Laserwire
The next generation of particle collider is likely to consist of two linear accelerators pointing towards each other. In this design the two bunches of particles will have only one opportunity to collide and any particles that don't collide will be wasted. In a circular collider such as the LHC, the particles circulate for many hours until the bunches are depeleted. In a linear collider such as the ILC or CLIC the particle bunches will have to be squeezed to around 5nm to achieve the high collision rate required. This requires keeping a very close eye on how particle beam is accelerated and transported throughout the whole machine.
Schematic of the International Linear Collider (ILC) with two linear accelerators.
Laser photons Compton scattered from a particle
beam producing gamma rays detected downstream.
Particle beam emittance is traditionally measured using a wire scanner wherein a tungsten wire is scanned through the particle beam. As the wire is scanned across the beam, the detected Bremsstrahlung radiation increases with the density of the beam and the overlap between the beam and the wire. Knowing the shape and size of the wire, the electron beam profile can be reconstructed. With high energy, high charge and small particle beams the wire is destroyed. Laserwire achieves the same result using a focussed laser beam to scan across the particle beam. Photons are Compton scattered from the particles and are detected further down the accelerator. Additionally, this is a non-destructive technique allowing further measurements downstrean, This technique can be used for electrons, positrons and hydrogen. The laser can be focussed to a micron level which is at least 10x smaller than the finest wire. Additionally, the laser only interacts with around 1 a million particles so most are left unaffected allowing further measurements of the particle beam downstream. A typical laserwire scan is shown below.
This technique could also be used on a light source accelerator such as Diamond where the stored electron beams have a very low vertical emittance that is beyond the resolution of normal wire scanners.
We are currently working at the ATF2 at KEK in Japan with Royal Holloway University of London. Our first results are presented in (1).
Typical laserwire scan of an electron beam.