Assessing the prospects for achieving double-shell ignition on the National Ignition Facility using vacuum hohlraums

Abstract

The goal of demonstrating ignition on the National Ignition Facility (NIF) has motivated a revisit of double-shell (DS) targets as a complementary path to the cryogenic baseline approach. Expected benefits of DS ignition targets include non-cryogenic deuterium-tritium (DT) fuel preparation, minimal hohlraum-plasma mediated laser backscatter, low thresholdignition temperatures (≈ 4 keV) for relaxed hohlraum x-ray flux asymmetry tolerances, and minimal (two-) shock timing requirements. On the other hand, DS ignition presents several formidable challenges, encompassing room-temperature containment of high-pressure DT (≈ 790 atm) in the inner shell, strict concentricity requirements on the two shells (< 3 µm), development of nano-porous (<100 nm) low-density (<100 mg/cc) metallic foams for structural support of the inner shell and hydrodynamic instability mitigation, and effective control of hydrodynamic instabilities on the high-Atwood number interface between the DT fuel and the high-Z inner shell. Recent progress in DS ignition designs and required materials-science advances at the nanoscale are described herein. Two new ignition designs that use rugby-shaped vacuum hohlraums are presented which utilize either 1 MJ or 2 MJ of laser energy at 3ω. The capability of the NIF to generate the requested reverse-ramp pulse shape for DS ignition is expected to be comparable to the planned high-contrast (≈100) pulse-shape at 1.8 MJ for the baseline cryogenic target. Nano-crystalline, high-strength, AuCu alloy inner shells are under development using electrochemical deposition over a glass mandrel, exhibiting tensile strengths well in excess of 790 atm. Novel, low-density (85 mg/cc) copper foams have recently been demonstrated using 10 mg/cc SiO 2 nano-porous aerogels with suspended Cu particles. A prototype demonstration of an ignition DS is planned for 2008, incorporating the needed novel nano-materials science developments and the required fabrication tolerances for a realistic ignition attempt after 2010.

Observations of Electromagnetic Fields and Plasma Flow in Hohlraums with Proton Radiography

“…ce is the electron gyrofrequency and is the collision time [10,11]. E fields may modify the plasma conditions and, if sufficiently large, could enhance thick-target bremsstrahlung at x-ray energies well above the Planckian background.For low-intensity laser drive, such as used in most hohlraum experiments [1][2][3][4][5][6][7][8][9], the dominant source for B-field generation is expected to be nonparallel electron density (n e ) and temperature (T e ) gradients (rn e  rT e ) [10,11]. The E field is expected to result from electron pressure gradients (rP e ) [10,11].…”

“…The use of hohlraums requires an understanding of physics details, such as coupling efficiency, plasma conditions, instabilities, radiation uniformity [1,2,5,6], and cavity shape [1,2,7,8]. Electric (E) and magnetic (B) fields generated by several processes may have important effects on hohlraum physics and overall performance [9].…”

“…In studies of laboratory astrophysics and highenergy-density (HED) physics [3,4], for example, hohlraums are used for creating and simulating various extreme HED conditions, including those of stellar and planetary interiors. The hohlraum radiation field is used to compress spherical capsules, through capsule ablation, to high temperature and density in indirect-drive inertial confinement fusion [1,2].The use of hohlraums requires an understanding of physics details, such as coupling efficiency, plasma conditions, instabilities, radiation uniformity [1,2,5,6], and cavity shape [1,2,7,8]. Electric (E) and magnetic (B) fields generated by several processes may have important effects on hohlraum physics and overall performance [9].…”

Some recent studies on hohlraum physics

“…As an example, here we use the extended plasma-filling model to give an initial design of an elliptical hohlraum [14], shorted as ellipraum, and pertinent laser to produce a 300 eV ignition radiation [7] in Fig. 5, with a 1 mm radius capsule inside.…”

Simulation of the hohlraum for a laser facility of Megajoule scale

“…For laser energy saving and improving of x-ray flux uniformity on the target the configuration of hohlraum in the form of a rugby ball [4] was used. The calculation variant with the laser entrance hole shields [5] was also performed.…”