HTS Magnet Ramping to Reduce Screening Currents

Abstract

A method of energizing or de-energizing a high temperature superconducting, HTS, coil, from an initial transport current to a final transport current. The HTS coil comprises a plurality of turns of HTS material. A transport current is supplied to the HTS coil, the transport current starting at the initial transport current and varying over time to the final transport current. Cooling is applied to the HTS coil. An operating condition of the HTS coil is monitored, wherein the operating condition is indicative of a ratio I/I.sub.c of the transport current, I, to a critical current, I.sub.c, of the HTS material in at least a part of the HTS coil. One or both of the transport current applied to the coil and a net cooling applied to the coil are controlled in a feedback loop responsive to the operating condition, in order to maintain the operating condition in a desired range during energisation or de-energisation, such that the indicated ratio I/I.sub.c is maintained above a threshold ratio (e.g. 0.7).

Claims

1. A method of energizing or de-energizing a high temperature superconducting, HTS, coil, from an initial transport current to a final transport current, the HTS coil comprising a plurality of turns of HTS material, the method comprising: supplying a transport current to the HTS coil, the transport current starting at the initial transport current and varying over time to the final transport current; applying cooling to the HTS coil; monitoring an operating condition of the HTS coil, wherein the operating condition is indicative of a ratio I/I.sub.c of the transport current, I, to a critical current, I.sub.c, of the HTS material in at least a part of the HTS coil; controlling one or both of the transport current applied to the coil and a net cooling applied to the coil in a feedback loop responsive to the operating condition, in order to maintain the operating condition in a desired range during energisation or de-energisation, such that the indicated ratio I/I.sub.c is maintained above a threshold ratio and below 1.

2. A method according to claim 1, wherein the threshold ratio is at least 0.7.

3. A method according to claim 1, wherein the operating condition is a non-inductive component of the start-to-end voltage across the HTS coil.

4. A method according to claim 3, wherein the non-inductive component of the start-to-end voltage across the HTS coil is monitored by monitoring a total start-to-end voltage across the HTS coil and determining the non-inductive component by one of: subtracting a pre-calculated inductive voltage dependent on the rate of change of transport current; modelling of the HTS coil, the model taking as input at least the total start-to-end voltage, the transport current, a monitored temperature of the HTS coil, and a monitored magnetic field produced by the HTS coil; or determining the non-inductive component as a difference between the total start-to-end voltage and a voltage across a pickup coil co-wound or inductively coupled with the HTS coil.

5. A method according to claim 1, wherein the operating condition is indicative that current in the HTS material of at least part of the coil is spilling into adjacent non-superconducting material.

6. A method according to claim 5, wherein the operating condition is a rate of change of temperature of the HTS coil with time, and the rate of change of temperature is maintained within a desired range dependent upon a rate of change of the transport current and/or the net cooling.

7. A method according to claim 1, wherein controlling the net cooling comprises one or both of: controlling cooling applied to the HTS coil; applying heating to the HTS coil.

8. A method of energizing or de-energizing a high temperature superconducting, HTS, coil, from an initial transport current to a final transport current, the HTS coil comprising a plurality of turns of HTS material, the method comprising: supplying a transport current to the HTS coil, the transport current starting at the initial transport current and varying over time to the final transport current; monitoring a non-inductive component of the start to end voltage across the HTS coil; applying cooling to the HTS coil to maintain the temperature of the HTS material at or below the zero field critical temperature of the HTS material; controlling one or both of: the rate of change of transport current; or the net cooling power applied to the coil, where the net cooling power comprises the applied cooling and any applied heating of the coil; such that the non-inductive component of the start-to-end voltage across the HTS coil remains above a threshold at least until the transport current is equal to the final transport current.

9. A method according to claim 8, wherein monitoring the non-inductive component comprises monitoring a total start to end voltage of the HTS coil and determining the non-inductive component by one of: subtracting a pre-calculated inductive voltage dependent on the rate of change of transport current; modelling of the HTS coil, the model taking as input at least the total start to end voltage, the transport current, a monitored temperature of the HTS coil, and a monitored magnetic field produced by the HTS coil.

