SUPERCONDUCTING QUICK SWITCH

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to magnet systems, and in particular to a non-

persistent switch for use with a superconducting magnet.

2. Discussion of the Related Art

As is well known, a magnet can be made superconducting by placing it in an

extremely cold environment, such as by enclosing it in a cryostat or pressure vessel

containing liquid helium or other cryogen. The extreme cold reduces the resistance in the

magnet coils to negligible levels. After the power source that is initially connected to the coil

is removed, the current will continue to flow through the magnet coils relatively unimpeded

by the negligible resistance, thereby maintaining a magnetic field.

To maintain current flow in the magnet coils after removal of power, it is typically

necessary to complete the electric circuit within the cryogenic environment with a

superconducting switch that is connected in parallel with the power supply and the magnet

coils. The superconducting switch generally consists of a superconducting conductor, which

when driven into the non-superconducting or normal state, has sufficient resistance so that

current from the power supply will essentially flow through the magnet coils during "ramp-

up." When the desired magnetic field current is achieved, the switch is returned to its

- superconducting state and the magnet current commutates out of the power supply and

through the switch when the power supply is ramped down. The magnet is now in what is

referred to as "persistent mode."

There are four characteristics that a superconducting switch typically exhibits. One, it

must be capable of easily and quickly being transformed (switched) from the superconducting

state to the normal state, and vice versa. Three ways this can be done are: a) thermally— by

heating the superconducting material above its transition temperature; b) magnetically— by

applying a magnetic field greater than the critical field of the material; or c) electrically— by

raising the current in the material above its critical current. The thermal method is the most

common. Two, it must have a high enough resistance in its normal state such that current

flow through the switch during ramp-up is negligible so that excessive heating in the cryogen

environment is not produced. Three, the switch must be stable. That is, other than during a

desired transition phase, it must not transition from the superconducting to normal state.

Four, it must be capable of carrying the same high currents as the magnet coils.

Conventional persistent switches of the thermal type operate by heating the

superconducting material to a temperature above its superconducting critical temperature.

One known thermal persistent switch includes a resistive wire wound about the

superconducting wire. Normalization of the superconducting material of the switch is

effected by applying electrical current to the resistive wire, thereby heating the

superconducting material to above its critical temperature. One of the challenges in

designing a superconducting switch is to balance the conflicting requirements of minimizing

transition time between a superconducting state and a resistive state, and the need for low

heat output to minimize cryogen boil-off.

SUMMARY OF THE INVENTION

In accordance with an embodiment, a magnet system for generating a magnetic field

includes a superconducting magnet, a switch, and a heater element thermally coupled to the

switch. The superconducting magnet is structured to generate magnetic fields, and the switch

includes a non-inductive superconducting current carrying path connected in parallel to the

superconducting magnet. In general, the switch is structured to only carry a level of current

that is a portion of the current required to obtain a full field by the superconducting magnet.

BRIEF DESCRIPTION OF THE DRAWING

The above and other aspects, features, and advantages of the present invention will

become more apparent upon consideration of the following description of preferred

embodiments, taken in conjunction with the accompanying drawing figures, wherein:

FIG. 1 is an electrical schematic diagram of a magnet system in accordance with an

embodiment of the present invention;

FIG. 2 is an electrical schematic diagram of a magnet system in accordance with an

alternative embodiment of the present invention;

FIG. 3 is a flowchart showing exemplary operations for generating magnetic fields in

accordance with an embodiment of the invention; and

FIG. 4 is an electrical schematic diagram of an alternative embodiment of a magnet

system implementing a protective element.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following detailed description, reference is made to the accompanying drawing

figures which form a part hereof, and which show by way of illustration specific

embodiments of the invention. It is to be understood by those of ordinary skill in this

technological field that other embodiments may be utilized, and structural, electrical, as well

as procedural changes may be made without departing from the scope of the present

invention. As a matter of convenience, various components of a magnet system and

associated superconducting switch will be described using exemplary materials, sizes, shapes,

and dimensions. However, the present invention is not limited to the stated examples and

other configurations are possible and within the teachings of the present disclosure.

