Execution and modeling of drive symmetry experiments in gas-filled hohlraums have been pursued to provide both a better understanding of radiation symmetry in such hohlraums and to verify the accuracy of the design tools which are used to predict target performance for the National Ignition Facility (NIF) [J. Lindl, Phys. Plasmas 2, 3933 (1995)]. In this paper, the results of a series of drive symmetry experiments using gas-filled hohlraums at the Nova laser facility [C. Bibeau et al., Appl. Opt. 31, 5799 (1992)] at Lawrence Livermore National Laboratory are presented. A very important element of these experiments was the use of kineform phase plates (KPP) to smooth the Nova beams. The effect of smoothing the ten Nova beams with KPP phase plates is to remove most of the beam bending which had been observed previously, leaving a residual bending of only 1.5°, equivalent to a 35 μm pointing offset at the hohlraum wall. The results show that the symmetry variation with pointing of implosions in gas-filled hohlraums is consistent with time integrated modeling.

An improved understanding of strongly driven laser-plasma coupling is important for optimal use of the National Ignition Facility (NIF) for both inertial fusion and for a variety of advanced applications. Such applications range from high-energy x-ray sources and high-temperature hohlraums to fast ignition and laser radiography. We discuss a novel model for the scaling of strongly driven stimulated Brillouin and Raman scattering. This model postulates an intensity-dependent correlation length associated with spatial incoherence due to filamentation and stimulated forward scattering. We first describe the model and then relate it to a variety of experiments. Particular attention is paid to high-temperature hohlraum experiments, which exhibit low to modest stimulated Brillouin scattering even though this instability is strongly driven. We also briefly discuss the strongly nonlinear interaction physics for efficient generation of high-energy electrons either by irradiating a large plasma with near quarter-critical density or by irradiating overdense targets with ultra-intense laser light.

This paper presents design calculations for experiments that measure electron densities of laser heated hohlraums with soft-x-ray moire deflectometry. Hydrodynamical simulations of the hohlraums are analyzed to obtain deflection angles of the probing beam and x-ray emission from the hohlraum. The deflection angles and resulting moire fringe shifts and fringe contrast are predicted to be sufficient to infer electron-density gradients from measurements. In addition, the self-emission is found to be much lower than that of the probing laser beam, giving a good signal-to-noise ratio. In conclusion, moire deflectometry with soft-x-ray lasers has the potential to give valuable information about the electron density in laser driven hohlraums.

The effect of beam structure on propagation through underdense plasma is examined in two different examples. First, it is shown that the distribution of intensities within a laser beam affects how the beam deflects in the presence of transverse plasma flow. A detailed analysis of beam deflection shows that the rate scales linearly with intensity and plasma density, and inversely with plasma temperature. When the plasma flow is subsonic, the deflection rate is proportional to the ion damping decrement, and scales as M/(1−M2)3/2, where M is the transverse flow Mach number. When the plasma flow is supersonic, the deflection rate scales as 1/[M(M2−1))1/2]. Next, the effect of beam structure on channel formation by very intense laser beams is studied. A diffraction-limited beam with ∼3 TW of input power forms a channel through 400 μm of plasma, whereas when this beam is phase aberrated, channel formation does not occur.

Gas-filled hohlraums are presently the base line ignition target design for the National Ignition Facility. Initial Nova [E. M. Campbell et al. Rev. Sci. Instrum. 57, 2101 (1986).] experiments on gas-filled hohlraums showed that radiation temperature was reduced due to stimulated Brillouin and stimulated Raman scattering losses and that implosion symmetry had shifted compared with vacuum hohlraums and calculations. Subsequent single beam experiments imaging thermal x-ray emission showed the shift is due to laser–plasma heating dynamics and filamentation in a flowing plasma. Experiments using a single beam have shown that scattering losses and effects of filamentation are reduced when the beam is spatially smoothed with a random phase plate or kinoform phase plate. Scattering is further reduced to less than 5% of the incident laser energy when temporal smoothing is added.

The paper describes the structure of the ICF program and highlights its latest results.The pre-construction activities of the National Ignition Facility (NIF) project: have increased their momentum as a result of the recent Congressional decision to appropriate the requested funds for Fiscal Year 1997 to continue the project. The facility is an essential part of United States nuclear defense program in the absence of underground testing. The near-term goal of the U.S. ICF program is to achieve ignition in the laboratory (i.e. NIF) and to achieve high yield in the future. This paper describes some current results in ignition physics and related areas, as well as the progress in some new concepts that may have application to high yield.International collaborations that have been developed in recent years have contributed to the increased rate of progress, The paper closes with a summary of current and planned international initiatives.

