TRANSPORT CURRENT SATURATED HTS MAGNETS

Abstract

A high temperature superconducting, HTS, magnet system. The HTS magnet system comprises an HTS field coil, a temperature control system, a power supply, and a controller. The HTS field coil comprises a plurality of turns comprising HTS material; and a resistive material electrically connecting the turns, such that current can be shared radially between turns via the resistive material. The temperature control system is configured to control the temperature of the coil, the temperature control system comprising at least a cryogenic cool system configured to keep the coil below a self-field critical temperature of the HTS material. The power supply is configured to supply current to the HTS field coil. The controller is configured to cause the power supply to provide a current greater than a critical current of all of the HTS material.

Claims

1. A high temperature superconducting, HTS, magnet system comprising: an HTS field coil comprising: a plurality of turns comprising HTS material; a resistive material electrically connecting the turns, such that current can be shared radially between turns via the resistive material; a temperature control system configured to control the temperature of the coil, the temperature control system comprising at least a cryogenic cool system configured to keep the coil below a self-field critical temperature of the HTS material; a power supply configured to supply current to the HTS field coil; a controller configured to: cause the power supply to provide a current greater than a critical current of all of the HTS material.

2. An HTS magnet system according to claim 1, and comprising: a sensor configured to measure a temperature of the coil and/or a magnetic field produced by the coil; wherein the controller is further conf3igured to adjust the magnetic field strength of the coil by: monitoring readings from the sensor in order to determine a magnetic field strength of the coil; causing the temperature control system to lower the temperature of the coil in the case that the measured magnetic field strength of the coil is less than a desired magnetic field strength of the coil, and to raise the temperature of the coil in the case that the measured magnetic field strength of the coil is greater than the desired magnetic field strength of the coil.

3. An HTS magnet system according to claim 2, wherein the temperature control system comprises the power supply, and is configured to increase the temperature of the HTS field coil by increasing the current supplied to the HTS field coil, and to decrease the temperature of the HTS field coil by decreasing the current supplied to the HTS field coil, such that the supplied current remains greater than the critical current of all of the HTS material.

4. An HTS magnet system according to claim 2, wherein the temperature control system comprises a heater in thermal contact with the HTS field coil.

5. A method of operating a high temperature superconducting, HTS, field coil, the HTS field coil comprising a plurality of turns comprising HTS material, and a resistive material electrically connecting the turns, such that current can be shared radially between turns via the resistive material; the method comprising: supplying current to the HTS field coil such that a transport current of the HTS field coil is greater than a critical current of all of the HTS material; controlling the temperature of the HTS field coil.

6. A method according to claim 5, further comprising; monitoring one of: a temperature of the HTS field coil; a magnetic field produced by the HTS field coil; controlling the magnetic field strength of the HTS field coil by: determining a magnetic field strength of the coil from results of said monitoring; decreasing the temperature of the coil when the measured magnetic field strength is less than a desired field strength of the HTS coil; increasing the temperature of the coil when the measured magnetic field strength is greater than a desired field strength of the HTS coil.

7. A method according to claim 6, wherein increasing the temperature of the HTS field coil comprises one or more of: increasing power supplied to a heater in thermal contact with the HTS field coil; decreasing cooling provided by a cooling system of the HTS field coil; and increasing the current supplied to the HTS field coil.

8. A method according to claim 6, wherein decreasing the temperature of the HTS field coil comprises one or more of: decreasing power supplied to a heater in thermal contact with the HTS field coil; increasing cooling provided by a cooling system of the HTS field coil; and decreasing the current supplied to the HTS field coil, such that the current remains greater than the critical current of the HTS material in all of the HTS material.

9. An HTS magnet system comprising: an HTS field coil comprising: a plurality of turns of HTS material separated by a resistive material which is sufficiently electrically conductive to allow radial sharing of current between the turns; a temperature control system comprising a cooling system configured to keep the temperature of the HTS field coil below a self-field critical temperature of the HTS material; a power supply configured to supply current to the HTS field coil; and a controller configured to: cause the power supply to provide current sufficiently high to saturate the HTS material in the coil so that it all operates at its critical current; reduce the magnetic field generated by the HTS field coil by increasing the current supplied by the power supply, and increase the magnetic field generated by the HTS field coil by decreasing the current supplied by the power supply.

