For the first time high areal-density ͑R͒ cryogenic deuterium-tritium ͑DT͒ implosions have been probed using downscattered neutron spectra measured with the magnetic recoil spectrometer ͑MRS͒ ͓J. A. Frenje et al., Rev. Sci. Instrum. 79, 10E502 ͑2008͔͒, recently installed and commissioned on OMEGA ͓T. R. Boehly et al., Opt. Commun. 133, 495 ͑1997͔͒. The R data obtained with the MRS have been essential for understanding how the fuel is assembled and for guiding the cryogenic program at the Laboratory for Laser Energetics ͑LLE͒ to R values up to ϳ300 mg/ cm 2 . The R data obtained from well-established charged particle spectrometry techniques ͓C. K. Li et al., Phys. Plasmas 8, 4902 ͑2001͔͒ were used to authenticate the MRS data for low-R plastic capsule implosions, and the R values inferred from these techniques are in excellent agreement, indicating that the MRS technique provides high-fidelity data. Recent OMEGA-MRS data and Monte Carlo simulations have shown that the MRS on the NIF ͓G. H. Miller et al., Nucl. Fusion 44, S228͑2004͔͒ will meet most of the absolute and relative requirements for determining R, ion temperature ͑T i ͒ and neutron yield ͑Y n ͒ in both low-yield, tritium-rich, deuterium-lean, hydrogen-doped implosions and high-yield DT implosions.

We present a magnetic force microscopy @IF&i) analysis of arrays of submicron-scale Co dots fabricated by interference lithography. The dots are thin (I%--300 A) and elliptical in shape. MFiM reveals that these structures relax into highly ordered remanent states whose symmetry and configuration are governed by their shape anisotropy. In particular, when the dots are saturated alon g their long-axis, a uniformly magnetized state persists at remanence. However, when the dots are saturated along their short-axis, they relax into a single-vortex state in which the circulation can have either sign. Both states are characterized by smoothly varying magnetization patterns and a high degree of uniformity across the array. We attribute the ordered behavior of these.structures to the film microstructure, which allows the shape anisotropy to dominate over magnetocrystalline anjsotropy. By imaging a series of minor-loop remanent states, we show that magnetization reversal in these structures occurs via the nucleation and annihilation of a single vortex. Magnetic hysteresis loop measurements are consistent with these observations and provide additional details. Furthexnore. we present the results ofmicromagneticsimulations.which are in excellent agreement with both the klFiL1 images and the hysteresis loop measurements. G

An accurate understanding of burn dynamics in implosions of cryogenically layered deuterium (D) and tritium (T) filled capsules, obtained partly through precision diagnosis of these experiments, is essential for assessing the impediments to achieving ignition at the National Ignition Facility. We present measurements of neutrons from such implosions. The apparent ion temperatures T ion are inferred from the variance of the primary neutron spectrum. Consistently higher DT than DD T ion are observed and the difference is seen to increase with increasing apparent DT T ion . The line-of-sight rms variations of both DD and DT T ion are small, ∼150 eV, indicating an isotropic source. The DD neutron yields are consistently high relative to the DT neutron yields given the observed T ion . Spatial and temporal variations of the DT temperature and density, DD-DT differential attenuation in the surrounding DT fuel, and fluid motion variations contribute to a DT T ion greater than the DD T ion , but are in a one-dimensional model insufficient to explain the data. We hypothesize that in a three-dimensional interpretation, these effects combined could explain the results. DOI: 10.1103/PhysRevE.94.021202 At the National Ignition Facility (NIF) [1], cryogenically layered capsules of deuterium (D) and tritium (T) fuel contained in 2-mm-diam carbon-based shells are imploded through laser irradiation of a surrounding high-Z hohlraum [2,3]. The imploding DT fuel assembles and "stagnates" in a configuration with a cold high-density shell surrounding a low-density hot spot. Efficient conversion of shell kinetic energy to hot-spot thermal energy is an essential requirement to achieving ignition at the NIF [4,5]. At peak convergence, this ideally results in a spherically symmetric, cold, dense DT fuel shell with an areal density ρR of ∼1.5 g/cm 2 surrounding a ∼5-keV hot spot with ρR ∼ 0.3 g/cm 2 . Although the word "stagnation" is often used for this phase of the implosion, it is inappropriate as the DT and DD neutron spectra indicate significant remaining kinetic energy. Neutron spectrometers [6][7][8][9][10][11][12][13][14][15] provide directional measurements of DT and DD neutron spectra from which yield, burn-averaged ion temperatures T ion and areal densities ρR are obtained. Neutron activation detectors (NADs) [16] measure the unscattered DT yield Y DT . In this paper we focus on the ion "temperatures" from a more extensive set of experiments than previously published [2] and conclude that the fuel assembly during burn in layered DT implosions is not well described by detailed one-dimensional (1D) physics models and simulations. The leading hypothesis for the observed discrepancy between the data and the 1D description is significant disordered motion and the highly 3D nature of the assembly at burn.For a homogeneous stationary DT plasma in thermal equilibrium at ion temperature T thermal , the variance of the * Corresponding author: gatu@psfc.mit.edu DT neutron spectrum (in units of neutron energy) is given bywhere E n is th...

The National Ignition Facility has been used to compress deuterium-tritium to an average areal density of ~1.0±0.1 g cm(-2), which is 67% of the ignition requirement. These conditions were obtained using 192 laser beams with total energy of 1-1.6 MJ and peak power up to 420 TW to create a hohlraum drive with a shaped power profile, peaking at a soft x-ray radiation temperature of 275-300 eV. This pulse delivered a series of shocks that compressed a capsule containing cryogenic deuterium-tritium to a radius of 25-35 μm. Neutron images of the implosion were used to estimate a fuel density of 500-800 g cm(-3).

The simulation of ion extraction from a quasineutral plasma is described using a two-dimensional time-dependent numerical fluid equation code, flow. Ion motion is treated using the cold ion continuity and momentum conservation equations in the potential field of complex structures. An upwind differencing scheme in Cartesian coordinates is used to solve these fluid equations. The plasma electrons are modeled as having a Maxwell–Boltzmann distribution, which allows for ion acoustic effects inside the plasma. The growth and evolution of two-dimensional ion sheaths is treated while evaluating the ion flux collected on the extractor surface. It is found that spatial grids very much coarser than the Debye length (of order ten times larger or greater) can be successfully used to accurately model ion extraction. Although the transition region between the high density quasi-neutral plasma and the ion sheath is not resolved, the ion flux across this region is still obtained correctly. The assumption of a Maxwell–Boltzmann distribution for the electrons requires the solution of a highly nonlinear Poisson equation. Previous approaches to this problem have relied upon a Newton–Raphson root search with standard iterative techniques for the resulting linearized equations. A new methodology is described here using the recently developed strongly implicit procedure (SIP), which has been optimized for the discretized two-dimensional Laplace operator.