AUTONOMOUS COOLING OF A SUPERCONDUCTIVE DRY-COOLED MR MAGNETIC COIL SYSTEM

Abstract

A method for autonomously cooling down a cryogen-free superconductive magnetic coil system includes: (a1) measuring the current temperature T.sub.actual at the magnet and comparing it to a temperature target value T1.sub.target; (a2) if T.sub.actual>T1.sub.target, actuating a vacuum pump and opening a barrier valve in a vacuum conduit that leads from the vacuum pump into a vacuum vessel containing the magnet; (b1) measuring the current pressure P.sub.actual in the vacuum vessel and comparing it to a pressure target value P1.sub.target; (b2) if P.sub.actual<P1.sub.target, activating a cold head for cooling a cooling arm; (c1) measuring T.sub.actual and comparing it to the first temperature target value T1.sub.target; (c2) if T.sub.actual<T1.sub.target, closing the barrier valve and switching off the vacuum pump; (d1) measuring T.sub.actual and comparing it to a second temperature target value T2.sub.target and maintaining the second temperature target value T2.sub.target.

Claims

1. A method of operating a magnetic resonance (MR) apparatus having a superconductive MR magnetic coil system which is disposed in a vacuum vessel and is cryogen-free in MR analysis operation, the apparatus having a cryostat for cooling the MR magnetic coil system which comprises a neck tube that leads through an outer shell of the vacuum vessel to the MR magnetic coil system, wherein a cooling arm of a cold head is disposed at least partly within the neck tube, and wherein a closed cavity which is formed around the cooling arm is sealed in a fluid-tight manner with respect to the MR magnetic coil system to be cooled and is at least partly filled with a cryogenic fluid during normal operation of the MR apparatus, wherein the method comprises: (a1) measuring a current temperature T.sub.actual in the MR magnetic coil system and comparing T.sub.actual with a definable first temperature target value T1.sub.target at which the vacuum vessel is to be pumped out; (a2) if T.sub.actual>T1.sub.target, activating a vacuum pump disposed outside the vacuum vessel and opening a first barrier valve in a vacuum conduit that leads from the vacuum pump into the vacuum vessel; (b1) measuring a current pressure P.sub.actual in the vacuum vessel and comparing P.sub.actual with a definable first target pressure P1.sub.target at which the MR magnetic coil system is to be cooled down to cryogenic temperature; (b2) if P.sub.actual<P1.sub.target, activating the cold head for cooling of the cooling arm; (c1) measuring the current temperature T.sub.actual in the MR magnetic coil system and comparing T.sub.actual with the first temperature target value T1.sub.target; (c2) if T.sub.actual<T1.sub.target, closing the first barrier valve and switching off the vacuum pump; and (d1) measuring the current temperature T.sub.actual in the MR magnetic coil system and comparing T.sub.actual with a definable second temperature target value T2.sub.target at which the operating temperature of the MR magnetic coil system for MR measurements has been attained and maintaining T.sub.actual at the second temperature target value T2.sub.target.

2. The method as claimed in claim 1, wherein the neck tube is connected via a first valve V1 to a helium feed, and wherein the method further comprises, in a step (e), feeding helium into the neck tube and liquifying said helium.

3. The method as claimed in claim 1, wherein a feed conduit to the neck tube is connected via a further valve V2 to the vacuum pump, and wherein the neck tube is pumped dry by the steps of: (a3) measuring the current temperature T.sub.actual in the MR magnetic coil system and comparing T.sub.actual with a definable third temperature target value T3.sub.target from which the neck tube is to be pumped out; and (a4) if T.sub.actual>T3.sub.target, actuating the vacuum pump and opening the second barrier valve V2 in a vacuum conduit that leads from the vacuum pump to the neck tube.

4. The method as claimed in claim 3, wherein, in the event of a failure of the cold head, in a step (d2), the vacuum pump is used to pump away the liquid helium in the neck tube in order to cool the MR magnetic coil system.

5. The method as claimed in claim 1, wherein the definable temperature target values T1.sub.target, T2.sub.target and T3.sub.target and the definable pressure target value P1.sub.target are selected from the following ranges of values: 5K≤T1.sub.target≤20K; 3K≤T2.sub.target≤5K; 250K≤T3.sub.target≤300K, and 10.sup.−4 mbar≤P1.sub.target≤10.sup.−3 mbar.

