Method and device for detecting a resistive transition zone in a superconductor

Abstract

Device for detecting a resistive transition zone in a superconductor (2) provided with a cooling system (3) which maintains the superconductor (2) at a temperature lower than the critical temperature comprising: an emitter device (5) configured to inject a pulse train T (x) into the superconductor (2); a first detector (7) for detecting signals resulting from the reflection of the pulse train T (x) in the superconductive medium of the superconductor (2); the first detector (7) is coupled to a first end (2-a) of the superconductor; a second detector (9) for detecting signals resulting from the refraction of the pulse train T (x) in the superconductive medium; the second detector (9) is coupled to a second end (2-b) of the superconductor (2); a processing unit (12) configured to detect the formation of a resistive transition zone in the superconductor (2) when : the first sensor (7) detects signals SR(x) resulting from the reflection of the pulse train T (x) in the superconductive medium; and / or the second sensor (9) detects signals SF(X) resulting from the refraction of the pulse train T (x) in the superconductive medium.

Description

[0001] 'Method, and device for detecting a resistive transition zone in a superconductor '

[0002] Cross-Reference to Related Applications

[0003] This Patent Application claims priority from Italian Patent Application No . 102024000018394 filed on August 5 , 2024 , the entire disclosure of which is incorporated herein by reference .

[0004] Technical Field

[0005] The present invention relates to a method and a device for detecting a resistive transition zone in a superconductor .

[0006] As is known, superconductivity is a physical phenomenon which results in null electrical resistance in a conductor .

[0007] The electrical resistance of a metallic conductor gradually decreases as the temperature decreases and, below a certain temperature , which depends on the impurities and defects in the conductor, the resistance does not further decrease . Therefore , even near absolute zero , metal conductors exhibit electrical resistance , albeit of small value .

[0008] In superconductors , the resistance value instead suddenly completely vanishes when the critical temperature for superconduction is reached and the magnetic field in the conductor is lower than a limit value ; the resistance value remains zero even i f it falls below the critical temperature .

[0009] Therefore , an electric current can flow indefinitely in a closed circuit in a superconductor without any generator to power it . This is why superconductors can be used to make powerful electromagnets since , even at very high currents , the heat dissipated by the Joule ef fect in the electromagnet is negligible with respect to that in a normal conductor . It is believed that in superconductors , the current consists of pairs of electrons called Cooper pairs . The coupling between the electrons is due to the fact that the electrons exchange phonons which cause an attraction therebetween .

[0010] However, it is necessary to keep the temperature of the superconductor below the critical temperature for the entire length of the superconductor itsel f .

[0011] When such a condition is not met in one zone of the superconductor, the resistance has a non-zero value and therefore the temperature in such a zone increases due to the Joule ef fect . Based on the high currents carried, the appearance of the Joule ef fect is a dangerous phenomenon which easily leads to the melting and destruction of the superconductor .

[0012] It is therefore necessary to detect in a timely manner the zone of the superconductor where the resistive transition is occurring in order to immediately interrupt the current in the superconductor and prevent the destruction of the conductor itsel f .

[0013] The known techniques for detecting resistive transition zone formation are as follows : a) use of voltage taps : a plurality of voltage taps are arranged along the superconductor and the voltage present between pairs of taps is measured . Under normal operating conditions , the voltage between a pair of taps is always zero because the resistance is indeed zero ; i f , however, a non- zero voltage is detected, this indicates the beginning of a resistive transition and the current is immediately interrupted . However, such a technique has a number of drawbacks and limitations . For example , potential drops induced by external electromotive forces can be produced, e . g . , the presence of plasma in the vicinity i f the superconductor belongs to a magnet used for plasma confinement . To prevent such false positives , this technique is also aided by the insertion of temperature sensors which measure the temperature of the cable coolant or superconductive magnet in real time . b ) Quench detection with fibre optics . Fibre optics arranged parallel to and in contact with the superconductor are used to detect local temperature variations ( approximation around a metre ) . The fibre is not af fected by external electromotive forces and therefore the presence of plasma does not alter the temperature measurement . However, the fibre is arranged in direct contact with the superconductive element and is subj ect , like the superconductor, to very high stresses due to Lorenz forces , and the fibre can easily be damaged by such stresses . Furthermore , in the field of fusion, superconductors are constantly bombarded by neutron fluxes which af fect the fibres with an obvious derating of the measurement associated with the fibre , which must be suitably shielded . c ) Quench detection with magnetic field measurement : in this case , field sensors arranged along the superconductor are used and configured to measure the magnetic field of the superconductor in real time , which under normal operating conditions is stationary and has a small value . When a resistive transition occurs , the magnetic field value also increases abruptly and such a change is used to control the interruption of the current through the conductor . Even this technique , however, suf fers from the fact that the magnetic fields , especially in fusion machines , can be deformed by external causes , resulting in a false positive or readings from magnetic field sensors which are not perfectly aligned with what is actually happening to the superconductor .

