Patent Description:
This invention was made with Government support under Grant Number <NUM>-<NUM>-<NUM> awarded by the National Science Foundation. The Government has certain rights in the invention.

This invention relates generally to high temperature superconducting (HTS) magnets and particularly to quench protection of HTS magnets.

High temperature conducting (HTS) magnets, such as HTS magnets formed from rare earth barium copper oxide (REBCO), bismuth strontium calcium copper oxide (BSCCO), yttrium barium copper oxide (YBCO) and magnesium diboride (MgB<NUM>), are designed to produce high magnetic fields to store large amounts of magnetic energy during operation. However, the stored energy in the HTS magnet may subject the magnet to a failure mechanism referred to as a "quench", in which the stored energy is suddenly converted into heat accompanied by the presence of large electrical voltages. A quench typically occurs in an HTS magnet when a conductor transitions from the superconducting state to the normal state in some region of one of the magnet coils. In the normal state, the non-superconducting region of the conductor exhibits an increasingly large electrical resistance, relative to the super-conducting state, resulting in excessive heating of the magnet. The excess temperature and voltage in the windings of the HTS magnet cause by a quench condition can potentially damage the magnet. In the event of a quench condition in a large superconducting magnet, the current in the superconducting magnet must be rapidly reduced to prevent damage to the magnet resulting from a peak hot-spot temperature or from a mechanical strain. Any method to reduce the magnet current by altering the magnet current through external means, such as a resistor, would involve large voltages, which is undesirable. Alternatively, by warming a large portion of the superconductor to above critical temperature (Tc), the resulting resistance may be distributed throughout the superconducting device, which can dramatically reduce the peak voltages experienced by a quenching coil.

It is known in the art to integrate a set of embedded heaters into the magnet windings to warm the superconductor, thereby distributing the thermal energy throughout the magnet. Additionally, a method is known in the art which relies on AC coupling, commonly referred to as Coupling Loss Induced Quench (CLIQ). In CLIQ, the magnetic energy is distributed by driving an imbalance in the transport current between two or more sections of the magnet. While these known techniques are adequate for low temperature superconducting magnets, they provide marginal quench protection for recently developed high temperature superconductors and are not effective in rapidly distributing a large amount of thermal energy to the windings of an HTS magnet. Additionally, in CLIQ based quench protection systems, to increase the available energy, either the capacitance of the system must be increased, which slows down the frequency, or the voltage must be increased, which results in safety concerns. Protection of HTS magnets for reliable operation has proven to be a challenge, particularly in Rare Earth Barium Copper Oxide (REBCO) superconductors, due to the large amount of energy that is required to get enough of the current into the metallic stabilizer to properly distribute the magnetic energy and to minimize peak hot-spot temperatures during a quench condition.

Accordingly, what is needed in the art is an improved quench protection system for high temperature superconductors (HTS).

<CIT> discloses an apparatus for quenching at least part of a superconductor in a superconducting state in reply to a quench signal to initiate a transition from the superconducting state into a normal-conducting state comprises: means for providing an alternating (AC) current of a predetermined strength and/or predetermined frequency to the at least part of the superconductor, wherein the means for providing the AC current comprises a control terminal configured to receive the quench signal. The means for providing the AC current is configured to be activated in response of receiving the quench signal at the control terminal so that the AC current flows through the at least part of the superconductor, wherein the predetermined strength and/or the predetermined frequency is selected such that the transition from the superconducting state into a normal-conducting state is triggered.

<CIT> discloses a passive superconductive sensor for quench detection in a superconducting coil. The sensor consists of a first circuit electrically connected to two voltage taps of superconducting coil. The first circuit uses nonsuperconducting components and preferably contains a resistive element. The first circuit is magnetically coupled to a second circuit by a hybrid transformer. The second circuit is superconducting and contains a readout coil and preferably a second hybrid transformer. The second hybrid transformer is magnetically coupled to a sense coil which detects flux changes in the superconducting coil. The readout coil is coupled to a readout device which measures changes in the current through the readout coil. The current in the readout coil can be made a function only of the resistance of the superconducting coil between the voltage turns and thus can be used to detect a quench.

<CIT> discloses a superconducting coil having at least two connections therebetween. A current source connected to the superconducting wires to form a loop via the superconducting wires and the connection to supply a current in the loop when a quench is detected. A superconducting magnet includes the superconducting coil, a persistent current switch connected to the superconducting coil, and a quench detector configured to detect quench occurring in the superconducting coil.

