23.08.2013

New emittance world record at PETRA III

The PETRA III machine physicists set a new world record. At the beam energy of 3 Giga electron volts (GeV), they achieved a horizontal beam emittance of 160 picometre radiant (pm rad) – smaller than ever. The emittance is the product of cross-section and bundling of a particle bunch in an accelerator and consequently a measure for the size and order of the bunch. It significantly determines the properties of an accelerator as a light source.

Screen shot for experts: With a synchrotron radiation interferogramme, measuring the synchrotron light from a dipole magnet, the scientist can deduce the beam size and the emittance.

“In our machine studies, we wanted to find out if our theoretical calculations are still valid for such small emittances,” says Alexander Kling, head of the study, “and with 160 pm rad, the experiment perfectly coincided with the expected 158 pm rad.”

For their experiment, the researchers’ group used a three weeks interruption of user operation at PETRA III. They ran the accelerator at energies of 3 and 5 GeV and with several hundred weakly charged electron bunches. With this, the scientists want to extend the application possibilities for PETRA III and to take further steps on the road to the “ultimate storage ring X-ray source”.

A smaller emittance, for example, produces a higher resolution at synchrotron experiments. “With a reduced emittance, the transversal coherence increases and the light becomes more laser-like,” King explains. The group of scientists presented the first results of this study beginning of August at the Three-way Workshop in the United States, a regular meeting of the operators of the world’s leading storage ring X-ray sources.

For the users of synchrotron radiation, this improvement of beam properties is of great advantage. “A higher coherence allows a significant improvement of imaging procedures, for example in X-ray holography,” says Jens Viefhaus, beamline scientist at the PETRA III beamline P04, whose group observed the studies from their user station. “Even in the soft X-ray range, in which PETRA III already has a relatively high degree of coherence compared to other sources; it is possible to gain factors which result in considerably shorter measuring times and more precise and sharper reconstructions.”

When a particle bunch is being stored in a synchrotron light source, the emittance of a particle bunch adjusts to an equilibrium: the regular bending of the electron bunches and the corresponding emittance of synchrotron light leads to an excitation, the acceleration of the particles to compensate the energy loss damps the system. In addition, there are disturbing effects which are caused by the charge and scattering of the particles in a bunch. At very small emittances and low energies, these disturbing effects play a dominant role.

“In the next PETRA machine studies, we will modify additional parameters to study these charge effects and observe their impact on the emittance,” says Alexander Kling. “The experimental verification of the physical models is an important component that helps coming closer to the dream of an ultimate storage ring with the smallest emittance. But I think that the presently achieved record will last for quite a while for the currently running light sources.”