[0014] Finally, it should be pointed out that the techniques a ) , b ) and c ) , illustrated above , all require the insertion of external sensors into the superconductive conductor . This introduces a series of construction di f ficulties related to the insertion of the sensors , which - especially in the field of fus ion - can give rise to secondary problems such as a lack of electrical insulation or shielding from neutron radiation, or the breakage of the sensors themselves during measurements due to the strong Lorenz currents .

[0015] Furthermore , the variation of magnetic fields within a fusion machine induces a whole series of eddy currents on metallic and conductive elements of the sensors . This leads to a number of issues to be solved, ranging from electrical insulation problems to noise cancellation in measurements .

[0016] The need is therefore felt for a technical solution which does not require the use of the external elements illustrated above (voltage taps , fibre optics , magnetic field sensors ) and, at the same time , allows rapid detection of the resistive transition in the superconductor .

[0017] Patent EP3602577B1 describes quench detection in superconductor magnets .

[0018] US2005286180A1 de scribes a system and method for the protection of a superconductor from quenches . The quench protection system comprises a voltage detector capable of detecting the voltage across the superconductor . The system further comprises a frequency filter coupled to the voltage detector . The frequency filter is able to couple voltage signals to a control circuit which are representative of an increase in superconductor voltage caused by a quench condition and block voltage signals which are not . The system can detect i f a quench condition exists in the superconductor based on the voltage signal received by means of the frequency filter and initiate a protective action in response .

[0019] Solution of the Problem

[0020] The previous aim is achieved by the present invention in that it relates to a device for detecting a resistive transition zone in a superconductor of the type described in claim 1 .

[0021] The present invention further relates to a method for detecting a resistive transition zone in a superconductor of the type described in claim 6 .

[0022] Brief Description of the Drawings

[0023] The invention will now be described with reference to the accompanying drawings which represent a non-limiting embodiment thereof in which :

[0024] Figure 1 schematically illustrates a device for detecting a resistive transition zone in a superconductor made according to the dictates of the present invention;

[0025] Figure 2 illustrates a physical phenomenon underlying the operation of the device of Figure 1 ; and

[0026] Figure 3 illustrates an electromagnet using a superconductor . Preferred embodiment example

[0027] With reference to Figure 1 , the numeral 1 indicates , schematically, a device for detecting a resistive transition zone in a superconductor .

[0028] The superconductor 2 is schematically indicated by the number 2 and is of a known type , e . g . , a metal compound of the niobium-tin and niobium-titanium type . However, it is clear that the superconductor can be made from di f ferent metal compounds , for example : niobium-3-tin, magnesium diboride and iron selenide .

[0029] The superconductor 2 is provided with a cooling system 3 also of known type which keeps the superconductor 2 in a cryogenic condition at a temperature lower than the critical temperature . For example , the cooling system uses liquid helium at -269 degrees Celsius .

[0030] Typically, the superconductor 2 belongs to a magnet ( illustrated in Figure 3 ) which can also be used in a Tokamak for nuclear fusion (partially illustrated in Figure 3 ) .

[0031] The device 1 comprises an emitter device 5 : in this case a pulse generator coaxial to the superconductor in question is configured to inj ect a voltage pulse train T (x) having a predefined waveform, frequency and amplitude . For example , the amplitude of the pulse train T (x) can vary between 1 and 10 mV and have a frequency comprised between 1 and 10 MHz .

[0032] The emitter device 5 is physically connected to the superconductor cable with superconductive couplings , not illustrated in the figures . The superconductive couplings are configured to connect the cable coming from the emitter device 5 with the superconductive cable or belt which must be kept under cryogenic conditions .

[0033] The pulse train T (x) travels the entire length of the superconductor 2 from a first end 2-a to which the emitter device 5 is coupled to a second opposite end 2 -b with a speed which is equal to the propagation speed of the electric field in the superconductive medium, which is close to the speed of light .

[0034] The device 1 further comprises a first detector 7 for detecting signals SR (X) resulting from the reflection of the always coaxial pulse train T (x) in the superconductive medium . Preferably, the first detector 7 is coupled to the first end 2 -a and can be integrated with the emitter device 5 and be a transceiver .

