Techniques of vacuum pumping in multi-layer insulated cryogenic vessels

The improvement in heat transfer properties obtained by use of Multi-Layer Insulation (MLI) is reviewed. The introduction of many closely spaced layers of material increases considerably the surface exposed to vacuum and flow restrictions. Experiments have been made to demonstrate that by using correct pump-down procedures the inherent difficulties can be dealt with and the improved insulation properties of MLI can be used to full advantage in laboratory cryostats which are vented and re-pumped frequently. The test apparatus, procedure and calculation are shown in detail. Test results show that the heat losses of MLI at 10⁻⁴ torr are about 10 per cent of the losses of vacuum insulation only at 10⁻⁶ torr. Heat transfer of MLI remains constant at pressures below 10⁻³ … 10⁻⁴ torr. Recommended pump down procedures are given for equipment which is occasionally vented. The importance of cleanliness during installation of MLI, of the use of heating and getter materials is demonstrated. Pump down times to 10⁻⁴ torr of 5 … 6 h after venting were easily obtained in the test system.