Comparative Study Of The Influence Of Natural Convection On Directional Solidification Of Al-3.5 Wt% Ni And Al-7 Wt% Si Alloys

We present numerical simulations of thermosolutal convection for directional solidification of Al-3.5 wt% Ni and Al-7 wt% Si. Numerical results predict that fragmentation of dendrite arms resulting from dissolution could be favored in Al-7 wt% Si, but not in Al-3.5 wt% Ni. Corresponding experiments are in qualitative agreement with the numerical predictions. Distinguishing the two fragmentation mechanisms, namely dissolution and remelting, is critical during experiments on earth, when fluid flow is dominant. (C) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved

Time resolved gain/loss studies under low and high power loading for the XeF C-A transition

The behavior of gain versus peak power deposition and the gain-length product versus total energy deposited are measured for devices with power deposition levels from 1 to 13 MW/cm{sup 3} under a variety of gas mixtures. The temporal correlation between the power deposition and gain shows that both the fluorescence and the gain occur after the power deposition has concluded for excitation pulses in the range of 10--30 ns. We find that both B-X and C-A fluorescence takes place in the afterglow and their temporal shapes are similar. This indicates that the C and B states are tightly coupled. This tight coupling has two detrimental consequences for C-state lasing. First is that the population of the B and C states are approximately equal rather than 90% in the C state believed to exist for short pulse electron beam excitation. We believe this close coupling is due to the presence of electric fields in the afterglow which keep the electron temperature relatively hot. The relative populations of the B and C states are determined by a Boltzman distribution governed by the electron temperature and their relative energy separation. Second is that with the C state lifetime approximately the same as the B state lifetime the C-A saturation intensity is very high and efficient energy extraction is substantially more difficult. 5 refs., 10 figs