6. An MR apparatus for performing the method as claimed in claim 1, wherein the MR apparatus further comprises a temperature sensor for measuring a current temperature T.sub.actual in the MR magnetic coil system and a first pressure sensor for measuring a current pressure P.sub.actual that are present in the vacuum vessel, and wherein a control unit is configured to detect and compare the current temperature T.sub.actual in the MR magnetic coil system with defined temperature target values, to detect and compare the current pressure P.sub.actual in the vacuum vessel with defined pressure target values, and to actuate the first barrier valve, the vacuum pump and the cold head.

7. The MR apparatus as claimed in claim 6, wherein the vacuum pump has an at least 2-stage construction and has a turbomolecular pump and a backing pump therefor.

8. A control unit for use in an MR apparatus that performs the method as claimed in claim 1, wherein the control unit is configured to detect and compare a current temperature T.sub.actual in the MR magnetic coil system with defined temperature target values, and to detect and compare a current pressure P.sub.actual in the vacuum vessel with defined pressure target values, the control unit comprising: a measurement unit in which a temperature sensor for measuring the current temperature T.sub.actual in the MR magnetic coil system and a pressure sensor for measuring the current pressure P.sub.actual in the vacuum vessel are connected; an actuation unit for opening and closing the first barrier valve, for activating and deactivating the vacuum pump and for activating and deactivating the cold head, and a processor unit which is disposed as an interface between the measurement unit and the actuation unit for comparison of the sensor parameters detected with the target parameters and the processing of the data for actuation of the cold head, vacuum pump and barrier valve.

9. The control unit as claimed in claim 8, further comprising a second pressure sensor for measuring the current pressure P.sub.HR in the cavity surrounding the cooling arm of the cold head that is connected to the measurement unit, wherein the actuation unit is configured to open and close the second and third barrier valves.

10. The control unit as claimed in claim 9, further comprising a connection from the neck tube to the vacuum pump, wherein the control unit implements control of V2 to the vacuum pump.

11. The control unit as claimed in claim 9, further comprising a connection from a helium feed to the neck tube, wherein the control unit implements control of the He feed via the valve V1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The invention is shown in the drawing and is elucidated in detail by working examples. The figures show:

[0058] FIG. 1 a schematic vertical section diagram of an embodiment of the magnetic resonance apparatus with just one valve for performance of the auto-cooling method of the invention;

[0059] FIG. 2 a flow diagram of the basic method sequence of the auto-cooling method of the invention in embodiments with just one valve as in FIG. 1;

[0060] FIG. 3 a schematic flow diagram with the method steps according to the claims in embodiments with just one valve as in FIG. 1;

[0061] FIG. 4 a flow diagram of the basic sequence of the auto-cooling method of the invention in the form of an algorithm as is to proceed in a corresponding program in the control unit, specifically for embodiments with just one valve as in FIG. 1;

[0062] FIG. 5 a schematic vertical section diagram of an embodiment of the magnetic resonance apparatus having three valves for performance of the auto-cooling method of the invention;

[0063] FIG. 6 a flow diagram of the basic method sequence of the auto-cooling method of the invention in embodiments with three valves as in FIG. 5;

[0064] FIG. 7 a schematic flow diagram with the method steps according to the claims in embodiments with three valves as in FIG. 5; and

[0065] FIG. 8 a flow diagram of the basic sequence of the auto-cooling method of the invention in the form of an algorithm as is to proceed in a corresponding program in the control unit, specifically for embodiments with three valves as in FIG. 5.

DETAILED DESCRIPTION

[0066] The present invention is fundamentally concerned with a specifically modified method of operating a magnetic resonance apparatus 10 (“MR apparatus”), especially comprising the autonomous cooling of a superconductive magnet arrangement.

[0067] FIG. 1 shows a schematic of a particularly simple embodiment of the magnetic resonance apparatus 10 with just one valve for implementation of the auto-cooling method of the invention.

[0068] The MR apparatus 10 comprises a vacuum vessel 11 with a superconductive MR magnetic coil system 12. In MR operation, this MR magnetic coil system 12 is dry, i.e. cryogen-free. Cooling is effected by means of a cryostat 13 which is connected to a compressor 13a. The cryostat 13 includes a neck tube 14 which is guided through an outer shell 15 of the vacuum vessel 11 to the MR magnetic coil system 12. Within the neck tube 14 of the cryostat 13 is a cooling arm 16 of a cold head 17. A cavity 18 is formed around the cooling arm 16. The cavity 18 is sealed in a fluid-tight manner with respect to the MR magnetic coil system 12 to be cooled. In the operating state shown here, the cavity 18 is partly filled with a cryogenic fluid 19 (e.g. liquid helium), and gaseous helium.

