Session CO6: Hohlraum and X-ray Cavity Physics

Updates to NIF ignition point design

The National Ignition Campaign (NIC) on the National Ignition Facility plans to use an indirectly driven spherical implosion to assemble and ignite a mass of DT fuel. The NIC is currently in the process of conducting a variety of experiments using surrogate targets, meant to define various aspects of the future ignition experiment. Also, cryogenic capsules containing a DT or hydrodynamically equivalent tritium-rich fuel layer are being fielded. These integrate the laser and target adjustments made during the tuning experiments and provide the $||$R and T distributions that result. In an activity ongoing with these experiments, the point design for ignition is being updated and modified as appropriate. This presentation will describe how the experiments are being integrated into updates to several aspects of the point design: (i) the target configuration; (ii) the laser pulse pulse shape; (iii) our modeling of the implosion; and (iv) its projected performance.   

Capsule Symmetry in Different Scale Size NIF hohlraums

A key requirement for the achievement of ignition on the National Ignition Facility (NIF) is to adequately control the low mode symmetry of the imploding capsule. The NIF's 192 beams are arranged in inner and outer cones of beams whose powers can be varied, either directly or by crossbeam energy transfer, to achieve a uniform drive. NIF experiments to date have predominantly used hohlraums with a diameter of 5.44mm that require an inner cone fraction of up to 50{\%} to achieve a round implosion. Lasnex hi-flux simulations of hohlraums with a diameter of 5.75 mm predict that symmetry can be achieved at a lower cone fraction closer to NIF's intrinsic 33{\%} split. This paper will review the necessary cone fraction to obtain a symmetric implosion and discuss the laser beam propagation and plasma conditions of hohlraums with diameters up to 6mm.   

Experimental evidence of early-time Power Transfer on NIF

The 192 beams in the National Ignition Facility (NIF) are arrayed in inner and outer cones whose powers are adjusted to achieve a symmetric capsule implosion. A decade ago, it was realized that energy exchange between cones with different wavelengths can occur in the laser entrance hole region, where the beams overlap, thereby affecting symmetry. During the NIF 2009 campaign this phenomenon was observed and successfully employed to tune the peak of the laser pulse to achieve a round implosion [1]. Power transfer can also be present during other periods of the laser pulse. In particular while the laser burns through the hohlraum window and fill gas when the plasma is dense and cold. We have used the theory presented in [1] to assess the amount of power transfer during the first 2 ns of the ignition laser pulse and found that the increase in inner beam power could be as large as 300\%. Accounting for this effect has brought our calculations into closer agreement with recent early-time symmetry tuning and shock timing experiments. \\[4pt] [1] P. Michel et al. Phys Plasmas 17, 056305 (2010)   

First early time symmetry tuning experiments for indirect drive ignition implosions on the National Ignition Facility

In ignition experiments on the National Ignition Facility (NIF), the symmetry of the hohlraum radiation drive for the first 2 ns is tuned using the re-emit technique [1]. To achieve this, the capsule is replaced by a high-Z ``reemit'' sphere so that the incident drive symmetry can be inferred by soft x-ray imaging of the sphere re-emission pattern [2]. We report on the first re-emit symmetry experiments performed on NIF in full ignition scale hohlraums that achieved 1{\%} low mode accuracy. We will discuss results demonstrating the sensitivity of the radiation symmetry incident on the capsule to inner-to-outer laser beams wavelength shift that influences the inter-cone energy transfer, as well as to inner/outer beams power fraction that is used to tune the P2/P0 Legendre polynomial of the radiation flux at the capsule. \\[4pt] [1] E.L. Dewald, \textit{et. al.,} Rev. Sci. Instrum. 79, 10E903 (2008).\\[0pt] [2] N. Delamater, G. Magelssen, A. Hauer, \textit{Phys. Rev. E} \textbf{53}, 5241 (1996).   

Effects of Hohlraum Plasma Filling on Implosion Symmetry

We describe a study with the design code \textsc{hydra} to understand how hohlraum plasma filling impacts symmetry control. The 2010 National Ignition Facility (NIF) symmetry campaign demonstrated symmetry tuning via cross-beam transfer in ignition-scale hohlraums driven by 1.3 MJ of laser energy. Following the subsequent NIF shock-tuning campaign, the implosion symmetry changed from prolate to oblate ($a_2/a_0 \approx -50 \%$). Optical and x-ray data suggested higher hohlraum plasma density than in previous experiments, impairing the inner laser beam propagation. Design calculations with \textsc{hydra} were consistent with this finding; however, they also predicted an increase in cross-beam transfer that would counteract the impaired propagation, resulting in round implosions. We can empirically restore symmetry control by changing the hohlraum geometry, fielding conditions, or laser pulse.   