10. A high temperature superconducting, HTS, magnet system comprising: a plurality of HTS field coils, each comprising: a plurality of turns comprising HTS material; a resistive material electrically connecting the turns, such that current can be shared radially between turns via the resistive material; a temperature control system configured to control the temperature of each coil, the temperature control system comprising at least a cryogenic cool system configured to keep each coil below a self-field critical temperature of the HTS material; a power supply configured to supply current to the HTS field coil; a controller configured to: cause the power supply to provide a current to each field coil greater than a critical current of all of the HTS material in the HTS field coils; cause the temperature control system to adjust the temperature of each HTS coil and thereby adjust the contribution of each HTS coil to the magnetic field.

11. A magnet system according to claim 10, and comprising a magnetic field sensor array configured to measure a magnetic field produced by the plurality of HTS field coils; wherein the controller is further configured to: determine a magnetic field profile of the HTS magnet system from the measured magnetic field; cause the temperature control system to adjust the temperature of each HTS coil in order to achieve a desired magnetic field profile.

12. A magnet system according to claim 10, wherein the power supply is configured to supply the same current to each HTS field coil, and wherein the controller is configured to adjust the temperature of all of the coils by adjusting the power supply current, while keeping the current greater than the critical current of all of the HTS material in all of the HTS field coils.

13. A magnet system according to claim 10, wherein the temperature control system comprises a heater for each HTS field coil, and wherein the temperature control system is configured to adjust the temperature of each of the HTS field coils individually by controlling the heat provided to the respective coil by each heater.

14. A method of operating a high temperature superconducting, HTS, magnet system, the HTS magnet system comprising a plurality of HTS field coils, each comprising a plurality of turns comprising HTS material and a resistive material electrically connecting the turns, such that current can be shared radially between turns via the resistive material, the method comprising: supplying current to each of the HTS field coils such that a transport current of the HTS field coil is greater than a critical current of all of the HTS material; controlling the HTS magnet system by controlling the temperature of each of the HTS field coils,

15. A method according to claim 14, and comprising: monitoring a magnetic field produced by the HTS magnet system; adjusting the temperature of each HTS field coil in order to achieve a desired magnetic field profile.

16. A method according to claim 14, wherein the same current is supplied to all of the HTS field coils, and controlling the temperature of each of the HTS field coils comprises adjusting the temperature of all of the HTS field coils by adjusting the supplied current.

17. A method according to claim 14, wherein controlling the temperature of each of the HTS field coils comprises controlling power supplied to a respective heater in thermal contact with each HTS field coil.

18. A method of determining the critical surface of a high temperature superconducting, HTS, conductor, the method comprising: forming the HTS conductor into an HTS field coil comprising: a plurality of turns comprising the HTS conductor; a resistive material electrically connecting the turns, such that current can be shared radially between turns via the resistive material; operating the HTS field coil with a transport current which is greater than the critical current of all of the HTS conductor; measuring a temperature of one or more points on the HTS field coil; measuring a magnetic field produced by the field coil; determining the critical surface of the HTS conductor from said measurements.

19. A method according to claim 18, and comprising determining the critical current of a sample of the HTS field coil, and using said determined critical current to set the transport current.

20. A method according to claim 18, wherein the transport current is set to a value greater than an expected peak critical current of the HTS tape.

21. A method according to claim 18, wherein the transport current is ramped up until a monotonic relationship between the measured temperature and magnetic field strength is observed, and the transport current at that point is determined to the greater than the critical current of all of the HTS tape.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 is a schematic illustration of HTS tape;

[0035] FIG. 2 shows the results of a ramp-up test on a partially insulated HTS coil maintained at 77 K;

[0036] FIG. 3 shows the results of a ramp-up test on a partially insulated HTS coil maintained at 40K;

[0037] FIG. 4 shows the results of ramp-up tests performed on an HTS coil at a variety of starting temperatures;

[0038] FIG. 5 is a schematic illustration of an exemplary HTS magnet system.