<CIT> discloses a coil system for inductively heating a superconducting magnet in order to provide an internal energy dump by uniformly quenching a high performance superconducting magnet. The quench-inducing system uses AC magnetic fields that require negligible reactive power. The system is especially suited for inducing a relatively uniform quench in dry superconducting magnets.

<CIT> discloses a magnet that can be used to guide a particle beam in a specific direction. It consists of two sets of coils that work together to create a magnetic field inside the magnet, which helps control the path of the particle beam. The first and second set of coils may be configured to generate a combined desired magnetic field within the bore and may be configured to generate a combined magnetic field weaker than the desired magnetic field outside the bore.

In various embodiments, the present invention provides an improved quench protection system for high temperature superconducting (HTS) magnet coils.

In one embodiment, the present invention provides a method for controlling a quench in a high temperature superconductor (HTS) magnet coil which includes, coupling a current imbalance source to at least one coil subsection of an HTS magnet coil, wherein the HTS magnet coil comprises a plurality of coil subsections, and operating the current imbalance source to induce a current imbalance in the at least one coil subsection of the HTS magnet coil to establish a high frequency change in a magnetic field of the HTS magnet coil resulting in inductive heating of the HTS magnet coil to control a quench in the HTS magnet coil.

In various embodiments, the current imbalance source of the present invention may be external to the HTS magnet coil or internal to the HTS magnet coil and the method may be applied to HTS magnet coils having insulated conductors and uninsulated conductors.

The invention is directed to a method for controlling a quench in a high temperature superconductor as set forth in claim <NUM>.

According to the claimed invention, the current imbalance source in a first alternative includes an alternating current (AC) voltage source and a capacitive element coupled in series with the AC voltage source, wherein the AC voltage source and the capacitive element are coupled across the at least one coil subsection of the HTS magnet coil.

According to the claimed invention, the current imbalance source in a second alternative includes a high impedance device, a capacitive element coupled in series with the high impedance device and a switch coupled across the high impedance device and the capacitive element, wherein the high impedance device, the capacitive element and the switch are coupled across the at least one coil subsection of the HTS magnet coil.

In another embodiment not being part of the claimed invention, the current imbalance source may include an alternating current (AC) voltage source, a capacitive element coupled in series with the AC voltage source and a coil element, wherein the AC voltage source and the capacitive element are inductively coupled to the at least one coil subsection of the HTS magnet coil through the coil element.

The present invention as set forth in claim <NUM> further provides a system for controlling a quench in a high temperature superconductor (HTS) magnet coil. The quench protection system includes a current.

imbalance source coupled to at least one coil subsection of an HTS magnet coil, wherein the HTS magnet coil comprises a plurality of coil subsections and quench actuation circuitry for operating the current imbalance source to induce a current imbalance in the at least one coil subsection of the HTS magnet coil to establish high frequency change in a magnetic field of the HTS magnet coil resulting in inductive heating of the HTS magnet coil to control a quench in the HTS magnet coil.

The quench protection system may further include a quench detection circuitry for detecting a quench condition in the HTS magnet coil prior to operating the current imbalance source. In a particular embodiment, the quench detection circuitry may include a non-contact current sensing transducer.

Accordingly, the present invention provides an improved system and method for controlling a quench condition in an HTS magnet coil.

For a fuller understanding of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which:.

The key to quench protection is to prevent the degradation of the HTS coil by limiting localized temperature growth. In various embodiments, the present invention provides a protection system capable of safely quenching an HTS coil. The protection circuit design of the present invention advances the protection technology for HTS magnet coils and is capable of quickly distributing the heat energy uniformly in all the coil sections when a localized hot-spot is formed.

In accordance with the present invention, by placing a current imbalance source on a subsection of the HTS coil, a high frequency imbalance current can be driven into the coil subsection, whereby the alternating imbalance current drives a high frequency change in the magnetic field of the HTS coil, thereby depositing thermal energy into the conductor of the HTS coil through several mechanisms. Through the proper placement of one or more capacitors, the coil subsections that are not currently connected to the voltage source will be exposed to the same rapid change in the magnetic field, but <NUM>° out of phase. A current measurement device, that can measure the imbalance current, controls the switch and the state of the imbalance voltage inducing the current imbalance changes when the imbalance current approaches zero.

