Mark Gunderson

The effects of pre-mix on burn in ICF capsules

Directly driven implosions at the Omega laser have tested the effects of pre-mix of Ar, Kr, and Xe in D2 + 3 He filled glass micro-balloons. Diagnostics included: D+D and D+T neutron yields, D+ 3 He proton yields and spectra, Doppler broadened ion temperatures, time dependent neutron and proton burn rates, and time gated, high energy filtered, X-ray images. Yields are better calculated by XSN LTE than by non-LTE. Yields with a small amount of pre- mix, atom fractions of ~5e-3 for Ar, 2e-3 Kr, and Xe for 5e-4, are more degraded than calculated, while the measured ion temperatures are the same as without pre-mix. There is also a decrease in fuel ρr. The neutron burn histories suggest that the early yield coming before the reflected shock strikes the incoming shell is un-degraded, with yield degradation occurring afterwards. Adding 20 atm % 3 He to pure D fuel seems to produce a similar degradation. Calculated gated X-ray images agree with observed when the reflected shock strikes the incoming shell, but are smaller than observed afterward. This partially explains yield degradation and both the low fuel and whole capsule ρrs observed in secondary T+D neutrons and slowing of the D+ 3 He protons. Neither LTE on non-LTE captures the degradation by 3 He or at low pre-mix levels, nor matches the large shell radii after impact of the reflected shock.

Detailed diagnosis of a double-shell collision under realistic implosion conditions

Double-shell implosions provide a noncryogenic path to inertial confinement fusion. In the double-shell target, the energy is absorbed in an outer shell that is accelerated inward and collides with an inner shell that implodes against the deuterium fuel. Symmetric collision of the shells requires that the shells be illuminated and built symmetrically. In reality, the targets are complicated and the construction is not symmetric, due to the seam that our current assembly method requires. Using the Omega laser [R. T. Boehly et al., Opt. Comm. 133, 495 (1997)], an illumination strategy was designed that uses 40 beams in an offset geometry, leaving 20 beams to perform radiography from two different directions. This places a significant nonsymmetric illumination challenge that may not exist in final targets shot on the National Ignition Facility. This paper presents a measurement of the time history of a collision of two shells in a double-shell capsule, briefly reviews the illumination geometry, gives the res...

A comparison of a second-order quantum mechanical and an all-order semi-classical electron broadening model

Abstract It has been argued previously that line profiles created using a quantum model to second order in the radiator-perturbing electron interaction are reasonable over a wide region of plasma temperatures and densities for K shell α, β, and γ lines of many different radiator species. Most of the results shown in this paper show that this, in general, is true. However, the results also show that when an accurate treatment of the wings of a line profile is of importance or when a significant portion of the line profile extends beyond twice the plasma frequency from line center, one must take care in using a second-order electron broadening model.

Radiation transport in inhomogeneous media

Calculations of radiation transport in heated materials are greatly complicated by the presence of regions in which two or more materials are inhomogeneously mixed. This phenomenon is important in many systems, such as astrophysical systems where density clumps can be found in star-forming regions and molecular clouds. Laboratory experiments have been designed to test the modeling of radiation transport through inhomogeneous plasmas. A laser-heated hohlraum is used as a thermal source to drive radiation through polymer foam containing randomly distributed gold particles. Experimental measurements of radiation transport in foams with gold particle sizes ranging from 5–9μm to submicrometer diameters as well as the homogeneous foam case are presented. The simulation results of the radiation transport are compared to the experiment and show that an inhomogeneous transport model must be applied to explain radiation transport in foams loaded with 5μm diameter gold particles.

Constraining fundamental plasma physics processes using doped capsule implosions

A standard technique in inertial confinement fusion research is the use of low levels of spectroscopic dopants as a passive diagnostic of fuel conditions. Using higher dopant levels it becomes possible to modify the plasma conditions. Doped capsule experiments may thus provide a way to control and study fundamental plasma physics processes in the inertial fusion regime. As a precursor to eventual experiments on the National Ignition Facility (NIF) we have performed a series of capsule implosions using the Omega laser. These are intended to guide the modelling of high-Z dopants and explore the feasibility of using such capsule implosions for quantitative physics experiments. We have fielded thin glass shells filled with D-He3 fuel and varying levels of Ar, Kr and Xe dopants. X-ray emission spectroscopy is combined with simultaneous measurements of primary neutron and proton yields and energy spectra in an attempt to fully constrain capsule behaviour.

