Undulators are magnetic devices consisting of an array of alternating magnets. They are usually installed on the straight sections of storage ring based light sources. When an electron beam passes through an undulator, it moves in a ‘wiggle’ shaped trajectory, emitting an intense beam of light.

Most undulators installed on light sources use permanent magnets. A novel type of undulator using superconducting wires to generate the magnetic fields has been developed by the MaRS group in collaboration with engineers and scientists at the Rutherford Appleton Laboratory’s Technology Department. The eventual aim of this project is to construct and install a 2m long superconducting undulator at Diamond Light Source.

A superconducting undulator has several advantages over conventional undulator technology. It can reach higher fields and use shorter periods, meaning that it can produce extremely intense beams of light at very short wavelengths. However, the technology required to build such a device is far from straightforward. In order for the wire to become superconducting, the undulator must be cooled to a temperature of 4.2K (-269°C) using liquid helium. To further enhance the field, it is planned to operate the undulator at an even lower temperature of 1.8K (-271°C). The superconducting wires are wound around a steel former to concentrate the field. This steel structure also presents some engineering challenges. In order to provide the highest quality magnetic field, the steel poles must be machined to a flatness of less than 20µm (0.02mm). Similar tolerances apply to the pitch (the distance between the poles) and to the alignment between formers.

The team at RAL, led by Tim Hayler and Tom Bradshaw, have been working on a short prototype of this undulator, using four 30 cm long steel formers. The superconducting wire has been painstakingly wound around these formers in a helical pattern to produce the undulator's alternating magnetic field. This undulator prototype was measured at RAL and then brought to Daresbury in 2018. As part of CLARA's exploitation programme, an electron beam was passed through it. This was the first test with beam of any undulator of this type. We were able to identify a field offset in the undulator which had a detrimental effect on the beam - future revisions of the planar concept will be modified to remove this field offset and improve performance.

The next part of the SCU programme is to develop a helical superconducting undulator, aimed towards parameters for the CompactLight design study. The first step will be to design and build a short 30cm prototype of a helical device. Winding trials are ongoing with copper wire; rapid development is made possible with some 3D printed parts (see attached photo). We will then develop a full-scale 2m undulator, complete with other components required for installation on a FEL facility such as inter-undulator quadrupoles and phase shifters.

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