We present the first results from an experimental campaign to measure the atomic ablator-gas mix in the deceleration phase of gas-filled capsule implosions on the National Ignition Facility. Plastic ...capsules containing CD layers were filled with tritium gas; as the reactants are initially separated, DT fusion yield provides a direct measure of the atomic mix of ablator into the hot spot gas. Capsules were imploded with x rays generated in hohlraums with peak radiation temperatures of ∼294 eV. While the TT fusion reaction probes conditions in the central part (core) of the implosion hot spot, the DT reaction probes a mixed region on the outer part of the hot spot near the ablator-hot-spot interface. Experimental data were used to develop and validate the atomic-mix model used in two-dimensional simulations.
Experiments have recently been conducted at the National Ignition Facility utilizing inertial confinement fusion capsule ablators that are 175 and 165 μm in thickness, 10% and 15% thinner, ...respectively, than the nominal thickness capsule used throughout the high foot and most of the National Ignition Campaign. These three-shock, high-adiabat, high-foot implosions have demonstrated good performance, with higher velocity and better symmetry control at lower laser powers and energies than their nominal thickness ablator counterparts. Little to no hydrodynamic mix into the DT hot spot has been observed despite the higher velocities and reduced depth for possible instability feedthrough. Early results have shown good repeatability, with up to 1/2 the neutron yield coming from α-particle self-heating.
Death and the Research Imperative Callahan, Daniel
The New England journal of medicine,
03/2000, Letnik:
342, Številka:
9
Journal Article
Recenzirano
For several years, there has been an awareness of the often harmful power of the “technological imperative” in the care of dying patients — that is, the compulsive use of technology to maintain life ...when palliative care would be more appropriate. There is another imperative that now deserves more attention in assessing the care of dying patients: the research imperative. It stems from the view that medicine has an almost sacred duty to combat all the known causes of death. Underlying this view is the assumption, usually tacit, that death is the principal evil of human life.
At the heart . . .
Background. The purpose of this study was to explore the relationship between muscle power output at different external resistances and performance of functional tasks. The authors hypothesized that ...power at 40% skeletal muscle 1 repetition maximum (1RM), in which contraction velocity is high, would explain more of the variability in tasks such as level walking than would peak power or 1RM strength, in which contraction velocity is lower. Methods. Participants were men and women (n = 48; ages 65–91 years) with physical disability as evidenced by 2 or more deficits on the Medical Outcomes Study Short Form physical function subscale or a score of 9 or less on the Established Populations for the Epidemiologic Studies of the Elderly short physical performance battery. Muscle strength (1RM) was measured using a bilateral leg press exercise, and power output was determined by selecting the highest power output from 6 different contraction velocities: 40%, 50%, 60%, 70%, 80%, and 90% 1RM. Functional performance tasks consisted of habitual gait velocity (HGV) and stair climb (SC) and chair rise (CR) performance. Separate linear regression models were fit for each of the 3 dependent variables (SC, CR, HGV) using 1RM strength, power at 70% 1RM, and power at 40% 1RM as independent variables. All models were adjusted for age, body mass, and sex. Results. Lower extremity power at 70% and 40% 1RM demonstrated greater associations with SC and HGV than did 1RM strength, whereas power at 40% 1RM demonstrated similar or stronger associations with all functional tasks compared with 1RM strength. Power at 40% 1RM explained the same or more of the variability in SC (R2 =.42 regression coefficient = −.169 ±.06 vs.43 −.206 ±.071), CR (R2 =.28 −.154 ±.057 vs.24 −.152 +.070) and HGV (R2 =.59 .214 +.37 vs.51 .223 ±.049) compared with power at 70% 1RM. Power at 40% 1RM explained more of the variability in the lower intensity (HGV) compared with the higher intensity (SC or CR) functions. Conclusions. Power output at 40% of 1RM explained more of the variability in HGV than did power at 70% 1RM, suggesting that measures such as HGV that require a lower percentage of maximal strength to perform might be more sensitive to differences in contraction velocity. Because HGV is highly predictive of subsequent disability, future studies should evaluate the determinants of muscle power output at low external resistances.
Hydrodynamic instabilities can cause capsule defects and other perturbations to grow and degrade implosion performance in ignition experiments at the National Ignition Facility (NIF). Here, we show ...the first experimental demonstration that a strong unsupported first shock in indirect drive implosions at the NIF reduces ablation front instability growth leading to a 3 to 10 times higher yield with fuel ρR>1 g/cm(2). This work shows the importance of ablation front instability growth during the National Ignition Campaign and may provide a path to improved performance at the high compression necessary for ignition.
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