DETAILED DESCRIPTION

[0039] Screening currents within an HTS magnet occur because the transport current, I, is less than the critical current I.sub.C of the conductor in large parts of the coil. The critical current I.sub.C is the maximum current which the HTS conductor can carry while superconducting, given the instantaneous environmental conditions (e.g. temperature, external magnetic field). The critical current varies across the magnet, because the magnetic field, temperature, and the HTS conductor itself will generally not be uniform. By contrast, the “peak critical current” of an HTS conductor is the current which that conductor can carry at a temperature of absolute zero, zero strain, and zero external magnetic field (i.e. in ideal conditions)—this is sometimes referred to simply as the “critical current” in the literature, but that meaning is not used here.

[0040] At present, superconducting magnets are operated such that the transport current is less than the minimum critical current in any part of the magnet coil, to prevent current leaking from the HTS conductor. This is done because any current leak from the HTS conductor will generate heat (as the current is now flowing through a resistive material), which will in turn locally raise the temperature of the HTS conductor, further reducing the critical current, and potentially starting a feedback cycle which may result in a quench (the HTS material heating to the point where it is no longer superconducting at the “hot spot”, and the magnet dumping its energy into the non-superconducting region—often causing damage to the magnet unless mitigated). It is important to note that magnets made from coils would with multi-tape cables can operate stably with localized hot spots where current deviates around local defects in individual tapes.

[0041] The majority of the magnet will have an “operating fraction” (the ratio between the transport current and the critical current, I/I.sub.C) less than unity, which provides “spare” current capacity in the HTS which becomes occupied either partially or fully by screening currents. Over time, if the transport current is kept steady, these will achieve equilibrium—but this typically happens over a very long time constant (on the order of minutes to months), in part because the screening currents are flowing through a zero resistance medium.

[0042] The proposal of the present disclosure is to operate an HTS magnet coil under a different regime—instead of the transport current being lower than the minimum critical current of the coil, the transport current is greater than the maximum critical current of the coil (for the entire period of operation). As a result, all of the superconducting material in the coil has an operating factor of unity, meaning that screening currents are excluded (there is no “spare” superconducting capacity). This state will herein be referred to as the “saturated” state. Conventional wisdom regarding HTS magnets would suggest that this is a terrible idea—all of the coil would in effect be one big hotspot, with current leaking into the resistive components of the magnet throughout the coil and causing the coil to heat up, requiring additional cooling for no practical benefit. However, it has been found to be possible if the turn-to-turn resistance is low enough, the thermal conductivity of the coil high enough and if sufficient cooling is provided to counteract the heating due to the current leak into the normal components. As a consequence several advantageous features result that enable a HTS magnet to operate without the influence of screening currents, with more uniform quench conditions (forces and temperatures), producing the maximum possible field from the conductor, and with a simple control mechanism.

[0043] The new operating mode is only possible in partially insulated (or non-insulated) coils. When current in a partially insulated coil leaves the HTS conductor, it will initially flow in a spiral path parallel to the HTS through the resistive components of the magnet (i.e. the stabiliser layers of the HTS tape, and any resistive components connecting the turns). However, this spiral path flow will decay quickly into the radial path (i.e. flowing radially through the resistive components) due to the high resistance of the non-superconducting spiral path. This means that, when operating with an in the saturated regime, the magnetic field produced by a coil is dependent only on the shape of the coil and the critical current of the HTS within the coil—as the radial current flow through the resistive components will not make a significant contribution to the magnetic field.

[0044] The critical current of the HTS is, in turn, dependent on: [0045] the temperature of the HTS; [0046] the external magnetic field (i.e. magnetic field not due to the current in the HTS) at the HTS; [0047] the strain on the HTS.
All of these factors will vary though the coil.

