Patent Application
20250029761
The present invention relates to superconducting magnets. In particular, the present invention relates to methods of ramping (i.e. energising or de-energising, changing the transport current of) a superconducting magnet and apparatus implementing those methods.
Superconducting materials are typically divided into “high temperature superconductors” (HTS) and “low temperature superconductors” (LTS). LTS materials, such as Nb and NbTi, are metals or metal alloys whose superconductivity can be described by BCS theory. All low temperature superconductors have a self-field critical temperature (the temperature above which the material cannot be superconducting even in zero external magnetic field) below about 30K. The behaviour of HTS material is not described by BCS theory, and such materials may have self-field critical temperatures above about 30K (though it should be noted that it is the physical differences in composition and superconducting operation, rather than the self-field critical temperature, which define HTS and LTS material). The most commonly used HTS are “cuprate superconductors”—ceramics based on cuprates (compounds containing a copper oxide group), such as BSCCO, or ReBCO (where Re is a rare earth element, commonly Y, Eu or Gd). Other HTS materials include iron pnictides (e.g. FeAs and FeSe) and magnesium diborate (MgB2).
ReBCO is typically manufactured as tapes, with a structure as shown in
In addition, “exfoliated” HTS tape can be manufactured, which lacks a substrate and buffer stack, but typically has a “surrounding coating” of silver, i.e. layers on both sides and the edges of the HTS layer. Tape which has a substrate will be referred to as “substrated” HTS tape.
An HTS cable comprises one or more HTS tapes, which are connected along their length via conductive material (normally copper). The HTS tapes may be stacked (i.e. arranged such that the HTS layers are parallel), or they may have some other arrangement of tapes, which may vary along the length of the cable. Notable special cases of HTS cables are single HTS tapes, and HTS pairs. HTS pairs comprise a pair of HTS tapes, arranged such that the HTS layers are parallel. Where substrated tape is used, HTS pairs may be type-0 (with the HTS layers facing each other), type-1 (with the HTS layer of one tape facing the substrate of the other), or type-2 (with the substrates facing each other). Cables comprising more than 2 tapes may arrange some or all of the tapes in HTS pairs. Stacked HTS tapes may comprise various arrangements of HTS pairs, most commonly either a stack of type-1 pairs or a stack of type-0 pairs and (or, equivalently, type-2 pairs). HTS cables may comprise a mix of substrated and exfoliated tape.
A superconducting magnet is formed by arranging HTS cables (or individual HTS tapes, which for the purpose of this description can be treated as a single-tape cable) into coils, either by winding the HTS cables or by providing sections of the coil made from HTS cables and joining them together. HTS coils come in three broad classes:
Non-insulated coils could also be considered as the low-resistance case of partially insulated coils.
One example use of HTS field coils is in tokamak plasma chambers, where any or all of the poloidal field coils or toroidal field coils may be implemented as HTS coils. This is of particular use in spherical tokamaks, where the current density in the central column of the toroidal field coils is a key design parameter.
In the following discussion a magnet is defined as comprising a number of HTS coils connected in series.
Energising or charging a non-insulated or partially insulated HTS magnet is more complex than energizing a fully insulated coil as the current can take two paths, either around the spiral high inductance path, or through the radial low inductance path. The spiral path has negligible resistance when the coil is fully superconducting, whilst the radial path is resistive. During energization (ie: ramping the coil by applying a voltage from a power supply to the terminals to drive a transport current), the inductive voltage developed by changing current in the spiral path will drive some of the power supply current into the radial path. The exact split in current can be calculated as known in the art. If the ramp rate is increased, more current flows in the radial path, causing more heating. In large coils, the maximum ramp rate will be set by the available cooling power, ie: the heating caused by radial current flow during ramping must not cause the coil temperature to increase so much that it become non-superconducting.
After ramping the power supply voltage drops to the level needed only to drive current through the residual resistance of the spiral path of magnet. The magnet then enters the “stabilisation phase”, where the magnet is maintained at the operating current for sufficient time for the magnetic field to stabilise.
The instabilities in the magnetic field arise from parasitic currents induced in the magnet (in addition to the desired transport current), which each contribute towards the magnetic field of the magnet. These currents come in three types:
The phrase “closed loop of current” means that the current flows entirely within the specified material(s), and does not start or terminate at the power supply or current leads.
In the case of HTS coated conductors, such as REBCO, the screening currents flow in each individual length of continuous defect-free tape, flowing along one edge and returning along the other edge. Therefore the loop current in each generates a small magnetic field, called a screening field because it is generated by supercurrents that are induced with orientation that tends to inhibit the magnet flux generated by the rest of the magnet from penetrating the tape.
