Members of the HeLiCal collaboration with the undulator ready for testing.

Duncan Scott from Daresbury Laboratory inspecting the superconducting undulator prior to testing.

A slice through one of the early prototypes showing the superconducting cable packing arrangement in close-up.

The ILC's positrons might be wondering where their second damping ring has gone and whether to get damped in the middle or at the end of the machine. Where they come from, however, is pretty clear, and teams in the UK and the US are making sure that their moment of creating is both well controlled and well understood. A 4th –generation undulator prototype has just been tested at Rutherford Appleton lab in the UK, and tests are looking very good.

Undulators, with their arrays of magnets arranged in alternating order, make electrons wiggle when they pass through. The electrons send out high-energy photons, these hit a target, and positrons come out at the other end. This sounds less miraculous when you're in the business. "Undulators as such are well understood," says Daresbury's Jim Clarke, "but quite unknown in high-energy physics. We're looking at their effect on the beam quality, emittance and trajectory. The good news is: it did what we hoped it would do!"

The HeLiCal Collaboration has already tested several prototypes that approach the baseline design of a superconducting undulator with a period as close to 10 mm as possible to produce 10 MeV-photons. The first prototype dates back to TESLA times, the second was engineered differently, and this summer prototypes 3 and 4, with a period of 12 mm and a length of about 30 cm each, yielded first results. Number 4 is special because it contains iron, and it's the iron's effect that the collaboration wanted to measure. The undulator was cooled down to 4 K and a Hall probe passed through to measure the magnetic field. The test results showed excellent agreement with the computer simulations. The peak on-axis magnetic field achieved was about 1.1 T which comfortably exceeds the design operating point of 0.8 T needed to generate 10 MeV photons.

The next step is to build a longer prototype – number 5 will measure 50 cm with a period of 11.5 mm and use a new construction technique. It will be ready in late autumn. By summer 2007 a 4-metre module, consisting of two undulators of 1.75 metres each is supposed to be complete. Daresbury is hoping to get funding for a new test beam so that the longer prototype can be tested by passing an electron beam through. "The most accurate probe of a magnetic field is an electron," explains Clarke. The collaboration plans to use a low energy electron beam so that visible light is produced rather than gamma photons. This would enable the group to easily measure the photon polarisation, which is a key indicator of the quality of the undulator. Eventually the ILC will need an undulator of about 100 m to generate the required number of positrons.

-- Barbara Warmbein