INTRODUCTION
Harvesting energy from nuclear fusion processes would be a major breakthrough in the search for clean energy and is therefore of paramount importance to the scientific community. There are several approaches to this problem, which include the utilization of heavy particle beams (Temporal et al., Reference Temporal, Lopez-Cela, Piriz, Grandjouan, Tahir and Hoffmann2005; Neff et al., Reference Neff, Knobloch, Hoffmann, Tauschwitz and Yu2006; Constantin et al., Reference Constantin, Dewald, Niemann, Hoffmann, Udrea, Varentsov, Jacoby, Funk, Neuner and Tauschwitz2004; Hoffmann et al., Reference Hoffmann, Blazevic, Ni, Rosmej, Roth, Tahir, Tauschwitz, Udrea, Varentsov and Weyrich2005), of high explosives (Winterberg, Reference Winterberg2008), magnetic fields (Kaw & Burkhard, Reference Kaw and Burkhard2005), and direct and indirect irradiation with lasers (Lindl, Reference Lindl1995; Hora, Reference Hora2007). In this article, we focus on the inertial confinement fusion (ICF) approach. In order to achieve ICF, a number of problems have to be addressed and solved; these are target design (Cook et al., Reference Cook, Kozioziemski, Nikroo, Wilkens, Bhandarkar, Forsman, Haan, Hoppe, Huang, Mapoles, Moody, Sater, Seugling, Stephens, Takagi and Xu2008), ignition (Deutsch et al., Reference Deutsch, Bret, Firpo, Gremillet, Lefebvre and Lifschitz2008; Ghoranneviss et al., Reference Ghoraneviss, Malekynia, Hora, Miley and He2008), material properties (Elezier et al., Reference Eliezer, Murakami and Val2007), diagnostic issues (Romagnani et al., Reference Romagnani, Borghesi, Cecchetti, Kar, Antici, Audebert, Bandhoupadjay, Ceccherini, Cowan, Fuchs, Galimberti, Gizzi, Grismayer, Heathcote, Jung, Liseykina, Macchi, Mora, Neely, Notley, Osterholtz, Pipahl, Pretzler, Schiavi, Schurtz, Toncian, Wilson and Willi2008), simulations (Rodriguez et al., Reference Rodriguez, Florido, Gil, Rubiano, Martel and Minguez2008), and symmetry of the implosion (Ramis et al., Reference Ramis, Ramirez and Schurtz2008) just to name the most prominent.
The first attempts to achieve ICF at the National Ignition Facility (NIF) will commence in 2010. These first experiments will use the indirect drive approach where laser beams heat a hohlraum that radiates soft X-rays, which then implode a spherical capsule containing deuterium and tritium fuel. The X-rays emitted by the heated hohlraum walls are absorbed by the capsule, creating shocks, and imploding the capsule that then ignite the compressed fuel. Although using a hohlraum to convert the laser energy into X-ray radiation introduces a loss in energy conversion, it has the advantage of reducing Rayleigh-Taylor instabilities by increasing the ablation velocity of the capsule, and providing a more symmetric implosion. However, “hot” spots on the hohlraum wall, created where the laser beams strike the hohlraum, impose low-mode perturbations in the capsule radiation drive. Small non-uniformities in the drive radiation field can seed instabilities and cause non-spherical, less-efficient implosions. Therefore, it is important to control the symmetry of these low-mode perturbations. This can be done by varying the laser power in the three laser beam cones, by varying the pointing of the laser beam cones on the hohlraum wall, or by varying the hohlraum dimensions.
In preparation for experiments on the NIF, experiments at the Omega laser demonstrate control of the implosion symmetry At the foot of the NIF laser pulse in 0.7-NIF-scale hohlraums and capsules. These experiments use a symmetry capsule (Symcap) (Hoffmann et al., Reference Hoffman, Wilson, Edwards, Kalantar, Kyrala, Goldman, Weber, Izumi, Callahan, Meezan, Delamater, Tregillis, Schmitt, Bradley, Seifter, Jones, Milovitch and Thomas2008) to diagnose the hohlraum radiation drive. The symcap is a gas-filled surrogate capsule designed to create X-rays that show the shape of the imploding capsule. The time sensitivity of the experiment is changed by using different capsule wall thicknesses to implode at various desired times. Measurements of the size, shape, and brightness of these X-rays as a function of time can then be used to deduce the time-integrated radiation drive. The experiments described in this article are the first to demonstrate the feasibility of using symcaps to diagnose the symmetry of the drive for the relatively low radiation temperatures (100–120 eV) expected at the foot (first 2 ns) of the NIF laser pulse.
EXPERIMENTAL SETUP
The experiments used vacuum-filled gold hohlraums surrounding capsules filled with deuterium (D2) gas. The gold hohlraums were cylindrical and had dimensions of 6.8 mm in length, 3.56 mm in diameter, a laser entrance hole (LEH) diameter of 1.90 mm, and 0.1 mm thick walls. These dimensions were selected to produce a 110 eV radiation drive at Omega using 15 kJ of laser energy. Furthermore, they are a scale 0.7 NIF Rev3.1 point design hohlraum, but without the gas fill and without the gas windows. The hohlraum axis is aligned along the Omega P6-P7 axis. Two 750 µm diameter X-ray diagnostic holes were drilled on the side of the hohlraum and filled with beryllium or aluminum plugs. These holes allowed viewing of the capsule implosion with X-ray framing cameras at directions perpendicular to the hohlraum axis. The plastic polystyrene (CH) capsule was placed in the center of the hohlraums, using a stalk to maintain exact centering and had a 1.387 ± 0.003 mm inside diameter, and a shell thickness of 19.6 ± 0.2 µm. The capsule was filled with 1 atm of D2 gas at 300 K. The capsules were centered within 35 µm of the hohlraum center. Figure 1 shows a photograph of one of the targets. In this figure, one can see the LEH, the stalk to mount the hohlraum in the experimental chamber, the window with the beryllium plug, and shields to block unwanted X-rays from diagnostics.
