Coil energization apparatus and method of energizing a superconductive coil

A coil energizing apparatus has a superconducting energization power supply having an output port. The power supply is arranged to generate, when in use, a pulsed output current signal at the output port.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a coil energization apparatus of the type that, for example, is used to provide a superconductive coil with electrical current in order to achieve generation of a magnetic field, such as a static magnetic field. The present invention also relates to a method of energizing a superconductive coil of the type that, for example, is used to provide the superconductive coil with electrical current in order to achieve generation of a magnetic field, such as a static magnetic field.

2. Description of the Prior Art

In the field of nuclear Magnetic Resonance Imaging (MRI), a magnetic resonance imaging system typically comprises a superconducting magnet, a gradient coil system, field coils, shim coils and a patient table. The superconducting magnet is provided in order to generate a strong uniform static magnetic field, known as the B0field, in order to polarize nuclear spins in an object under test.

Presently, it is known to make the coils forming the superconducting magnet from metals that exhibit the property of superconduction at very low temperatures. To achieve superconduction, the superconducting magnet is therefore cooled to the very low temperatures. One known cryogen-cooled superconducting magnet unit includes a cryostat including a cryogen vessel. A cooled superconducting magnet is provided within the cryogen vessel, the cryogen vessel being retained within an outer vacuum chamber (OVC). One or more thermal radiation shields are provided in a vacuum space between the cryogen vessel and the OVC. In some known arrangements, a refrigerator is mounted in a refrigerator sock located in a turret towards the side of the cryostat, the refrigerator being provided for the purpose of maintaining the temperature of a cryogen provided in the cryogen vessel. The refrigerator also serves sometimes to cool one of the radiation shields. The refrigerator can be a two-stage refrigerator, a first cooling stage being thermally linked to the radiation shield in order to provide cooling to a first temperature, typically in the region of 80-100K. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K.

During manufacture of the superconducting magnet, at maintenance intervals and/or when installing the superconducting magnet, it is necessary to energize the superconducting magnet to generate the static magnetic field mentioned above, typically using a Direct Current (DC) power supply. The power supply is therefore connected to the magnet coils forming the superconducting magnet via a so-called “Current Lead”. The Current Lead is a pair of conductors, approximately 1 meter long, one end of which is at room temperature while the other is at about 4.2K. The design of the Current Lead is constrained by mutually exclusive properties of the Current Lead: electrical resistance and thermal conductivity. Consequently, design of the superconducting magnet is constrained by limitations associated with the Current Lead in the following ways.

Firstly, the Current Lead limits the maximum current at which the superconducting magnet can operate. In this respect, as the static magnetic field generated by the superconducting magnet is a function of the operating current of the superconducting magnet and/or the number of turns of superconducting wire used to form the coils, it therefore follows that if the superconducting magnet is energized using the maximum current allowed by the Current Lead, the number of turns of superconducting wire used must be relied upon in order to achieve a desired static field strength from the superconducting magnet. Reliance on the number of turns of superconducting wire used is, however, undesirable because the superconducting wire is costly and manufacturing process time is, in part, a function of the number of turns of the coils of the superconducting magnet that has to be provided during manufacture.

Furthermore, when the Current Lead is a so-called fixed Current Lead, i.e. a Current Lead that remains in place after the superconducting magnet has been energized, heat leak is known to occur through the persistent Current Leads. Since the maximum allowable heat leak from the superconducting magnet unit is limited by the refrigeration power of the refrigerator, the more heat leak that occurs through the fixed Current Lead, the less heat leak margin that is available for other parts of the superconducting magnet unit. From a design perspective, the amount of heat leak that can therefore be tolerated from other parts of the superconducting magnet unit, for example suspension components of the superconducting magnet unit, is limited. The use of more thermally efficient materials for the other parts of the superconducting magnet unit is therefore necessitated and this is costly, because whilst, in some cases, the cost of the materials is low, complexity of fitting the materials can be high.

