Heat transfer properties of polymer target shells at temperatures between 250K and 350K

The emissivities and accommodation coefficients for helium and deuterium gas were measured for polystyrene target shells from several production batches. The shells varied in wall thickness, diameter and surface conditions as viewed with an optical microscope. For emissivity measurements, it is desirable for the radiative heat transfer to dominate over conductive heat transfer via the surrounding gas and the sample (and thermometer) mount. This is achieved by maintaining very low gas pressure (free molecular conduction regime) and by a novel contact-less thermometric measurement, in which the temperature of the shell is determined from the strongly temperature-dependent shell outgassing rate. The accommodation coefficient is also obtained in the process. Emissivity and accommodation coefficient results are reported in the temperature range 250 - 350K. The values are very low, in the 0.01 range for the former, and 0.003 range for the latter, which augurs well for thermal stability after the shroud removal prior to a target shot. For measurements at lower temperatures (down to 4K), other contact-less thermometry methods are proposed, with electronic magnetic susceptibility shown to be very favorable.