In “steady state” applications, where the magnetic field of the magnet does not change quickly, the eddy currents and coupling currents will decay quickly (exponentially, with a time constant on the order of a few seconds), due to the resistance of the materials they travel through. However, screening currents will persist indefinitely, and change over long timescales (with a time constant on the order of minutes, hours, or even months). The screening currents also depend on the ramping history of the magnet-meaning that a magnet ramped up quickly will have different screening currents (and therefore a different magnetic field quality) to an identical magnet ramped up slowly, and that a magnet configured to produce 5T which is ramped-up to 5T from a zero-current state will have different field quality to the same magnet ramped up to 5T from a previous steady 3T state.
The magnetic field generated by a superconducting magnet therefore depends on its previous ramp history. It is possible to reset the magnet to a virgin state with no screening currents by raising its temperature above the superconducting transition temperature.
The effect of screening currents is particularly pronounced in HTS magnets using ReBCO or BSCCO tapes, as the large dimension of the superconducting filaments allows larger screening currents to form. The polluting magnetic “screening field” created by screening currents is a severe problem for application of existing HTS tape and coil technology in applications that demand high field homogeneity and stability, such as nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI).
There are a number of methods to reduce the impact of screening currents. The magnet may be ramped up in an oscillatory manner, the magnet may be “shimmed” (i.e. applying a corrective field using a secondary coil), etc.
A further problem created by screening currents is the local stress created by the screening currents flowing in the edges of tapes. It has been theorized that this stress is enough to degrade the critical current of the tapes by locally crushing the REBCO crystal structure. It is therefore possible for the ultimate performance of a magnet to be degraded on its first ramp by the generation of screening currents.
Recent research has found that screening current contribute massively to the stress and strain experienced by an HTS coil, both during stable operation and during ramp-up (Jing Xia et al 2019 Supercond. Sci. Technol. 32 095005), with forces being as much as five times higher than they would be in the absence of screening currents. While many of the above techniques can achieve a reduction in screening currents during steady-state operation, they still result in the potential for large screening currents during ramp-up, which could cause excessive force on the coil.
Therefore there exists a need for a better method of reducing or ideally eliminating formation of screening currents in an HTS magnet during ramp-up.
According to a first aspect, there is provided 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/Ic of the transport current, I, to a critical current, Ic, 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/Ic is maintained above a threshold ratio (e.g. above 0.7) and preferably less than 1.
According to a second aspect, there is provided 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 a final transport current. A non-inductive component of the start to end voltage across the HTS coil is monitored. Cooling is applied to the HTS coil to maintain the temperature of the HTS material at or below the zero field critical temperature of the HTS material. One or both of the rate of change of transport current; or the net cooling power applied to the coil are controlled, where the net cooling power comprises the applied cooling and any applied heating of the coil. The control is 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.
According to a third aspect, there is provided 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 a final transport current. Temperature of the HTS coil is monitored. Cooling is applied to the HTS coil to maintain the temperature of the HTS material at or below the zero field critical temperature of the HTS material. One or both of the rate of change of transport current or the net cooling power applied to the coil are controlled, where the net cooling power comprises the applied cooling and any applied heating of the coil. The control is such that the rate of change of temperature 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.
According to a fourth aspect, there is provided a high temperature superconducting, HTS, magnet system. The HTS magnet system comprises and HTS coil, a power supply, a cooling system, a voltage monitoring system, and a controller. The HTS coil comprises a plurality of turns of HTS material. The power supply is configured to supply a transport current to the HTS coil. The cooling system is 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. The voltage monitoring system is configured to monitor the start-to-end voltage of the HTS coil. The controller is configured to control ramping of the magnet. The controller controls ramping of the magnet by:
According to a fifth aspect, there is provided a high temperature superconducting, HTS, magnet system. The HTS magnet system comprises an HTS coil, a power supply, a cooling system, and a controller. The HTS coil comprises a plurality of turns of HTS material. The power supply is configured to supply a transport current to the HTS coil.
The cooling system is 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. The controller is configured to control ramping of the magnet. The controller controls the ramping of the magnet by:
As previously described in WO 2020/178594 A1, screening currents arise in the “spare” capacity of an HTS magnet—i.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.
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 saturation—above 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 IC 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”.
Saturation may be detected in several ways:
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 saturation—i.e. with current shedding radially between turns, driven by non-inductive voltage due to the HTS being run close to or above critical current—for 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.
As illustrated in
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.
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).
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
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.
While
As in the previous examples, the I/Ic 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.
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/IC ratio above a given threshold would require IC 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/IC 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/IC will be above the threshold at a predetermined small but non-zero current I0. The coil is then ramped up from zero transport current to the point where the transport current equals the predetermined current I0, while maintaining that initial temperature. The ramp is then continued from the predetermined current I0 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/Ic, they may be non-zero currents.
The modelled results of an alternative control scheme are illustrated in
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).
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.
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.
Another alternative control scheme is illustrated in
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 IC 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/IC above the desired threshold.
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/IC, and thereby the “spare capacity” in which screening currents may form, rather than the particular nature of the ramp.
Other measures for controlling screening currents in steady state coils may be used following the ramp—e.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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2117049.3 | Nov 2021 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/083314 | 11/25/2022 | WO |