[0069] Outside the vacuum vessel 11 is disposed a vacuum pump 20, in the form here of a 2-stage vacuum pump 20. Vacuum pump 20 here encompasses a turbomolecular pump 20a with a backing pump 20b (for example a membrane pump). The vacuum pump 20 is connected by a first barrier valve 21 (barrier valve V3) via a vacuum conduit 22 to the vacuum vessel 11.

[0070] In the MR magnetic coil system 12, there is a temperature sensor 23 for measuring a current temperature T.sub.actual in the MR magnetic coil system 12. A first pressure sensor 24 connected via the vacuum conduit 22 to the vacuum vessel 11 measures a current pressure P.sub.actual in the vacuum vessel 11. A control unit 40 detects and compares the current temperature T.sub.actual in the MR magnetic coil system 12 with defined temperature target values. In addition, the control unit 40 detects and compares the current pressure P.sub.actual in the vacuum vessel 11 with defined pressure target values. The control unit 40 is also configured such that it can actuate the first barrier valve 21, the vacuum pump 20 and the compressor 13a of the cold head 17.

[0071] The control unit 40 comprises a measurement unit, an actuation unit and a processor unit. The temperature sensor 23 is connected to the measurement unit and is used to measure the current temperature T.sub.actual in the MR magnetic coil system 12. Additionally connected to the measurement unit is the pressure sensor 24, with which the current pressure P.sub.actual in the vacuum vessel 11 is measured. The actuation unit is connected to the first barrier valve 21, which can open and close the actuation unit. In addition, the actuation unit is connected to the vacuum pump 20 and the cold head 17 (or the compressor 13a), which can activate and deactivate the actuation unit. The processor unit is disposed between the measurement unit and the actuation unit. The processor unit compares the parameters detected by the temperature sensor 23 and the pressure sensor 24 with the target parameters. These data are processed by the processor unit and utilized for actuation of the cold head 17, the vacuum pump 20 and the barrier valve 21.

[0072] By means of a second pressure sensor 25 connected to a connection 28, which leads from a helium feed 29 into the cavity 18 of the cryostat 13, it is possible to measure the current pressure P.sub.HR in cavity 18. The second pressure sensor 25 is likewise connected to the measurement unit of the processor unit.

[0073] The basic sequence of the cooling method of the invention for embodiments with just one valve is shown as a flow diagram in FIG. 2 and discussed in the example below.

[0074] The sequence of the autonomous cooling method, in execution variants, may comprise the following steps:

[0075] 1. The operation of the MR apparatus is started. In this state of operation, the barrier valve V3 is closed and the vacuum pump is deactivated. The helium feed is connected directly to the neck tube. The gaseous helium is liquefied, and the liquid helium cools the MR magnetic arrangement. The liquefaction proceeds until the system is at thermodynamic equilibrium.

[0076] 2. The temperature sensor measures the current temperature T.sub.actual in the MR magnetic coil system.

[0077] 3. The temperature T.sub.actual is transmitted to the control unit, where it is compared in the processor unit with a defined temperature target value T1.sub.target, which is 8 K here. When T.sub.actual≥T1.sub.target, the control unit becomes active. The control unit opens the barrier valve V3 and activates the vacuum pump. An increase in temperature occurs when, for example, the compressor fails, and hence the cooling of the MR magnetic coil system. There is an increased input of heat from the environment into the MR magnetic coil system. However, this does not immediately result in heating of the MR magnetic coil system. First of all, the liquid helium evaporates, and hence the MR magnetic coil system is cooled. The evaporated helium is removed via a pressure release valve. In the event of a failure of the compressor for a prolonged period, however, it may be the case that the helium evaporates completely and the MR magnetic coil system is quenched and heats up. The heating releases gases in the vacuum vessel from the MR magnetic coil system. These gases worsen the vacuum, which in turn has an adverse effect on thermal insulation. There is a risk of excessively rapid and uncontrolled heating. In order to prevent this, the control unit becomes active at an early stage in order to open the barrier valve V3 and to activate the vacuum pump, in order that the vacuum in the vacuum vessel is maintained for as long as possible and hence the input of heat into the MR magnetic coil system can be reduced to such an extent that uncontrolled heating of the MR magnetic coil system is prevented.

