The finest beam of the world

Picture of the pixel camera, which is used to measure the emittance of the PETRA III beam.

Shortly after start-up, the PETRA III accelerator already set a world record in “lowest emittance”. The measure of the size of the light spot is a key parameter of the facility, making PETRA III the best storage ring light source worldwide.

In this summer, a total of 20 so-called wiggler magnets were built in and gradually started operation in the north and west sector of the 2.3-kilometre-long PETRA III accelerator. The wigglers “damp” the positron beam to guarantee the reduction of the beam’s transverse size in longitudinal direction. Only with a small beam it is possible to provide the experimenters with small and highly intense light spots.

Now that all damping wigglers are in operation, the PETRA III team was able to carry out measurements, with the result that PETRA III achieved its horizontal design emittance of 1 nm rad. With this, PETRA III holds the world record of the smallest horizontal emittance ever measured at a storage ring light source.

The measurement of the emittance at PETRA III is carried out at an extra beamline at the end of the new experimental hall. At this beamline, the synchrotron light is read out with a pixel camera from a bending magnet. The diameter size of the light spot gives information about the cross section area of the particle bunch.