REGAE is a joint project of the CFEL partners Max Planck Society, University of Hamburg and DESY. The approximately ten metres-long facility will generate highly coherent ultra-short electron pulses to carry out time resolved structural investigations of crystallised materials and possible push the boundaries to in situ studies of liquids, surface and solution phase chemistry on the nanoscale. In the same way as scientists shoot light from storage rings or in free-electron lasers at materials to deduce the molecular structure from the diffraction patterns, these experiments are also possible with electrons. In this respect, there is a long history of complementarity between X-ray and electron probes. This facility offers a new tool to explore systems that is best suited for nanoscale materials due to the short penetration depth of electrons relative to X-rays.
In terms of time resolution for tracking atomic motions,, REGAE will produce electron bunches with only about 10 femtoseconds length (one millionth of one billionth of a second), thus allowing experiments with an extremely high time resolution. The diameter of an electron bunch is half a millimetre; the length however is only one fifth of the diameter of a hair. When the electrons hit the object of investigation, they are diffracted by the molecular structure of the sample. The angles of diffraction – a measure for the interatomic distances in the sample – are measured with high precision with an innovative CCD detector. This experimental method is comparable to the one of a transmission electron microscope.
Dwayne Miller (CFEL), head of the project, enthusiastically says: “With our newest developments in the production of intensive electron pulses lasting only femtoseconds, REGAE will enable direct observation of the movements of atoms at this time scale – molecular movies can now capture all the actors (i.e. atoms), even the very fastest, in full motion!”
REGAE has an electron source built similar to that one in FLASH, but working with a radio frequency of 3 Gigahertz. The electron bunches, each filled with an amount of electrons a thousand times less than in FLASH, are accelerated to a total energy of 5 Mega electronvolts (MeV) and packed tightly together with a special accelerating unit: a so-called buncher cavity accelerates the particles at the end and slows down those in front of the bunch. The optical laser which triggers the electron bunch in the source can be used simultaneously to excite the sample, allowing so-called pump-probe experiments.
Although REGAE on first sight clearly differs from FLASH and the European XFEL, not only in size, there are considerable synergies with the free-electron lasers. “At REGAE, we profit very much from the experience we gathered at FLASH,” accelerator physicist Klaus Flöttmann (DESY) explains. Indeed, the parameters of REGAE are very sophisticated, but on the basis of our experience we are confident that very soon we will have a very good method for time-resolved electron diffraction, which will also take care that the samples remain undamaged.” At the same time, the scientists will use their REGAE experience for FEL operation – synchronising and timing for example.
After commissioning of the 2-million-euro facility that took place recently, the working group headed by Dwayne Miller will put into operation the diagnosis systems, optimise the synchronisation software and complete the vacuum system. When all tests are running successfully, the first experiments with REGAE could be carried out in January next year.
As a further highly interesting future perspective, it is planned to use the femtosecond beam from REGAE to explore the injection of electrons into a plasma wave excited by a high power laser and test ultra-high gradient acceleration.