Martin Wilson BSc, DSc, FInst P, MIEEE.
After obtaining a BSc with first class honours in Physics from Manchester University, Martin Wilson started work in the Nuclear Power Industry, but shortly moved to the Rutherford Laboratory where he initially worked on producing pulsed high magnetic fields by capacitor discharge. He then joined the Superconductivity Group working on the idea of building a superconducting synchrotron. In order to avoid flux jumping, minimize ac losses and reduce field perturbations, the Group developed (in collaboration with IMI Titanium) the filamentary superconducting wires which are now used universally for magnet making. Rutherford Cable was also developed by this group and has been used in every superconducting synchrotron built so far - most recently the LHC. For his work on fine filaments, Rutherford Cable and his book 'Superconducting Magnets', Martin was awarded the IEEE Award for contributions in Applied Superconductivity and (jointly with Al Tollestrupp) the Robert R Wilson Prize of the American Institute of Physics. On the basis of his published work in superconductivity, he was awarded a DSc by Manchester University in 1983.
Moving to Oxford Instruments, Martin supervised two industrial accelerator projects using superconductivity. Firstly Helios was a £10m compact superconducting storage ring X-ray source for use in microchip lithography. Secondly Oscar was a compact medical cyclotron for producing PET isotopes and weighing only ~ 1/10 as much as a conventional machine.
At CERN, he worked on the stability of Rutherford cables for use in the LHC. Returning to Oxford he spent his final years with the company trying to commercialize the use of high temperature superconductors HTS in magnets.
He is the author of 74 scientific papers and published lectures.
Following his retirement, Martin has continued to work as a consultant and an educator in the field of applied superconductivity
The School features 17 lectures on the application of superconductivity – 5 on electronics and 12 on heavy current applications. In this brief introduction I will present the overall commercial picture and then give some attention to those applications not covered by other speakers. Market surveys show that activity in superconductivity worldwide is dominated by magnets and high current applications, with superconducting electronic devices so far claiming only a small share of the market. Scientific research was the first application to use superconductivity, with NMR spectroscopy becoming the first area for commercial products. This research naturally led to the use of NMR for medical imaging, now known as MRI, and MRI scanner magnets now dominate the market sector with ~ 75% of the total worldwide business in superconducting products. Large scale applications are dominated by particle accelerators and thermonuclear fusion. Other heavy current applications include magnetic separation, maglev, induction heating, bearings, power generation, energy storage, transformers and fault current limiters.
Some 35 years after their discovery, high temperature superconductors HTS have finally started to make a commercial impact and their use is now growing. Prospects for the application of HTS will be discussed.
Because they have no Ohmic dissipation, superconducting magnets are able to reach high fields, but unlike conventional electromagnets they cannot use an iron yoke to shape the field because iron saturates at ~ 1.8T. In the absence of iron, the field must be shaped entirely by the winding configuration. The different windings needed to produce solenoid, dipole and toroidal field shapes will be described, together with the electromagnetic forces and resulting stresses produced in the windings and their supporting structure.
Quenching occurs when a point within the magnet windings goes from superconducting to resistive state. Intense Ohmic heating ensues and the resistive zone grows by thermal conduction so that the magnet current decays via the growing internal resistance. If the current does not decay quickly enough, the temperature at the point where the quench started may be high enough to destroy the magnet. Methods of calculating quench behaviour and of protecting against damage by quenching will be described. The resistive zone in an HTS winding grows much more slowly than in an LTS winding, which makes the quench protection problem much more difficult and requires new ideas to protect the magnet from damage.
A common experience in building LTS magnets is that they quench before reaching the current and field expected from the critical properties of the superconductor. Furthermore, the magnet performance usually improves after repeated energizations, an effect known as 'training'. The problem is thought to be caused by sudden small releases of energy within the winding which, combined with the extremely low specific heat of materials at low temperature, can produce temperature rises of several degrees. The magnetic instability, known as 'flux jumping', can release enough energy to trigger a quench, but can be cured by making the superconductor in the form of fine filaments. Conductor motion can also release energy and must be avoided by careful mechanical design of the magnet winding. Techniques for improving performance are generally known as stabilization. HTS magnets are more sable because specific heats are much higher at higher temperatures.
Because flux motion through hard superconductors is a dissipative process, all superconductors suffer from ac losses in changing magnetic fields. HTS and LTS conductors are affected equally, but the problem is more serious for LTS because refrigeration costs are higher at low temperatures. Fine filaments can reduce ac losses by reducing the distance that the flux moves through the superconductor. For convenient handling, many fine filaments are embedded in a resistive matrix and made into a composite wire. If high currents are needed, many filamentary wires are made into a cable. In both these situations, the filaments are coupled together magnetically, which causes undesirable behaviour. The steps needed to minimize coupling in filamentary composite wires and cables will be described.