Session GO6: ICF/HEDP Diagnostics

Magnetic Field Measurements in Wire-Array Z-Pinches using Magneto-Optically Active Waveguides

Understanding the magnetic field topology in wire-array Z-pinches as a function of time is of great significance to understanding these high-energy density plasmas. We are developing techniques to measure magnetic fields as a function of space and time using Faraday rotation of a single longitudinal mode (SLM) laser through a magneto-optically active bulk waveguide (terbium borate glass) placed adjacent to, or within, the wire array in experiments on the COBRA pulsed power generator [1]. We have measured fields $>$10 T with 100 ns rise times outside of a wire-array for the entire duration of the current pulse and as much as $\sim$2 T inside a wire-array for $\sim$40 ns from the start of current. This is the first time that such rapidly varying and large fields have been measured using these materials. We will also present our progress on field measurements using an optical fiber sensor and a very small ``thin film waveguide'' coupled to a fiber optic system. In a dense Z-pinch, these sensing devices may not survive for long but may provide the magnetic field at the position of the sensor for a greater fraction of the current pulse than magnetic probes, with which we compare our results. This research was sponsored by NNSA under SSAA program via DOE Coop Agreement DE-F03-02NA00057. \newline [1] W. Syed, D. A. Hammer, {\&} M. Lipson, 34$^{th}$ ICOPS {\&} 16$^{th}$ PPPS, Albuquerque, NM, June 2007.   

Time-Gated X Pinch Backlighting of Imploding Wire Array Z Pinches Using MCP

X-ray backlighting of imploding Z pinches is difficult because the Z pinch generally produces much more radiation than the backlighting source, thus saturating the film. Monochromatic x-ray backlighting is currently the only way to image the axis of an imploding Z pinch. Other techniques exist, but require modifying the Z pinch in a way that prevents it from radiating strongly. One promising technique is to use a microchannel plate (MCP) detector to gate the exposure. This way, the Z pinch can be allowed to form and radiate without modification. A 12.5 micron Ti filter allows energies in the range of 3-5 keV to reach the MCP which acts as a shutter to stop exposure of the film before the Z pinch radiates in that range. Presented here are the results of sensitivity measurements of the MCP as well as preliminary experiments imaging imploding Al and W arrays on the COBRA pulser.   

Electron density determination in a MgFe dense plasma via Stark-broadening analysis

Recent measurements of L-shell Fe opacity at temperatures above 100eV were performed at Sandia National Laboratories by recording x-ray transmission through a MgFe two-element plasma. While the observation of line absorption in Fe L-shell transitions was an important goal of the experiment, the Stark-broadened line absorption in Mg K-shell transitions provided an independent spectral signature for electron density determination. Indeed, at the expected 10$^{21}$ cm$^{-3 }$to 10$^{22}$ cm$^{-3 }$electron density range, Stark-broadening dominates the line width of several Mg K-shell transitions. We discuss the Mg K-shell line absorption spectrum, the calculation of Stark-broadened absorption line shapes, and their application to data analysis and electron density extraction.   

Experimental Stark widths of the 3p-3s transition of Ne VIII

We report measurements of the line width of the 3p-3s transition in Ne VIII in laser-produced gas jet plasmas. The density is inferred from the Paschen-alpha line in He II, the temperature from Thomson scattering. Preliminary results confirm measurements in pinch and pulsed-arc plasmas indicating that newest quantum-mechanical calculations underestimate the line width by a factor of two.   

Development of Compton radiography using multi-pulse high-Z backlighters

Radiography of the cold dense fuel will be a valuable diagnostic for imploding inertial confinement fusion targets at the National Ignition Facility (NIF). For x-rays with energies between 30 and 200 keV, the main opacity will be Compton scattering. We report on experiments to characterize x-ray emission from low- to high-Z planar foils irradiated by intense picosecond laser pulses. Spectra generated by a sequence of elements from Mo to Pb, with line and continuum emission up to 100 keV, have been recorded using a Charged Coupled Device (CCD) in single hit regime and a crystal spectrometer. We discuss the results and implications for the design of the experiments at the NIF.   

