Rutherford Appleton Laboratory has a long history of particle physics contributions. With over 80 years of history​, this laboratory has conducted experiments with onsite accelerators, manufactured particle detectors as well as coordinating the UK effort at CERN. A highlight among this pioneering work is Rutherford Cable, a configuration of wires that enabled the practical use of superconducting magnets in particle accelerators for the first time. This innovation - developed by the predecessor ​to STFC’s Technology Department​ - allowed for accelerators to achieve ultra-high particle energies whilst remaining both compact and cost effective.  ​

Superconductivity

In 1961, almost 50 years following its discovery, materials capable of superconductivity in high fields had been discovered. If the low temperatures required for superconductivity could be maintained, then powerful magnetic fields could be generated with no power consumption. An alluring alternative to the iron core electromagnets used at the time, which required high currents to pass through coils of copper conductor. The discovery came at a time where power consumption through conductor resistance and cooling was becoming unsustainable.   

Unfortunately, hopes for application of these materials were dashed, with small coil prototypes being sensitive to a thermodynamic instability known as flux jumping. A small team headed by Dr Peter Smith, a physicist at Rutherford Appleton Laboratory, RAL (then Rutherford High Energy Laboratory) began conducting theoretical study into a coil design that would prevent flux jumps. This team would later expand to the ‘superconducting applications group’, who’d work to engineer complex magnet designs and their associated cryogenics.  At the time, RAL was primarily devoted to Particle Physics. Whilst this R&D program received dedicated funding, it was relatively modest compared to major projects. Despite these limited means, a small and determined team made an extraordinary impact on particle physics.  

Transmission Electron Microscope image of Superconducting Alloy, Niobium Titanium: 

Niobium Titanium was discovered as having favourable properties for superconducting magnet applications in 1962. It was selected among others due to its ductility - it was able to be drawn out into long, thin wires without cracking​. Whilst it could not sustain as high fields or current densities as other materials - this mechanical robustness made it the most viable option for magnets at the time.

It gained the nickname 'Workhorse of Superconductivity'

Development at RAL

Physicists, Dr Peter Smith and Dr Martin Wilson, proposed dividing the superconductor into filaments of >50µm to resolve this instability. A conductive matrix, typically copper, would surround these fine filaments to ensure any temperature rise was mitigated and superconductive state was maintained. 10s or even 100s of twisted filaments would collectively form a single strand. These strands were termed 'superconducting filamentary composites'. RAL would engineer their first prototype in collaboration with Imperial Metal Industries in 1967.  

Prototype coil manufactured in 1967 by the IMI and RAL collaboration:

This successful prototype test verified years of their theoretical study, demonstrating that the design prevented flux jumps and limited sources of loss within time varying magnetic fields, such as those used within synchrotrons. Synchrotrons are particle accelerators that use a ramping magnetic field with increasing pulses that correspond to the greater energy of the accelerating beam.

Cross section of a 1000 filament conductor (1971):

20 - 50 of these strands were flattened to form a cable. The superconducting applications group identified transposition as a fundamental feature of this cable. Transposition ensured equal distribution of the current and limited losses in changing fields - complimenting the reduction of losses achieved by twisting. 

Strand transposition in Rutherford Cable. 

The position of the strand at the cross section will change across the cables length.

 This can be seen if you follow a strand from its initial position and trace it along the cable. 

Image Courtesy of Fermilab

The Superconducting Applications group experimented with different transposition arrangements. Whilst each were stable, reducing sources of loss in ramping fields to tolerable levels, they varied in packing factor - the ratio between superconducting wires and the conductive matrix. Higher density of superconducting wire permits a greater current density and thus greater magnetic fields for a given volume of cable. ​'Rutherford Cable', first implemented into RAL's AC5 prototype (pictured below), was announced to widespread acclaim within the field. Its namesake pays tribute to the laboratory where it was conceived.  

Diagram showing different forms of transposition

​From the left to right depicts braid, rope and Rutherford (as it would later be termed).

All were assessed in magnet prototypes developed at RAL or at Laboratories in Europe, particualrly Karlsruhe and Saclay.

Rutherford Transposition was selected due to its high packing factor.