It is also known to use a so-called demountable Current Lead, i.e. a Current Lead that is only connected to the superconducting magnet during charging. An advantage of using the demountable Current Lead is that heat leak is minimized. However, the minimization of the heat leak is at the expense of providing access to a connector arrangement to connect the demountable Current Leads to the superconducting magnet and hence the need to expose the superconducting magnet to atmosphere. Consequently, problems arise when trying to connect the demountable Current Lead to the superconducting magnet due to formation of ice in the connector arrangement provided. Formation of ice within the superconducting magnet unit as a result of the superconducting magnet being exposed to atmosphere is an even greater and particularly costly problem associated with the use of the demountable Current Lead, because the ice formed influences interaction between the coils of the superconductive magnet and a supporting former of the coils resulting in the superconductive magnet being unable to achieve a desired operating field. In more extreme cases, the ice can block the exit of the cryogen with undesirable effects.

Use of High-Temperature Superconductor (HTS) Current Leads has also been suggested as an alternative solution for providing leads to enable the superconducting magnet to be energized, the HTS Current Lead remaining in place after charging the superconducting magnet. In this respect, the cross-sectional area of an HTS Current Lead is less than the respective cross-sectional areas of the conventional fixed and demountable Current Leads described above. Consequently, the HTS Current Leads exhibit less heat leak when in use. However, as the HTS Current Leads are formed from a superconducting material, the use of the HTS Current Leads introduces a risk of the HTS material forming the Current Lead “quenching”, if the current flowing through the HTS Current Leads results in the so-called “critical surface” (characterizing the superconducting material from which the HTS Current Leads are formed) being reached or exceeded. When quenching occurs in one or both of the HTS Current Leads, the energy used to energize the superconducting magnet is dissipated as heat.

In this respect, any heating in the HTS Current Lead (or indeed the non-superconductive leads described above) can result in heating of the superconductive magnet and so part of the superconductive magnet can cease to exhibit the property of superconduction and ohmic heating in the superconducting magnet takes place. The ohmic heating causes so-called “boil-off” of the cryogen used. Boil-off is undesirable as the cryogen is wasted if no recovery system is in place, the cryogen being an expensive commodity.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a coil energization apparatus for a superconducting magnet, and a method for energizing such a coil, wherein energization efficiency is improved.

This object is achieved in accordance with the present invention by a method and arrangement for introducing current into a superconducting magnet having a superconducting switch connected across terminals of the superconducting magnet, wherein a controlled switch is connected across the superconducting switch, the controlled switch, the controlled switch being configured to respond more rapidly to control signals than the superconducting switch. The controlled switch is controlled to be in an OFF state, and a progressively increasing continuous current is supplied to the terminals of the superconducting magnet, until a first limiting value is reached. A pulsed current is then applied to the terminals of the superconducting magnet, the pulses of the pulsed current being of a larger magnitude than the first limiting value. The controlled switch is controlled to be in an ON state when current is not flowing through the terminals. Once a desired current is flowing in the superconducting magnet, application of the pulsed current ceases, and the controlled switch is maintained in an ON state until the superconducting switch becomes superconducting. The controlled switch is then controlled to an OFF state, so that the current in the magnet flows through the current leads. The current flowing through the current leads is reduced so that an increasing current flows through the superconducting switch.

According to the present invention, a coil energization apparatus, a superconductive magnet apparatus and a method of energizing a coil are provided that exhibit improved energization efficiency, resulting in less current loss during energization of the coil, for example of a low-temperature superconductive magnet. Also, an operating current of the magnet is increased during the energization process and hence less superconducting wire is required to form coils that maintain the same B0field strength, thereby reducing materials costs and process time associated with manufacture of the superconductive magnet. Furthermore, it is possible to reduce the cross-sectional area of the current leads used to energize the magnet and so reduce heat leak through the current leads employed, thereby making use of fixed Current Leads more attractive. The availability of a greater heat leak margin thus also permits the use of cheaper, less thermally efficient, materials for certain parts of the superconductive magnet, for example the suspension components of the superconductive magnet. Additionally, it is possible to reduce current loss between completion of energization of the superconducting magnet and efficient conduction by a cryogenic switch. Also, by provision of the switching device in parallel with the superconductive magnet, it is possible to protect a Current Lead employed under various fault conditions, for example a power failure while energizing the superconductive magnet, without the need to quench the superconductive magnet.

