Session JO2: ICF II: Implosions, Diagnostics, and Magnetic Fields

Developmental Status of a Liquid-Freon Bubble Chamber for Neutron Imaging

In inertial confinement fusion (ICF) ignition experiments it is important to distinguish failure mechanisms of the imploding capsule and unambiguously diagnose compression and hot-spot formation in the burning fuel. A neutron image of the imploded core can be used to infer both drive symmetry and final core compression/convergence. To provide additional options for imaging on the NIF, a high-resolution, reduced line-of-sight detector is being developed at LLE. The detector is based on a high-pressure freon bubble chamber. 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. The status of the bubble chamber and the optical-readout-system development will be discussed. Initial bubble growth data will be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Diagnosing Cryogenic DT Implosions at OMEGA and the NIF using Magnetic Recoil Spectrometry (MRS)

Spectral measurements of deuterons and tritons elastically scattered (``knock-ons'') by DT primary neutrons have been used at OMEGA to determine the $\rho R$ of several types of implosions. But these techniques will fail for $\rho R$s larger than $\sim $200 mg/cm$^{2}$, which is expected in the OMEGA cryogenic DT campaign and at the NIF. We are therefore building a Magnetic Recoil Spectrometer (MRS), at both OMEGA and the NIF, to probe these large $\rho R$ implosions using the down-scattered neutrons that originate from the same scattering process. In this presentation, the similarities of the two neutron spectrometers, as well as some important differences, will be discussed. The OMEGA MRS will be interfaced and qualified early in 2007, which will be invaluable for the risk reduction of the NIF MRS. This work is supported in part by LLE, LLNL, the U.S. DoE, and the N.Y. State Energy Research and Development Authority.   

Layering and Characterization of Cryogenic-DT Targets for OMEGA

The Laboratory for Laser Energetics has begun layering, characterizing, and imploding DT-ice-layer cryogenic targets on OMEGA. Shadowgraphic characterization protocols for D$_{2}$ ice layers in OMEGA cryogenic targets developed over several years have been modified for DT. In D$_{2}$ layers, three-dimensional, ice-layer analysis identifies the major cause of nonuniformity as temperature perturbation from IR absorption in the target support structures. D$_{2}$ surface-averaged, rms-roughness ranges from 1.7 to 8 \textit{$\mu $}m were observed. DT layers are formed using beta layering, eliminating IR heating and its absorption by the target support structures. Initial results indicate that the major nonuniformities observed in D$_{2}$ targets have been eliminated and layers with surface-averaged rms roughness $<$2 \textit{$\mu $}m have been formed. New challenges have arisen for DT layering, requiring changes in the layering protocols. Three-dimensional, DT-ice-layer characterization has identified a new dominant perturbation in ice-layer uniformity consistent with fractionation at the point of initial freezing. Automation of the layering process has been initiated to minimize the time and effort required to form quality layers. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Quantitative Radiography for Inertial Confinement Fusion (ICF) Capsule Metrology

Film-based radiography can be accurate enough to satisfy the demanding requirements in ICF capsule metrology. We (1) built a precision digitizer to non-destructively resolve the structures in an ICF capsule with sub-micron resolution and (2) developed a film model to measure the impurity elements in each layer to better than a fraction of an atomic percent. We achieved submicron dimension measurement accuracy by developing a wave propagation model and physical standards to calibrate the offsets due to X-ray diffraction and lens distortion, and an edge detection routine to mitigate film noises. We also developed a quantitative film model to measure the Cu or Ge dopant concentration profiles in ICF capsules to better than 0.1 atomic percent, with the results in agreement with those from destructive techniques. The technique can estimate the hard-to-measure oxygen which is valuable to process development. Furthermore, it can be useful as a quality control tool to cap the maximum tolerable impurity levels.   

Proton Deflectometry of Electric and Magnetic Fields

Highly penetrating proton beams, generated by irradiating a thin foil target with picosecond laser pulses at intensities up to 10$^{20}$ Wcm$^{-2}$ have been used to detect electric and magnetic fields in laser-produced plasmas [1]. More recently experiments to characterize the transient electric and magnetic fields using mono-energetic protons from imploding D$^{3}$He-filled capsules have been performed [2]. In the deflectometry technique a beam of protons is passed through a mesh to generate a grid of proton beamlets. This proton beam is passed through the laser-generated plasma and the grid deflection can be measured. We report on LASNEX calculations of such laser-produced plasmas, which predict electric fields of the order of 10$^{9}$V/m and 1MG magnetic fields. We will show \textsc{Lsp} calculations of proton transport through such electric and magnetic fields and compare them to the experimental data. [1] A. J. Mackinnon, \textit{et al.}, Rev. Sci. Instrum. \textbf{75}, 3531 (2004). [2] C. K. Li, \textit{et al.}, Bull. Am. Phys. Soc \textbf{50}, 266 (2005). This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. The experimental work was performed at the LLE National User's Facility under grant number DE-SC52-04NA25436.   

