMEGA laser facility for inertial confinement fusion experiments. This diagnostic uses a coded imaging technique with a penumbral or an annular aperture. The sensitiveness of these techniques to misalignment was pointed out with both experiments and simulations. Results obtained during OMEGA shots are in good agreement with calculations performed with the Monte Carlo code GEANT4. Both techniques are sensitive to the relative position of the source in the field of view. The penumbral imaging technique then demonstrates to be less sensitive to misalignment compared to the ring. These results show the necessity to develop a neutron imaging diagnostic for megajoule class lasers taking into account our alignment capabilities on such facilities.

Cryogenic Target-Implosion Experiments on OMEGA

The University of Rochesters Laboratory for Laser Energetics has been imploding thick cryogenic targets for six years. Improvements in the Cryogenic Target Handling System and the ability to accurately design laser pulse shapes that properly time shocks and minimize electron preheat, produced high fuel areal densities in deuterium cryogenic targets (202±7 mg/cm2). The areal density was inferred from the energy loss of secondary protons in the fuel (D2) shell. Targets were driven on a low final adiabat (α = 2) employing techniques to radially grade the adiabat (the highest adiabat at the ablation surface). The ice layer meets the target-design toughness specification for DT ice of 1-μm rms (all modes), while D2 ice layers average 3.0-μm-rms roughness. The implosion experiments and the improvements in the quality and understanding of cryogenic targets are presented.

Vulnerability of fiber-optic links for high-speed diagnostics to pulsed-power facilities

The vulnerability of fiber-optic links for high-speed diagnostics to the harsh environments (short pulse of 14 MeV nneutrons, x- and γ-rays) associated with the future Megajoule class lasers (LMJ, NIF) is investigated through nexperiments at both ASTERIX and OMEGA facilities. Radiation responses of a Mach Zehnder modulator and single-mode nfiber optics have been characterized with high dose rate x-ray pulses to simulate the conditions expected for the nnext generation of pulsed-power facilities. The tolerance of a fiber-optic link to the mixed environment to the present nworldwide most powerful laser neutron source facility (OMEGA, Rochester) is also presented.

CR-39 (PADC) Reflection and Transmission of Light in the Ultraviolet–Near-Infrared (UV–NIR) Range:

The spectral reflection (specular and diffuse) and transmission of Columbia Resin 39 (CR-39) were measured for incoherent light with wavelengths in the range of 200–2500u2009nm. These results will be of use for the optical characterization of CR-39, as well as in investigations of the chemical modifications of the polymer caused by ultraviolet (UV) exposure. A Varian Cary 5000 was used to perform spectroscopy on several different thicknesses of CR-39. With proper analysis for the interdependence of reflectance and transmittance, results are consistent across all samples. The reflectivity from each CR-39–air boundary reveals an increase in the index of refraction in the near-UV. Absorption observations are consistent with the Beer–Lambert law. Strong absorption of UV light of wavelength shorter than 350u2009nm suggests an optical band gap of 3.5u2009eV, although the standard analysis is not conclusive. Absorption features observed in the near infrared are assigned to molecular vibrations, including some that are new to the literature.