General Description
This system comprises a special horizontal field 5T split pair magnet mounted in the tail section of a large capacity, liquid nitrogen shielded, vacuum insulated cryostat. The magnet has a cold split of 100 mm x 40 mm and a cold bore diameter of approximately 120 mm, accessed through aluminium windows. Alternatively, room temperature tubes may be inserted to allow the magnetic field to be plotted. The vertical split access allows a large cooling power helium-4 insert to be fitted from above.
The cryogenic efficiency of the cryostat is very high due to the small helium neck diameters and the way in which the exhausting helium gas is used to cool them. The helium can heat load is minimized by the use of a nitrogen cooled shield which will minimize both the conducted and radiated heat to the minimum levels possible.
The current leads are all fixed for high reliability and safe operation.
The magnet is 5 T with a homogeneity of better than 1 in 10-4 over a length of 20 mm and a diameter of 80 mm. See the (insert link to Test Results) for more detailed information. Access is provided through aluminium windows which may be demounted if required for access to the magnet cold bore.
The weight of the system is approximately 1,000 kg. Appropriate lifting gear must be used to move the cryostat.
This system, when energized to full field has a considerable stray field, extending over many meters, and the system stored energy is approximately 0.5 MJ. It is therefore VITAL that the safety section is read by ALL personnel coming near the system.
PARTICULAR CARE MUST BE TAKEN TO ENSURE THAT THE SYSTEM IS WELL ANCHORED TO THE FLOOR, AND ANY STEEL OBJECTS IN THE VICINITY ARE SIMILARLY WELL BOLTED TO THE FLOOR.
Cryostat Description
The cryostat is of a vacuum insulated, all metal construction with intermediate temperature radiation shielding. The outside surfaces of the helium and nitrogen vessels are wrapped with single or multi-layer super-insulation to reduce emissivity. The outer vacuum case (OVC) of the dewar will be fitted with an evacuation valve incorporating a pressure relief safety feature that will operate in the event of a cryogen leak to the vacuum space. In addition there is a drop-off plate at the base or side of the dewar.
The siphon entry port has an associated cone located within the cryostat. A tube runs from the cone to the bottom of the cryostat and ensures that all liquid nitrogen can be removed from the helium reservoir after pre-cooling the magnet and that filling with helium is from the bottom.
All cryomagnet service ports should be sealed with the plugs provided when not in use. In all cases, the boil-off of cryogens is minimized by taking great care in the design to prevent heat entering from the following main sources:
Gaseous conduction. An evacuation / pressure relief valve allows the insulating vacuum space to be evacuated to less than 10-4 torr.
Metallic conduction.Great care is always taken to use materials of low thermal conductivity combined with mechanical strength to support the cryogens in their vacuum. The supports (usually tubes) are of minimum cross sectional area and maximum effective length within overall size constraints. Neck tubes are thermally anchored with a copper thermal link to the top of the nitrogen vessel and good use is made of the enthalpy of the exhausting gas to minimize incoming conducted heat.
Radiation. The radiation load is reduced to reasonable values by the introduction of intermediate temperature radiation shields. These are usually cooled by a reservoir of liquid nitrogen surrounding the helium bath. The enthalpy of the exhausting helium gas is sometimes used to cool a radiation shield inside the nitrogen shield. The emissivity of cold surfaces can also be reduced. This is achieved using many interleaved layers of aluminium and insulation known as super-insulation.
Ohmic heating. The principal sources of ohmic heating are the current leads and the superconducting switch. In some systems the current leads are made demountable to minimize the cryogen boil-off with a persistent magnet, the remainder of systems feature carefully designed current leads which do not impose a significant heat load. All systems now feature low-loss switches.
Evacuating the Cryostat OVC
In order to maintain the thermal isolation of the liquid helium it is necessary that a high vacuum be maintained within the cryostat outer vacuum case.
IMPORTANT In many cases the thin wall construction of the helium reservoir will not support an external pressure differential of one atmosphere. The helium reservoir must therefore NEVER be evacuated unless the OVC is first evacuated. The recommended pumping equipment consists of an oil diffusion pump of 50 mm (2 inch) diameter or, even better, a turbomolecular pump fitted with a liquid nitrogen cold trap. This pump should be backed by a rotary pump of not less than 12-15 m3/hr pumping speed, fitted with a gas ballast facility. All connecting lines should have an internal diameter of not less than 50 mm and be as short as possible. Tubes must NOT have been used previously to carry or pump helium.
a. Connect the valve on the cryostat top flange to the pumping equipment. Using the rotary pump, evacuate the cryostat slowly (approximately half hour) to prevent any possible collapse of internal shielding, until the pressure is less than 0.05 mB.
b. Switch over to the diffusion pump and evacuate the cryostat to less than 5 x 10-4 mB. Continue pumping at least overnight to ensure the removal of residual gases trapped in the super-insulation.
Inspecting the vacuum:-
If the cryostat is already evacuated and it is desired to inspect the pressure only, the pumping tube should be evacuated and the diffusion pump operating before the OVC valve is opened. If the pressure is greater than 10-3mB with the system warm, the cryostat should be evacuated overnight with the diffusion pump to less than 5 x 10-4 mB. It is recommended that the cryostat is always pumped overnight before use.
Flushing the vacuum space:-
If the vacuum space has been accidentally contaminated with helium gas or moisture evacuation can be improved by flushing the space. NOTE: Never vent cryostats with helium gas as this will 'stick' in the super-insulation.
- Using a rotary pump, evacuate cryostats to less than 1 mB.
- Admit an atmosphere of DRY nitrogen gas, preferably through a 1 mm orifice, and pump out to less than 1 mB.
- Repeat (2) several times, then pump to less than 0.05 mB.
- Switch over to the diffusion pump as in (b) above.
Precooling the Magnet
Before filling the cryostat with liquid helium, the magnet and system must be cooled to a temperature below 100 K, this will save a considerable amount of liquid helium which is much more expensive than liquid nitrogen. To perform the precool, fill the liquid helium container with liquid nitrogen, completely above the magnet. Use a length of 9.6 mm diameter stainless steel tubing inserted into the transfer tube entry port (this is the 'blow-out' tube supplied with the system). Ensure that the tube is located in the cone fitting below the siphon entry port inside the cryostat, the liquid nitrogen storage dewar should be conveniently positioned and connected to the blow-out tube with flexible plastic tubing (once the transfer has started this should not be moved as it is very brittle and will break easily). Allow the liquid nitrogen to remain for one or two hours and then fill it completely again.