Hard x-ray (>100 keV) imager to measure hot electron preheat for indirectly driven capsule implosions on the NIFa) Rev. Sci. Instrum. 83, 10E508 (2012); K and Lshell xray spectroscopy of indirectly driven implosions (invited) Rev. Sci.The performance of indirectly driven fusion capsules has been improved by mid Z doping of the plastic capsule ablator. The doping increases x-ray preheat shielding leading to a more isentropic compression, higher convergence, and higher neutron yield. A 4ϫ increase in neutron yield is both calculated and observed as the Ge doping level is increased from 0% to 3% by atomic fraction. A predicted 40% decrease in x-ray image core size with increasing Ge content is confirmed.
Backlighting of low-density Si02 aerogel balls (0.3 g/cc) in Nova hohlraums enables direct imaging of the indirectly driven ablative shock trajectory versus time. An immediate application of the technique is a new method for measuring drive temperature and asymmetry in Nova hohlraums. Measured drive asymmetries are exaggerated due to an increased shock speed in low-density foam balls. From the backlighter-imaged distortion of the radially converging shock, a potentially useful time-dependent tlux asymmetry diagnostic is obtained. Results from a 3:l peak-to-foot contrast ratio pulse shape (~~22) are analyzed and shown to agree with modeling. The current goal of the witness-ball effort on Nova is to identify the transition in flux on capsule from waist high (sausaging) to pole high (pancaking), thereby validating the technique as a promising symmetry diagnostic for the proposed National Ignition Facility. 0 1995 American Institute of Physics.
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