10. A method according to claim 8, wherein monitoring the non-inductive component comprises: monitoring a total start to end voltage of the HTS coil; monitoring a pickup voltage which is the voltage between a first end of the HTS coil and first end of a co-wound pickup coil, wherein the pickup coil is directly electrically connected at a second end to the second end of the HTS coil, and is not directly electrically connected to the HTS coil elsewhere; determining the non-inductive component as the difference between the start to end voltage and the pickup voltage.

11. A method according to claim 8, wherein the non-inductive start-to-end voltage of the HTS coil remains above 0.5 micro Volts per meter of length of the HTS material of the HTS coil, V/m, more preferably above 1 V/m, more preferably above 10 V/m, more preferably above 100 V/m.

12. A method of energizing or de-energizing a high temperature superconducting, HTS, coil, from an initial transport current to a final transport current, the HTS coil comprising a plurality of turns of HTS material, the method comprising: supplying a transport current to the HTS coil, the transport current starting at the initial transport current and varying over time to the final transport current; monitoring temperature of the HTS coil; applying cooling to the HTS coil to maintain the temperature of the HTS material at or below the zero field critical temperature of the HTS material; controlling one or both of: the rate of change of transport current; or the net cooling power applied to the coil, where the net cooling power comprises the applied cooling and any applied heating of the coil; such that the rate of change of temperature over time of the HTS coil does not exceed a predetermined threshold dependent upon the controlled rate of change of transport current and/or net cooling power.

13. A method according to claim 12, wherein the magnet is being energised and comprising maintaining the rate of change of temperature at a constant negative value.

14. A method according to claim 12, and comprising heating the magnet with heaters, and wherein controlling the net cooling power comprises adjusting the power of the heaters.

15. A method according to claim 12, wherein the magnet is being energised, the initial transport current is less than the final transport current, and the predetermined threshold is negative.

16. A high temperature superconducting, HTS, magnet system, the HTS magnet system comprising: an HTS coil comprising a plurality of turns of HTS material; a power supply configured to supply a transport current to the HTS coil; a cooling system configured to apply cooling to the HTS coil to maintain the temperature of the HTS material at or below the zero field critical temperature of the HTS material; a voltage monitoring system configured to monitor the start-to-end voltage of the HTS coil; a controller configured to control ramping of the magnet by: causing the power supply to provide a transport current which starts at an initial transport current and varies over time to a final transport current; and determining the non-inductive component of the monitored start-to-end voltage of the HTS coil; controlling one or both of: the rate of change of transport current; the net cooling power applied to the coil, where the net cooling power comprises the cooling applied by the cooling system and any applied heating of the coil; such that the non-inductive component of the start-to-end voltage of the HTS coil remains above a threshold at least until the transport current is equal to the final transport current.

17. An HTS magnet system according to claim 16, wherein the HTS coil comprises a co-wound pickup coil; wherein the pickup coil is directly electrically connected at a second end to the second end of the HTS coil, and is not directly electrically connected to the HTS coil elsewhere, and wherein the voltage monitoring system is further configured to monitor a pickup voltage which is the voltage between a first end of the HTS coil and first end of the co-wound pickup coil, and the controller is configured to determine the non-inductive component of the start-to-end voltage based on the monitored start-to-end voltage of the HTS coil and the pickup voltage.

18. A high temperature superconducting, HTS, magnet system, the HTS magnet system comprising: an HTS coil comprising a plurality of turns of HTS material; a power supply configured to supply a transport current to the HTS coil; a cooling system configured to apply cooling to the HTS coil to maintain the temperature of the HTS material at or below the zero field critical temperature of the HTS material; a controller configured to control ramping of the magnet by: causing the power supply to provide a transport current which starts at an initial transport current and varies over time to a final transport current; controlling one or both of: the rate of change of transport current; or the net cooling power applied to the coil, where the net cooling power comprises the applied cooling and any applied heating of the coil; such that the rate of change of temperature over time of the HTS coil does not exceed a predetermined threshold dependent upon the controlled rate of change of transport current and/or net cooling power.