Referring now to FIG. 1, an electrical schematic diagram of an embodiment of a

magnet system of the present invention is shown. In particular, magnet system 10 is shown

having switch assembly 15 and superconducting magnet 20. The switch assembly includes

switch 25, which is in electrical communication with the magnet, and thermally coupled to

heater element 30. Optional radio frequency (RF) shield 35 is shown positioned relative to

the switch and. heater elements, and effectively reduces the RF coupling between these

elements. The various components of the switch assembly are shown contained within

housing 40.

Current supply 45 may be used to supply electrical current to magnet 20, and heater

power source 50 provides electrical current to heater element 30. In an embodiment, the

magnet and various components of the switch assembly may be located in a suitably cooled

environment, such as vessel 55, which enables the superconducting properties of the magnet and switch to be exploited.

Vessel 55 may be implemented using any suitable container or structure designed to

contain liquid helium or other cryogen. Housing 40 may be formed from materials that are

not electrically conductive, and is typically used to contain the various components of switch

assembly 15. In embodiments in which the housing and included components are subjected

to a cryogenic environment, such as that illustrated in FIG. 1, the housing may additionally

include thermal materials. Such thermal materials are structured to inhibit heat transfer, from

switch 25 and heater element 30, to the surrounding cryogen contained with the vessel.

Reducing heat transfer to the cryogen reduces costly boil-off during the magnet charging and

discharging processes (discussed in detail below).

In an embodiment, magnet 20 and switch 25 may include coils wound from a suitable

superconducting wire formed from, for example, NbTi, Nb ₃ Sn, and the like. The magnet is

generally capable of providing a range of magnetic fields, as controlled by current supplied

by current supply 45, and operating in conjunction with the switch.

Switch 25 includes a non-inductive current carrying path connected in parallel to the

magnet. Optimally, the switch is structured to only carry a level of current that is a portion

(that is, less than 100%) of the current required by magnet 20 to obtain a full field. By way of

non-limiting example, the switch maybe structured so that it is capable of only carrying a

level of current that is about 1% -20%, or more preferably about 2%-7%, of the current

required by magnet 20 to obtain a full field.

In general, the superconducting wire used to form magnet 20 may have a diameter

that ranges from about 25μm - 125μm, to about six inches, or more. The magnet may be

implemented using conventional magnet technologies, the specifics of which are not essential to the present invention. Switch 25 may be formed from superconducting wire (for example,

non-clad, bifilar wound wire) having a diameter that provides the above-described current

carrying levels. There is generally no minimum wire diameter required for switch 25. In a

typical embodiment, the superconducting wire used for switch 25 has a diameter of about

5μm - 125μm, but greater diameters are possible.

With further reference to FIG. 1, operation of a magnet system in accordance with an

embodiment of the present invention will now be described. Initially, the cryogen contained

within vessel 55 cools magnet 20 and switch 25 so that they are in a superconducting state.

At this point, the magnet system may generate a magnetic field in accordance with the level

of current supplied by current source 45.

At some point, a change in the magnetic field generated by the magnet system is

desired. This change in magnetic field may be accomplished by heater power source 50

supplying current to heater element 30, thereby heating switch 25 to above its

superconducting critical temperature. Once the critical temperature has been reached, the

switch transitions from a superconducting state (closed state) to a non-superconducting

resistive state (open state). When the switch reaches the resistive state, current supply 45 may modify or otherwise change the amount of current supplied to magnet 20.

When the supplied electrical current reaches a particular or desired current value, as

determined by the desired field to be produced by the magnet, power from the heater power

source is turned off. This allows heater element 30, and consequently switch 25, to cool. As

the switch cools, it falls below the superconducting critical temperature and transitions back

to the superconducting state (closed state).

