Reaching ignition in direct-drive (DD) inertial confinement fusion implosions requires achieving central pressures in excess of 100 Gbar. The OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] is used to study the physics of implosions that are hydrodynamically equivalent to the ignition designs on the National Ignition Facility (NIF) [J. A. Paisner et al., Laser Focus World 30, 75 (1994)]. It is shown that the highest hot-spot pressures (up to 40 Gbar) are achieved in target designs with a fuel adiabat of α ≃ 4, an implosion velocity of 3.8 × 107 cm/s, and a laser intensity of ∼1015 W/cm2. These moderate-adiabat implosions are well understood using two-dimensional hydrocode simulations. The performance of lower-adiabat implosions is significantly degraded relative to code predictions, a common feature between DD implosions on OMEGA and indirect-drive cryogenic implosions on the NIF. Simplified theoretical models are developed to gain physical understanding of the implosion dynamics that dictate the target performance. These models indicate that degradations in the shell density and integrity (caused by hydrodynamic instabilities during the target acceleration) coupled with hydrodynamics at stagnation are the main failure mechanisms in low-adiabat designs. To demonstrate ignition hydrodynamic equivalence in cryogenic implosions on OMEGA, the target-design robustness to hydrodynamic instability growth must be improved by reducing laser-coupling losses caused by cross beam energy transfer.

Improving the hot-spot pressure and demonstrating ignition hydrodynamic equivalence in cryogenic deuterium–tritium implosions on OMEGAa)

Three recent developments in direct-drive target design have enhanced the possibility of achieving high target gain on the National Ignition Facility (NIF): (1) Laser absorption was increased by almost 50% using wetted-foam targets. (2) Adiabat shaping significantly increased the hydrodynamic stability of the target during the acceleration phase of the implosion without sacrificing target gain. (3) Techniques to reduce laser imprint using pulse shaping and radiation preheat were developed. These design features can be employed for direct-drive-ignition experiments while the NIF is in the x-ray-drive configuration. This involves repointing some of the beams toward the equator of the target to improve uniformity of target drive. This approach, known as polar direct drive (PDD), will enhance the capability of the NIF to explore ignition conditions. PDD will couple more energy to the fuel than x-ray drive. The compressed fuel core can be more easily accessed for high-ρR diagnostic development and for fast-ign...

/pdf/polar-direct-drive-on-the-national-ignition-facility-2s5ptml1n7.pdf

Polar Direct Drive on the National Ignition Facility

Direct-drive-implosion experiments on the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] have showed discrepancies between simulations of the scattered (non-absorbed) light levels and measured ones that indicate the presence of a mechanism that reduces laser coupling efficiency by 10%-20%. This appears to be due to crossed-beam energy transfer (CBET) that involves electromagnetic-seeded, low-gain stimulated Brillouin scattering. CBET scatters energy from the central portion of the incoming light beam to outgoing light, reducing the laser absorption and hydrodynamic efficiency of implosions. One-dimensional hydrodynamic simulations including CBET show good agreement with all observables in implosion experiments on OMEGA. Three strategies to mitigate CBET and improve laser coupling are considered: the use of narrow beams, multicolor lasers, and higher-Z ablators. Experiments on OMEGA using narrow beams have demonstrated improvements in implosion performance.

/pdf/crossed-beam-energy-transfer-in-direct-drive-implosionsa-12o94a89al.pdf

Crossed-beam energy transfer in direct-drive implosionsa)

Theoretical opacities are required for calculating energy transport in plasmas. In particular, understanding stellar interiors, inertial fusion, and Z pinches depends on the opacities of mid-atomic-number elements over a wide range of temperatures. The 150–300 eV temperature range is particularly interesting. The opacity models are complex and experimental validation is crucial. For example, solar models presently disagree with helioseismology and one possible explanation is inadequate theoretical opacities. Testing these opacities requires well-characterized plasmas at temperatures high enough to produce the ion charge states that exist in the sun. Typical opacity experiments heat a sample using x rays and measure the spectrally resolved transmission with a backlight. The difficulty grows as the temperature increases because the heating x-ray source must supply more energy and the backlight must be bright enough to overwhelm the plasma self-emission. These problems can be overcome with the new generation...