19. An HTS magnet system according to claim 18, wherein the controller is configured to control the rate of change of transport current and/or net cooling power, such that the rate of change of temperature is maintained at a constant negative value.

20. An HTS magnet system according to claim 16, wherein the controller is configured to control the net cooling power applied to the coil by one or more of: causing the cooling system to adjust cooling provided to the HTS coil; supplying current to one or more heaters in thermal contact with the HTS coil, wherein the HTS magnet system comprises the heaters.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG. 1 is a diagram of a ReBCO tape;

[0046] FIG. 2 is a graph of critical current and transport current for a magnet ramped up according to a conventional technique;

[0047] FIG. 3A is a graph of critical current and transport current for an exemplary method of ramping an HTS coil;

[0048] FIG. 3B is a graph of temperature during the ramp-up method of FIG. 3A;

[0049] FIGS. 4A, B, and C show an alternative exemplary method for ramping up a magnet;

[0050] FIG. 5 is a series of graphs showing modelled performance of an HTS magnet comprising several field coils during a further alternative exemplary method of ramping the magnet;

[0051] FIG. 6A is a graph of temperature against time for an exemplary method of energising or de-energising a magnet; and

[0052] FIG. 6B is a graph of current against time for the same method as FIG. 6A.

DETAILED DESCRIPTION

[0053] As previously described in WO 2020/178594 A1, screening currents arise in the spare capacity of an HTS magneti.e. the maximum amplitude of the screening currents is the difference between the critical current of the HTS, and the transport current in the HTS.

[0054] The critical current of an HTS conductor is conventionally defined as the current at which the HTS conductor generates 100 microvolts over 1m of length. The critical current of the HTS will depend on the temperature, external magnetic field intensity and orientation, and strain on the HTS. As an alternative definition, screening currents will exist in the HTS whenever it is below saturationabove saturation (i.e. in the conditions where screening currents are eliminated), the HTS will shed current into electrically connected non-superconducting conductors. Within an HTS magnet this will arise as current flow in the cladding of the HTS or in the radial current path between turns in a non-insulated or partially-insulated HTS coil. Where the critical current I.sub.C is used in the below description, this may be replaced with the current at which the HTS enters the saturated state and begins to shed current into non-superconducting elements.

[0055] Saturation may be detected in several ways: [0056] Where the critical current of the HTS in given conditions is known theoretically or experimentally, saturation may be determined by comparing the transport current and the known critical current; [0057] The non-inductive start-to-end voltage of the HTS coil may be measured, with a non-zero voltage indicating that the coil is approaching saturation and a voltage above 100 microvolts per meter of HTS conductor indicating that the coil is at or above its critical current. One way to measure the non-inductive component of the start-to-end voltage is to measure the total start-to-end voltage of the coil, and subtract the start-to-end voltage of an open circuit co-wound coil. Some correction may be applied for the minor deviation in self-inductance of the co-wound coil and the HTS coil due to imperfect coupling, but for co-wound coils this deviation is generally insignificant. Another approach that does not require a co-wound coil is to measure the total start-to-end voltage and to determine the non-inductive component based on a model or other knowledge about the magnet. [0058] In non-insulated and partially insulated magnets, saturation will be characterised by onset of rollover of the magnetic field, where the rate of change of the magnetic field with the transport current decreases due to shedding of current into the non-superconducting radial path. [0059] In non-insulated and partially insulated magnets, saturation will be characterised by a change in the rate of change of coil temperature, due to the additional Joule heating in those turns that are shedding current to adjacent turns.

