Variable pressure and temperature liquid nitrogen cryostat for optical measurements with applied electric fields

Abstract

A cryostat with a high-voltage bushing, optical observation ports, variable controlled temperature and pressure has been designed to further the study of liquid nitrogen as a dielectric medium. The novelty in this design lies in the simultaneous achievement of these functions in a single design with a sufficiently large vessel to accommodate realistic geometries for high temperature superconducting power cable termination prototypes. In addition, a commercial single-stage helium expander cryo-cooler is integrated into the apparatus to achieve steady state temperatures down to 63.5 K, without the need for sacrificial loss of liquid nitrogen to maintain vessel temperature. The cryostat inner vessel is certified for operating pressures up to 2 MPa. A custom-made filled-resin bushing provides an electrical feed-through rated to 76.2 kV ac. For optical measurements with a range of sample geometries four optical ports are incorporated into the vessel utilizing sapphire windows and indium seals to form the inner pressure vessel. A technique employing a copper-vapour laser light source and high-speed digital camera for stroboscopic image capture of density change streamers and bubble dynamics with synchronized collection of electrical discharge data has been developed. This design has been used to study pre-breakdown phenomena, bubble dynamics with applied electric fields and electrical breakdown. General construction, mode of operation and initial results are presented.