More light thanks to coherence

In storage rings designed to generate synchrotron radiation, the electrons circulate in individual clusters ("bunches"). Conventional electron-optical methods do not allow the length of these bunches (typically a few millimeters) to be kept so short as to be of the same order of magnitude as the wavelength of the synchrotron radiation (from a few 10 nm to a few 100 nm), which would allow coherent emission with much higher intensities.

It has now been experimentally demonstrated for the first time that structures as short as 1 μm (micro bunches), which had been generated in an electron bunch of a few millimeters in length by means of a superimposed laser beam, still emit coherent radiation even after a full revolution around the storage ring. The intensity of this coherent synchrotron radiation is scaled to the squared number of electrons involved, contrary to non-coherent radiation, whose intensity is only linear with the number of electrons. Maintaining such short microstructures during their revolution in the electron storage ring is a technically very demanding task, since such structures normally disintegrate within a few meters of rotating in the ring due to the radiation generated in the magnetic fields – which leads to an energy loss in the electrons.

This first piece of evidence of how to maintain an imprinted microstructure over a whole revolution is an important step towards an SSMB synchrotron radiation source. For this purpose, the MLS was operated with special optomagnetic settings. Hereby, the length of the revolution of an individual electron depends only very little on the electron's energy. The MLS is the first – and currently the only – electron storage ring worldwide which has been optimized for this specific operating mode.

After a revolution, the electron bunches microstructured by the laser radiation emit coherently elevated radiation compared to the bunches that have not interacted with the laser radiation. The MLS undulator was used to generate the radiation.

The experiments on SSMB, which are being coordinated from Tsinghua University, can be continued and enhanced at the MLS. These experiments aim to obtain a so-called steady state of the microstructures imprinted onto the electron bunches.