[0060] As described in the above referenced document, screening currents can be eliminated from some or all turns in some or all coils in the magnet during steady-state operation (or other normal operation) by running some or all of the magnet's coils (and the turns in those coils) at saturationi.e. with current shedding radially between turns, driven by non-inductive voltage due to the HTS being run close to or above critical currentfor certain magnets. However, this operation mode is not suitable for many magnets. For example, large partially insulated magnets may overheat due to Joule heating and quench, and there would be a tradeoff between the low turn-to-turn resistance needed for stable saturated operation and the low time constant L/R needed to allow the magnet to be ramped up in reasonable time. In addition, screening currents and the stresses they induce would still present a problem during ramping up of the magnet, when the transport current has not yet reached saturation, or during ramping down of the magnet where the transport current will fall below saturation.

[0061] As illustrated in FIG. 2 (not to scale), for a magnet at constant temperature, the critical current I.sub.C 201 will reduce as the transport current 202 of the magnet is ramped up (due to the effect of the increasing magnetic fields, stress, and strain on the magnet). This means that at the initial stages of the ramp, the ratio I/I.sub.C is very small-allowing for large screening currents to arise in the unused current region 203. As described in Jing Xia et al (see background) such large screening currents can cause large forces on the HTS coil during ramp-up, potentially damaging the coil and reducing performance.

[0062] As such, a method of ramping a magnet between a first transport current and a second transport current is proposed below where the magnet is kept at or near saturation during the ramp of the magnet, at least until the second transport current is reached. In this way, the potential screening currents induced in the magnet during the ramp are limited (difference between the transport current and the critical current will be small), and thereby the stresses on the magnet during the ramp are reduced.

[0063] FIG. 3A shows a graph of critical current 301 and transport current 302 against time during ramp-up of an example magnet. Again, this is for illustration purposes and is not to accurate scale. As can be seen, the critical current is initially low, and is increased during the ramp-up by varying environmental factors as described in more detail below. This is done such that the ratio I/I.sub.c between the transport current I and the critical current I.sub.c is always above a threshold ratio (e.g. greater than 0.7, greater than 0.75, greater than 0.8, greater than 0.9, or greater than 0.95), and the unused current region 303 is significantly reduced when compared to FIG. 2. During the ramp up, the ratio I/I.sub.c between the transport current and the critical current is less than 1, or in cases where current can flow in the HTS even with an I/I.sub.c ratio above 1 (e.g. as disclosed in WO 2020/178594 A1, in a partially insulated magnet with sufficient cooling), sufficiently low that current can flow in the HTS material.

[0064] Achieving this requires controlling the critical current of the magnet with some level of independence from the transport current. This can be achieved by controlling the temperature of the magnet (i.e. of the HTS material).

[0065] FIG. 3B shows a graph of temperature 311 against time during ramp-up of the toroidal field coil used in FIG. 3A, with the same time axis scale. As can be seen, the control over the critical current 301 can be implemented by gradually reducing the temperature 311 of the magnet as the magnet ramps up, starting from a value below the critical temperature T.sub.C of the magnet, and finishing at the intended operating temperature of the magnet.

[0066] Temperature control of the magnet, or in general control of the net cooling power applied to the magnet, can be achieved by varying the cooling provided by the magnet cooling system, and/or by the use of additional heating during the early stages of ramping-up. The additional heating may be provided by resistive heaters placed adjacent to the magnet or integrated within it. Alternatively, or in addition, additional heating may be provided by adjusting the ramp rate (i.e. the rate of change of transport current), so that the inductive voltage developed drives a proportion of transport current in the radial path. This will result in heating of the magnet due to the radial path being resistive. The heat resulting from driving current through the radial path will generally be evenly distributed (to the extent that resistive connections between turns are evenly distributed in the magnet), and unlike the use of integrated resistive heaters does not require additional space within the magnet coils. This applies to the example of FIGS. 3A and B, as well as to other examples in this disclosure involving control of temperature and/or net cooling power.

[0067] The required values of the temperature to achieve a particular critical current at a particular transport current will depend on the magnet, and can be predetermined by simulation as known in the art, or can be calibrated on-the-fly by a suitable feedback mechanism. In general, the critical current will increase as the temperature decreases, and therefore the ramp-up process will involve cooling the magnet as the transport current is increased.

