The Radiation Source ELBE |
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At the Forschungszentrum Rossendorf (FZR) a major new installation is approaching its
completion - the ''Strahlungsquelle ELBE''. This radiation source will use the high brilliance
electron beam from a linac with superconducting rf-cavities to produce various secondary
beams for experiments in nuclear, solid-state, environmental and bio-medical physics as well
as in various other fields. To present a comprehensive status report of this important and
innovative project, the present Annual Report combines contributions made to it by several
groups and departments of the FZR. The Departments ''Experimental Facilities and Information
Technology'' and ''New Accelerators'' as well as the ELBE-beam-diagnostics-group have carried out
numerous developments within the ELBE project; the most important are described mainly in the
beginning of this Report. The subsequent contributions from groups of the IKH are more strongly
focused on the production and future use of the different kinds of secondary radiation. Physics with IR-FEL's and their radiation
The high brilliance electron beam produced by a super-conducting linac like ELBE allows
the investigation of various processes which produce high quality electromagnetic radiation
in various energy (i.e.) ranges. An especially illustrative and
theoretically clear example for the generation of radiation is the Free-Electron-Laser (FEL).
With the electron energy range available at ELBE coherent infrared radiation from 300 mm
(corresponding to 1 THz) to 3 mm (equivalent to 0.4 eV) can be produced in short pulses of a
few ps and with a large repetition rate finally allowing tens of watts average intensity. At the
long wavelength side the limitation results mainly from the difficulties in handling the strong
diffraction effects. Various theoretical and numerical studies have been performed on this
problem and on the resulting IR-beam properties; laboratory experiments to test some of
these calculations are presently being set up. For the short wavelengths the quality of the
electron pulses and the undulator field is crucial. Our newly perfected pulsed-wire field scan
allows to cross-check the detailed field maps obtained with Hall-probes and it will be used to
document 'in situ' future variations in magnetic field strength. Research making use of IR-
radiation in the 10 mm-range has been performed at existing FEL facilities and with Fourier-
transform (FTIR)-spectroscopy methods. A FTIR-spectrometer operable in conjunction with a
reflection-optic microscope is newly available at the institute to be used in conjunction with the
IR-studies on biomedical probes to be performed at the FEL in the next years by scientists
from the IKH. X-rays, Bremsstrahlung Photons and Neutrons
Quasi-monochromatic X-rays can be generated from the fast electrons either by using the
channeling process in single crystals (esp. diamond) or by Compton scattering of intense
laser light from the electrons. For both processes a very high brilliance e-beam has to be
realized which requires a beam transport system allowing for the reduction of unwanted
emittance growth effects. At ELBE a decrease of the transverse emittance beyond present
values is expected from improvements of the e-gun and finally the use of a photo cathode
gun directly coupled to a superconducting cavity. Space charge effects can be minimized by
enlarging the repetition rate from 13 up to 260 MHz - at constant average beam intensity.
Studies will be performed to enhance the yield of channeling X-rays by resonant variation of
the crystal parameters using a variable frequency ultra-sound field; additionally an improvement
of the X-ray-to-bremsstrahlung-ratio will be strived for by inserting polycristalline carbon
segments between radiator and probe. To promote the radiation damage studies planned by
the IKH for the X-ray beam a cell-laboratory has been set up in the ELBE building.
Collaborations Within the FZR a close collaboration was established with the two departments responsible for the installation and operation of ELBE (i.e. Experimental Facilities / Information Technology and New Accelerators / ELBE-beam-transport and -diagnostics). Intense contacts also exist with the other Rossendorf institutes planning to use ELBE:
Various collaborative efforts have been or are being formed to make use of the various novel research possibilities offered by ELBE and its secondary beams. As a typical example the EU-project THz-Bridge is mentioned here; it brings together scientists from IKH, from Frankfurt and Stuttgart universities and from four other EU-countries in a study on the interaction of far-IR and THz radiation with living cells.
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