[0048] For a magnet isolated from other variable magnetic field sources, the external magnetic field on each turn of the coil will be dependent only on the magnetic field produced by each other turn, and if the magnet is also isolated from other variable sources of strain, then the strain on the tape is only dependent on strains which are a result of the magnetic field produced by the magnet.

[0049] FIG. 2 illustrates the behaviour of a small non-insulated pancake coil wound using a pair of tapes with all turns soldered together when it is ramped into the unity operating fraction regime, with the temperature maintained at 77K by a liquid nitrogen bath. The power supply unit (PSU) current (top graph) is ramped from 0 to 400 A, and when it hits approximately 200 A the HTS of the coil becomes saturated—the central magnetic field (middle) levels off, and the voltage across the coil (bottom) begins to rise with the PSU current. The central magnetic field remains approximately constant during the rest of the ramp-up, and during the subsequent ramp-down, until the transport current falls below approximately 200 A and the coil is no longer saturated.

[0050] FIG. 3 shows the results of a similar test performed on a magnet comprising a pair of pancake coils coil that is conduction cooled with a cryocooler, and controlled with a temperature control system configured to maintain the coil temperature at 40K. The magnetic field of the coil increases during the ramp up, until a current of approx 1.1 kA is reached. Above this, the magnetic field remains approximately steady, until the PSU current exceeds about 2.6 kA, at which stage the temperature control system is overwhelmed by the excess heat caused by the radial current leak. The coil's temperature increases gradually, causing the critical current of the coil to diminish, and the magnetic field produced by the coil to diminish. This occurs in a steady manner over ˜1000 s until the self-field critical temperature of the coil is reached and the magnetic field has reached zero. The power supply is then turned off.

[0051] FIG. 4 shows plots of several ramps of the same magnet with a temperature control system configured to maintain the coils respectively at base temperature (heater turned off), 20 K, 30 K, and 40 K until the coils saturate (at which point they heat up under the excess current provided by the power supply, which continues to ramp up). The ramp-up is shown in the central magnetic field—coil temperature (B-T) plot. In each case, the ramp begins at low magnetic field (bottom of the substantially vertical line), and magnetic field increases as the transport current increases, while remaining below the critical current of the HTS. In the upper portion of the graph, the transport current is beginning to saturate the HTS, and the magnetic field “rolls over” as the coil enters the saturated regime. In this regime, each of the tests shown follows the same B-T relationship between central magnetic field (B) and coil temperature (T), regardless of the ramping history of the coil and the exact value of the current supplied (the “loops” at the right hand extreme of each graph are artefacts resulting from the end of the test). This lack of any hysteresis effect arises because the central magnetic field is determined solely by the critical current of the HTS in the coil, with no interference from screening currents which would be present in a typical scenario.

[0052] The temperature will tend to vary through the magnet—e.g. regions with lower critical current will experience more current passing through the nearby resistive material, and hence more heating, and the cooling will depend on the heat conductance of the materials forming the coil and the layout of the cooling system, but this pattern will generally result in a consistent temperature profile.

[0053] If a characteristic temperature is chosen to represent the temperature profile throughout the magnet (e.g. the temperature at a specific point on the magnet, or an average of the temperature at several such points), then it can be shown (and demonstrated experimentally, see FIG. 4) that the field produced by a magnet in the saturated regime depends only on this temperature.

[0054] While the HTS material remains superconducting throughout the magnet (i.e. the minimum critical current of the HTS does not drop to 0), the relationship between the characteristic temperature and the magnetic field strength is such that an increase in temperature results in a decrease in magnetic field, as shown in FIG. 4.

[0055] When operating in saturated mode the field of the HTS magnet can be decreased monotonically by warming the coil from low temperature (maximum field) towards the critical temperature of the magnet (zero field). The field sweep rate, dB/dt, is set by the rate of warming, dT.sub.magnet/dt. Under this condition, the field can be changed quicker than the magnet's electromagnetic time constant, τ=L/R, where L is the inductance of the magnet and R is the radial resistance, which is often prohibitively long. In this regime the stored energy of the magnet is dissipated as heat in the coil, and the maximum field sweep rate permitted is determined entirely by thermal design (i.e. how quickly the temperature can be changed). Similarly, accelerated field sweep rates can be achieved for a monotonic increase of the magnetic field, by rapidly cooling the magnet and simultaneously providing surplus power supply current so that the magnet remains in the saturated regime.