[0068] While FIGS. 3A and B and other examples in this disclosure may use a TF coil for illustrative purposes, the same principles apply to any HTS coil.

[0069] FIGS. 4A, B, and C show an alternative scheme for ramping up a magnet, where the temperature is varied in a stepwise fashion. As FIG. 4A is a graph of transport current I 410 and critical current I.sub.C 420 against time, FIG. 4B is a graph of coil temperature 430 against time, and FIG. 4C is a graph of the magnetic field 440 produced by the coil against time. All three graphs use the same time-axis scale. These are intended as illustrative only, and are not presented to exact scale. The coil is initially at a transport current I1 and temperature T.sub.1, and the transport current is ramped up while holding the temperature steady and monitoring the magnetic field of the coil. This continues until roll over 441 of the magnetic field is detected, i.e. the rate of change of the coil magnetic field with current decreases, indicating that the coil approaching saturation. The temperature of the coil is then decreased 431 in order to increase the critical current I.sub.C 421 and hence decrease the ratio I/I.sub.C to the threshold described in the previous examples. The transport current is then ramped up again until a further rollover 441 is detected, at which point the temperature is decreased again as described above, and the cycle continues until the transport current reaches the desired final transport current I.sub.2.

[0070] As in the previous examples, the I/I.sub.c threshold may be, for example, greater than 0.7 times the critical current, greater than 0.75 times the critical current, greater than 0.8, greater than 0.9, or greater than 0.95.

[0071] For the initial ramp-up of a coil, i.e. where the initial transport current for the ramp is zero or near-zero, maintaining an I/I.sub.C ratio above a given threshold would require I.sub.C to also be zero or near-zero. One possible method for initiating the ramp-up is to provide a small amount of initial current to the coil before cooling it below the critical temperature of the HTS (i.e. while the HTS is not superconducting), and then cool the HTS below the critical temperature to a point where the ratio between the initially supplied current and the critical current is above the desired I/I.sub.C ratio. An alternative which avoids the need to supply current to the coil when the HTS is not superconducting is to set the initial temperature of the coil such that the ratio I/I.sub.C will be above the threshold at a predetermined small but non-zero current I.sub.0. The coil is then ramped up from zero transport current to the point where the transport current equals the predetermined current I.sub.0, while maintaining that initial temperature. The ramp is then continued from the predetermined current I.sub.0 to the final target transport current using one of the methods previously described, i.e. with the predetermined current acting as the initial current for those methods. Similar measures may be taken for the final ramp-down of a coil, i.e. where the final transport current is equal to zero. As such, where initial transport current and final transport current are used, particularly for examples relying on the ratio I/I.sub.c, they may be non-zero currents.

[0072] The modelled results of an alternative control scheme are illustrated in FIG. 5. FIG. is a sequence of graphs of different properties of a magnet over time during energisation of the magnet. In this example, a magnet comprising several HTS field coils is ramped up such that the non-inductive start-to-end voltage is kept approximately constant around 1 mV. The non-inductive start-to-end voltage is the voltage across all HTS coils, disregarding contributions due to inductive voltages on the coils (ie: voltages induced by a rate of change of current). This is measured by, for example, monitoring the total voltage across the coils, and then correcting this by vector subtracting the voltage across a co-wound or otherwise inductively coupled secondary coil. The net cooling power applied to the coil (which may be controlled by varying the output of coil heaters), or ramp rate (i.e. the rate of change of transport current), are controlled to maintain the target non-inductive voltage. This method of ramping keeps the coil near-saturation during the ramp, effectively maintaining the I/I.sub.C ratio above a threshold as described earlier. As can be seen from the graphs in FIG. 5, the I/I.sub.C ratio in each of the HTS coils remains relatively high after the initial portion of the ramp, and the current margin (i.e. spare current in which screening currents could form) remains low throughout the ramp. It will also be appreciated from FIG. 5 that the critical current of the HTS material varies in different parts of the magnet, due to factors including local temperature and magnetic field, but that the I/I.sub.C ratio can be maintained above a threshold in at least a part of the coil. This control scheme can be implemented as a feedback loop which monitors the non-inductive start-to-end voltage and controls the ramp current to maintain that non-inductive start-to-end voltage above a threshold (or at a constant value). Example thresholds may at least 0.5 microvolts per meter of HTS conductor length (V/m), at least 1 V/m, at least 10 V/m, or at least 100 V/m (which will approximately correspond to running at I>I.sub.C).