[0035] The device 1 further comprises a second detector 9 for detecting signals SF (X) resulting from the refraction of the pulse train T (x) in the superconductive medium . Preferably, the second detector 9 is coupled to the second end 2-b by means of a coupling identical to the first so that the first and second detectors 7 , 9 are coupled to opposite ends of the superconductor 2 . Typically, the ends 2-a and 2-b are connected with a current generator 10 ( Figure 3 ) , which runs an electric current through the superconductor 2 and which has an intensity lower than the typical current of the superconductor at the cryogenic temperature to which it is cooled .

[0036] The first and second detectors 7 , 9 cooperate with a processing unit 12 which receives the signals SR (X) SF (X) and operates according to the dictates of the method of the present invention : i . e . , by detecting the reflected and refracted voltage wave, respectively . The physical basis of operation is described with reference to Figure 2 . Under normal operating conditions , for temperatures lower than the critical temperature and for currents in which the superconductor is used below the critical temperature at the established cryogenic temperature , the superconductor 2 carries a current I ( the current value is indicated on the x-axis ) during normal operation, the superconductor in this case has zero resistance to the passage of current and therefore operates in short-circuit conditions , i . e . , at a voltage V of almost zero ( the voltage value is indicated on the y-axis ) , which is slightly above zero , because it has no resistance therein .

[0037] As soon as a transition zone TZ ( also called Quench, see Figure 1 ) arises in the superconductor, it can be seen that the characteristic curve VI of the superconductor begins to steepen according to a trigonometric tangent given by the establishment of a characteristic impedance of the superconductive line in question, and a transition zone TZ is established, which depends on the time in which this transition zone persists . A potential drop AV is created in the transition zone due to the establishment of a resistance R presented by the transition zone itsel f TZ . Based on the applicant ' s studies and based on electric field theory, the transition zone reflects part of the pulse train towards the first detector 7 ( and thus sends the pulses back to the device 5 ) while part of the pulses from the train T (x) are refracted and detected by the detector 9 .

[0038] The electronic unit 12 is there fore configured to detect the formation of a resistive transition zone when : the first detector 7 detects signals SR (X) resulting from the reflection of the pulse train T (x) in the superconductive medium; and / or the second detector 9 detects signals SF (X) resulting from the refraction of the pulse train T (x) in the superconductive medium . The reflected pulses SR (X) are reversed in modulus and also have a change in frequency with respect to the frequency of the pulse train T (x) . The refracted signals SF (X) also have a change in frequency with respect to the frequency of the pulse train T (x) . The electronic processing unit 12 is also configured to detect the absence of a resistive transition zone when : the first detector 7 does not detect signals resulting from the reflection of the pulse train T (x ) in the superconductive medium; and / or the second detector 9 detects signals corresponding exactly in modulus to those emitted by the device 5 .

[0039] In more detail , the electronic unit 12 is configured to detect the refraction of the pulse train when the amplitude of the pulses detected by the second detector 9 is less than a predetermined percentage with respect to the pulse amplitude of the pulse train T (X ) ; such a quantity is called the reflection coefficient of the superconductive line itsel f being examined .

[0040] Furthermore , the electronic processing unit 12 is configured to detect the absence of a resistive transition zone when the amplitude of the pulses detected by the second sensor 9 is equal to the pulse amplitude of the pulse train T (X ) .

[0041] The electronic processing unit 12 is configured to detect the time tV between the emission of the pulse train and the reception of the reflected signal SR (X) for the calculation of the position of the resistive transition zone based on the propagation speed of the pulse train in the superconductive medium . Furthermore , the amplitude of the reflected train only depends on the impedance of the anomaly which has arisen due to the quench, and never by the characteristic impedance of the superconductive line as per the telegrapher ' s theory by calculating the reflection coef ficient on the load ( in this case quench) .

[0042] Furthermore , the absence of a resistive transition zone is ascertained when the pulse amplitude detected by the second detector is equal to the pulse amplitude of the pulse train T (X ) . Upon detecting the formation of a resistive transition zone , the electronic unit can carry out a correction action consisting of interrupting the current flow in the superconductor or reducing the temperature of the superconductor by acting on the cooling system .

[0043] The advantages of the device and method according to the present invention are mani fold .

[0044] The device and method allow the transition zone to be identi fied from the very first moments of its formation, this is because the total impedance of the superconductive line changes and a reflection parameter is created based on the resolution of the telegrapher ' s equations set as a lossless line when there is no quench and on impedance when a quench is established .