[0078] 4. The pressure sensor measures the current pressure P.sub.actual of the vacuum vessel.

[0079] 5. The pressure P.sub.actual is transmitted to the control unit, where it is compared in the processor unit with a defined pressure target value P1.sub.target, which is 10.sup.−3 mbar here. When P.sub.actual<P1.sub.target, the control unit becomes active. The control unit activates the cold head for cooling of the cooling arm. The pressure target value P1.sub.target ensures that there is a sufficiently high vacuum and the vacuum vessel and hence the input of heat into the MR magnetic coil system is reduced. Once this value has been attained, the cold head is cooled down again in order to convert the MR apparatus back to normal operation.

[0080] 6. The temperature sensor measures the current temperature T.sub.actual in the MR magnetic coil system.

[0081] 7. The temperature T.sub.actual is transmitted to the control unit, where it is compared in the processor unit with the defined temperature target value T1.sub.target. When T.sub.actual<T1.sub.target, the control unit becomes active. The control unit closes the barrier valve V3 and deactivates the vacuum pump.

[0082] 8. The temperature sensor also measures the current temperature T.sub.actual in the MR magnetic coil system. The temperature T.sub.actual is transmitted to the control unit, where it is compared in the processing unit to a defined temperature target value T2.sub.target, which is 4.2 K here. When T.sub.actual≤T2.sub.target, the control unit becomes active. The control unit sends the message to the cold head to maintain the temperature.

[0083] 9. Gaseous helium is supplied again to the neck tube. The temperature T.sub.actual is sufficient to liquefy the gaseous helium. The liquid helium cools the MR magnetic coil system, and the liquefaction proceeds until the system is at thermodynamic equilibrium.

[0084] FIG. 3 shows a schematic flow diagram comprising the method steps according to the claims for embodiments with just one valve, and is discussed in the example below.

[0085] Start: The MR apparatus is put into operation.

[0086] a1: The current temperature T.sub.actual in the MR magnetic coil system is measured and compared with the definable first temperature target value T1.sub.target from which the vacuum vessel is to be pumped out.

[0087] a2: When T.sub.actual>T1.sub.target, the vacuum pump disposed outside the vacuum vessel is activated, and the first barrier valve in the conduit leading from the vacuum pump into the vacuum vessel is opened.

[0088] b1: The current pressure P.sub.actual in the vacuum vessel is measured and compared with a definable first pressure target value P1.sub.target at which the MR magnetic coil system is to be cooled down to cryogenic temperature.

[0089] b2: When P.sub.actual<P1.sub.target, the cold head for cooling of the cooling arm is activated.

[0090] c1: The current temperature T.sub.actual in the MR magnetic coil system is measured and compared with the first temperature target value T1.sub.target.

[0091] c2: When T.sub.actual<T1.sub.target, the first barrier valve is closed and the vacuum pump is switched off.

[0092] d1: The current temperature T.sub.actual in the MR magnetic coil system is measured and compared with the second temperature target value T2.sub.target at which the operating temperature of the MR magnetic coil system for MR measurements is attained. The second temperature target value T2.sub.target is maintained.

[0093] e: Helium is fed into the neck tube and liquefied.

[0094] The basic sequence of the cooling method of the invention in the form of an algorithm as is to proceed in a corresponding program in the control unit for embodiments with just one valve is shown as a flow diagram in FIG. 4 and discussed further below.

[0095] The algorithm can be divided into the following steps:

[0096] S1: The control unit is started.

[0097] S2: The temperature sensor of the MR magnetic coil system which is connected to the measurement unit measures the current temperature T.sub.actual.

[0098] S3: The temperature T.sub.actual is compared in the processor unit with the value T1.sub.target, and it is verified whether T.sub.actualT1.sub.target. If the test is negative, the algorithm is continued at S7. If the test is positive, the actuation unit activates the vacuum pump and opens the barrier valve V3.

[0099] S4: The actuation unit activates the vacuum pump and opens the barrier valve V3.

[0100] S5: The pressure sensor for the vacuum vessel which is connected to the measurement unit measures the current pressure P.sub.actual.

[0101] S6: The pressure P.sub.actual is compared in the processor unit with the value P1.sub.target, and it is verified whether P.sub.actual<P1.sub.target. If the test is negative, the pressure P.sub.actual is measured again (and the algorithm is continued at S5). If the test is positive, the actuation unit activates the cold head.