Measuring E and B fields in Laser-Produced Plasmas through Monoenergetic Proton Radiography

Electromagnetic (E/B) fields generated by the interaction with plasmas of long-pulse, low-intensity laser beams relevant to inertial confinement fusion have been measured for the first time using novel monoenergetic proton radiography methods. High-resolution, time gated radiography images of a plastic foil driven by a 10$^{14}$ W/cm$^{2}$ laser implied B fields of $\sim $ 0.5 MG and E fields of $\sim $ 1.5$\times $10$^{8}$ V/m. Complete simulations of these experiments with LASNEX+LSP have been performed and are quantitatively consistent with the data both for field strengths and for spatial distributions; this is the first direct experimental test of the B-field generation package in LASNEX. The experiments also demonstrated that laser phase plates substantially reduce small-scale chaotic field structure. Future experiments designed for characterizing the field formation and evolution due to the interactions of multiple laser beams with a foil will be discussed. The work described here was performed in part at the LLE National Laser User's Facility (NLUF), and was supported in part by US DOE (Grant No. DE-FG03-03SF22691), LLNL (subcontract Grant No. B504974), and LLE (subcontract Grant No. 412160-001G).   

Monoenergetic Particle backlighter for Radiography and measuring E and B fields and Plasma Areal density

J.COBBLE, N.M. HOFFMAN, G.A. KYRALA, D.C. WILSON$, $LANL; R. STEPHENS, J. KILKENNY, GA--A novel monoenergetic particle backlighter source (14.7 and 3.0 MeV P, 3.5 and 0.8 MeV $\alpha $, and 1.0 MeV T), has been utilized at OMEGA to quantitatively measure the evolution of electric (E) and magnetic (B) fields generated by laser-plasma interactions, and will be utilized in the near future to radiograph plasmas and fields generated from fast ignitor implosions, magnetic compression experiments, magnetized foil experiments, hohlraums and, more generally, from 2- and 3-D laser-plasmas. The backlighter consists of an exploding-pusher glass micro-balloon, filled with D$^{3}$He, in which a fraction of the OMEGA 60-beams are used to drive the implosion. The width of the spectral line, $\sim $ 3{\%}, is determined by the burn temperature ($\sim $ 10 keV); the monoenergetic character, as well as multiple particles, enables unique discrimination between E and B fields; it further allows for the probing of cold (warm) plasmas with areal densities of order 1 (10) to 200 (1000) mg/cm**2. New results and planned experiments will be presented.   

Measurements of magnetic fields in two beam laser-solid interactions using proton grid deflectrometry

Measurements of the self-generated magnetic fields around the focal spots of two laser heater beams ($1 \: \rm{ns}$ pulse duration, $1 \: \mu \rm{m}$ wavelength, $1 \times 10^{15} \: \rm{Wcm}^{-2}$) on a planar target were made. Face-on proton grid deflectometry was used to measure the magnetic field at the focal spot edges to be $0.7 - 1.3$ MG. The proton deflectometry also provides evidence that the plasma flows from the focal spots are magnetized. The formation of a driven magnetic reconnection geometry with varying focal spot separation will be discussed.   

A simple model for the generation and detection of a poloidal magnetic field in laser-target interactions

When a linearly-polarized ultra-intense laser beam interacts with a target, it may generate not only toroidal but also poloidal non-oscillating magnetic field (D.D. Ryutov, B.A. Remington. AIP Conf. Proc., v. 827, p. 341, 2006; Astrophys. Space Sci., submitted, 2006). The poloidal field has a structure resembling the field of a group of four sunspots of alternating polarity. Its magnitude may reach the magnitude of an oscillating magnetic field in the incident wave. Effects of a pulse duration and ion expansion are discussed. Scaling laws determining this field are established. Detection of this field is feasible with side-on ion deflectometry. An optimum orientation of the probe beam is shown to form a 45-degree angle with the polarization plane. Examples of the distortion of an image of a rectangular grid are presented. It is concluded that the poloidal field can be identified even in the presence of the toroidal field of a comparable magnitude. Work performed for US DoE by UC LLNL under contract \#W-7405-Eng-48.   

