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discovered_01_2013

FOCUS// The HZDR Research Magazine WWW.Hzdr.DE 12 13 by an entire order of magnitude and is meant to significantly expand experimental opportunities for internal and external researchers. Why is terahertz radiation so popular? At wavelengths of 0.03 to three millimeters, this light is particularly well suited to trigger or study reactions of matter. Researchers from the life sciences are dreaming about using terahertz radiation to study key organismal processes like the conduction of nerve impulses or the processing of different stimuli. And accelerator experts are hoping to use terahertz facilities to measure the temporal characteristics (e.g. arrival time and duration) of the ultra-short electron pulses. Accordingly, the new HZDR facility is set up as a precise diagnostic laboratory for electron pulses that are produced by the ELBE accelerator, given that the latter will soon yield more highly-charged and shorter electron pulses than was previously the case. Electron pulses with those features are also the reason why a superradiant terahertz source is even possible in the first place at a comparatively small electron accelerator like ELBE. Simply put, superradiance means high-intensity light is produced in an avalanche-like, short process without the need for mirrors as resonators - as is the case with a free electron laser. Provided that the accelerator physicists are able to reach the predetermined parameters, going forward, TELBE will cover the entire spectrum all the way from the gigahertz range up to 3 terahertz or from three millimeter to 0.1 millimeter wavelength with still more intense pulses. In case they are successful, TELBE is supposed to become an international user facility for materials and life sciences researchers in the future. Here, another benefit TELBE has to offer comes into play: it was conceived for continuous operation. "With the help of this facility, and in contrast to all the other comparable machines, a continuous- wave operation is a real possibility," explains physicist Michael Gensch. "Other sources have to take a break on the order of a few thousandths of a second after every short series of ultrashort flashes. Our superconducting electron accelerator, however, renders these interruptions superfluous and allows for a continuous-wave operation." Successful start-up of operations Michael Gensch’s machine passed its first radiation time in March 2013 with flying colors. Physicist colleagues from DESY had traveled all the way from Hamburg where one of TELBE’s "competitors" is located: The FLASH free electron laser’s terahertz source. Gensch views this pilot facility as a model for TELBE, however, his radiation source - and this is the main challenge - is supposed to be powered by electrons from a much smaller accelerator across a very short distance. But although, just like the costs, the dimensions of the Dresden project are also moving within entirely different orders of magnitude, the plan is to be able to keep up with the global accelerator community after all. "In the Helmholtz accelerator research and development program, we are collaborating with the big players DESY and KIT, the Karlsruhe Institute of Technology. What’s also important for us is the close collaboration with international partners from the two big US national accelerator labs, SLAC and Jefferson Lab, as well as with the PTB, the German national metrology institute. Our common goal with the PTB is to develop methods that will allow us to precisely determine our terahertz flashes’ energy. Because in this spectral range, this is a thus far unresolved task." For Gensch and his HZDR colleagues, the next important step on the road to a prestigious light source of the future is to shorten and intensify ELBE’s electron pulses by more than one order of magnitude. As such, they want to gradually start operations and test TELBE with the support of partners and pilot users. "We have a long road ahead of us in terms of development and are hovering at the border of that which is technologically feasible. Our hope is to be able to decide, by 2015, whether or not TELBE is actually operational as a regular experimentation facility," explains Gensch. And, he adds, "either way, we’re looking at an internationally unique facility for the study of electron pulses and development of concepts for the operation of large-scale research facilities like the European XFEL in Hamburg." LITERATURE F. Tavella, N. Stojanovic, G. Geloni, M. Gensch: "Few femtosecond timing at 4th generation X-ray lightsources", in Nature Photonics, vol. 5 (2011), p. 162 (DOI: 10.1038/ NPHOTON.2010.311) M. Foerst et al.: "THz control in correlated electron solids: sources and applications", in K.-E. Peiponen et al. (eds.), Terahertz Spectroscopy and Imaging, vol. 1, Springer-Verlag Berlin Heidelberg 2012 (DOI: 10.1007/978-3-642-29564- 5_23) TERAHERTZ: The new facility emits radiation over a wide range of wavelengths, expanding the electromagnetic spectrum available at the ELBE. Photo: Frank Bierstedt CONTACT _Institute of Radiation Physics & Institute of Ion Beam Physics and Materials Research at HZDR Dr. Michael Gensch m.gensch@hzdr.de

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