[0102] S7: The actuation unit activates the cold head.

[0103] S8: If necessary, the actuation unit closes the barrier valve V3 and deactivates the vacuum pump.

[0104] S9: The temperature sensor of the MR magnetic coil system which is connected to the measurement unit measures the current temperature T.sub.actual.

[0105] S10: The temperature T.sub.actual is compared in the processor unit with the value T2.sub.target, and it is verified whether T.sub.actual≥T2.sub.target. If the test is negative, the algorithm is continued at S3. If the test is positive, the actuation unit signals that the cold head is to maintain the temperature T2.sub.target.

[0106] S11: The actuation unit signals to the cold head that it should maintain the temperature T2.sub.target.

[0107] FIG. 5 shows a schematic of a particularly simple embodiment of a magnetic resonance apparatus 10 (MR apparatus) having three valves for implementation of the auto-cooling method of the invention.

[0108] The MR apparatus 10 comprises the vacuum vessel 11 containing the superconductive MR magnetic coil system 12. In MR analysis operation, this MR magnetic coil system 12 is dry, i.e cryogen-free. Cooling is effected by means of a cryostat 13 connected to the compressor 13a. The cryostat 13 comprises a neck tube 14 which is guided through the outer shell 15 of the vacuum vessel 11 to the MR magnetic coil system 12. The cooling arm 16 of the cold head 17 is disposed in the neck tube 14 of the cryostat 13. The cavity 18 is formed around the cooling arm 16. The cavity 18 is sealed in a fluid-tight manner with respect to the MR magnetic coil system 12 to be cooled. In the state of operation shown here, the cavity 18 is partly filled with cryogenic fluid 19 (e.g. liquid helium), and gaseous helium.

[0109] The vacuum pump 20 is disposed outside the vacuum vessel 11 and is designed here as a 2-stage vacuum pump 20. Vacuum pump 20 here comprises the turbomolecular pump 20a with the backing pump 20b (for example a membrane pump). The vacuum pump 20 is connected by the first barrier valve 21 (barrier valve V3) via the vacuum conduit 22 to the vacuum vessel 11. In addition, the vacuum pump 20 is connected by a second barrier valve 27 (barrier valve V2) via a connection 26 to the neck tube 14.

[0110] In the MR magnetic coil system 12, the temperature sensor 23 is present for measurement of a current temperature T.sub.actual in the MR magnetic coil system 12. The first pressure sensor 24, which is connected via the vacuum conduit 22 to the vacuum vessel 11, measures a current pressure P.sub.actual in the vacuum vessel 11. The second pressure sensor 25, which is connected via the connection 26 to the neck tube 14, measures a current pressure P.sub.HR in the neck tube. The control unit 40 detects and compares the current temperature T.sub.actual in the MR magnetic coil system 12 with defined temperature target values. In addition, the control unit 40 detects and compares the current pressure P.sub.actual in the vacuum vessel 11 with defined pressure target values. The control unit 40 is also configured such that it can actuate the first barrier valve 21, the vacuum pump 20 and the compressor 13a of the cold head 17.

[0111] The control unit 40 comprises the measurement unit, the actuation unit and the processor unit. Connected to the measurement unit is the temperature sensor 23, with which the current temperature T.sub.actual in the MR magnetic coil system 12 is measured. Additionally connected to the measurement unit is the first pressure sensor 24, with which the current pressure P.sub.actual in the vacuum vessel 11 is measured. Likewise connected to the measurement unit is the second pressure sensor 25, with which the current pressure P.sub.HR in the neck tube 14 is measured. The actuation unit is connected to the first barrier valve 21, the second barrier valve 27 and a third barrier valve 30 (barrier valve V1), which can open and close the actuation unit. Barrier valve 30 is integrated into connection 28 that connects the helium feed 29 and the neck tube 14. In addition, the actuation unit is connected to the vacuum pump 20 and via the compressor 13a to the cold head 17, which can activate and deactivate the actuation unit. The processor unit is disposed between the measurement unit and the actuation unit. The processor unit compares the parameters detected by the temperature sensor 23, the pressure sensor 24 and the pressure sensor 25 with the target parameters. These data are processed by the processor unit and utilized for control of the cold head 17, the vacuum pump 20 and the barrier valves 21, 27, 30.

