No abstract

The effects of shock dynamics on compressibility of indirect-drive ignition-scale surrogate implosions, CH shells filled with D 3 He gas, have been studied using charged-particle spectroscopy. Spectral measurements of D 3 He protons produced at the shock-bang time probe the shock dynamics and in-flight characteristics of an implosion. The proton shock yield is found to vary by over an order of magnitude. A simple model relates the observed yield to incipient hot-spot adiabat, suggesting that implosions with rapid radiation-power increase during the main drive pulse may have a 2× higher hot-spot adiabat, potentially reducing compressibility. A self-consistent 1-D implosion model was used to infer the areal density (ρR) and the shell center-of-mass radius (R cm ) from the downshift of the shock-produced D 3 He protons. The observed ρR at shock-bang time is substantially higher for implosions where the laser drive is on until near the compression bang time ('shortcoast'), while longer-coasting implosions have lower ρR. This corresponds to a much larger temporal difference between the shock-and compression-bang time in the long-coast implosions (∼ 800ps) than in the short-coast (∼ 400ps), which is shown in Fig. 17; this will be verified with a future direct bang-time diagnostic. This model-inferred differential bang time contradicts radiation-hydrodynamic simulations, which predict constant 700 − 800ps differential independent of coasting time; this result is potentially explained by uncertainties in modeling late-time ablation drive on the capsule. In an ignition experiment, an earlier shock-bang time resulting in an earlier onset of shell deceleration, potentially reducing compression and thus fuel ρR.

Current two-dimensional X-ray imaging at the National Ignition Facility (NIF) uses time resolved pinhole cameras with ∼10-25 μm pinholes. This method has limitations in the smallest resolvable features that can be imaged with reasonable photon statistics for inertial confinement fusion (ICF) applications. ICF sources have a broadband self-emission spectrum that causes the pinhole images obtained, through thin foil filters, to contain a similarly broadband spectrum complicating the interpretation of structure in the source. In order to study phenomena on the scale of ∼5 μm, such as dopant mix in the ICF capsule, a narrow energy band, higher spatial resolution microscope system with improved signal/noise has been developed using X-ray optics. Utilizing grazing incidence mirrors in a Kirkpatrick-Baez microscope (KBM) configuration [P. Kirkpatrick and A. V. Baez, J. Opt. Soc. Am. 38, 766-774 (1948)], an X-ray microscope has been designed and fielded on NIF with four imaging channels. The KBM has ∼12 × magnification, <8 μm resolution, and higher throughput in comparison to similar pinhole systems. The first KBM mirrors are coated with a multilayer mirror to allow a "narrow band" energy response at 10.2 keV with ΔE ∼ 3 keV. By adjusting the mirror coating only, the energy response can be matched to the future experimental requirements. Several mirror packs have been commissioned and are interchangeable in the diagnostic snout.

Photoconductive detectors (PCDs) fabricated from natural IIa diamonds have been used to measure the x-ray power emitted from laser-produced plasmas. The detector was operated without any absorbing filters which distort the x-ray power measurement. The 5.5 eV band gap of the detector material practically eliminates its sensitivity to scattered laser radiation thus permitting filterless operation. Excellent agreement was achieved between a diamond PCD and a multichannel photoemissive diode array in the measurement of radiated x-ray power and energy.

Photoconductive devices have been fabricated from type IIa diamonds. The sensitivity of these devices is independent of photon energy from 200 to 2200 eV. The dynamic range is 105. The large band gap of the diamond greatly reduces the sensitivity to photons with an energy less than 5.5 eV which is an attractive feature for many applications. The carrier lifetime in the material is 90 ps and the mobility is 1650 cm2/V/s at 106 V/m.