Neutron-Induced Signal Measurements in Cables on OMEGA

The National Ignition Facility (NIF) and the Laser Megajoule Facility (LMJ) are currently under construction in the U.S. and France, respectively. Ignited targets at these facilities are anticipated to produce up to 10$^{19}$ DT neutrons. For approximately 500 ns after ignition, the NIF and LMJ target diagnostics and control systems will work under extremely harsh radiation conditions. In particular, neutron-induced signals in cables can compromise or destroy diagnostic instruments and control systems. Recent results of neutron-induced signal measurements at 30 kJ in different cables at the 60-beam OMEGA Laser Facility will be reported. Based on these results, specific recommendations on cable selection for the NIF and LMJ will be given. Neutron-background mitigation techniques in the NIF neutron time-of-flight diagnostics will be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under the Cooperative Agreement No. DE-FC52-92SF19460.   

EMP Measurements on the Omega Laser System at LLE

We present the result of Electromagnetic Pulse measurements on the 30kJ, 60-beam, OMEGA laser system at the University of Rochester Laboratory for Laser Energetics. The suite of EMP probes includes free field electric and magnetic field sensors located internal and external to the OMEGA target chamber (1.5-2.3 m from target chamber center range). Measurements were taken under typical shot conditions for OMEGA and include variations in drive intensity, target size and mass and with neutron yields varying from zero to 1E13. Correlations of the EMP measurements with shot conditions will be presented. Such correlations are expected to form part of the basis for predictive EMP models on the National Ignition Facility. This work was supported by the U.S. DOE Office of Inertial Confinement Fusion cooperative agreement DE-FC52-92SF19460, NNSA Nevada Operations, contract DE-AC52-06NA25946 and by the University of California, LLNL, contract W-7405-Eng-48.   

First Tests on OMEGA of a Bubble Chamber for Neutron Detection

To provide additional options for imaging at NIF, a high-resolution, reduced-line-of-sight detector was developed and tested at LLE. The detector is based on a high-pressure freon, 115-bubble chamber with an expansion mechanism controlled by a linear motor. A CCD camera is used to photograph the bubbles in parallel, monochromatic light, while a Schlieren disk is used to enhance the contrast of the image. With bubble diameters in the range of 100 \textit{$\mu $}m, the achieved spatial resolution is significantly better than more-conventional pixilated arrays. The higher spatial resolution can be utilized to significantly shorten the neutron flight path. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement DE-FC52-92SF19460.   

First Measurements of Neutron Spectra using the Magnetic Recoil Spectrometer (MRS) at OMEGA

A new type of neutron spectrometer, a Magnetic Recoil Spectrometer (MRS), is being implemented at OMEGA for measurements of the scattered-DT-neutron spectrum, from which \textit{$\rho $R} can be inferred. Implementing an MRS at OMEGA is important for several reasons. First, it allows comprehensive tests of the different MRS parameters. Second, \textit{$\rho $R} of cryogenic DT implosions can be inferred from both the MRS and charged-particle spectrometry for moderate \textit{$\rho $R} implosions (\textit{$\rho $R}$<$200 mg/cm$^{2})$; this allows for a definitive check of the MRS. Third, as there is no other way to determine \textit{$\rho $R }when it exceeds 200 mg/cm$^{2}$, the MRS will bring a required diagnostic to the OMEGA cryogenic program. Fourth, the experience with MRS implementation and resulting neutron data at OMEGA will enable us to implement an optimal MRS for the NIF. The results from the first MRS measurements performed at OMEGA are presented. This work was supported in part by the U.S. DOE (Grant No. DE-FG03-03SF22691), LLE (subcontract Grant No. 412160-001G), LLNL (subcontract Grant No. B504974).   

Multiview observation and analysis of OMEGA direct-drive implosion cores

We discuss the observation and data analysis of OMEGA direct-drive implosion cores based on data recorded with three identical multi-monochromatic x-ray imagers. These instruments observed the implosion core along three quasi-orthogonal lines-of-sight, and recorded gated images of the core. The targets were plastic shells filled with deuterium gas and a tracer amount of argon for diagnostic purposes. Core imaging was based on argon Ly$\alpha $, He$\beta $ and Ly$\beta $ line emission. The data analysis rely on detailed spectral models that take into account non-equilibrium atomic kinetics, Stark broadened line shapes, and radiation transport calculations and a search and reconstruction technique based on a novel application of Pareto genetic algorithms to plasma spectroscopy. The spectroscopic analysis yields the spatial profiles of temperature and density in the core at the collapse of the implosion.   