[0073] Equivalently, this may be performed for a single-coil system, or the measurements may be made separately for each coil in a multi-coil system (with the control system balancing the need to keep the currents in the coils balanced, with the need to keep the non-inductive start-to-end voltage of each coil above the chosen threshold).

[0074] The secondary coil may be co-wound with the HTS coil, e.g. as backing wire on the HTS conductors of the coil, or it may be provided in close proximity to the HTS coil, e.g. integrated into assemblies bonded onto the side of the coil to provide connection to other HTS coils, such as those disclosed in WO 2020/079412 A1.

[0075] The secondary coil may be connected at one end to a corresponding end of the HTS coil, and the voltage measured between the other end of the secondary coil and the other end of the HTS coil (hereafter the pickup voltage). The non-inductive component of the start-to-end voltage across the HTS is then the difference between the measured start-to-end voltage of the HTS coil and the pickup voltage.

[0076] Another alternative control scheme is illustrated in FIGS. 6A and 6B. In this example, a magnet comprising several HTS field coils is ramped up and the coil temperatures accurately monitored (FIG. 6A). The coil heaters, or ramp rate, are controlled to maintain the target rate of coil temperature decrease as indicated by slope 601. At time t1, the rate of decrease slows to a slope 602, this indicates some parts of the magnet are entering saturation. The heater power or the coil ramp rate is decreased by the control system to keep the (negative) rate of coil temperature change within the desired range, e.g. returning to a temperature decrease with slope 601. This method of ramping keeps the coil near-saturation during the ramp, effectively maintaining the I/I.sub.C ratio above a threshold as described earlier. As can be seen from FIG. 6B, the I/I.sub.C ratio in each of the HTS coils remains relatively high after the initial portion of the ramp, and the current margin (i.e. spare current in which screening currents could form) remains low throughout the ramp. This control scheme can be implemented as a feedback loop which monitors the coil temperatures and controls the ramp current or net cooling power to maintain the rate of change of temperature below a threshold, within a given range, or at a constant value. Suitable thresholds (or ranges or values) depend on the particular characteristic of the magnet, but the expected temperature decrease for given known heat inputs (i.e. ramp rate, cooling power, and any applied heating from heaters or the like) can be readily determined by the skilled person by modelling or simulation methods as well known in the art, or determined through calibration of the HTS coil, and may then be used to calculate the expected rate of change of temperature for a given saturation and thereby determine an appropriate threshold or range.

[0077] While the above examples have focused on ramping-up of the coil, i.e. where the initial transport current is less than the final transport current, it will be appreciated that the same principles apply to ramp-down of the coil, i.e. where the initial transport current is greater than the final transport current. In this case, the critical current I.sub.C is reduced as the magnet is ramped down (e.g. by increasing the coil temperature), in order to maintain the ratio of the transport current and the critical current/I.sub.C above the desired threshold.

[0078] The variation in the transport current between the initial transport current and the final transport current may be a monotonic variation (i.e. always increasing or steady for ramp-up, or always decreasing or steady for ramp-down), or it may be a non-monotonic variation. The important characteristic for the above examples is the ratio I/I.sub.C, and thereby the spare capacity in which screening currents may form, rather than the particular nature of the ramp.

[0079] Other measures for controlling screening currents in steady state coils may be used following the rampe.g. oscillating the transport current or applying an oscillating external magnetic field to scramble the screening currents and reduce their net effect on the final field.