Experimental investigation of opacity models for stellar interior, inertial fusion, and high energy density plasmas

This paper explores the role of electro-thermal instabilities on the dynamics of magnetically accelerated implosion systems. Electro-thermal instabilities result from non-uniform heating due to temperature dependence in the conductivity of a material. Comparatively little is known about these types of instabilities compared to the well known Magneto-Rayleigh-Taylor (MRT) instability. We present simulations that show electrothermal instabilities form immediately after the surface material of a conductor melts and can act as a significant seed to subsequent MRT instability growth. We also present the results of several experiments performed on Sandia National Laboratories Z accelerator to investigate signatures of electrothermal instability growth on well characterized initially solid aluminum and copper rods driven with a 20 MA, 100 ns risetime current pulse. These experiments show excellent agreement with electrothermal instability simulations and exhibit larger instability growth than can be explained by MRT theory alone.

Electrothermal instability growth in magnetically driven pulsed power liners

http://www.prism-cs.com/Software/Spect3D/SPECT3D_RPHDMpaper_2007.pdf

SPECT3D - A Multi-Dimensional Collisional-Radiative Code for Generating Diagnostic Signatures Based on Hydrodynamics and PIC Simulation Output

Tin, lithium, and xenon laser-produced plasmas are attractive candidates as light sources for extreme ultraviolet lithography (EUVL). Simulation of the dynamics and spectral properties of plasmas created in EUVL experiments plays a crucial role in analyzing and interpreting experimental measurements, and in optimizing the 13.5 nm radiation from the plasma source. Developing a good understanding of the physical processes in EUVL plasmas is challenging, as it requires accurate modeling for the atomic physics of complex atomic systems, frequency-dependent radiation transport, hydrodynamics, and the ability to simulate emergent spectra and images that can be directly compared with experimental measurements. We have developed a suite of plasma and atomic physics codes to simulate in detail the radiative properties of hot plasmas. HELIOS-CR is a 1-D radiation-magnetohydrodynamics code used to simulate the dynamic evolution of laser-produced and z-pinch plasmas. Multi-frequency radiation transport can be computed using either flux-limited diffusion or multi-angle models. HELIOS-CR also includes the capability to perform in-line non-LTE atomic kinetics calculations at each time step in the simulation. Energy source modeling includes laser energy deposition, radiation from external sources, and current discharges. The results of HELIOS-CR simulations can be post-processed using SPECT3D to generate images and spectra that include instrumental effects, and therefore can be directly compared with experimental measurements. Results from simulations of Sn laser-produced plasmas are presented, along with comparisons with experimental data. We discuss the sensitivity of the 13.5 nm conversion efficiency to laser intensity, wavelength, and pulse width, and show how the thickness of the Sn radiation layer affects the characteristics of the 13.5 nm emission.

/pdf/radiation-hydrodynamics-spectral-and-atomic-physics-modeling-3ivk2182x8.pdf

Radiation-hydrodynamics, spectral, and atomic physics modeling of laser-produced plasma EUVL light sources

Laser-produced plasmas (LPPs) are being studied as potential extreme ultraviolet (EUV) and soft x-ray (SXR) sources for a wide variety of applications in commercial, defense, and medical research. For radiation sources to be of practical use in these systems, they must very efficiently emit light at the desired wavelength. EUV lithography, a viable approach in the manufacture of next-generation semiconductor chips, requires radiation sources that efficiently emit light at a wavelength of 13.5 nm, while producing relatively little radiation at other wavelengths in order to avoid damaging the wafer. Developing highly efficient plasma radiation sources requires a good understanding of critical physics issues that influence the plasma emission, including laser heating, plasma hydrodynamics, radiation transport, and atomic physics. We have developed a suite of well-tested plasma hydrodynamics, atomic physics, and plasma radiation simulation tools that are being used to simulate in detail the key physical processes in LPPs, and guide the development of higher efficiency plasma radiation sources. These tools include: 1-D and 2-D radiation-hydrodynamics codes, multidimensional spectral analysis tools, and a suite of atomic physics codes used to generate accurate atomic databases for radiation source simulations. Here, we discuss results from 2-D simulations of tin spherical droplets irradiated on one side by 0.35 μm laser beams. In particular, we examine the angular dependence of the 13.5 nm flux from the Sn plasma, and the sensitivity of the 13.5 nm conversion efficiency (CE) to the laser spot size and laser pulse width.

P. Wang

Papers

SPECT3D - A Multi-Dimensional Collisional-Radiative Code for Generating Diagnostic Signatures Based on Hydrodynamics and PIC Simulation Output

Radiation-hydrodynamics, spectral, and atomic physics modeling of laser-produced plasma EUVL light sources

Optimization of EUV/SXR plasma radiation source characteristics