Daresbury Laboratory Wiggler II Repairs

The Need
The Daresbury Laboratory operate a Second Generation Light Source Synchrotron with superconducting wiggler magnets generating high intensity X-rays for two beam lines. The wiggler magnets are cooled by liquid helium circulating in a closed loop from a liquefier. After twenty years operation the system was proving vulnerable to slight disturbances which would overload the helium liquefier and disrupt the operation. The Laboratory were trying to identify the cause of their problems and to effect a solution to improve the system reliability. They came to Monroe Brothers Ltd for technical assistance.

 

 

The Service Provided
Monroe Brothers Ltd examined the operation of the system. There was a known anomaly in the operation of the Wiggler II magnet and close inspection of the engineering drawings identified a design error with the thermal links between the nitrogen cooled radiation screen and the helium vessel which would account for this anomaly and the unreliability of operation.

Above: Schematic figure of the superconducting wiggler magnet

Thermal links are often made between the liquid nitrogen cooled radiation screen and the neck of a cryostat to reduce the heat load on the liquid helium: low cost liquid nitrogen is sacrificed to intercept the heat which would otherwise boil liquid helium. The link is typically braided copper which has a high thermal conductivity to transfer the heat and flexibility to allow for any thermal contractions. Typically the thermal links are connected to the neck 20% of the distance from the warm end. In this design the connections were made 20% from the cold end! Close examinati-on of the internal design of the neck showed that the problem was worse: the cold helium gas was flowing part way up and cooling the bottom of the neck. Hence the position where the thermal links were connecting to the neck could easily be 4 K. Far from liquid nitrogen being sacrificed to save the liquid helium, liquid helium was in fact being lost to cool the radiation screen and reduce the nitrogen consumption!

Initially the helium liquefier had enough spare capacity to overcome the problem. However as the age of the plant increased, this extra heat load was enough to compromise the performance and cause the liquefier to trip out.

Monroe Brothers Ltd devised a process of work to open the wiggler cryostat, remove the links and seal the system. The work had to be done in a confined space and with precautions to ensure that vacuum space of the cryostat was not compromised by the dirt from the surrounding environment.

Above: The Wiggler Magnet Cryostat Neck

A S Scientific Products Ltd were selected as the chosen contractor to make the repairs and worked carefully under the supervision of Monroe Brothers Ltd.

Above: The Wiggler Magnet Cryostat Neck with the vacuum casing raised

All twelve thermal links were removed, the insulation replaced to provide improved thermal radiation shielding on the neck and the cryostat closed.

Above: The braided copper thermal links are shown linking the aluminium liquid nitrogen radiation screen and the copper ring on the neck of the helium vessel.

After welding the cryostat closed it was successfully leak tested and the wiggler magnet put back in operation.

At the same time as the work on the wiggler magnet, Monroe Brothers contributed to the specifications for refurbishing the liquefier to increase its operational life. The major changes were:

Replacing the reciprocating compressor with a screw compressor having a greater gas flow rate and a reduced vibration signature
Replacing the valve stems and valve actuators on the Cold Box
Installing a new gas control panel and a new PLC control system.

The Benefit
Monroe Brothers Ltd identified a design problem which had compromised the operation of the complete cryogenic system. The error was rectified and, with technical support to refurbish the helium liquefier, the system was put back in operation. With both wiggler magnets now operating as originally intended, and the improvements to the helium liquefier, the complete cryogenic system has operated satisfactorily since January 2003.

The two wiggler magnets and the cryogenic system are producing synchrotron radiation for the cutting edge research which takes place at the Daresbury Laboratory.

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