In one embodiment, the source of the current imbalance may be external. As shown in <FIG>, a system for providing quench protection in an HTS coil <NUM> includes an external current imbalance source <NUM> which includes an alternating current (AC) voltage source <NUM> in series with a capacitive element <NUM>. The HTS magnet comprises of a plurality of coil subsections <NUM>, <NUM> and includes a magnet power supply <NUM> coupled across the plurality of coil subsections <NUM>, <NUM>. The current imbalance source <NUM> of the present invention is positioned across a first coil subsection <NUM> of the HTS coil. In operation, when a quench is detected in one of the subsections of the HTS coil, the current imbalance source <NUM> is actuated to induce a current imbalance in the first coil subsection <NUM> of the HTS magnet coil, thereby establishing a high frequency change in a magnetic field of the HTS magnet coil which results in inductive heating of the HTS magnet coil subsections <NUM>, <NUM> to control the quench in the HTS magnet coil.

In another embodiment, as shown in <FIG>, a system for providing quench protection in an HTS coil <NUM> includes an internal current imbalance source <NUM> which includes a high impedance device <NUM> in series with a capacitive element <NUM> and an imbalance switch <NUM> across the high impedance device <NUM>. The HTS magnet comprises of a plurality of coil subsections <NUM>, <NUM> and includes a magnet power supply <NUM> coupled across the plurality of coil subsections <NUM>, <NUM>. In this embodiment, the source of the current imbalance is provided by placing the high impedance device <NUM> on one of the leads of the HTS magnet and rapidly running the transport current through the high impedance device <NUM> or by a rapidly switching short using the imbalance switch <NUM>. In operation, when a quench is detected in one of the subsections of the HTS coil, the current imbalance source <NUM> is actuated to induce a current imbalance in the first coil subsection <NUM> of the HTS magnet coil, thereby establishing a high frequency change in a magnetic field of the HTS magnet coil which results in inductive heating of the HTS magnet coil subsections <NUM>, <NUM> to control the quench in the HTS magnet coil. In this embodiment, the high impedance device <NUM> comprising a plurality of diodes in series, however this is not intended to be limiting and our means for providing a high impedance device are within the scope of the present invention.

In the embodiment illustrated in <FIG> and <FIG>, the conductor of the HTS coils are wound with insulation, thereby allowing the capacitor to resonate in series with the capacitive element. Due to internal shorting, HTS coils that are not wound with insulation cannot resonate in series with a capacitor.

However, as shown in the embodiment of <FIG> (not claimed embodiment), by inductively coupling an external pulse coil <NUM> to the HTS magnet subsections <NUM>, <NUM>, large amounts of thermal energy can be deposited directly into the windings (coil subsections) <NUM>, <NUM> of the HTS magnet to provide a quench protection system <NUM>. The HTS magnet comprises of a plurality of coil subsections <NUM>, <NUM> and includes a magnet power supply <NUM> coupled across the plurality of coil subsections <NUM>, <NUM>. In this embodiment, the imbalance current source <NUM> includes an AC voltage source <NUM> in series with a capacitive element <NUM> that is coupled across an external pulse coil <NUM>. In operation, when a quench is detected in one of the subsections of the HTS coil, the current imbalance source <NUM> is actuated to inductively induce a current imbalance in the coil subsections <NUM>, <NUM> of the HTS magnet coil through the external pulse coil <NUM>, thereby establishing a high frequency change in a magnetic field of the HTS magnet coil which results in inductive heating of the HTS magnet coil subsections <NUM>, <NUM> to control the quench in the HTS magnet coil.

In general, the present invention provides a quench protection system implementing a Frequency Loss Induced Quench (FLIQ) which relies on AC losses to distribute the magnetic energy in the HTS magnet and to minimize peak hot-spot temperatures in the HTS magnet coil during a quench condition.

The FLIQ system of the present invention utilizes the heat generated by AC losses in the high temperature superconductor magnet windings, which is deliberately achieved by applying electrical current oscillating at the frequency of the load. The FLIQ system uses induction heating methods, which change the local current distribution and magnetic fields in the HTS magnet coil. Based on Faraday's law, changes in current induce voltages caused by the created magnetic field. The change in magnetic field results in both eddy currents and magnetization losses in the HTS magnet, which increases the temperature of the superconductor. With eddy current losses, the deposited thermal energy decreases, as penetration depth increases, due to the fact that the induced currents do not have an ohmic loss component in the superconductor, assuming the currents do not exceed the local critical current, Ic. The penetration depth is dependent upon the resistance of the metallic stabilizer on the superconductor and the frequency. A higher frequency is desirable for the quench protection system and penetration depth of the induced currents is given by: <MAT>.

Where, ω=<NUM>π, σ= conductivity, µ= magnetic permeability and f= frequency.

In a specific embodiment of the quench protection system of the present invention, the design characteristics of the FLIQ system include a remote trigger using transistor-transistor logic (TTL), adjustable activation time, adjustable frequency, zero-crossing capability, current feedback control and self-oscillation.