[0112] The basic sequence of the cooling method of the invention for embodiments having three valves is shown as a flow diagram in FIG. 6, and discussed further below.

[0113] The sequence of the autonomous cooling method, in execution variant, may comprise the following steps:

[0114] 1′. The operation of the MR apparatus is started. In this state of operation, the barrier valves V1, V2 and V3 are closed and the vacuum pump is deactivated. The helium feed is connected to the neck tube via barrier valve V1. The gaseous helium is liquefied and the liquid helium cools the MR magnetic arrangement. The liquefaction proceeds until the system is at thermodynamic equilibrium.

[0115] 2′. The temperature sensor measures the current temperature T.sub.actual in the MR magnetic coil system.

[0116] 3′. The temperature T.sub.actual is transmitted to the control unit, where it is compared in the processing unit to a defined temperature target value T1.sub.target, which is 8 K here. When T.sub.actual≥T1.sub.target, the control unit becomes active. The control unit opens the barrier valve V3 and activates the vacuum pump.

[0117] 3.1′. At the same time, the processor unit compares the temperature T.sub.actual with a defined temperature target value T3.sub.target, which is 280 K here. When T.sub.actual>T3.sub.target, the control unit becomes active. The control unit opens the barrier valve V2 and activates the vacuum pump. An increase in temperature occurs when, for example, the compressor fails, and hence the cooling of the MR magnetic coil system. There is an increased input of heat from the environment into the MR magnetic coil system. However, this does not immediately result in heating of the MR magnetic coil system. The control unit becomes active when the temperature target value T3.sub.target is exceeded. The barrier valve V2 is opened and the helium gas is pumped out, as a result of which the MR magnetic coil system is actively cooled. In the event of a failure of the compressor for a prolonged period, however, it may be the case that the helium evaporates completely and the MR magnetic coil system is quenched and heats up. The heating releases gases in the vacuum vessel from the MR magnetic coil system. These gases worsen the vacuum, which in turn has an adverse effect on thermal insulation. There is a risk of excessively rapid and uncontrolled heating. In order to prevent this, the control unit becomes active at an early stage in order to open the barrier valve V3 and to activate the vacuum pump, in order that the vacuum in the vacuum vessel is maintained for as long as possible and hence the input of heat into the MR magnetic coil system can be reduced to such an extent that uncontrolled heating of the MR magnetic coil system is prevented.

[0118] 4′. The pressure sensor measures the current pressure P.sub.actual of the vacuum vessel.

[0119] 5′. The pressure P.sub.actual is transmitted to the control unit, where it is compared in the processing unit to a defined pressure target value P1.sub.target, which is 10.sup.−3 mbar here. When P.sub.actual<P1.sub.target, the control unit becomes active. The control unit activates the cold head for cooling of the cold arm. The pressure target value P1.sub.target ensures that a sufficiently large vacuum exists in the vacuum vessel, and hence the input of heat into the MR magnetic coil system is reduced. Once this value has been attained, the cold head is cooled down again in order to be able to put the MR apparatus back into normal operation.

[0120] 6′. The temperature sensor measures the current temperature T.sub.actual in the MR magnetic coil system.

[0121] 7′. The temperature T.sub.actual is transmitted to the control unit, where it is compared in the processor unit with the defined temperature target value T1.sub.target. When T.sub.actual<T1.sub.target, the control unit becomes active. The control unit closes the barrier valve V3 and deactivates the vacuum pump.

[0122] 8′. The temperature sensor also measures the current temperature T.sub.actual in the MR magnetic coil system. The temperature T.sub.actual is transmitted to the control unit, where it is compared in the processing unit to a defined temperature target value T2.sub.target, which is 4.2 K here. When T.sub.actual≤T2.sub.target, the control unit becomes active. The control unit sends the message to the cold head to maintain the temperature.

[0123] 9′. Barrier valve V2 is closed and barrier valve V1 is opened in order to supply gaseous helium to the neck tube again. The temperature T.sub.actual is sufficient to liquefy the gaseous helium. The liquid helium cools the MR magnetic arrangement, and the liquefaction proceeds until the system is at thermodynamic equilibrium.

[0124] 10′. Alternatively, in the event of a failure of the cold head, the control unit can reactivate the vacuum pump and open the barrier valve V2.

[0125] FIG. 7 shows a schematic flow diagram comprising the method steps according to the claims for embodiments having three valves, which are discussed further below.

