Development of vapour shielded liquid helium dewar

This paper describes the development of a 100L nominal capacity liquid helium (He) Dewar which uses multi-shield insulation system utilizing the cold from evaporated vapour of the stored cryogenic liquid. Heat in-leak design calculations are done using Finite Difference Method (FDM) by discretisation of Dewar neck into smaller elements and solving the energy balance taking care of variable neck material properties, variable vapour properties, heat in-leak through super-insulation, conductive heat transfer through neck and vapour, convective heat transfer from neck to vapour etc. A computer program is developed using Visual Basic Script of Microsoft Excel coupled with Gas property software package. Positions of He vapour cooled thermal shields are optimized for minimum heat in-leak to liquid helium (LHe). Mechanical design and fabrication of Dewar is done using ASME Boiler and Pressure Vessel code Section II, V, VIII Division-1 and section IX. Vacuum leak tightness of the Dewar is evaluated using He mass spectrometric leak detector (MSLD). Performance evaluation is done using liquid nitrogen (LN₂).