At least one embodiment of the invention will now be described, as an example only, with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Throughout the following description identical reference numerals will be used to identify like parts.

Referring toFIG. 1, a tomography system, for example a Magnetic Resonance Imaging (MRI) system, comprises a superconductive magnet unit100having a cryostat including a cryogen vessel102. A cooled superconductive magnet104is provided within the cryogen vessel102, the cryogen vessel102being retained within an Outer Vacuum Chamber (OVC)106. One or more thermal radiation shields108are provided in a vacuum space between the cryogen vessel102and the OVC106. In this example, a refrigerator110is mounted in a refrigerator sock112located in a turret114towards the side of the cryostat, the refrigerator110being provided for the purpose of maintaining the temperature of a cryogen provided in the cryogen vessel102. An access turret116is provided at the top of the cryostat, the access turret116retaining a vent tube118that serves as an access neck. The refrigerator110is, in this example, a two-stage refrigerator, a first cooling stage being thermally linked to the radiation shield108in order to provide cooling to a first temperature, typically in the region of 80-100K. A second cooling stage provides cooling of the cryogen to a much lower temperature, typically in the region of 4-10K. The cryogen used in this example is helium, but the skilled person will appreciate that other suitable cryogens can be employed.

A negative electrical connection120is provided to the superconductive magnet104through the body of the cryostat. A positive electrical connection122, only schematically represented inFIG. 1, is provided by a conductor passing through the vent tube118. In this example, a demountable Current Lead (not shown inFIG. 1) is used to couple a Direct Current (DC) power supply unit (also not shown inFIG. 1) to the negative and positive connections120,122. For fixed Current Lead designs, a separate, auxiliary, vent path (not shown inFIG. 1) is provided in the cryostat as a fail-safe vent in case of blockage of the vent tube118.

A gradient coil system is also provided and comprises, in this example, three paired orthogonal gradient coils124disposed within the superconductive magnet104in order to produce gradient magnetic fields when in use.

Referring toFIG. 2, a current flow control circuit apparatus200comprises a first positive lead terminal202and a first negative lead terminal204, a first terminal of a bidirectional diode pack206being coupled to the first positive lead terminal202via the positive electrical connection122described above and a second terminal of the diode pack206being coupled to the first negative lead terminal204via the negative electrical connection120described above. The diode pack206is provided to protect a cryogenic switch208. As the manner of protection provided by the diode pack206is known in the art, the protection provided need not be described further herein.

In this example, the cryogenic switch208is a heating element, or a plurality of series-coupled heating elements, disposed adjacent a coil formed from a superconducting material, for example a Niobium-Titanium alloy. The skilled person should, however, appreciate that any suitable arrangement can be provided to serve as the cryogenic switch208and capable of fulfilling the function associated with the cryogenic switch208described later herein. The cryogenic switch208has a latency, or delayed response, associated with actuation of the cryogenic switch208and, depending upon the arrangement employed, can be perceived as a response-delayed selective conduction device. In this example, the cryogenic switch208is an example of a temperature-dependent switching device.

Coils of the superconductive magnet104are coupled to the current flow control circuit apparatus200. In this respect, a first terminal of the superconductive magnet104is coupled to the first positive lead terminal202and a second terminal of the superconductive magnet104is coupled to the first negative lead terminal204. The cryogenic switch208is coupled in parallel with the superconductive magnet104, and so a first terminal of the cryogenic switch208is coupled to the first positive lead terminal202and a second terminal of the cryogenic switch208is coupled to the first negative lead terminal204.

