3 Dismantling and Re-assembling the System

Important:

• Before disassembly all cryogen's should be removed from the cryostat and all parts of the system should be allowed to warm to room temperature.
• Take care not to trap cryogen's in closed off volume - high pressures will result causing possible damage to the equipment.
• Particular care should be taken with regard to the VTI and lambda point refrigerator. Take care not to isolate cold gas in lines that are closed off between shut-off valves, far better review operating procedures so that one of the valves can be omitted from the gas handling circuit.

3.1 Dismantling the system

The internal parts of the cryostat can be accessed for removal or refit of the VTI or for limited inspection of the cryostat suspension and superinsulation. No direct access is possible to the magnet or protection circuit as these are welded into their helium vessels; it is possible to separate the magnet from the cryostat as there are indium sealed flanges between the two items.

To gain access to the internal parts of the cryostat, release the vacuum in the OVC by opening the vac/vent valve to air or preferably dry nitrogen gas. Only do this if the system has been allowed to warm thoroughly to room temperature. A rubber tube connected to the valve with its free end immersed in liquid nitrogen is the best way to introduce an atmosphere of dry gas into the OVC, if no other supply is available.

Remove the screws that hold the OVC tail of the cryostat to the main OVC body. Lift the cryostat away from the OVC tail using a hoist.

Remove the radiation shield; this will allow access to the bottom of the VTI.

The VTI can be removed by separating the two halves of the small indium seal in the capillary line, the capillary can then be carefully straightened. Remove the screws that hold the VTI in place at the top of the cryostat and carefully extract the inner parts of the VTI upwards. Take great care not to bruise the wiring during extraction. Direct access is then available to the temperature sensors and the heater.

No further disassembly of the cryostat should be undertaken as all other parts are secured by welds.

3.2 Re-assembling the System

Re-assembly is essentially the reverse of the above procedures. Points to note are:

1. When replacing assemblies, take care not to damage any electrical wiring. Check wiring at each stage of assembly for continuity at short circuits. One area where this is particularly important is the fitting of the VTI inner parts into the bore of the magnet.
2. Make certain that all flanges are replaced with the same relative orientation they had when removed. Alphabet characters have been punched on mating flanges and these should be placed together for correct orientation.
   
   
3. Ensure that all O-rings are clean and lightly greased with vacuum grease.
   
   
4. Avoid grease and finger marks on any superinsulated surfaces. Clean with a degreasing fluid if necessary.
   
   
5. When remaking indium seals use new or re-extruded 1 mm diameter indium wire. Ensure that all mating surfaces are clean and dry. Wrap a single turn around the male half of the jointing flanges; the ends of the wire should cross each other. Assembled the joint and fit the bolts. Tighten the bolts evenly and gradually, in a methodical diagonal order until the indium is fully compressed. Check that the joint is leak tight before proceeding further.
   
   
6. Make sure that the radial spacing on the VTI and its radiation shield are set so that they cannot touch each other or the helium reservoir bore tube. Also check the capillary tube for touches.
   
   
7. If the magnet has been removed take care to solder the leads from the magnet to their correct cryostat current leads (check continuity and identity using a multimeter). Insulate the soldered joints against high voltage. Connect all electrical connectors, and connect the lower fill/blow-out tube to the upper using the compression union. All these connections are made at the magnet indium seal level.