Using hard-X-ray images of ignition hohlraums on NIF to characterize hot electrons generated by laser-plasma interaction

Using multi-channel hard X-ray images of an ignition hohlraum taken along two axis and the time-integrated FFLEX broadband spectrometer that measures Bremsstrahlung hard X-rays emitted in the hohlraum wall by energetic electrons, we characterize the location of hot electron generation. We distinguish two hot electron components of the spectrum: a 20 keV thermal-like component related to Stimulated Raman scattering and a ``super ho'' ($>$ 60 keV) component due to LPI at higher density. In addition, the effect of the hohlraum thermal emission (with Thot $\approx $ 2-4 keV) on this analysis will be assessed.   

Measurement of hot electron preheat during capsule implosions on the NIF with hard x-ray imaging

Hot electrons of energies between 170 and 250 keV can penetrate the capsule ablator and preheat the DT fuel in indirect-drive ICF implosions, reducing the final compressed fuel area density and ignition margin. We have fielded a high aspect ratio pinhole imager with 400 $\mu$m resolution, 0.9x magnification viewing through a Laser Entrance Hole to measure the $50 - 125$ keV hard x-ray Bremsstrahlung emission from hot electrons slowing in the capsule. The absolutely calibrated, time-integrating image plate detector allows inferring an upper limit of 150 J in hot electrons with E $>$ 170 keV impinging on the fusion capsule in a 1.3 MJ experiment with a 20 ns laser drive. Time-resolved, spatially integrated hard x-ray measurements confirm that these hot electrons are generated close to the end of the laser pulse. Based on measured hot-electron energy and time history, simulations predict a degradation of implosion performance by $<$ 10\% due to hot electron preheat.   

Comparison of measured soft x-ray drive with shock and capsule implosion velocity for ignition tuning experiments on NIF

Indirect drive inertial confinement fusion experiments use high-Z hohlraums to convert laser energy to soft x-ray energy. The soft x-rays then drive the capsule via material ablation to compress the fuel payload and heat the central hot spot to initiate ignition. To achieve the highest fuel compression, a shaped radiation drive is used launching multiple shocks timed minimizes fuel entropy. The strength and velocity of these shocks depend directly on the radiation drive. The main laser pulse is then used to drive the implosion such that the PdV work can heat the central core to fusion conditions. To diagnose the soft x-ray drive in the hohlraum, Dante, an 18 channel soft x-ray spectrometer, measures the flux escaping the laser entrance hole. Measurements of this flux are used to assess the conditions for the capsule implosion. In this presentation, we will examine correlations between the soft x-ray measurements and shock velocity, as well as implosion velocity for recent ignition tuning experiments on NIF.   

Results from Recent NIF Shock Timing Experiments

Experiments are underway to tune the shock timing of capsule implosions on the National Ignition Facility (NIF). These experiments use a modified cryogenic hohlraum geometry designed to precisely match the performance of ignition hohlraums. The targets employ a re-entrant Au cone to provide optical access to multiple shocks as they propagate in the liquid deuterium-filled capsule interior. The strength and timing of all four shocks is diagnosed with VISAR (Velocity Interferometer System for Any Reflector). The tuned pulse shape resulting from these experiments has been tested in ignition capsule implosions and demonstrates a considerable improvement in fuel adiabat. Experimental results and comparisons with numerical simulation are presented.   

Spontaneously-Generated Strong Fields around the Hohlraum Laser-Entrance Holes in ICF experiments at the NIF and OMEGA

Comprehensive spectra and spatially-resolved fluence images of 14.7-MeV D$^{3}$He protons from indirectly-driven implosions at the NIF uniquely revealed the occurrence of strong fields around the laser-entrance hole (LEH) of ignition scale hohlraums. Such fields persist even at $\sim $0.5-1 ns after the laser has turned off, generating large deflections and fluence inhomogeneities in the hohlraum polar direction. The occurrence is unpredictable, and the spatial distributions suggest that such strong fields likely result from the outward flow of the on-axis stagnated plasmas from the LEHs. To quantitatively understand the generation, evolution, interaction, and dissipation of such spontaneously-generated, as well as their effects on hohlraum plasma dynamics, new and relevant experiments probed with backlighting protons are performed at the OMEGA laser facility. These measurements provide novel physics insight and are relevant to ongoing experiments at the NIF. This work was supported in part by the U.S. DOE, LLNL, GA and LLE.   