[0056] There are no screening currents in the coil when operating in this regime, so the only delays in changing the magnetic field are the time taken for the magnet to heat up or cool down, and the time taken for currents in the resistive spiral path to decay into the radial path. Both of these are parameters that can be controlled by appropriate thermal and electrical coil designs, and in the examples shown have a timescale of tens of minutes at 20 K.

[0057] The magnet can therefore be controlled by monitoring either a characteristic temperature of the magnet or monitoring the magnetic field directly, and heating or cooling the magnet to achieve the desired magnetic field. Heating the magnet will reduce the critical current of the HTS, and hence the magnetic field strength, and cooling the magnet will increase the critical current of the HTS, and hence the magnetic field strength.

[0058] Where only the temperature is monitored, the relationship between the characteristic temperature and the magnetic field may be determined based on a pre-calibrated lookup table or formula. It will be appreciated that the control of the magnet is equivalent whether this is used to relate the measured temperature to the instantaneous magnetic field, and determine the difference between the instantaneous and desired magnetic field, or to relate the desired magnetic field to a desired temperature, and determine the difference between the desired and measured temperatures.

[0059] Heating of the magnet may be achieved by increasing the transport current (thereby causing more current to enter the resistive portions of the magnet), by the use of dedicated heaters provided in thermal contact with the coils, or by reducing the cooling (e.g. flow rate) provided by the cryogenic cooling system of the magnet. Cooling of the magnet may be achieved by increasing the cooling of the cryogenic cooling system, or by reducing the transport current (while still remaining in the saturated range) or the power supplied to heaters.

[0060] In the first case mentioned above (heating the magnet by increasing the transport current), it will be noted the outcome is highly non-intuitive, ie: to increase the magnetic field one would reduce the power supply current, and vice versa. This is only the case when the magnet is being operated in the saturated regime.

[0061] A feedback system is implemented to control the measured temperature/field by heating and cooling—i.e. when the measured temperature is too high, or the measured field too low, then the magnet is cooled down (or the heat applied is reduced), and when the measured temperature is too low, or the field too high, then the magnet is heated up (or the cooling applied is reduced). Any suitable feedback scheme as known in the art may be used for this purpose.

[0062] When operating with magnetic field monitoring, the control scheme outlined above may be used even in situations where the external strain and/or magnetic field on the magnet is variable. This could also be done with temperature monitoring if strain and/or field sensors were included, and the lookup table or formula contained terms to account for the effects of strain and/or field. Alternatively (in either the constant or variable background field case), a lookup table between temperature and desired field could be used to obtain an initial estimate for the heating required, and then a feedback loop based on the monitored magnetic field used to reach the desired magnetic field.

[0063] When operating in the saturated regime, field stability is determined only by the stability of the critical current of the HTS—i.e. by the stability of the external magnetic field, strain, and temperature.

[0064] For multi-coil systems the same principle applies—each individual coil can be operated in saturated mode. Furthermore, it is possible to control the homogeneity of the magnetic field by independently controlling the temperature of each individual coil, based on spatially distributed measurements of the magnetic field. The control feedback loop will be more complicated—an array of sensors should be positioned in a way that allows the homogeneity of the magnetic field produced by all coils to be determined, and the temperature of each coil can then be individually controlled to adjust the field homogeneity by adjusting the field contributed by each individual coil. The shape of the magnetic field can conveniently be described using a weighted sum of spatial harmonics, such as Legendre polynomials, as described in the prior art of shimming. However, many other ways to determine the field homogeneity exist.

