Thermal design considerations for the Cryogenic High Accuracy Refraction Measuring System (CHARMS)

摘要

Accurate, high precision, refractive index data for infrared (IR) optical materials at cryogenic temperatures is scarce. CHARMS has been designed to take advantage of the conceptually simplest and most accurate method for determining the real part of refractive index, namely minimum deviation prism refractometry, in order to populate the world's database of optical constants for IR materials with accurate index values, measured over the full range of practical operating temperatures. Chief among the challenges in obtaining high accuracy index measurements - to levels of 2 to 3 parts in the fifth decimal place of index - at the lowest cryogenic temperatures are satisfactory cooling of the sample and accurate measurement and maintenance of its temperature. We discuss the thermal design aspects of our refractometer system which enable cooling of prismatic samples to temperatures near that of liquid helium (LHe) in a vessel which provides a windowless optical path for the highest accuracy refractometry.