ILC NewsLine
Getting Wired for Beam Diagnostics

The laser for the laser wire in the ATF extraction line.

Laser-wire interaction chamber, where electrons enter through the top hole and exit at the bottom. The green cone shows the laser light coming in and traversing the focusing lens and the vacuum window.
[F. Gannaway - D. Howell]

Luminosity is the ILC's magical word. The higher it is, the better the collisions, the better the results, the higher the chance to find the answer to some of the open questions the ILC community hopes to answer. The particle beams need to be long and thin, and the scientists have to know exactly what they looked like before they collided with their counterparts: if they know what went in, they can understand what comes out. The laser wire system, led by a team of universities in the UK, measures the beam size and its location and thus helps to know just what 'went in'.

Looking up from the metal cartridge to which he is soldering wire, Laurent Millischer grins, "This is not like at school. The French system is very much about learning loads of things and doing hand calculations, but I wanted to do some research, too. I love the system where you work in a small area doing lots of different things, and that's really the case here at Oxford." Laurent is both French and Austrian, in his third year of physics at the Ecole Centrale in Paris and in Orsay and works as a summer student for Nicolas Delerue, laser wire expert in Oxford University. He also compared temperature probes of the laser wire results, designed electrical circuits and programmed computer-probe interfaces during his placement.

Oxford is developing a stable laser that exactly follows the ILC pulse structure. Work started about half a year ago, and right now the team is finding out which parts to purchase for the laser that will be shared between several laser wires. Laurent has worked on a laser project before and his summer student placement has him convinced: "It was my first real experience where I saw that physics is not necessarily Nobel Prize orientated theoretical calculations, but I also saw the human aspects in a research group. I would really like to work in fundamental research." The team he works with in Oxford is testing lenses at the moment to demonstrate that it is possible to do a beam scan with a very small laser beam. They are working on a lower-energy prototype at the moment and will move on to a full test once all results are understood.

Laurent Millischer from the Ecole Centrale in Paris worked as a summer student in on the laser wire in Oxford.

The laser wire analysis system uses the Compton effect: the fact that photons interact with electrons, exchanging momentum and thus changing energy and consequently wavelength. A laser sweeps across the electron beam, measuring how many Compton photons are produced. Because the density of the laser is constant, the number of photons tells the scientists how dense the beam is at the location of the laser wire. From this they know the shape of the beam – and the smaller it is vertically (and longer horizontally), the higher the magical luminosity.

About 70 laser wire analysis systems will be installed in the ILC at 35 different locations, including near the electron gun, in the damping rings and of course in the main linac. This has been done before at other accelerators, but for the ILC the resolution needs to be much higher. "We use a green laser beam that is much smaller than the electron beam. It's a big challenge," says Nicolas Delerue. The laser used for a laser-wire is located outside the vacuum and the laser light must be brought inside the accelerator vacuum through a window. To allow tight focusing of the laser light on the electron beam, it is important to have a window of very good quality. The laser-wire team has designed a special chamber onto which a very good quality (fused silica) window can be sealed. It is in this chamber that the compton interactions take place. The UK team – about 15 people from Royal Holloway University London and Oxford – performs tests at KEK's ATF and at DESY, in close collaboration with Japanese teams and a group from SLAC.

-- Barbara Warmbein