GB2174247A

GB2174247A - Superconducting coil

Info

Publication number: GB2174247A
Application number: GB08606844A
Authority: GB
Inventors: Martin Norman Wilson; John Sterrey Hawley Ross; Peter Simon Aptaker
Original assignee: Oxford Instruments Ltd
Current assignee: Oxford Instruments Ltd
Priority date: 1985-03-19
Filing date: 1986-03-19
Publication date: 1986-10-29
Also published as: GB8507083D0; GB8606844D0

Abstract

A superconducting solenoid magnet has a shorted secondary winding 12 positioned on the opposing side of support shell 11 from a coil 10, the secondary winding is coupled magnetically to the coil and current is induced in the secondary winding at the onset of quench. The secondary winding heats up uniformly along its length when current is induced in it and in turn this heats the coil all along its length thus propagating quench in the whole coil and preventing localised excessive temperatures in the coil. The secondary winding may be hoops, a cylinder, or a shorted spiral coil. The secondary winding 12 is positioned on the opposing side (either inside or outside) of the support shell 11 from the coil 10 and is thus shielded from, and does not interfere with, alternating current tests for shorted turns in the coil. <IMAGE>

Description

SPECIFICATION Improvements in superconducting coils This invention relates to superconducting magnets and their protection against damage by quenching. Such magnets can consist of a solenoid coil wound on or within a coil former or shell and housed within a cryostat so that they can operate at temperatures of, typically, 4 degrees Kelvin (-269 degrees centigrade).

When the magnet is operating, quenching can occur if a localised source of heat causes a small region of the magnet to become resistive. This region can propagate until the entire magnet is resistive and all of the stored magnetic energy is dissipated as heat within the magnet.

To protect this type of magnet at quench it is desirable for this region to propagate as quickly as possible throughout the magnet so as to reduce the peak temperature occuring at the origin of the resistive region.

One technique used to achieve this is called "quench-back". A secondary winding comprising one or more shorted turns, is positioned so as to be closely coupled magnetically to the primary winding.

Any change in current in the primary winding due to a local resistive region, for example, will cause a current to flow in the secondary winding. As the secondary winding is resistive, it will be heated uniformly through its length and the heat will flow quickly to the primary winding causing it to revert to the resistive state, i.e. to "quench-back" The quench-back will benefit the primary winding both by spreading the heat uniformly through its length and also by adding to the thermal mass of the coil thereby reducing the peak and average temperatures caused by a quench.

The coil former or shell would itself act as a secondary winding but the material from which it is made has to be chosen to provide mechanical support for the primary winding and this prevents the former having the resistance required to provide the necessary heating. The secondary winding has to be made of a material having an optimum resistance between being too low to cause appreciable heating and so high that insufficient current will flow, which also leads to insufficient heating.

Conventional designs place this secondary winding very close to the primary winding in order to minimise the thermal resistance between the two. This, however, can make the detection of faults in the winding, e.g. shorted turns, impossible as the secondary will appear to the test equipment to be a shorted turn.

The present invention overcomes this problem whilst retaining the benefits provided by the use of a secondary coil, in relation to the quench-back.

According to the present invention, there is provided a superconducting solenoid magnet comprising a cylindrical shell, a coil composed of superconducting material, means to operate said coil at superconducting temperature and a shorted secondary winding, one of said coil and secondary winding being wound inside said shell and the other being wound outside said shell.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing which is a cross-section through a superconducting solenoid magnet.

In the drawing the magnet comprises a superconducting coil 10 wound within a shell 11 made of aluminium alloy. The aluminium alloy shell 11 serves as a support for the magnetic forces on the coil. The drawing does not show the cryostat details of which are well known in the art. The cryostat enables the coil 10 to operate as a superconducting coil.

A secondary winding 12 made from high purity aluminium, is wound around the outside of shell 11, winding 12 being in the form of circular hoops wound close to or embedded in the shell 11. As an alternative, the secondary winding 12 could be in the form of a cylinder fitted closely over the shell 11 or in the form of a shorted spiral coil again wound closely against shell 11.

With this arrangement, when quenching occurs excellent quench-back performance is achieved as the thermal diffusivity of the materials concerned is very low at these temperatures and the heat generated in the second any can be transferred to the primary in a very short time.

When detection equipment is used to test for shorted turns in the superconducting coil, the test is performed at ambient temperature using alternating current. A fairly high frequency alternating current is used in this test and it is attenuated by the coil shell and cannot detect the shorted turns of the secondary winding positioned on the opposing side of the coil shell. Thus, the turns of the secondary winding are completely shielded by the coil shell from the detector(s). In this way the secondary winding will not interfere with the shorted turn testing.