In contrast to systems that utilize persistent switches, the current supplied by current supply 45 is not removed from magnet 20 after the switch transitions back to the

superconducting state. Instead, current supply 45 maintains current to the magnet, thereby

producing a stable field. Power to the magnet must generally be maintained since switch 25

is not designed to sustain the magnet in the persistent mode. This is because the switch

cannot carry the full field current level of the magnet. Without maintaining the current

supply to the magnet, the generated magnetic fields would decay, and the magnet would

eventually demagnetize. Note that it is possible that continually applying current to the

magnet may affect the resultant field because of current-induced noise. However, switch 25

shorts out any noise that would otherwise be introduced into the magnet.

The field produced by magnet 20 may again be changed by essentially repeating the

above-described operations. For instance, heater power source 50 may again supply current

to heater element 30, causing switch 25 to be heated to above its superconducting critical

temperature. While the switch is in the resistive state, the current supplied to magnet 20 by

current supply 45 is changed according to the desired field to be generated. When the

supplied electrical current reaches the desired value, the heater power source is turned off,

and the temperature of the switch falls below the superconducting critical temperature.

Switch 25 ultimately transitions back to the superconducting state (closed state). Again,

current supply 45 continues to provide current to the magnet.

Benefits that may be realized by the various switches disclosed herein include

relatively faster charge times and decreased cryogen boil-off. Overall magnet charge time

may be improved by reducing the amount of time required for transitioning the switch

between superconducting and resistive states. Switch 25 experiences transition times that are

significantly lower than those possible by a conventional persistent switch, for example.

Switch 25 experiences transition times (from either the cooled or the heated state) on the

order of 0.5 - 1.5 seconds. These transition times are possible because of the relatively small

size of the wire that forms the switch.

Another reason for faster charge time is because switch 25 may be implemented with

a higher resistance than typically present in a conventional persistent switch. For example, in

an embodiment, switch 25 may have a resistance between about 60 ohms - 500 ohms, or

higher. This higher switch resistance allows for higher charge voltage to be applied to the

magnet during a charge phase. Higher charge voltage translates to a decrease in charge time

to achieve a desired current level in the magnet.

Minimizing cryogen boil-off in a magnet system is also desirable since it is a costly

and time-consuming process to maintain cryogen in the containment vessel. Boil-off occurs

as a result of the heat generated by the switch heating device, such as heating element 30.

Boil-off also occurs during the magnet charging phase since switch 25 is in a resistive (non-

superconducting) state during this phase. When the switch is in the resistive state, there is a

voltage across the switch. This voltage generates heat, which consequently results in the undesirable cryogen boil-off.

The amount of boil-off generated by switch 25 may be reduced, as compared to

conventional persistent switches, for several reasons. First, switch 25 is typically much

smaller than conventional persistent switches, thereby requiring less heat for the switch to

reach the superconducting critical temperature. Less heat translates to decrease boil-off.

Furthermore, the decreased charge time minimizes the length of time that the switch remains

in the resistive state. This reduces the length of time that there is a voltage across the switch,

which reduces the amount of generated heat and corresponding boil-off.

Another benefit provided by switch 25 is that the connection requirements between

the switch and magnet 20 are not as strict, as compared the requirements of conventional

persistent switches. In general, conventional persistent switches require care in being

connected to a magnet since excessive amounts of resistance resulting from this connection

would be undesirable. However, the present invention does not have any such requirements

and higher resistance caused by the switch-to-magnet connection can be factored into the

overall resistance of the switch.

In accordance with an embodiment, an additional advantage relates to the relatively

higher resistance of switch 25. For instance, when charging a typical magnet, any current

flowing in the switch may represent a reduction in the true field. Such measurements may be

inferred by monitoring the current in the leads. However, it is somewhat difficult to ascertain

the effective resistance of a warm persistent switch, so this effect cannot be compensated for

very accurately. Switch 25 minimizes this problem in proportion to its higher resistance.

To illustrate the various benefits provided by an embodiment of the switch and

magnet system of the present invention, the following is presented. A magnet system

operating with a conventional persistent switch is compared with the same system operating

with switch 25. For both types of switches, magnet 20 is ramped from 0 tesla - 9 tesla,

stopping every 0.01 tesla. A magnet system utilizing the conventional persistent switch was

operated in known fashion. The resistance of the conventional persistent switch is 30 ohms,

and the power provided by the heater power source is 75 mW (35 mA across 60 ohms). For

both setups, magnet 20 had a charging voltage of 5 V, an inductivity of 10 henries (H), and a

current at 9 tesla of 50 A.

