Modeling Density Changes Inside a Cryogenic Target using a Fabry-Perot Interferometer: A Feasibility Study

The use of a Fabry-Perot interferometer has been proposed as a method of measuring changes in deuterium gas density inside a cryogenic target. It is important to determine whether rapidly decreasing the temperature of the helium gas surrounding the target results in the required change in deuterium gas density for use on the National Ignition Facility. A program, SAKURA, has been written to investigate the feasibility of this proposal. SAKURA calculates the intensity of the interference fringes based on the distance between the two mirrors that make up the interferometer, the temperature of the target, and other parameters. Multiple bounces between the mirrors and refraction through the target layers are taken into account and interference fringes inside the target as well as background fringes are modeled. SAKURA has shown that the interference fringes are experimentally detectable and that a high mirror reflectivity is required. Experiments using the interferometer, along with SAKURA, will be used to study the thermal lag of heat out of the target following a sudden drop in ambient temperature. 2