Additionally, a Field Effect Transistor (FET) device210is coupled in parallel with the cryogenic switch208and so a first conduction terminal of the FET210is coupled to the first positive lead terminal202and a second conduction terminal of the FET210is coupled to the first negative lead terminal204. FET210is schematically represented as a switch inFIG. 2. The skilled person should appreciate that although use of a field effect transistor is described herein, any suitable threshold-dependent conduction device can be employed and that the field effect transistor is one example of the threshold-dependent conduction device arrangement. Indeed, any suitable switching device can be employed, for example any suitable solid state device, the field effect transistor being one example of the solid state device. In this example, the FET210is implemented by way of 100 so-called “D2 Pak” (10 mm×15 mm) surface mount FETs on an A4 sized Printed Circuit Board (PCB). Implemented in this exemplary manner, the FET210typically drops 10 mV at 1000 A, dissipating 10 W of energy. Hence, energy loss is relatively small and acceptable on a temporary basis.

A processing resource, for example a controller212, is operably coupled to a respective control terminal (not shown) of the cryogenic switch208and a respective control terminal, for example a gate terminal, of the FET210in order to coordinate, for example synchronize, actuation of the cryogenic switch208and/or the FET210in the manner described herein. In this example, a single controller is employed. However, the controller is simply an example of the processing resource and other suitable arrangements can be used. Indeed, control of the cryogenic switch208and the FET210need not be provided by a single functional element and responsibility for control of the cryogenic switch208and the FET210can be distributed over any appropriate number of processing elements.

The demountable Current Lead214comprises a first conductor216and a second conductor218, a first end of the first conductor216being coupleable to the first positive lead terminal202and a first end of the second conductor218being coupleable to the first negative lead terminal204. Although not shown, a coupling, for example a plug and socket arrangement, is provided in order to achieve temporary coupling of the demountable Current Lead214to the lead terminals202,204. It should be appreciated that reference herein to “conductors” in relation to Current Leads should not necessarily be interpreted as implying any particular degree of flexibility. Indeed, the “conductors” can be, for example, rigid, resiliently deformable, or flexible to any necessary degree.

A second end of the first conductor216is coupled to a positive output terminal220of the power supply unit222and a second end of the second conductor218is coupled to negative output terminal224of the power supply unit222. Again, although not shown, another coupling, for example another plug and socket arrangement, is provided in order to achieve temporary coupling of the demountable Current Lead214to the output terminals220,224of the power supply unit222.

In operation, the demountable Current Lead214is coupled at the second end thereof to the output terminals220,224of the power supply unit222and at the first end thereof to the positive and negative lead terminals202,204in order to couple electrically the power supply unit222to the current flow control circuit apparatus200and the superconductive magnet104.

Turning toFIG. 3, in particularFIGS. 3(a) and3(b), the power supply222, by virtue of a current source (not shown) thereof, provides an output current signal300over an energization period for energizing the superconductive magnet104. InFIG. 3(a), Vpsu shows the output voltage of the power supply unit222, and Vmag shows the voltage across the magnet coils. InFIG. 3(b), Ipsu shows the current supplied by the power supply unit, and Icryo shows the current flowing through the cryogenic switch208. In this respect, from an initial current of 0 A, the output current signal is continuously incremented at a nominal voltage302, for example between about 2 volts and about 20 volts, such as about 10 volts, over a first duration304in order that the output current signal constitutes a continuous incrementing current signal306until a maximum operating current of the demountable Current Lead214is reached. The maximum operating current of a given Current Lead is dictated by a number of factors, including electrical conductivity of the material from which the given Current Lead is formed, cross sectional area of the given Current Lead, and temperature coefficient of the material, a thermal time constant of the Current Lead being a function of the material from which the Current Lead is formed and the cross-sectional area of the Current Lead. The maximum operating current of the Current Lead is rated by the manufacturer of the demountable Current Lead214and so the power supply unit222can be configured by an engineer, charged with the task of energizing the superconductive magnet104, to respect the maximum operating current of the Current Lead214. The process of supplying electrical current to the coils of the superconductive magnet104in this manner is known as “ramping”.

During the first duration304, the FET210is in an “off” state (FIG. 3(d)), i.e. is not actuated, and the cryogenic switch208is open (FIG. 3(e)), i.e. constitutes a resistance in the circuit, and so does not conduct electricity. Consequently, the output current signal serves to energize the coils of the superconductive magnet104.

