Wiggler II Repairs
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.
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
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
The Wiggler Magnet Cryostat Neck with the vacuum
All twelve thermal links were
removed, the insulation replaced to provide improved thermal
radiation shielding on the neck and the cryostat closed.
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
||Installing a new gas control panel and a new PLC control
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
The two wiggler magnets
and the cryogenic system are producing synchrotron radiation
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