H. N. Kornblum

Radiation drive in laser‐heated hohlraums

Nearly 10 years of Nova [E. M. Campbell, Laser Part. Beams 9, 209 (1991)] experiments and analysis have lead to a relatively detailed quantitative and qualitative understanding of radiation drive in laser‐heated hohlraums. Our most successful quantitative modeling tool is two‐dimensional (2‐D) LASNEX numerical simulations [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)]. Analysis of the simulations provides us with insight into the physics of hohlraum drive. In particular we find hohlraum radiation conversion efficiency becomes quite high with longer pulses as the accumulated, high‐Z blow‐off plasma begins to radiate. Extensive Nova experiments corroborate our quantitative and qualitative understanding.

Drive characterization of indirect drive targets on the Nova laser (invited)

The indirect drive method of inertial confinement fusion uses a high‐Z radiation case to convert energy from high‐powered laser beams to x rays which implode fusion capsules. Experiments have been performed on the Nova laser to characterize the x‐ray production in high‐Z cavities for studying the efficiency for x‐ray production using two methods for characterization. One method measures the shock velocity produced in low‐Z materials by the radiation. The shock velocity is measured by observing the optical signal from the rear of a stepped or continuously varying thickness of Al placed over a hole in the cavity wall. The other method measures the reradiated x‐ray flux from the cavity wall viewing through a hole in the cavity. Both methods have been shown to provide a consistent characterization of the x‐ray drive in the cavity target.

Inertial confinement fusion ablator physics experiments on Saturn and Nova

The Saturn pulsed power accelerator [R. B. Spielman et al., in Proceedings of the 2nd International Conference on Dense Z-pinches, Laguna Beach, CA, 1989, edited by N. R. Pereira, J. Davis, and N. Rostoker (American Institute of Physics, New York, 1989), p. 3] at Sandia National Laboratories (SNL) and the Nova laser [J. T. Hunt and D. R. Speck, Opt. Eng. 28, 461 (1989)] at Lawrence Livermore National Laboratory (LLNL) have been used to explore techniques for studying the behavior of ablator material in x-ray radiation environments comparable in magnitude, spectrum, and duration to those that would be experienced in National Ignition Facility (NIF) hohlraums [J. D. Lindl, Phys. Plasmas 2, 3933 (1995)]. The large x-ray outputs available from the Saturn pulsed-power-driven z pinch have enabled us to drive hohlraums of full NIF ignition scale size at radiation temperatures and time scales comparable to those required for the low-power foot pulse of an ignition capsule. The high-intensity drives available in t...

Diagnostic systems for the National Ignition Facility (NIF) (invited)

A tentative schedule of experiments for the ignition campaign on the National Ignition Facility (NIF) has been developed. These experiments will be used to validate beam pointing and balance, to tune time history and symmetry of drive of NIF hohlraums, and to implode subignition and igniting targets. The initial target diagnostics are designed to validate beam pointing and to demonstrate the properties of the hohlraums.

The Rosseland mean opacity of a composite material at high temperatures

The Rosseland mean opacity can be used to describe radiation transport through high‐opacity materials. This mean opacity is dominated by the minima in the frequency‐dependent opacity. By mixing appropriate materials, we can fill in the low opacity regions of one material with the high opacity regions of another material, resulting in a material with a Rosseland mean opacity higher than either of the constituents. This composite material can be used to improve the energy balance in indirect‐drive inertial confinement fusion. For a given laser energy, this can raise the temperature of the laser heated hohlraum, or for a given desired temperature, require less laser energy.

Dynamics of gas‐filled hohlraums

In order to prevent high‐Z plasma from filling in the hohlraum in indirect drive experiments, a low‐Z material, or tamper is introduced into the hohlraum. This material, when fully ionized is typically less than one‐tenth of the critical density for the laser light used to illuminate the hohlraum. This tamper absorbs little of the laser light, thus allowing most of the laser energy to be absorbed in the high‐Z material. However, the pressure associated with this tamper is sufficient to keep the hohlraum wall material from moving a significant distance into the interior of the hohlraum. In this paper we discuss measurements of the motion of the interface between the tamper and the high‐Z hohlraum material. We also present measurements of the effect the tamper has on the hohlraum temperature.

High convergence, indirect drive inertial confinement fusion experiments at Nova

High convergence, indirect drive implosion experiments have been done at the Nova Laser Facility. The targets were deuterium and deuterium/tritium filled, glass microballoons driven symmetrically by x rays produced in a surrounding uranium hohlraum. Implosions achieved convergence ratios of 24:1 with fuel densities of 19 g/cm3; this is equivalent to the range required for the hot spot of ignition scale capsules. The implosions used a shaped drive and were well characterized by a variety of laser and target measurements. The primary measurement was the fuel density using the secondary neutron technique (neutrons from the reaction 2H(3H,n)4He in initially pure deuterium fuel). Laser measurements include power, energy and pointing. Simultaneous measurement of neutron yield, fusion reaction rate, and x‐ray images provide additional information about the implosion process. Computer models are in good agreement with measurement results.

Recent experimental results on Nova

Summary form only given. A National Academy of Science Review Committee on inertial confinement fusion has endorsed a 12-point technical contract for the Nova program. Recent experiments have achieved a substantial number of these goals. The ten-beam laser has now been operated with the rms power balance among the ten beams as low as 8% in the foot and 5% in the peak of a high-energy, temporally shaped pulse. A decrease in the level of plasma instabilities from large scale length, low density plasma has been demonstrated by the use of random phase plates. Neutron measurements have been used to demonstrate high densities in well-understood implosions. A detailed understanding of the Rayleigh-Taylor instability at the ablation front of X-ray drive planar foil targets, with large hydrodynamic growth factors, has been demonstrated.