In an exemplary hardware design of the system, the FLIQ device consists of different sections that support the generation of AC loss in the HTS magnet coil. In one embodiment, FLIQ device includes four Insulated Gate Bipolar Transistors (IGBTs) connected in an H-bridge circuit and a driving circuit. A capacitor is also connected at the center of the H-bridge circuit, in series with the HTS coil. The H-bridge mechanism was chosen because it allows the flow of current in two directions and supports the zero switching capability which generates the signal used to control to the gate driver input. Switching of the H-bridge generates the energy needed to quench the superconducting coil. The FLIQ system optimizes frequency, as a result of the current resonating at the frequency of the inductive/capacitive (LC) network across the H-bridge.

The system and method for quench protection includes circuitry <NUM> that further detects a quench condition in the HTS coil. As shown in <FIG>, in one embodiment, quench detection circuitry may include a non-contact current sensing transducer <NUM> that uses the Hall Effect measuring principle to measure the current across the load. This circuitry may be effective over a sensing range between -<NUM> A to +<NUM> A, which further improves the reliability of the FLIQ system due to its wide measurement range.

In various embodiments, the current transducer <NUM> is powered between 0V ground <NUM> and +<NUM> V supply <NUM>.

Capacitors <NUM>, <NUM>, <NUM> are connected across the input and output terminals of the current sensor to further filter unregulated signals. The output voltage <NUM> and the reference voltage <NUM> are supplied to a comparator (not shown). The comparator compares the output voltage <NUM> of the current sensor to the reference voltage <NUM> and produces an output that serves as the input signal to the gate drivers. A diode may be connected to the output of the comparator in order to prevent backward flow of current or high voltage surges from other components to the comparator and invariably protect the current sensor as well.

The quench protection system further comprises quench actuation circuitry <NUM> to provide actuation of the FLIQ system via a remote Transistor-Transistor Logic (TTL) signal <NUM>, as shown in <FIG>. The pulse <NUM> has a maximum output threshold to determine when the signal is high and a minimum output threshold level to determine when the signal is low. The <NUM> V Zener diode <NUM> acts as a transient suppressor in case an overvoltage is supplied. The Zener <NUM> swings into a breakdown mode of operation and instantly clamps the overvoltage to a safe level to limit the voltage to ≤<NUM> V signal. An optoisolator <NUM> is connected between the supplied TTL signal <NUM> and an enable signal <NUM> to the gate driver. The optoisolator <NUM> protects and electrically isolate the supplied voltage from the rest of the circuit in case an unregulated high voltage transient is supplied. The <NUM> kΩ resistor <NUM> connected across the enable signal <NUM> makes the system active only when the enable signal <NUM> is high because the enable input of the gate driver has a <NUM> kΩ resistor drawn to VDD (voltage supply).

In a particular embodiment, the quench protection system of the present invention further comprises an H-bridge circuit <NUM> for providing the high frequency and high current power needed to drive the superconducting coil, as shown in <FIG>. The H-bridge <NUM> can be divided into two-half sections, the top half of the bridge has two IGBTs labelled IGBT <NUM> <NUM> and IGBT <NUM><NUM> and the bottom half of the bridge has IGBT <NUM><NUM> and IGBT <NUM><NUM>. In the H-bridge circuit <NUM>, current flows in two directions across the bridge. IGBT <NUM> <NUM> and IGBT <NUM> <NUM> are controlled with the inverting gate driver <NUM> and IGBT <NUM> <NUM> and IGBT <NUM> <NUM> are controlled with the non-inverting gate driver <NUM>. This is to ensure that only two adjacent IGBTs are powered at the same time, as voltage is applied in two directions. Based on <FIG>, none of the top IGBTs <NUM>, <NUM> or the bottom IGBTs <NUM>, <NUM> should be powered at the same, otherwise the entire system will be shorted.

The gate drivers <NUM>, <NUM> are integrated circuits that has an enable input that is supplied from the remote TTL signal and the input signal is supplied from the comparator output, which is a function of the current sensed across the superconducting magnet and capacitor.

The output of the gate drivers supplies the gate-emitter voltage(VGE) needed to control which of the IGBTs <NUM>, <NUM>, <NUM>, <NUM> are operating at any given time. The primary function of the gate drive circuit is to convert logic level control signals into the appropriate voltage and current for efficient, reliable, switching of the IGBT module. The VGE for IGBT <NUM> <NUM> and IGBT <NUM> <NUM> is supplied through an optocoupler, thereby creating a voltage difference or floating potential to the gate and emitter terminal of the top two IGBTs <NUM>, <NUM>.