Operation of the magnet system utilizing switch 25 is as follows. Each time the ramping process is stopped, switch 25 is heated so that it transitions from a superconducting

state (closed state) to a resistive state (open state). Current supply 45 then supplies additional

current to magnet 20 until the current in the magnet reaches the desired value. Then the

current supplied to heater element 30 is shut off, and the switch is allowed to cool,

transitioning back to the superconducting state (closed state). Once again, current supply 45

maintains current to the magnet, even after the switch has reached the superconducting state.

Stopping the ramping process permits measurements to be taken at the generated field. In

this scenario, 900 separate measurements were taken. By way of non-limiting example, the

resistance of switch 25 is 250 ohms, and the power provided by heater power source 50 is 20

mW (30 mA across 20 ohms).

Table 1 below provides an example of measuring time for both a conventional

persistent switch and switch 25. More specifically, Table I depicts the time necessary for

ramping the magnet to the desired magnet field, the time for opening and closing the switch

(that is, the time it takes for the switch to transition from a superconducting state (closed

state), to a resistive state (open state), and back to a superconducting state (closed state)), and

the total time necessary for obtaining 900 separate measurements. Note that the illustrated times are approximate.

Table 1

The- foregoing results illustrate that the above-described switch, according to

embodiments of the invention, allows for significantly faster measurement times. As noted

in this table, a typical persistent switch may take about 60 seconds for opening and closing.

This results in a total time of about 900 minutes for obtaining 900 measurements. In such a

scenario, switch 25 performs over 50 times faster than a conventional persistent switch.

As noted above, the various embodiments of the switch and magnet systems of the

present invention also produce reduced amounts of cryogen boil-off, as compared to

conventional persistent switches. Table 2 below provides an example of various parameters

relating to the boil-off of liquid Helium for both types of switches. The same switch setup, as

described above in conjunction with Table 1, was used for the data for Table 2.

Table 2

These results illustrate the significant savings that may be realized in terms of the

amount of energy required to operate switch 25. This energy savings translates to a reduction

of liquid Helium boil-off.

FIG. 2 is an electrical schematic diagram of an alternative embodiment of a magnet

system. In this figure, magnet system 100 includes switch assembly 15 and superconducting

magnet 20 positioned within isolation vacuum 105. Cooler 1 10 may be used to cool switch

25 and magnet 20 to a desired superconducting temperature. In particular, the cooler is

shown having thermal link 1 15, which is in thermal contact with the switch, and thermal link

120, which is in thermal contact with the magnet. Cooler 1 10 maybe implemented using

known cooling systems, such as a compressed gas cooler, which can provide the necessary

cooling to the magnet and switch to make these elements superconducting. Isolation vacuum

105 is a structure typically used to thermally isolate the various components contained within

the isolation vacuum from the outside environment.

Operation of magnet system 100 may occur as follows. Initially, cooler 110 cools

magnet 20 and switch 25 so that they are in a superconducting state. At this point, the

magnet system may generate a magnetic field in accordance with the level of current supplied

by current source 45. As before, the power supplied by the current supply is not removed

from magnet 20 while the magnet generates the magnetic field.

At some point, a change in the magnetic field generated by the magnet system is

desired. This change in magnetic field may be accomplished in a manner similar to that

described in conjunction with FIG. 1. That is, switch 25 maybe heated to above its

superconducting critical temperature. When the switch reaches the resistive state, current

supply 45 may modify or otherwise change the amount of current supplied to magnet 20.

When the supplied electrical current reaches a particular or desired current value, as

determined by the desired field to be produced by the magnet, the switch is allowed to cool

and fall below the superconducting critical temperature and transition back to the

superconducting state (closed state). As before, the power supplied by current supply 45 is

not removed from magnet 20 after the switch transitions back to the superconducting state.

Current supply 45 typically maintains current to the magnet, thereby producing a stable

magnetic field. The field produced by magnet 20 may be changed by essentially repeating the

above-described operations.