The characterization of laser‐produced plasmas has required the application of spectroscopic techniques to non‐standard conditions where kinetics models have not been extensively tested. The plasmas are produced by the Nova laser for the study of inertial confinement fusion, can be mm in size, and evolve on sub‐nanosecond time scales. These targets typically achieve electron temperatures from 2–4 keV and electron densities of 1020–1022 cm−3. We have measured the electron temperature of two types of targets: bags of gas and hohlraums, Au cylinders with laser entrance holes in the flat ends. By comparing data from different targets, we examine the time‐dependence of spectroscopic plasma diagnostics.

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.

Understanding and controlling capsule implosion symmetry is a key requirement for inertial confinement fusion (ICF). Symmetry was specifically called out in the Nova Technical Contract (NTC) as the HLP4 task. Later, elements of HLP3 were expanded to include symmetry work. For nearly a decade and a half it has been recognized that the fundamental asymmetry in a laser-heated hohlraum is a long-wavelength pole-waist radiation flux variation that varies like the P{sub 2} Legendre polynomial. It has also been recognized that one can control this asymmetry and achieve nearly symmetric implosions by appropriately pointing the laser beams.

Understanding drive symmetry in gas-filled hohlraums is currently of interest because the baseline design of the indirect drive ignition target for the planned National Ignition Facility uses a gas-filled hohlraum. This paper reports on the results of a series of experiments performed at the Nova laser [C. Bibeau et al. Appl. Opt. 31, 5799 (1992)] facility at Lawrence Livermore National Laboratory with the goal of understanding time-dependent drive symmetry in gas filled hohlraums. Time-dependent symmetry data from capsule implosions and reemission targets in gas-filled hohlraums are discussed. Results of symmetry measurements using thin wall gas-filled hohlraums are also discussed. The results show that the gas is effective in impeding the motion of the wall blowoff material, and that the resulting implosion performance of the capsule is not significantly degraded from vacuum results. The implosion symmetry in gas differs from vacuum results with similar laser pointing indicating a shift in beam position on the hohlraum wall and hotter drive at the capsule’s poles than at the equator. A theory has been proposed to explain the observed shift as a plasma physics effect: beam steering due to filamentation and transverse plasma flows.

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.

Indirect-drive targets planned for the National Ignition Facility (NIF) laser consist of spherical fuel capsules enclosed in cylindrical Au hohlraums. Laser beams, arranged in cylindrical rings, heat the inside of the Au wall to produce x rays that in turn heat and implode the capsule to produce fusion conditions in the fuel. Detailed calculations show that adequate implosion symmetry can be maintained by filling the hohlraum interior with low-density, low-Z gases. The plasma produced from the heated gas provides sufficient pressure to keep the radiating Au surface from expanding excessively. As the laser heats this gas, the gas becomes a relatively uniform plasma with small gradients in velocity and density. Such long-scale-length plasmas can be ideal mediums for stimulated Brillouin Scattering (SBS). SBS can reflect a large fraction of the incident laser light before it is absorbed by the hohlraum; therefore, it is undesirable in an inertial confinement fusion target. To examine the importance of SBS in NIF targets, the authors used Nova to measure SBS from hohlraums with plasma conditions similar to those predicted for high-gain NIF targets. The plasmas differ from the more familiar exploding foil or solid targets as follows: they are hot (3 keV); they have high electron densities (n{sub e}=10{sup 21}cm{sup {minus}3}); and they are nearly stationary, confined within an Au cylinder, and uniform over large distances (>2 mm). These hohlraums have <3% peak SBS backscatter for an interaction beam with intensities of 1-4 x 10{sup 15} W/cm{sup 2}, a laser wavelength of 0.351{micro}m, f/4 or f/8 focusing optics, and a variety of beam smoothing implementations. Based on these conditions the authors conclude that SBS does not appear to be a problem for NIF targets.

We report on the use of x-ray spectroscopy of mid-Z dopants to measure the electron temperature of hohlraum targets. The hohlraums are gas-filled Au cylinders and, when irradiated with 20 kJ of 0.35 mu m laser light, they become mm-sized plasmas bathed in a radiation field. The peak temperatures achieved by the target are 3.7 keV. In addition to being the first electron temperature measurements of the hohlraum itself, these measurements enable an investigation of the thermal energy and electron conduction of the target.