However, once the maximum operating current of the Current Lead214has been reached, the output current signal is pulsed (FIG. 3(b)) by the power supply unit222over a second duration308in order that the output current signal constitutes a pulsed output current signal310. In this example, each pulse has a respective duration of 1 second, though the duration can be longer provided the duration is less than the thermal time constant of the Current Lead. A duty cycle of the pulse output current signal310is controlled by the power supply unit222so that a Root Mean Square (RMS) value of the pulsed output current signal310does not exceed a maximum RMS operating current of the Current Lead214. The duty cycle and amplitude of the pulses generated by the PSU222is configurable through the provision of suitable controls (not shown). Those skilled in the art will appreciate that the RMS value of the operating current is only one example of a predetermined average operating parameter not to be exceeded. In this respect, the predetermined average operating parameter can be a predetermined average operating current and/or correspond to a mean power dissipation in the Current Lead.

During the second duration308, the cryogenic switch208remains open. During periods when the power supply unit222is not providing current, for example between pulses, the FET210is actuated (FIG. 3(d)) so that the FET210conducts electricity in order to maintain the flow of electrical current around the coils of the superconductive magnet104, thereby minimizing loss of charge by the superconductive magnet104.

To further demonstrate the principle of controlling the RMS operating current with respect to the Current Lead214, over a third duration312, the amplitude of the current pulses is increased, but the duty cycle of the output current signal is reduced in order to ensure that the RMS value of the output current signal does not exceed the maximum RMS operating current of the Current Lead214. In this example, pulses, 1 second in duration, are separated by a duration of 3 seconds.

Again, during the third duration312, the cryogenic switch208remains open, but during periods when the power supply unit222is not providing current, for example between pulses, the FET210is actuated so that the FET conducts electricity in order to maintain the flow of electrical current around the coils of the superconductive magnet104, thereby minimizing loss of charge by the superconductive magnet104. Since the inter-pulse duration has been increased, the duration of the periods when the FET210is in the conducting state is correspondingly increased.

For illustrative purposes, a fourth duration314ofFIG. 3contains a single 1 second pulse316having a rising edge318and corresponding rise-time of 0.1 second. The single pulse316also has a falling edge320having a corresponding fall-time of 0.1 second. The single pulse316is an example of the pulses that can be employed in the pulsed output current signal310, which comprises a plurality of pulses.

Once an operating current of the superconductive magnet104is reached, for example between about 300 A and about 700 A, the operating current corresponding to the superconductive magnet104being capable of generating, in this example, a desired static magnetic field for tomography purposes, the FET210is actuated so that the FET210is in a conductive state322over a temporary period326. Closure324of the cryogenic switch208is then, subsequently, commenced. While the cryogenic switch208is a more efficient device than the FET210for closing the circuit of the superconductive magnet104in order to maintain the flow of electrical current around the coils of the superconductive magnet104until energization of the superconductive magnet104is next required, the cryogenic switch208does not conduct instantaneously. Nevertheless, the conduction of the FET210serves as a temporary measure and closes the circuit of the coils of the superconductive magnet104whilst the cryogenic switch208cools over an associated cooling period328, the cryogenic switch208reaching an optimum level of efficiency, i.e. when superconductivity is exhibited, when cooled to a temperature at which the superconductive material from which part of the cryogenic switch208is formed is known to be superconductive. The temporary period326commences before and overlaps with the cooling period328. By virtue of the temporary provision of a conductive path through the FET210whilst the cryogenic switch208is cooling, loss of charge by the superconductive magnet104is minimized. Once the cryogenic switch208is closed, the time taken to close being known from a performance characteristic of the cryogenic switch208, the power supply unit222supplies a current resulting in the current flowing through the FET210reducing to 0 A and hence the voltage across the FET210and the cryogenic switch208is 0V. The current supplied by the power supply unit222is then decreased over a fifth period330, during which the current in the superconducting magnet104increasingly flows through the cryogenic switch208. Once the current provided by the power supply unit reaches 0 A, the power supply unit222is disconnected from superconductive magnet104and removed.

Although in the above examples reference has been made to a demountable Current Lead, those skilled in the art will appreciate that use of other types of electrical conductor is contemplated, for example a permanent Current Lead.