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ENEA - Fusion division
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Liquid Nitrogen sub-plant of FTU
The liquid nitrogen plant serves to cool the FTU windings to the cryogenic
temperature of -192°C so as to considerably lower the values of Ohmic
resistance and, consequently, obtain high magnetic field values.
The plant
(called LN2 in the following) was built by RIVOIRA and consists
basically of:
- three RIVOIRA storage tanks, with a capacity of 30,000 litres, each one
operating at a pressure of 2.5 bar;
- two cryogenic pumps fabricated by the French company of CRIOSTAR, lubricated
with the same liquid nitrogen, each one with a flow rate of about 30 m3/h;
- two evaporators which operate alternately to avoid saturation during
operations at high gas flow;
- tanks, valves and the typical equipment of a standard hydraulic plant.
View of out-door part of LiquidNitrogen sub-plant (tanks)
At the start of operations, the FTU machine (i.e., the vacuum chamber +
magnets + support structure) is cooled from room temperature to -180°C. This
operation is performed very slowly (about 4 days) and is done by using appropriate
injectors to trickle liquid nitrogen inside the cryostat which encloses the
machine. Once the whole machine has reached -180°C, the copper windings are
cooled further to a temperature of -190°C. This cooling is carried out by
means of one of the two cryogenic pumps. At the end of cooling, the second
pump is used to send gaseous nitrogen under pressure inside the cooling channels
of the windings so as to eliminate any residual liquid nitrogen, which could
create unexpected evaporation with a dangerous increase in pressure when
a plasma shot takes place.
At this point, experimentation can begin with
the plasma shots.
Between one plasma shot and the next, the magnet windings have to be cooled because they heat up, on average, by about 35°C.
Another important operation
of the LN2 plant is to constantly send a flow of gaseous nitrogen inside
the cryostat enclosing the FTU machine. This flow serves to keep the cryostat
in slight overpressure (20 mm H2O) to avoid air from the outside
penetrating the cryostat and giving rise to ice formation due to the humidity
contained in the air.
The LN2 plant also provides gaseous nitrogen flows to
the FTU diagnostic instruments, where required.
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