It can be seen that the coil can be wound inside the shell with the shorted secondary winding around the outside, as shown in the drawing or, alternatively, the coil can be wound around the outside of the shell with the shorted secondary on the inside.

1. A superconducting solenoid magnet comprising a coil made of superconducting material, means to operate said coil at superconducting temperature, a cylindrical shell arranged to provide mechanical support for said coil against operating forces and a shorted

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in superconducting coils This invention relates to superconducting magnets and their protection against damage by quenching. Such magnets can consist of a solenoid coil wound on or within a coil former or shell and housed within a cryostat so that they can operate at temperatures of, typically, 4 degrees Kelvin (-269 degrees centigrade). When the magnet is operating, quenching can occur if a localised source of heat causes a small region of the magnet to become resistive. This region can propagate until the entire magnet is resistive and all of the stored magnetic energy is dissipated as heat within the magnet. To protect this type of magnet at quench it is desirable for this region to propagate as quickly as possible throughout the magnet so as to reduce the peak temperature occuring at the origin of the resistive region. One technique used to achieve this is called "quench-back". A secondary winding comprising one or more shorted turns, is positioned so as to be closely coupled magnetically to the primary winding. Any change in current in the primary winding due to a local resistive region, for example, will cause a current to flow in the secondary winding. As the secondary winding is resistive, it will be heated uniformly through its length and the heat will flow quickly to the primary winding causing it to revert to the resistive state, i.e. to "quench-back" The quench-back will benefit the primary winding both by spreading the heat uniformly through its length and also by adding to the thermal mass of the coil thereby reducing the peak and average temperatures caused by a quench. The coil former or shell would itself act as a secondary winding but the material from which it is made has to be chosen to provide mechanical support for the primary winding and this prevents the former having the resistance required to provide the necessary heating. The secondary winding has to be made of a material having an optimum resistance between being too low to cause appreciable heating and so high that insufficient current will flow, which also leads to insufficient heating. Conventional designs place this secondary winding very close to the primary winding in order to minimise the thermal resistance between the two. This, however, can make the detection of faults in the winding, e.g. shorted turns, impossible as the secondary will appear to the test equipment to be a shorted turn. The present invention overcomes this problem whilst retaining the benefits provided by the use of a secondary coil, in relation to the quench-back. According to the present invention, there is provided a superconducting solenoid magnet comprising a cylindrical shell, a coil composed of superconducting material, means to operate said coil at superconducting temperature and a shorted secondary winding, one of said coil and secondary winding being wound inside said shell and the other being wound outside said shell. An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing which is a cross-section through a superconducting solenoid magnet. In the drawing the magnet comprises a superconducting coil 10 wound within a shell 11 made of aluminium alloy. The aluminium alloy shell 11 serves as a support for the magnetic forces on the coil. The drawing does not show the cryostat details of which are well known in the art. The cryostat enables the coil 10 to operate as a superconducting coil. A secondary winding 12 made from high purity aluminium, is wound around the outside of shell 11, winding 12 being in the form of circular hoops wound close to or embedded in the shell 11. As an alternative, the secondary winding 12 could be in the form of a cylinder fitted closely over the shell 11 or in the form of a shorted spiral coil again wound closely against shell 11. With this arrangement, when quenching occurs excellent quench-back performance is achieved as the thermal diffusivity of the materials concerned is very low at these temperatures and the heat generated in the second any can be transferred to the primary in a very short time. When detection equipment is used to test for shorted turns in the superconducting coil, the test is performed at ambient temperature using alternating current. A fairly high frequency alternating current is used in this test and it is attenuated by the coil shell and cannot detect the shorted turns of the secondary winding positioned on the opposing side of the coil shell. Thus, the turns of the secondary winding are completely shielded by the coil shell from the detector(s). In this way the secondary winding will not interfere with the shorted turn testing. It can be seen that the coil can be wound inside the shell with the shorted secondary winding around the outside, as shown in the drawing or, alternatively, the coil can be wound around the outside of the shell with the shorted secondary on the inside. CLAIMS

1. A superconducting solenoid magnet comprising a coil made of superconducting material, means to operate said coil at superconducting temperature, a cylindrical shell arranged to provide mechanical support for said coil against operating forces and a shorted secondary winding, characterised in that one of said coil (12) and said shorted secondary winding (10) being wound inside said shell (11) and the other being wound outside said shell.

2. A superconducting solenoid magnet as claimed in claim 1, characterised in that said coil (12) is wound inside said shell (11).