Hades magnet in steady state operation
Once the liquid He level reaches 61%, a heater is switched on with increasing heating power
of 6 Watt / 1% level. Since the excess cooling power provided by the refrigerator was adjusted to only
about 3 Watt, the liquid level stays about 50% of the time above 61% (heater on), the remaining time below 61% (heater off).
With the current lead temperature at 96K (i.e. more He extracted to the current leads), the excess cooling
power of the fridge is slightly larger since less heat is injected into the liquid by the leads.
The pressure of the liquid He increases when the heater is on due to the additional cold gas
produced by the heater. At a higher vapour pressure the temperature of the liquid is slightly higher.
This results in further condensation of gas and the level rises. In particular, the single phase He used for cooling
the coils expands slightly, is pushed through a JT valve and gets liquefied.
The instreaming He is cooled by the returning cold gas. Its temperature depends on the flow and temperature
of the returned cold gas.
The He inlet valve keeps the pressure of the single phase He (which cools the coils) constant (2.96B).
Since the flow resistance of the cold, single phase He depends strongly on temperature,
the valve position follows the temperature variation of the system.
Due to heat exchangers, the temperatures of the coils are determined by the temperature of the liquid
The liquid Nitrogen pressure undergoes fast cycles induced by vaopur bubbles in the
instreaming liquid. Shown is the pressure of the instreaming liquid as well as the pressure behind a
pressure reduction valve. The pressure oszillations of the instreaming Nitrogen are additionally affected by the
refrigerator which gets refills in regular periods.
The pressure reduction results in a cooling of liquid Nitrogen by nearly 10 K. The
cooling takes place via evaporation of liquid Nitrogen. The vapour is extracted via a phase separator.
The latter was operated in manual mode. Every now and then a gas bubble went through the cooling
pipe resulting in temperature increases at low pressure.
expanded scale:
The pressure of the instreaming liquid shows deep dips when the FOPI plant gets refills.
last modified 26.12.98
by Wolfgang Koenig