The ITER Magnet System comprises 18 superconducting Toroidal Field and 6 Poloidal Field coils, a Central Solenoid, and a set of Correction coils that magnetically confine, shape and control the plasma inside the vacuum vessel.

The superconducting material for both the Central Solenoid and the Toroidal Field coils is designed to achieve operation at high magnetic field (13 Tesla), and is a special brittle intermetallic made of Niobium and Tin (Nb3Sn). The Poloidal Field coils and the Correction coils operate at lower magnetic fields use a ductile Niobium-Titanium (NbTi) alloy. In order to achieve superconductivity, all coils are cooled with supercritical Helium in the range of 4 Kelvin (-269°C). Superconductivity offers an attractive ratio of power consumption to cost for the long plasma pulses envisaged for the ITER machine.*

Niobium tin strand testing

Durham University has been undertaking research into the properties of superconducting materials under strain (tensile and compressive) for a number of years, as part of a research programme sponsored by EPSRC. The research is aimed at understanding the fundamental mechanics and utility of superconducting material in high field applications such as medical scanners, particle accelerators and fusion reactors.

Typically the strand is tested in large magnetic fields at cryogenic temperatures similar to those encountered in fusion magnets.   The test results are used to measure the strain caused by:

- thermal mismatch of materials going from room temp to 4/5 Kelvin, and

- the effect of current and the enormous Lorentz forces produced during high-field operation.

F4E approached Durham University because they had demonstrated the ability to achieve accurate strain measurements, based on their experience with earlier prototype magnets.

The Durham University team is led by Professor Damian Hampshire, Head of Superconductivity Group, Dept of Physics, “Our research allows us to understand how strain effects the material's superconducting properties. The F4E project is to measure how much current the wire can carry as a function of strain, magnetic field and temperature, and in so doing mimic the conditions likely to be experienced in the ITER tokamak. A commercial fusion reactor will need to use superconducting magnets to be economic – after ITER the Japanese are intending to build a 2 GW system that will be on the grid.”

The three year test programme is being undertaken in collaboration with Twente Universty in Holland. Twente are looking at other types of complementary strain measurements including period strain variations.

“The team at Twente is expert on measuring large conductors and investigating periodic strain variations.  We are expert at performing in-depth strain measurements on single strands (wires and tapes)” noted Professor Hampshire.

* source: www.iter.org

For more information on Durham University's research into superconductivity visit  http://www.dur.ac.uk/superconductivity.durham/