X-ray Thomson scattering on compressed Be plasmas

X-ray Thomson scattering measurements have provided much insight into characterization of dense plasmas such as in determining electron temperature, density, and ionization state [1]. We performed spectrally resolved 6 keV x-ray scattering on laser shock compressed Be plasmas in both the non-collective and collective forward scattering regime at the Omega laser facility. From the forward scattering geometry, we have measured a plasmon peak whose frequency is sensitive to the electron density. We will present first data that indicate a density of n$_{e}$=8$\times $10$^{23}$/cc can be reached for a pressure of 10-15 Mbar. \newline [1] S. H. Glenzer \textit{et al}., Phys. Rev. Lett. \textbf{98}, 065002 (2007); Phys. Rev. Lett. \textbf{90}, 175002 (2003). G. Gregori \textit{et al}., Phys. Plasmas \textbf{11}, 2754 (2004).   

Low-Aberration Diffraction Mirror to Focus X-Rays

Radiography of laser-driven implosions is important for the development of inertial confinement fusion, and it is challenging to obtain adequate diagnostic radiographs of the compression of the fuel capsule. X-ray imaging, through the use of x-ray mirrors, can potentially increase the resolution and accuracy of radiographs, for instance by allowing radiography at locally normal incidence through an imploding spherical capsule. Mirrors for kilovolt x-rays usually work through crystal diffraction. Focusing mirrors have been demonstrated using planar crystals, bent to form a toroidal or spherical surface. We show that the correct profile for a focusing diffraction mirror requires the diffracting planes to change orientation with respect to the surface of the mirror, and suggest how such a bent crystal could be formed.   

Characterization of High-Current Relativistic Electron Beam Transport Through Solid-Density Matter Using High-Resolution Imaging of Coherent Transition Radiation

A diagnostic has been developed to measure the emission of coherent transition radiation produced by relativistic electrons emerging from the rear side of laser-illuminated targets. Experiments have been conducted on the Multi-Terawatt (MTW) Laser Facility at the University of Rochester's Laboratory for Laser Energetics. The MTW laser is capable of producing 10-J, 500-fs pulses of 1053-nm-wavelength radiation, which are focused using an $f$/2.5, off-axis parabolic mirror to an intensity in excess of 10$^{19}$ W cm$^{-2}$. The initial experimental campaign used Al, Fe, Cu, Au, and CH foils of varying thickness, which were shot with varying laser energy. High-resolution images of the rear-side emission show evidence of both electron-beam filamentation and electron-beam annular propagation. In this talk we will present the most recently acquired data and provide a brief description of the diagnostic characteristics and capabilities. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement DE-FC52-92SF19460.   

Quasi-DC Terahertz Electrical Conductivity Measurements of Dense Aluminum Plasma

We report on our investigation of electrical transport in ultrashort laser-heated aluminum. By measuring the complex electrical conductivity at terahertz (THz = 10$^{12}$ Hz) frequencies, we explore the dependence of electrical transport across the material phase transition from the cold solid to the dense plasma state. Using optical-pump, terahertz-probe spectroscopy, we measure the phase shifts and absorption of terahertz probe pulses that are reflected from the warm (0.1$\sim $3 eV) dense plasma. To characterize the THz field, we develop and use a single-shot, high temporal-resolution THz diagnostic capable of measuring free-space electromagnetic pulse fields in time and space. In contrast to the previous measurements of conductivities at optical frequencies, our THz non-contact probe method can directly measure quasi-DC electrical conductivities, providing insight into the transport nature of warm dense matter without dependence on conductivity models for extrapolation. Full hydrodynamic laser-foil calculations and THz Helmholtz wave equation calculations of the THz probe field show that deep penetration across the plasma gradient and into the dense solid is achieved with the THz probe. The technique demonstrates a new promising ultrafast time-resolved diagnostic capability for extracting conductivity transport.   

Short pulse laser coupling efficiency to hot electrons for fast-ignition studies.

Experiments were performed at the Titan laser facility at LLNL to study energy coupling efficiency to hot electrons as a function of irradiation conditions and target geometry. Hot electron spectra from cone and slab targets are compared and correlation with their K shell emission spectra is examined.