Quench protection in superconducting magnets

Abstract

A method of protecting a superconducting magnet from quenches, the superconducting magnet having at least one primary coil comprising high temperature superconductor, HTS, material. A secondary HTS tape is provided, the secondary HTS tape being in proximity to and electrically insulated from the primary coil, and being configured to cease superconducting at a lower temperature than the primary coil during operation of the magnet. A loss of superconductivity in the secondary HTS tape is detected. In response to said detection, energy is dumped from the primary coil into an external resistive load.

Claims

1. A method of protecting a superconducting magnet from quenches, the superconducting magnet having at least one primary coil comprising high temperature superconductor, HTS, material, the method comprising: providing a secondary HTS tape, the secondary HTS tape being in proximity to and electrically insulated from the primary coil, and being configured to cease superconducting at a lower temperature than the primary coil during operation of the magnet; detecting a loss of superconductivity in the secondary HTS tape; and in response to said detection, dumping energy from the primary coil into an external resistive load.

2. The method according to claim 1, wherein during operation of the magnet a current in the secondary HTS tape is a higher fraction of its critical current than a current in the primary coil.

3. The according to claim 1, wherein the secondary HTS tape is provided as a pair of tapes, with the tapes carrying current in opposite directions and being laid parallel and adjacent to each other.

4. The according to claim 1, wherein detecting a loss of superconductivity comprises detecting a voltage difference in excess of a predetermined value between two points of the secondary HTS tape.

5. The according to claim 1, wherein the difference between the temperature at which the secondary HTS tape ceases superconducting and the temperature at which the HTS tape of the primary coil ceases superconducting is about 5 Kelvin to about 80 Kelvin.

6. A quench protection system for use with a superconducting magnet having at least one primary coil comprising high temperature superconductor, HTS, material, the system comprising: a secondary HTS tape configured to be positioned in proximity to and insulated from the primary coil of the magnet, and to cease superconducting at a lower temperature than the primary coil during operation of the magnet; a detection unit configured to detect a loss of superconductivity in the secondary HTS tape; and a quench protection unit configured to cause energy to be dumped from the primary coil to an external resistive load in response to said detection.

7. The quench protection system according to claim 6, wherein the secondary HTS tape comprises a pair of HTS tapes laid parallel and adjacent to each other and configured to carry current in opposite directions.

8. A superconducting magnet comprising a primary coil and the quench protection system according to claim 7, the primary coil comprising high temperature superconductor, HTS, material.

9. A toroidal or poloidal field coil for a nuclear fusion reactor, the toroidal or poloidal field coil comprising high temperature superconductor, HTS, material and a quench protection system according to claim 7.

10. The method according to claim 1, wherein the secondary HTS tape is configured to be disposable or replaceable with another secondary HTS tape.

11. The quench protection system according to claim 7, wherein the secondary HTS tape is configured to be disposable or replaceable with another secondary HTS tape.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a circuit diagram of a part of a quench protection system;

(2) FIG. 2 is a schematic illustration of a hot spot in an HTS conductor;

(3) FIG. 3 is a graph of temperature against time for a hot spot in an exemplary HTS conductor;

(4) FIG. 4 is a schematic illustration of a superconducting magnet comprising a primary coil and a quench protection system.

DETAILED DESCRIPTION

(5) In order to provide faster and more reliable quench detection, either the noise which obscures the quench detection signal must be reduced, or the signal itself must be increased. A solution is proposed below, which provides greatly improved detection speed and reliability compared to conventional methods.

(6) The second solution involves the use of “canary tape”. A secondary HTS tape is provided in proximity to the (“primary”) coil of the magnet. The secondary tape is configured such that it will cease superconducting at a lower temperature during operation of the magnet, for example at a temperature about 5K to about 80K lower than the temperature at which the primary coil ceases superconducting. The secondary HTS tape may be physically weakened e.g. by neutron irradiation or chemical etching, or may be of an alternative HTS material with lower critical temperature than the HTS of the primary coil.

(7) Since the secondary HTS tape ceases superconducting at a lower temperature than the primary coil, it will generally become resistive sooner than the primary coil, or will quickly become resistive if a hotspot forms in the primary coil. Therefore, a voltage across the secondary tape can be taken as an indication that a quench is likely in the primary tape. Since the secondary tape is not required for the functioning of a magnet, detection of the quench can wait until the voltage drop across the secondary HTS tape is high enough to easily stand out from the electrical noise, which greatly reduces the risk of false alerts, and thus the processing (and time) needed to detect the signal. Once a quench is detected, the quench detection system causes the energy in the magnet to the dumped to an external resistive load, mitigating the effects of the quench.

(8) In one embodiment, the secondary HTS tape is provided as a pair of tapes which are laid parallel and adjacent to each other, and which carry current in opposite directions. This ensures that the field produced by the secondary tapes is substantially cancelled out, and that the size of the loop enclosed by the secondary tape is minimised, which reduces inductive effects.

(9) The secondary tape may carry a much lower current than the primary coil, so that the secondary tape needs very little copper to protect it in the event of a quench. In one embodiment, the secondary tape may be treated as disposable, and provided with little or no protection, but this requires that the energy released by a quench in the secondary tape is not sufficient to cause damage to other components of the magnet. The magnet may be configured such that the secondary tape can be easily replaced after the energy is dumped from the magnet.

(10) Multiple secondary tapes may be provided in different regions of the magnet, with properties dependent on that region. For example, the tape used in the central column of a toroidal field coil at 18 T might not detect a quench in the return limb at 2 T until it was too late, therefore a more sensitive tape may be used in the lower magnetic field.

(11) FIG. 4 shows a superconducting magnet having a primary coil 401 and a quench protection system. The primary coil 401 comprises HTS material. The system comprises a secondary HTS tape 402, a detection unit 403, and a quench protection unit 404. The secondary HTS tape 402 is configured to be positioned in proximity to and insulated from the primary coil 401 of the magnet, and to cease superconducting at a lower temperature than the primary coil during operation of the magnet. The detection unit 403 is configured to detect a loss of superconductivity in the secondary HTS tape 402. The quench protection unit 404 is configured to cause energy to be dumped from the primary coil 401 to an external resistive load 405 in response to said detection.