Magnetic Field Measurements in Wire-Array Z-Pinches

Understanding the evolution of the magnetic field topology and magnitude in the high energy density plasmas produced by wire-array Z-pinches is of critical importance for their ultimate application to stockpile stewardship and inertial confinement fusion$^{1}$. A method to determine the magnetic field profile in megampere level wire-array Z-pinches with high spatial and temporal resolution is under development. An ideal method would be passive and non-perturbing, such as Faraday rotation of laser light. We are developing a method involving temporally-resolved Faraday rotation through a sensing waveguide placed in the vicinity of, and eventually in, a wire-array Z-pinch$^{2}$. We present measurements of the magnetic field outside of a wire-array, and progress on measurements within the array. Our ideal device is a ``thin film waveguide'' coupled to an optical fiber system. While these sensing devices may not survive for long in a dense Z-pinch, they may provide useful information for a significant fraction of the current pulse. We present preliminary theoretical and experimental results. 1. M. Keith Matzen, M. A. Sweeney, R. G. Adams et al., Phys. Plasmas \textbf{12}, 055503 (2005). 2. W. Syed, D. A. Hammer, M. Lipson, R. B. van Dover, AIP Proceedings of the 6th International Conference on Dense Z-Pinches, University of Oxford, UK, July 25-28, 2005. *This research was sponsored by the National Nuclear Security Administration under the Stockpile Stewardship Academic Alliances program through DOE Cooperative Agreement DE-F03-02NA00057.   

A Compact, TIM-Based, Pulsed-Power System for Magnetized Target Experiments on OMEGA

By magnetizing the target and then compressing the magnetic flux to levels sufficient to inhibit thermal transport in the hot spot, one can trigger ignition in massive cryogenic shells imploded with low velocity. The reduction in thermal-conduction losses leads to increased hot-spot temperatures at lower implosion velocities, thus relaxing the energy requirements for ignition. This work describes a compact, pulsed-power system for the generation of a macroscopic seed magnetic field and its integration into such flux-compression experiments on OMEGA. Magnetohydrodynamic simulations\footnote{ N. W. Jang \textit{et al}., ``Theory and Simulation of Laser-Driven Magnetic Field Compression,'' this conference.} predict compression of a 10-T seed field to multimegagauss values. A fast (100-ns) current pulse (up to 60 kA), driven by a TIM-based energy-delivery system, is discharged into a low-mass, double coil that surrounds the laser target. A working prototype has generated a $>$11-T seed field utilizing a $<$100-J capacitor bank, laser-triggered spark gap, and a low-impedance ($<$1-$\Omega )$ stripline. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Theory and Simulation of Laser-Driven Magnetic-Field Compression

During the compression of the ICF target, the magnetic diffusivity is greatly reduced in the hot, low-density halo---the region of unloaded shell material on the inner surface of the imploding shell. Thus, the imploding cold shell acts like a piston compressing the magnetic field to ultrahigh field strengths. This results in increased hot-spot temperatures due to inhibition of thermal transport in the radial direction. Implosions of cylindrical targets seeded with an axial magnetic field of 10 T\footnote{ O. V. Gotchev \textit{et al}., ``A Compact, Tim-Based, Pulsed-Power System for Magnetized Target Experiments on OMEGA,'' this conference.} are studied with a radiation magnetohydrodynamic simulation code, \textit{LILAC}-MHD. An 860-\textit{$\mu $}m-diam, 1.5-mm-long cylindrical CH target is filled with D$_{2}$ and driven by 40 beams of the OMEGA laser. The value of the magnetic field at peak compression exceeds 10$^{4}$ T, while the peak temperature in the core exceeds 10 keV, an almost tenfold increase over the case with no seed field. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-92SF19460.   

Numerical Modeling Studies of Plasma Driven Magnetoinertial Fusion

Plasma liner driven magnetoinertial fusion is studied using a new 1D hydro code based on Smooth Particle Hydrodynamics (SPH) and with MACH2, a 2D magnetohydrodynamic code. The effort was undertaken as a step in studying the feasibility of using high-velocity plasma jets to implode a magnetized plasma as a stand-off-driver embodiment of magnetoinertial fusion. The 1D and 2D models were compared with the normal shock relations and results from self-similar analytical solutions of converging shocks in cylindrical and spherical geometries, allowance being made for the simplifying approximations made in the analytical results.   

Parameter Space for Plasma Liner Driven Magnetoinertial Fusion

Lindl-Widner diagrams are used in inertial confinement fusion for identifying the region in $\rho $R-T parameter space in which heating power in the fusion target exceeds the power losses. These diagrams have been recently applied to magnetoinertial fusion (MIF). Typically, in MIF, a magnetized target is compressed by an imploding solid liner. In plasma-driven magnetoinertial fusion (PLMIF), a plasma liner compresses the target. PLMIF has some potential advantages including repeatable formation of the liner in a standoff manner and secondary fusion burn of the liner material. Following the basic approach in computation of MIF-based Lindl-Widner diagrams, we have extended these studies to PLMIF. We developed a new alpha deposition model from Monte Carlo simulations for spherical magnetized targets and a finite difference thermal conduction model for heat transfer from the target to the liner in order to construct the diagrams. The motivation for this study is two-fold. First, we use this approach to identify possible PLMIF ignition regions. Second, we investigate conditions in which the plasma liner may contribute significantly to the fusion burn power.