[0126] Start: The MR apparatus is put into operation.

[0127] a1/a3: The current temperature T.sub.actual in the MR magnetic coil system is measured and compared with the definable first temperature target value T1.sub.target from which the vacuum vessel is to be pumped out. The current temperature T.sub.actual is also compared with the definable third temperature target value T3.sub.target.

[0128] a4: When T.sub.actual>T3.sub.target, the vacuum pump disposed outside the vacuum vessel is activated, and the second barrier valve V2 in the vacuum conduit leading from the vacuum pump to the neck tube is opened.

[0129] a2: When T.sub.actual>T1.sub.target, the vacuum pump disposed outside the vacuum vessel is activated, and the first barrier valve in the conduit leading from the vacuum pump into the vacuum vessel is opened.

[0130] b1: The current pressure P.sub.actual in the vacuum vessel is measured and compared with a definable first pressure target value P1.sub.target at which the MR magnetic coil system is to be cooled down to cryogenic temperature.

[0131] b2: When P.sub.actual<P1.sub.target, the cold head is activated for cooling of the cooling arm.

[0132] c1: The current temperature T.sub.actual in the MR magnetic coil system is measured and compared with the first temperature target value T1.sub.target.

[0133] c2: When T.sub.actual<T1.sub.target, the first barrier valve is closed in the vacuum pump is switched off.

[0134] d1: The current temperature T.sub.actual in the MR magnetic coil system is measured and compared with the second temperature target value T2.sub.target at which the operating temperature of the MR magnetic coil system for MR analyses has been attained. The second temperature target value T2.sub.target is maintained.

[0135] e: Helium is fed into the neck tube and liquefied.

[0136] d2: In the event of failure of the cold head, liquid helium in the neck tube is pumped away by the vacuum pump in order to cool the MR magnetic coil system.

[0137] The basic sequence of the cooling method of the invention in the form of an algorithm as is to proceed in a corresponding program in the control unit for embodiments having three valves is shown as a flow diagram in FIG. 8 and discussed further below.

[0138] The algorithm can be divided into the following steps:

[0139] S′1: The control unit is started.

[0140] S′2: The temperature sensor of the MR magnetic coil system which is connected to the measurement unit measures the current temperature T.sub.actual.

[0141] S′3: The temperature T.sub.actual is compared in the processor unit with the value T1.sub.target, and it is verified whether T.sub.actual≥T1.sub.target. If the test is negative, the algorithm is continued at S7′. If the test is positive, the actuation unit activates the vacuum pump and opens the barrier valve V3.

[0142] S′4: The actuation unit activates the vacuum pump and opens the barrier valve V3.

[0143] S′4.1: The temperature sensor of the MR magnetic coil system which is connected to the measurement unit measures the current temperature T.sub.actual.

[0144] S′4.2: The temperature T.sub.actual is compared in the processor unit with the value T3.sub.target, and it is verified whether T.sub.actual>T3.sub.target. If the test is positive, the actuation unit activates the vacuum pump and opens the barrier valve V2.

[0145] S5′: The pressure sensor for the vacuum vessel connected to the measurement unit measures the current pressure P.sub.actual.

[0146] S6′: The pressure P.sub.actual is compared in the processor unit with the value P1.sub.target, and it is verified whether P.sub.actual<P1.sub.target. If the test is negative, the pressure P.sub.actual is measured again (and the algorithm is continued at S5′). If the test is positive, the actuation unit activates the cold head.

[0147] S7′: The actuation unit activates the cold head.

[0148] S8′: If necessary, the actuation unit closes the barrier valve V3 and the barrier valve V2 and deactivates the vacuum pump.

[0149] S9′: The temperature sensor of the MR magnetic coil system which is connected to the measurement unit measures the current temperature T.sub.actual.

[0150] S10′: The temperature T.sub.actual is compared in the processor unit with the value T2.sub.target, and it is verified whether T.sub.actual≤T2.sub.target. If the test is negative, the algorithm is continued at S3′. If the test is positive, the algorithm is continued at S11′.

[0151] S11′: The processor unit checks whether, in normal operation, T2.sub.target=OK. If the test is positive, the algorithm is continued at S12′. If the test is negative, the algorithm is continued at S13′.

[0152] S12′: The actuation unit opens valve V1, and gaseous helium is guided into the neck tube and liquefied.

[0153] S13′: The actuation unit activates the vacuum pump and opens the barrier valve V2.