Switch 25 has been described as being formed from superconducting wire. However,

this is not a requirement and other techniques and structures that are capable of providing a

non-inductive current carrying path may alternatively or additionally be used. For instance,

switch 25 may be implemented using a device containing integrated circuitry such that the

current carrying path includes a thin-film current carrying path.

The various magnet systems disclosed herein include a single magnet 20 and a single

switch assembly 15. However, a magnet system having a plurality of magnets, each having a

separate switch assembly, is also possible and within the teachings of the present disclosure.

Various embodiments have been disclosed in which a superconducting magnet has

been utilized to generate a desired field. It is to be understood that such a magnet may be

implemented with one or more superconducting coils, or with one or more solenoids.

FIG. 3 is a flowchart showing exemplary operations for generating magnetic fields in

accordance with an embodiment of the invention. Block 300 includes maintaining electrical

current supplied to a superconducting magnet, which may be structured to generate magnetic fields. If desired, the magnetic fields generated by the superconducting magnet may be

changed according the operations of blocks 305, 310, and 315. For instance, at block 305, a

non-persistent switch may be heated to a critical temperature. Typically, such a non-

persistent switch operates in a superconducting mode and is connected in parallel to the

superconducting magnet. Such heating causes the non-persistent switch to transition to a

non-superconducting mode. At block 310, electrical current provided to the superconducting

magnet may be changed to generate a desired magnetic field. Next, the switch may be

allowed to cool below the critical temperature, thus causing the switch to transition back to

the superconducting mode (block 315). If desired, operations of blocks 300, 305, 310, and

315 may be repeated with different electrical current values to generate correspondingly different magnetic fields.

Although various processes and methods according to embodiments of the present

invention maybe implemented using the series of operations shown in FIG. 3, those of

ordinary skill in the art will realize that additional or fewer operations may be performed.

Moreover, it is to be understood that the order of operations shown in FIG. 3 is merely

exemplary and that no single order of operation is required.

It is possible that the switch may be damaged as a result of a critical event such as, for

example, a quench or a sudden or unexpected loss of power to the magnet current supply.

During a quench, the magnet may generate high internal voltages and locally elevated

temperatures. This causes electrical and mechanical stresses in the windings, and may also

damage the switch. A quench may occur for a variety of reasons. For example, the magnet

system may suffer a loss of cooling power because of insufficient amounts of cryogen in the

vessel, or a failure in' the active cooler.

Regardless of the cause of the critical event, a potentially large voltage may develop

across the magnet and switch since these elements are connected in parallel. Since the switch

is typically implemented so that it only carries a fraction of the magnet current while

superconducting, and has a high resistance in its normal state, a relatively large voltage across

the switch may result in a large amount of power being dissipated in the switch. This could

cause significant damage to the switch.

To prevent or minimize damage to the switch as a result of a critical event, such as

those described above, the magnet system may be implemented with a suitable protective

element, device, or circuit. For example, FIG. 4 is an electrical schematic diagram of an

alternative embodiment of a magnet system implementing a protective element. In this figure, magnet system 400 includes many of the same components as system 10 of FIG. 1.

However, magnet system 400 includes protective element 405, which is electrically

connected in parallel to magnet 20 and switch 25.

One purpose of the protective element is to limit the power being dissipated through

the switch in case of a failure or critical event, such as those described above. In particular,

the protective element may limit the maximum voltage across the switch. The protective

element may be implemented using, for example, a pair of diodes such as those depicted in

FIG. 4. Operation of magnet system 400 occurs in a manner similar to that of the system of

FIG. 1, but would of course have the added protection provided by protection element 405.

Note that any of the various switch and magnet systems presented herein may also be

configured with one or more protective elements.

While the invention has been described in detail with reference to disclosed

embodiments, various modifications within the scope of the invention will be apparent to

those of ordinary skill in this technological field. It is to be appreciated that features

described with respect to one embodiment typically may be applied to other embodiments.

Therefore, the invention properly is to be construed only with reference to the claims.