The output of the H-bridge <NUM> is coupled to the HTS coil <NUM> and the frequency of the load is based on the equation: <MAT>.

An exemplary embodiment of the quench protection system <NUM> of the present invention includes a REBCO (<NUM>-<NUM>) coil in liquid Nitrogen at a temperature of <NUM>. As shown in the block diagram in <FIG> (not claimed embodiment), the superconducting coil <NUM> and a capacitor <NUM> are connected in series across the centre of the H-bridge <NUM>. In this exemplary embodiment, a fuse <NUM> limits the supplied current to its current rating. When testing the FLIQ system for the first time, a dump resistor (Rdump) <NUM> was used to act as a preventive protection method to further ensure the safe quenching of the coil <NUM>. The dump resistor <NUM> is connected across the LC load to redirect current in a fault condition. In this exemplary embodiment, the system <NUM> was operated at a frequency of <NUM> produced by an external signal generator. In this embodiment, a DC current is supplied to the load via the two switches <NUM>, <NUM> with a <NUM> Ohms resistor <NUM> connected across the DC current supply <NUM> to sense the imbalance. The sense resistor <NUM> is used to measure the FLIQ current across the H-bridge <NUM>. In this embodiment, when quench is detected, the direction of current flow across the H-bridge <NUM> generates the imbalance in the transport current of the coil <NUM>, thereby safely controlling the quench in the coil <NUM>.

VAC, <NUM> ADC at a temperature of <NUM>. The graph in <FIG> shows that the voltage across the HTS coil oscillates at the frequency of the current and that the voltage across the top half and the bottom half of the coil are opposite to each other.

In various embodiments, the present invention provides a protection system capable of safely quenching an HTS coil. The protection circuit design advances the protection technology for high temperature superconducting magnet coils and provides a protection system that is capable of quickly distributing the heat energy uniformly in all the coil sections when a localized hot-spot forms. The experimental validation of the performance of the circuit demonstrates that the circuit causes a safe quench in a model coil.

The present invention may be embodied on various computing platforms that perform actions responsive to software-based instructions. The following provides an antecedent basis for the information technology that may be utilized to enable the invention.

The computer readable medium described in the claims below may be a computer readable signal medium or a computer readable storage medium.

Program code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wire-line, optical fiber cable, radio frequency, etc., or any suitable combination of the foregoing. Computer program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, C#, C++ or the like and conventional procedural programming languages, such as the "C" programming language or similar programming languages.

Aspects of the present invention are described below with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention.

It will be seen that the advantages set forth above, and those made apparent from the foregoing description, are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claim 1:
A method for controlling a quench in a high temperature superconductor, HTS, magnet coil, the method comprising:
coupling a current imbalance source to at least one coil subsection of an HTS magnet coil, wherein the HTS magnet coil comprises a plurality of coil subsections (<NUM>, <NUM>; <NUM>, <NUM>), wherein the HTS magnet coil comprises an insulated conductor, and wherein the current imbalance source comprises an alternating current (AC) voltage source or a high impedance device, and wherein the current imbalance source further comprises a capacitive element coupled in series with the AC voltage source or the high impedance device, and wherein the AC voltage source or the high impedance device and the capacitive element are coupled across the at least one coil subsection of the HTS magnet coil; and
operating the current imbalance source to induce a current imbalance in the at least one coil subsection of the HTS magnet coil to establish a high frequency change in a magnetic field of the HTS magnet coil resulting in inductive heating of the HTS magnet coil to control a quench in the HTS magnet coil;
wherein the HTS magnet coil comprises a magnet power supply (<NUM>; <NUM>) coupled across the plurality of coil subsections (<NUM>, <NUM>; <NUM>, <NUM>), and
wherein the HTS magnet coil comprises an insulated conductor, and
wherein the current imbalance source comprises: an alternating current, AC, voltage source (<NUM>); and a capacitive element (<NUM>) coupled in series with the AC voltage source, wherein the AC voltage source and the capacitive element are coupled across the at least one coil subsection (<NUM>) of the HTS magnet coil;
or
wherein the current imbalance source comprises: an high impedance device (<NUM>); a capacitive element (<NUM>) coupled in series with the high impedance device; and a switch (<NUM>) coupled across the high impedance device and the capacitive element, wherein the high impedance device, the capacitive element and the switch are coupled across the at least one coil subsection (<NUM>) of the HTS magnet coil, the high impedance device (<NUM>) being placed on one of the leads of the HTS magnet power supply (<NUM>) leading to the at least one coil subsection.