Please activate JavaScript!
Please install Adobe Flash Player, click here for download

discovered_01_2013

FOCUS// The HZDR Research Magazine WWW.Hzdr.DE 10 11 CONTACT _Institute of Radiation Physics at HZDR Axel Jochmann a.jochmann@hzdr.de successfully realized. By now, they have moved into their own experimentation room - a "Cave" - in the new ELBE building. They also expanded their facilities by adding a second, new and improved experiment chamber. X-ray light can be produced by alternately accelerating and decelerating electrons; during the process, the electrons give off a small portion of their energy in the form of photons, or light particles. Current light sources continually repeat this process by getting the electrons to vibrate regularly. To do so, they guide them through specially arranged magnets called undulators. An alternative to using these devices is to use short, intense light pulses like the ones produced by the HZDR’s DRACO and PENELOPE lasers to "optically" vibrate the electrons. The laser pulses’ short wavelength determines the electrons’ movements and they in turn determine the wavelength of the X-ray light emitted by the electrons every time they change direction. Its wavelength is around 800 nanometers but relativistic effects act to further decrease it to 0.1 nanometers. Since this corresponds to the intramolecular distance between atoms, researchers are able to study the structure of matter at the atomic level. The light pulses’ extremely short duration, which is transmitted equally to the X-ray photons - and which is found in the range of one picosecond to one hundred femtoseconds, one trillionth and one hundred quadrillionths of a second, respectively - makes fast processes accessible. Just like synchrotron radiation "The X-ray light we produce has properties that are very similar to modern-day synchrotron light," explains Thomas Cowan, Director of the Institute of Radiation Physics. Since interactions between photons and electrons are occurring across much shorter distances - that is, the wavelengths of light - than the mechanically delimited distances between the undulator’s magnets, the overall dimensions of the HZDR’s X-ray source PHOENIX (PHOton Electron Collider for Narrow Bandwidth Intense X-Rays) are noticeably smaller. Axel Jochmann points to a tiny spec on a blueprint of the not-quite-finished experiment chamber in the new Cave. Here, the electron beam enters from the side and the laser light from below in order to get them to collide. "What we’re talking about here is an area of a few square micrometers," says Jochmann. The chamber itself will be approximately six meters in length and less than two meters in width, and will be cloaked in aluminum with several windows that can be opened. That is the scale of the experiments the laser researchers are looking at - they are referring to their setup as "table-top" - experimental stations on more or less large laboratory tables. The high degree of precision the scientists are having to work with is almost unfathomable: "We have to precisely focus both research facilities spatially onto a spot the diameter of a single hair and with a precision on the order of one picosecond," Jochmann explains. The laser pulse and electron beam, which in turn is made up of even smaller electron bundles, are very densely packed. "They each contain some 500 million electrons and over one quadrillion light particles," says the Ph.D. student. "With each pulse, we are able to generate some 100 million X-ray photons. That is, if we use the new superconducting electron source, which allows for a higher electron density than previous sources." Preliminary experiments for Helmholtz Beamline at XFEL X-ray laser Going forward, the perfect synchronization of laser and electron beam at the HZDR is also supposed to be used to prepare experiments at the planned Helmholtz International Beamline for Extreme Fields (HIBEF) at the European X-ray laser XFEL, which is currently under construction at the Hamburg-based accelerator center DESY. The goal for the beamline is to combine brilliant X-rays with high-power laser light. "This would allow us to conduct experiments in new research areas, giving us unprecedented scientific opportunities," says Cowan, who heads the international user consortium in charge of setting up the beamline. This way, certain structural biological or geophysical processes can be examined by activating probes using intense laser light and, immediately thereafter, examining them under bright X-ray light. By the time XFEL will be sending out its first X-ray flashes in 2015, the HZDR’s X-ray source is also supposed to be operational again. "We expect that the demand will be high for conducting experiments at both the XFEL laser and at the Helmholtz beamline – plus it’ll be very costly. Which is why we want to prepare it perfectly by pouring our collective know- how into the whole affair," says Cowan. Jochmann is already thinking one step ahead. "At our X-ray source, we have the option of combining the laser beam with laser-accelerated electrons." What this means is entirely new technical dimensions because in that case you could do without large-scale accelerator facilities, and that vision is appealing from a business point of view as well. In other words, for laser researchers like Jochmann the playing field remains impressively big. LITERATURE A. Jochmann, A. Irman et al.: "Operation of a picosecond narrow-bandwidth Laser-Thomson-Backscattering X-Ray source", in Nuclear Instruments and Methods in Physics Research (2013), Section B: Beam Interactions with Materials and Atoms (DOI: 10.1016/j.nimb.2013.01.065)

Pages