The effects of laser absorption on direct-drive capsule experiments at OMEGA

Abstract

The yield of an inertial confinement fusion capsule can be greatly affected by the inclusion of high-Z material in the fuel, either intentionally as a diagnostic or from mixing due to hydrodynamic instabilities. To validate calculations of these conditions, glass shell targets filled with a D 2 and 3 He fuel mixture were fielded in experiments with controlled amounts of pre-mixed Ar, Kr, or Xe. The experiments were fielded at the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] using 1.0 ns square laser pulses having a total energy 23 kJ and direct drive illumination of shells with an outer diameter of $925 lm and a thickness of $5 lm. Data were collected and compared to one-dimensional integrated models for yield and burn-temperature measurements. This paper presents a critical examination of the calculational assumptions used in our experimental modeling. A modified treatment of laser-capsule interaction improves the match to the measured scattered laser light and also improves agreement for yields, burn-temperatures, and the fuel compression as measured by the ratio of two yields. Remaining discrepancies between measurement and calculation will also be discussed. V C 2012 American Institute of Physics. [http://dx

Analysis of mix experiments on Omega

“…This code can run the implosions through burn, but it lacks laser beams and laser energy deposition physics. Because of this, we use an energy source in a 7 m layer on the outside of the capsule, which gives the correct amount of energy absorbed (typically about 60 to 65% of the total laser energy [11]) and correct bang time. The source is spherically symmetric and does not include any beam imprinting or illumination irregularities (typically ∼2%).…”

“…The laser pulse was a 1 ns square shape with a nominal energy of 23 kJ. Capsules like these typically absorb about 60 to 65% of the laser drive energy [11]. The targets were mounted on a SiC stalk that had a diameter of 17 to 20 m and was glued to the capsule with epoxy.…”

Thermo-diffusion in inertially confined plasmas

“…Observation of the resulting fuel stratification in the DT implosion, which upsets the initially optimal arrangement of equal number densities of D and T, in experiments [11] and kinetic simulations [12][13][14][15] have recently been reported. The targets with high-Z dopants show a particularly strong yield anomaly [6,7,16], suggesting that even stronger fuel stratification may take place.Perhaps the most intriguing physics aspect of interion-species diffusion in a collisional plasma is the role of thermo-diffusion, which, as its name suggests, is driven by the gradients of ion and electron temperatures. The novelty comes through as a sharp contrast to the better-known case of a neutral mixture, where thermo-diffusion is substantially less important than baro-diffusion, though often counteracts it [9].…”

Kinetic studies of ICF implosions

“…Equally important, in low-Z/high-Z mixtures, k T were found to attain large absolute values and slopes at small concentrations of the high-Z component [20]. Plasma thermo-diffusion may thus be potentially responsible for the anomalous yield reduction in doped capsule implosions [8,9] in addition to the laser absorption effects considered earlier [18].…”

“…Observation of the resulting fuel stratification in experiments [14,15,16,17] and fully kinetic simulations [3,4] have recently been reported. The targets with high-Z dopants show particularly pronounced yield anomalies [8,9,18], suggesting that even stronger fuel stratification may take place. In addition, this is the same diffusion process that underlies the atomic mixing at the shell/fuel interface.…”