Observation, interpretation, and diagnostic utility of plasma-induced line shifts

Abstract Time-resolved Ar K-shell spectra from directly driven microballoon implosions are presented, where the position of the He- and H-like resonance lines and their associated satellites apparently shift to lower energy as the implosion proceeds. This apparent shift is explained as being solely the result of the plasma-induced line shifts long predicted by Stark broadening theory. The properties of the line shift and the line shape are exploited in a proposed time-dependent diagnostic of plasma composition which may be useful in determining time-dependent ablator-fuel mix amounts in inertial confinement fusion experiments.

Plasma Induced Line Shifts: New Light on an Old Controversy

It appears that the theoretical lines shifted according to calculations provide substantially better fits than the theoretical line shapes which exclude line shifts. The differing principal quantum numbers associated with the Ly-β and He-γ lead to significantly differing shifts. Thus, the use of unshifted lines, shifted arbitrarily, en masse, would not lead to the same quality of fit observed when using the lineshapes shifted according to calculation.

Plasma induced line shifts and their effects on line merging and population kinetics

Plasma-induced line shifts to the red have been observed in a number of laser-produced plasmas. After briefly reviewing some of the more recent of these observations and a method of calculating their magnitude, two implications of the existence of these shifts are explored. Firstly, calculations show for plasma conditions such as those in the reviewed experiments the redshifts are larger for lines originating from manifolds with higher principle quantum numbers. This leads to line merging at lower densities than would be expected without considering shifts. Secondly, even though the observed shifts are small with respect to kTe they can have a substantial effect on population kinetics calculations for loosely bound states which approximate the effects of continuum lowering on level populations through the use of effective statistical weights.

Design considerations for indirectly driven double shell capsules

Double shell capsules are predicted to ignite and burn at relatively low temperature (∼3 keV) via volume ignition and are a potential low-convergence path to substantial α-heating and possibly ignition at the National Ignition Facility. Double shells consist of a dense, high-Z pusher, which first shock heats and then performs work due to changes in pressure and volume (PdV work) on deuterium-tritium gas, bringing the entire fuel volume to high pressure thermonuclear conditions near implosion stagnation. The high-Z pusher is accelerated via a shock and subsequent compression of an intervening foam cushion by an ablatively driven low-Z outer shell. A broad capsule design parameter space exists due to the inherent flexibility of potential materials for the outer and inner shells and foam cushion. This is narrowed down by design physics choices and the ability to fabricate and assemble the separate pieces forming a double shell capsule. We describe the key physics for good double shell performance, the trade-offs in various design choices, and the challenges for capsule fabrication. Both 1D and 2D calculations from radiation-hydrodynamic simulations are presented.Double shell capsules are predicted to ignite and burn at relatively low temperature (∼3 keV) via volume ignition and are a potential low-convergence path to substantial α-heating and possibly ignition at the National Ignition Facility. Double shells consist of a dense, high-Z pusher, which first shock heats and then performs work due to changes in pressure and volume (PdV work) on deuterium-tritium gas, bringing the entire fuel volume to high pressure thermonuclear conditions near implosion stagnation. The high-Z pusher is accelerated via a shock and subsequent compression of an intervening foam cushion by an ablatively driven low-Z outer shell. A broad capsule design parameter space exists due to the inherent flexibility of potential materials for the outer and inner shells and foam cushion. This is narrowed down by design physics choices and the ability to fabricate and assemble the separate pieces forming a double shell capsule. We describe the key physics for good double shell performance, the trade-...

Shifting and Merging of Ar+17 Spectral Lines Due to Electron Collision Broadening

As the electron density in hot, dense plasmas increases above 1 × 1024/cm3, the spectral lines of argon from argon‐doped deuterium ICF capsules undergo a significant amount of shifting and broadening due to the effects of electron collisions. The shifts of these spectral lines are large enough so that adjacent spectral lines in a spectral line series begin to significantly merge together before the lines have been broadened beyond recognition or have been ionized. With the significant amount of merging of the spectral lines as the electron density increases, the width of these merged features requires that the validity of electron broadening models to second‐order in the perturbing electron‐radiator interaction be examined because these models are valid out to about twice the plasma frequency from line center. Thus, we present the results from an electron‐broadening model to all‐order in the perturbing electron‐radiator interaction and compare them to results from a second‐order model. Comparisons of line...