Laser-Plasma Interaction Campaign in Hohlraums

The major goal of the Laser MegaJoule (LMJ) [1], currently under construction in France, is to achieve fusion ignition and thermonuclear gain from a target driven with a laser. In order to check the efficiency of the laser beam conditioning a new laser-plasma interaction campaign has been conducted at the LIL facility in february 2011, using various gas-filled hohlraums. The LIL facility is a prototype of one quadruplet of the LMJ. Three differents gas-filled hohlraums have been designed in order to mimic plasma conditions that are expected along two particular beam paths in ignition hohlraums. The targets consist of 3- or 4-millimeters long, 1 atm neo-pentane gas-filled hohlraums. We will present and discuss hydrodynamic calculations together with preliminary results of the LPI campaign. Calculated plasma conditions allow to evaluate SBS and SRS linear gains. Finally, we use the 3D paraxial code HERA [3,4] to investigate the propagation of the LIL quad, by means of massivelly parallel simulations. \\[0pt] [1] J. Ebrardt and J. M. Chaput, J. Phys.: Conf. Ser. 112, 032005 (2008). [2] S. Laffite and P. Loiseau, Phys. Plasmas 17, 102704 (2010). [3] Loiseau et al., Phys. Rev. Lett. 97, 205001 (2006). [4] Ballereau et al., J. Scient. Comput. 33, 1 (2007).   

Characterization of a halfraum x-ray drive using VISAR at the National Ignition Facility

Laser hohlraums driven from one side, or ``halfraums,'' are a convenient method for obtaining a planar x-ray drive in order to study radiation-hydrodynamics phenomena in simplified geometries. The VISAR diagnostic at the National Ignition Facility (NIF) was recently used to characterize the x-ray drive at the mid-plane of a NIF vacuum halfraum. The experiment used a 400 micron thick quartz window with a 70 micron aluminum ablator located at the mid-plane of a 5 mm diameter halfraum driven by 80 beams from the NIF laser. The pulse shape was a ramp with a peak power of 35.5 TW delivering a total of 210 kJ. The VISAR data details the speed of the shock resulting from the 9 ns laser pulse as it traverses the quartz window. The spatial dimension of the VISAR field of view captures the radial uniformity of the drive pressure over 1.5 mm from the center of the halfraum. 2-D integrated simulations calculating the drive temperature, shock speed and pressure uniformity, and results will be compared with data. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Plasma Adiabatic Lapse Rates for ICF

The adiabatic lapse rate (or degree of decreasing temperature with increasing altitude) is a well-known phenomenon in atmospheric physics. An analogous effect in plasma physics or inertial confinement fusion (ICF) exists for an accelerating capsule and leads to self-consistent temperature gradients. An analysis is performed for an adiabatic, binary mixture of fuel ions in an ICF capsule to obtain the plasma analogue of an adiabatic lapse rate. A novel source term for generating a plasma temperature gradient that is proportional to a difference in ionization states between the two species of ions is identified. For high- and low-$Z$ mixtures as in the mix layer between the hydrogen fuel and gold inner shell of an ignition double-shell target or between the ablating gold wall and the low-$Z$ tamping gas (helium) in a hohlraum, an associated strong GVolt/m -scale thermoelectric field is predicted that can promote runaway populations and species separation. The consequences of this analysis on the ICF database are presented.   

Plasma Effects in Spherical Implosions

A remarkable self-similar solution to the problem of a spherically converging shock was published by Guderley in 1942 [1]. Being applicable to an ideal gas, this solution neglects viscosity, thermal conduction and radiation losses and presents singularities when the shock reaches the origin. Radiation hydrodynamic codes include the effects of non-ideality (with artificial viscosity in place of real viscosity), ensuring that the solution is well-behaved at all times. However during an ICF implosion, separation of the electron and ion species occurs at the shock front. For the high Mach number ($M>10$) incoming (coalesced) shock that is typical of ICF scenarios, the width of the plasma shock front is comparable to the ion-ion mean-free-path $\lambda_{ii} \sim 1 \ \mu$m and much larger than the shock front width in an unionized gas at the same density ($\sim 10^{-2} \ \mu$m). Ahead of the plasma shock front, electrons pre-heat the inner gas over distances $\lambda_{ei} \approx(m_i/ m_e)^{1/2} \lambda_{ii} \sim 70 \mu$m. This decreases the strength of the incoming shock and lowers the temperature behind the rebound shock, a phenomenon analogous to the non-ideal gas effects found in hydro-codes. \\[4pt] [1] Zel'dovich and Raizer, Physics of shock waves and high-temperature phenomena, edited by Hayes and Probstein (Dover, Mineola, NY 2002).