[0045] The method described also provides the exact positioning and magnitude (based on the reflection coef ficient o f the incident pulse voltage wave ) of the transition zone within the superconductor without installing any type of sensor or fibre optics inside the superconductor, which would represent a construction complication and frequently result in untrue detection or failure of the devices themselves , adding to the hassle of troubleshooting and maintenance itsel f . This quench detection method in a superconductor can be defined ' non- invasive ' . numerals

[0046] 1 device for detecting a resistive transition zone in a superconductor

[0047] 2 superconductor

[0048] 3 cooling system

[0049] 5 emitter device configured to inj ect a pulse train T (x )

[0050] 2-a first end

[0051] 2-b second end

[0052] 7 first detector for detecting signals resulting from the reflection of the pulse train T (x ) in the superconductive medium

[0053] 9 second detector for detecting signals resulting from the refraction of the pulse train T (x ) in the superconductive medium

[0054] 10 current generator

[0055] 12 processing unit

Claims

CLAIMS1.- Device for detecting a resistive transition zone in a superconductor (2) provided with a cooling system (3) which maintains the superconductor (2) at a cryogenic temperature lower than the critical temperature characteristic of the superconductor and comprising:- an emitter device (5) configured to inject a voltage pulse train T (x) having a predefined waveform, frequency and amplitude into the superconductor (2) ;- a first detector (7) for detecting signals resulting from the reflection of the pulse train T (x) in the superconductive medium of the superconductor (2) ; the first detector (7) is coupled to a first end (2-a) of the superconductor;- a second detector (9) for detecting signals resulting from the refraction of the pulse train T (x) in the superconductive medium; the second detector (9) is coupled to a second end (2-b) of the superconductor (2) ;- a processing unit (12) configured to detect the formation of a resistive transition zone in the superconductor (2) when: the first detector (7) detects signals SR(X) resulting from the reflection of the pulse train T (x) in the superconductive medium; and / or the second detector (9) detects signals SF(X) resulting from the refraction of the pulse train T (x) in the superconductive medium; the electronic processing unit (12) is also configured to detect the absence of a resistive transition zone when:the first detector (7) does not detect signals resulting from the reflection of the pulse train T (x) in the superconductive medium; and / or the second detector (9) detects signals corresponding to those emitted by the emitter device (5) .2.- Device according to claim 1, wherein the emitter device (5) and the first detector (7) are integrated into a single transceiver device.3.- Device according to claim 1 or 2, wherein the electronic unit (12) is configured to detect the refraction of the pulse train when the amplitude of the pulses detected by the second sensor (9) is lower than the amplitude of the pulses of the pulse train T (X) by a predetermined percentage.4.- Device according to any one of the preceding claims, wherein the electronic processing unit (12) is configured to detect the absence of a resistive transition zone when the amplitude of the pulses detected by the second sensor is equal in modulus to the pulse amplitude of the pulse train T (X) .5.- Device according to any one of the preceding claims, wherein, upon detecting the formation of a resistive transition zone, the electronic unit is configured to carry out a correction action consisting in interrupting the current flow in the superconductor or in reducing the temperature of the superconductor by acting on the cooling system.6.- Method for detecting a resistive transition zone in a superconductor (2) provided with a cooling system (3) that maintains the superconductor (2) at a temperature lower than the critical temperature, comprising the steps of:- injecting a voltage pulse train T (x) having a predetermined waveform, frequency and amplitude into the superconductor (2) ;- detecting signals resulting from the reflection of the pulse train T (x) in the superconductive medium of the superconductor (2) at one first end (2-a) of the superconductor;- detecting the signals resulting from the refraction of the pulse train T (x) in the superconductive medium at a second end (2-b) of the superconductor (2) ;- detecting the formation of a resistive transition zone in the superconductor (2) when: signals SR(X) resulting from the reflection of the pulse train T (x) in the superconductive medium are detected at the first end; and / or signals SF(X) resulting from the refraction of the pulse train T (x) in the superconductive medium are detected at the second end;- detecting the absence of a resistive transition zone when: signals resulting from reflection of the pulse train T (x) in the superconductive medium are not detected at the first end; and / or signals that correspond to those emitted are detected at the second end.7 Method according to claim 6, wherein the refraction of the pulse train T (x) is detected when the amplitude of the pulses at the second end is lower than the amplitude of the pulses of the pulses train T (X) by a predetermined percentage.8.- Method according to claim 6 or 7, the absence of a resistive transition zone is detected when the amplitude of pulses at the second end is equal in modulus to the pulse amplitude of the pulse train T (X) .9 . - Method according to any one of claims 6 to 8 , wherein, upon detecting the formation of a resistive transition zone , a correction action is carried out consisting in interrupting the current flow in the superconductor or in reducing the temperature of the superconductor by acting on the cooling system .

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