[0065] It should be noted that to adjust the field homogeneity for a set of coils connected in series it is necessary to adjust the contribution of each coil independently. This cannot be done by adjusting the transport current, which affects the temperature of all coils operating in saturated mode. It is therefore necessary to adjust the temperature of each coil independently. The coils therefore need to be at least partially thermally isolated from each other. Their temperatures can then be adjusted either by controlling the cooling of each coil or adding additional heating to each coil, for example, with a heater.

[0066] Alternatively, the magnet may have a mix of coils operated in the conventional regime and coils operated in the saturated regime, with the latter adjusted to ensure field homogeneity.

[0067] While the above refers to field homogeneity, it will be appreciated that other field profiles can be achieved by adjustment of the magnet coils, where needed.

[0068] The saturated regime also provides a convenient way to test the quality of HTS tape—for a given coil temperature, environment, and coil geometry, the magnetic field is entirely determined by the critical current of the HTS tape—so an HTS tape can be tested by measuring the magnetic field produced by a coil of that tape running in the saturated regime at different temperatures, and determining the critical current response. The magnetic field provides a measure of the integrated critical current density of the tape throughout the coil—and further magnetic field sensors can be used to determine how the critical current varies through the coil, and hence obtain the critical surface of the HTS tape (a profile of the temperature and/or magnetic field variance of critical current in the tape). Operating in the saturated regime with HTS of unknown critical current would either require first determining an estimate or upper limit for the critical current, or simply supplying a very high transport current such that it is unlikely that the critical current is below the transport current. Alternatively, the coil transport current can be ramped up until the temperature/magnetic field relationship characteristic of the saturated regime is observed (i.e. the “roll over” shown in FIG. 4), and then measurements are taken as the coil's temperature is raised towards the self-field critical temperature to determine the integrated critical current and/or critical surface (i.e. the variation of the critical current with temperature, field, and strain).

[0069] Operating at saturation will increase the likelihood of a quench compared to operating in the conventional regime—if the cooling system is not able to counteract the additional heating from the current flow in the resistive material in any part of the magnet, then a thermal runaway may occur. However, since all of the HTS will be operating at saturation, it will all be equally susceptible to thermal runaway (i.e the thermal margin is uniform). This means that any quench will propagate quickly, causing the energy of the magnet to be dumped throughout the volume of the magnet. This will cause significantly less damage than a quench in a conventionally operating HTS magnet, where a hotspot will tend to be only a small portion of the magnet, into which all of the magnet's stored energy is then dumped unless countermeasures are taken. The minimum quench energy would still be much higher for a saturated HTS magnet than for an equivalent LTS magnet, allowing an HTS magnet to be operated with many of the advantages of HTS while also having the resilience during quenches of an LTS magnet. In summary, quenches are more likely in the new regime, but damage from quenches is less likely.

[0070] The new regime applies to any non-insulated or partially insulated coil. The performance of a coil in the new regime may be optimised by providing material between the turns with high electrical and thermal conductivity (to reduce heating from excess current, and increase the ability to transport that heat to the cooling system), but these are not strictly necessary—it would be equally valid to run a coil with lower electrical and thermal conductivity in the saturated regime, and provide additional cooling power to ensure that the coil does not quench. This would cause a temperature gradient across the coil—but as noted previously this does not change the predictability of the temperature/magnetic field relationship provided a representative temperature is chosen for the temperature profile of the coil.

[0071] FIG. 5 shows an exemplary HTS magnet system using the above described control scheme. The system comprises two partially insulated coils 501, formed into a double pancake coil, each of which is monitored by temperature sensors 502 and magnetic field sensors 503. Cooling plates 504 are provided on the side of the double pancake, to ensure good heat conduction from the HTS coils, and a heater 505 is provided to heat the coils. The HTS magnet system has a power supply (not shown) which provides a transport current to the HTS coils, and a controller (not shown) which receives input from the temperature sensors 502 and magnetic field sensors 503, and adjusts the magnetic field strength of the magnet by controlling the temperature using the heater 505, and by adjusting the PSU current (while keeping the magnet in the saturated regime).