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MAIN MAGNET CRYOSTAT FOR K 500
SUPERCONDUCTING CYCLOTRON
Introduction The
main magnet cryostat houses the superconducting Nb-Ti coil immersed inside
a liquid helium (LHe) bath operating at 1.2 bar absolute pressure. The
coil produces maximum 5.5 Tesla magnetic fields at the central region of
the Cyclotron. Description The
main magnet cryostat is a fully confined annular chamber. It consists of 3
major subassemblies – the liquid helium chamber (bobbin), liquid
nitrogen cooled radiation shield and vacuum chamber (coil tank).
The bobbin,
weighing about 7 ton, is supported inside the coil tank by three
horizontal and six vertical support links made of glass epoxy material
(scotch ply). There are three ports at the top of the cryostat for
connecting the current leads, helium lines and safety devices to the
helium chamber. There are also three ports at the bottom for liquid
nitrogen supply, vacuum pump connection and over-pressure safety flange
connection. There
are 20 numbers of radial penetrations in the median plane of cryostat to
access the beam space from outside. Various beam extraction and diagnostic
devices like magnetic channels, deflectors, beam
probe etc. pass through these penetrations to guide the beam in to the
desired path and finally extract from cyclotron through exit port. Fabrication and assembly The inner walls of
bobbin and inner & outer walls of the coil tank were made of ring
rolled forgings to avoid longitudinal joints. They were rough machined and
welded with the end flanges by TIG process. Overall welding length is
about 200 M. Each assembly was finally machined to achieve dimensions
within specified tolerances. The bobbin assembly was cryoshocked and
subsequently vacuum-leak tested before it was taken for coil winding.
After the coil winding was over, the outer walls were welded to make it a
closed chamber for the liquid helium. The chamber was helium-leak checked
and afterwards insulated with multi-layer insulations. It was then covered
with the copper radiation shield, insulated again with more multi-layer
insulation and placed inside the vacuum chamber supported with the
vertical links. Finally, all the links, ports, connections and median-
plane-penetrations were welded to complete the assembly work. The whole
assembly was vacuum-leak tested successfully. Through
out the fabrication process an exhaustive quality control plan (QCP) was
followed to ensure the quality as per the specification of the main magnet
cryostat. The following is a general list of hold-points:
To simulate the
severe cooling condition of the bobbin during operation, it was cooled to
liquid nitrogen temperature from room temperature keeping it inside a
chamber and filling the chamber with liquid nitrogen. When the temperature
was stabilized, it was taken back to near room temperature. This is called
one cycle and three such cycles were performed before the bobbin was
vacuum-leak tested. Integration at site The complete
cryostat assembly was assembled with the magnet iron in VECC project site.
The alignment of the cryostat with the magnet iron is checked for correct
positioning and alignment of the cryostat.
Commissioning The
commissioning work of the cryostat was started by cooling down the bobbin
and filling it up with liquid helium. The temperature at four different
places of the coil was monitored online as a check to keep their maximum
difference within 50K so that the thermal stress is kept under acceptable
limit. A typical steady state temperature distribution within coil was
calculated along with the deformations. As
the temperature of the bobbin reduces, the forces on the support links
increases. These forces were monitored continuously during the cool down
process and the horizontal links were adjusted to keep the forces within
3200 Kg (~7,000 lbs). Graph 1 shows the increase in the forces in the
support links with temperature and the sudden drops of forces are the
adjustment done. A sudden drop in link #7 (e7) force is due to an
accidental locking of the safety nut.
Graph
1. Horizontal link force during cool down A
boil off measurement was taken to estimate the performance of the helium
chamber. Liquid helium supply was stopped and the level drop was monitored
continuously. The measured boil off rate varies with the height of liquid
in the bobbin and it is always less than 40 litres/ hr. When
a steady state condition achieved, the α and β coils were
energized up to maximum 550A current on both coils. It produced maximum
magnetic field of 4.8 Tesla at the median plane of the magnet bore. The
coil was centered with respect to the magnet iron by adjusting the
horizontal support links in such a way that all horizontal link forces
drop down approximately by the same rate as the current increase. Graph 2
shows a typical behaviour of these forces in the three support links
during energisation.
Graph
2: Horizontal link forces vs. current in both a
& b
coils At the end of the energisation process, magnetic field measurement was started and analysis of the data showed that the coil centering was done satisfactorily. The successful commissioning of main magnet cryostat was completed in February 2006 and magnetic field measurement work continued.
Cryostat assembly with the magnet iron
Cool
down calculation for the bobbin to estimate the amount of deformations
take place and corresponding support link loads after it reaches
to the liquid helium temperature
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