Fig. 1. Photograph of one of the targets. One can see the LEH, the stalk to mount the hohlraum in the experimental chamber, the window with the beryllium plug and shields to block unwanted X-rays from diagnostics.
Three cones of laser beams enter the hohlraum from each end at angles of 21°, 42°, and 59° with respect to the hohlraum axis (Fig. 2). The 42° cone consists of 10 beams while the 21° and 59° cones consist of five beams each. The cones are pointed in a configuration that creates two rings in the hohlraum, with the beams distributed uniformly in azimuth. This is done by pointing the 21° beams to the same axial point on the hohlraum wall as the 44° beams from the opposite side of the hohlraum. The laser is fired with a 2 ns square pulse with ~375 J in each beam. The 21° cone had an elliptic phase plate (EIDI-300) that was designed to have a circular cross-section at the LEH. The other cones did not use phase plates. The pointing and defocusing of the laser beams for each of the three cones as well as the irradiant power on the walls of the hohlraum is given in Table 1.
Fig. 2. (Color online) Sketch of the pointing of the three different cones from each side of the hohlraum. Blue, Cone 3 (59° with respect to hohlraum axis); red, cone 2 (42°); green, cone 1 (21°). F is the distance of defocusing, x is the distance where the beam intersects the axis with respect to the laser entrance hole.
Table 1. Pointing, focusing and irradiation of the laser beams of the three cones for shot #45797
The hohlraums were vacuum hohlraums, in contrast with the gas-filled hohlraums to be used at NIF. The short laser pulse eliminated the effect of wall on capsule uniformity and obviated the need for a gas fill. This simplified the experiment, minimized the effects of laser-plasma instabilities, and eliminated the presence of the LEH windows that can affect laser beam propagation. We used up to 16 fast filtered X-ray diodes (the DANTE spectrometer) to measure the time-resolved brightness temperature between 0 and 5000 eV (Kornblum et al., Reference Kornblum, Kauffman and Smith1986, Seifter & Kyrala, Reference Seifter and Kyrala2008) emitted through the LEH at 37° with respect to the axis.
EXPERIMENTAL RESULTS
The measured peak radiation drive at the end of the laser pulse is ~118 eV (see Fig. 3), consistent with the scaling of the drive with laser energy and hohlraum dimensions, and matches the point design NIF foot. Processed images of the X-ray measurements, made by the X-ray framing camera at different times are shown in Figure 4. The images show the X-ray emission at different times during the implosion (the intensity for each image is normalized to its peak). The signal levels are clearly strong enough to diagnose the implosion symmetry under NIF foot radiation drive conditions. The time dependence of the shape of the core can be determined, for each image, along with the peak emission time for the implosion (6.03 ns). These quantities will be compared with simulations to benchmark their predictive accuracy.
Fig. 3. (Color online) Hohlraum brightness temperature as measured by DANTE shows reproducibility of the laser drive as the desired peak temperature of 118 eV was achieved.
Fig. 4. (Color online) Processed images of the capsule X-ray self emission of shot #45797, where the intensity for each image is normalized to its peak. The implosion occurs at t = 6.03 ns (peak X-ray emission time), the hohlraum axis is horizontal. The brightest emission is shown by the darkest colors.
To quantify each of the image shapes, a Legendry polynomial is fit to a brightness contour of the X-ray emission. The fitting algorithm uses singular value decomposition (SVD) to determine the best Legendry fit. Figure 5 shows a sample fit to the 30% brightness contour of the implosion image at 6.03 ns (peak emission time). In this case, the implosion has a positive 20% P2 asymmetry meaning the radiation drive is larger near the ends of the hohlraum than at the center or equator. The analysis also shows a small P4 asymmetry of 8% as well.
Fig. 5. (Color online) Detailed image of the capsule implosion of shot #45797 at peak emission time, integrated over 35 ps (FWHM). The hohlraum axis is horizontal. The parameters a(0) to a(4) are the fractional amplitudes of the Legendry-fit parameters normalized to the fitted zero order amplitude (a(0); measured to be 53.7 µm) at the 30% brightness contour.
CONCLUSIONS
The present work demonstrates the ability to use specially designed symcaps to diagnose the low radiation temperature drive symmetry at the foot of the NIF laser pulse. This is the first time this technique has been applied to large hohlraums with relatively low radiation drive. Under these conditions, the core X-ray emission is of sufficient brightness to measure the size, and shape of the core in addition to the X-ray peak emission time. Future experiments will use this technique and similar symcaps made of Be at the Omega laser to verify the ability to tune experimentally the symmetry of the implosion and validate the predictive capability of simulations.
ACKNOWLEDGMENTS
The authors would like to thank the LANL personnel who supported these experiments, T. N. Archuleta, J. S. Cowan, S. C. Evans, and T. J. Sedillo in the Physics Division, as well as the target fabrication team of E. Breden, D. Capelli, R. D. Day, K. A. Defriend Obrey, D. J. Hatch, R. V. Lucero, B. M. Patterson, R. B. Randolph, D. W. Schmidt, and A. C. Valdez. We also thank J. Schein and C. Sorce of Lawrence Livermore National Laboratory for operating the Dante Spectrometer and the Omega operations crew for their efforts during the experiments, especially S. Regan for help with the EIDI phase plate use. This work was performed by Los Alamos National Laboratory under the auspices of University of California and later the Los Alamos National Security, LLC, for the Department of Energy under contract number DOE-AC52-06NA25396.
