Researchers at Berkeley Lab’s Superconducting Magnet Group have created a record breaking niobium-tin dipole electromagnet with a field-strength of 14.7 Tesla, which is more than 300,000 times the strength of Earth’s magnetic field.
Dubbed RD-3, the new World record-holding magnet is one metre long, weighs several tons and consists of three magnetic coil modules that were wound from more than eight miles worth of niobium-tin wire.
Dipole magnets are used to bend and maintain the path of accelerating particle beams. The higher the field strengths of the magnets, the tighter the arc of the beam. With stronger dipole magnets, an accelerator can push particles to much higher relativistic energies around the same-sized circular beam path.
Today’s most powerful particle accelerators, including the Tevatron and the Large Hadron Collider, rely on dipoles fashioned out of a niobium-titanium alloy.
This material, whilst ductile, is said to be limited to a field strength no greater than 10 Tesla. The niobium-tin superconductor was believed in theory to be capable of reaching field strengths in excess of 14 Tesla.
However, until the 1997 record-setting performance of the Superconducting Magnet Group’s D20 magnet, niobium-tin was considered too brittle and fragile to be able to withstand the forces that threaten to push the coil windings apart.
‘These forces are enormous, about 3 million pounds or more than the combined thrust of more than a dozen 747 planes,’ said Steve Gourlay, a physicist with the Accelerator and Fusion Research Division (AFRD). ‘To withstand this force, we needed a really good support structure design.’
The design Gourlay and his colleagues employed is centred on a ‘common-coil racetrack’ geometry, in which a pair of coils shaped like an oval racetrack are shared between two apertures to produce opposing magnetic fields.
To overcome the brittleness factor of niobium-tin, Gourlay and his team made their coil modules using a ‘wind and react’ technique.
The cable was made from separate strands of niobium and tin and was then wound around an iron ‘pole piece’ and impregnated with epoxy filler to make each coil module.
Not until after the cable was wound into the three coil modules were the strands ‘reacted’ to make the superconducting alloy. This reaction was accomplished by heating the cable to about 950 Kelvin (680 degrees Celsius) and baking it at that temperature for two weeks.
To complete the magnet, the coils were encased in an iron yoke then wrapped in a 40-millimetre-thick aluminium shell.
For the coils to become superconducting, they had to be cooled to a temperature of about 4.2 Kelvin (-270 degrees Celsius), which started the process whereby a magnet is ‘trained’ to attain its peak field strength.
The magnet is then chilled to make its coils superconducting, then energised up in field strength until an inadvertent warming along some part of the coils causes the magnet to lose its superconductivity.
This temporary loss of superconductivity is called ‘quenching.’ After quenching occurs, the magnet is re-cooled and training resumes. The process is repeated until the magnet reaches the field-strength limit dictated by the properties of its superconductor.