The invention relates to a method for cooling a cryostat configuration during transport, wherein the cryostat configuration comprises a superconducting magnet coil in a helium tank containing liquid helium, which is surrounded by at least one radiation shield, wherein the cooling inside the cryostat configuration in stationary operation is performed entirely without liquid nitrogen using a refrigerator. The invention also relates to a cryostat configuration with an outer shield within which a helium tank for liquid helium is installed, which contains a superconducting magnet coil and with at least one radiation shield surrounding the helium tank, wherein the cryostat configuration is cooled by a refrigerator entirely without liquid nitrogen during stationary operation.
Such a configuration is known, for example, from U.S. Pat. No. 714,190 B2.
Superconducting magnets consist of windings made of superconducting wire that are cooled down to temperatures of 4.2 Kelvin using liquid helium. The principal task of the cryostat configuration is to maintain the superconducting magnet at the specified operating temperature with liquid helium while consuming as little coolant as possible.
The main components of known cryostat configurations are the helium tank, which contains the superconducting magnet coil and liquid helium, the outer jacket, which acts as an outer vacuum vessel, and a neck tube, which connects the helium tank to the outer shield. The helium tank is surrounded by a vacuum space, which is delimited by the helium tank itself, the neck tube, and the outer jacket.
To reduce the heat input into the helium tank and thus into the superconducting magnet coil, it was previously common practice to surround the helium tank not only with radiation shields but also with a nitrogen tank containing liquid nitrogen, which was permanently installed in the cryostat. However, such configurations require frequent maintenance as the nitrogen tank usually has to be refilled every two weeks due to evaporation of the liquid nitrogen.
EP 0 468 425 A2 describes a cryostat configuration with a nitrogen tank that surrounds the helium tank, which is cooled by means of a refrigerator. It is thereby possible to reduce the nitrogen consumption and, therefore, the number of maintenance interventions in this way. However, all cryostat configurations with a nitrogen tank have the disadvantage that the resulting cryostat configuration is very large due to incorporation of the nitrogen tank and therefore occupies a large amount of space in the laboratory.
Modern cryostats of superconducting magnets therefore use a refrigerator to directly cool the radiation shield. Depending on the type of construction, the refrigerator is also able to prevent evaporation of helium by recondensing the helium gas in the helium tank. Such a cryostat configuration is known, for example, from U.S. Pat. No. 7,140,190 B2 and www.bruker-biospin.com/biospec, www.bruker-biospin.com/mri_usr_technology. Such cryostat configurations, which use refrigerators for the direct cooling of radiation shields, can be operated fully nitrogen free. This permits more compact cryostat configurations to be implemented and maintenance costs to be saved because maintenance interventions, such as replenishing the nitrogen and vacuum testing of the nitrogen vessel, are no longer necessary.
Transport of such cryostat configurations to their destination is preferably performed in the cold state because, in many installation locations, no service personnel with expert knowledge are available, and the cooling time of a superconducting magnet configuration disposed in a cryostat configuration as described above can take up to three weeks, during which time specialist personnel must be available on site.
However, the transport of cryostat configurations in the cold state is a time-critical and expensive undertaking. Transport by airfreight, although less problematic, is, however, very expensive. Transport by sea is considerably cheaper but very time consuming. Since the refrigerator consumes a large amount of electrical energy, it is almost logistically impossible to run it while the cryostat configuration is being transported: The compressor of the cold head requires up to 15 kW of electrical power, a large part of which is converted to heat and usually has to be dissipated by water-cooling. For this, a water circuit has to be connected to a water cooler during transport, which consumes roughly the same amount of electrical power as the compressor. The entire system must be monitored by a control unit, which also requires electrical energy. Furthermore, the large quantity of waste heat must be allowed to escape from the transport system which will otherwise heat up. All these constraints would make operating a refrigerator during transport extremely complicated and therefore very expensive.
However, if the refrigerator is not run, the radiation shield continues to heat up until the dissipating helium cools down the radiation shield in accordance with the energy that is incident upon the radiation shield, and an equilibrium temperature is established. This is usually around 100-150 Kelvin. The thermal losses are then considerable and can result in evaporation rates of 5 liters of LHe (liquid helium)/hour. The residence time (i.e. the time until no more helium is to be found in the magnet) sinks to well below one month and makes transport to remote regions or by ship practically impossible without the magnet becoming dry and therefore heating up during transport, that is, there is no more liquid helium in the helium tank. In such cases, only expensive airfreight transport is possible.
The object of this invention is therefore to suggest a method by which the consumption of liquid helium can be greatly reduced during transport and the possible transport time of a cooled superconducting magnet configuration therefore increased.