Lehnert, U.

Paper	Title	Page
MOPPH016	Gain Deterioration at Particular Wavelengths in a Partially Waveguided FEL	36
	U. Lehnert, P. Michel, G. Staats, J. Teichert, R. Wuensch FZD, Dresden V. B. Asgegar University of Pune, Pune
	At ELBE certain wavelengths within the working range of the FIR-FEL have been found to be inaccessible. The FEL either completely stops lasing or shows marked drop-outs in the observed optical spectra. The reason of this behaviour is sought in the use of a partial waveguide through the undulator to the downstream mirror combined with free optical propagation to the upstream mirror. The light pulse from the upstream mirror couples into the lowest transverse mode of the waveguide with minor contributions of other modes. The light generated in the gain process, however, is distributed over some of these modes and experiences dispersion over the waveguided propagation length. At the exit of the waveguide the different modes recombine with certain phase shifts. Depending on the amount of phase shift and the mode composition of the light a gain drop or even inversion is possible if a major part of the stimulated emission is out of phase to the primary beam. This work attempts to compute the mode distribution of the stimulated light emission and to translate this into a prediction of those wavelengths where the gain is markedly reduced by destructive interference of different modes.
TUAAU05	Design of the Nijmegen High-Resolution THz-FEL	200
	R. T. Jongma Radboud University Nijmegen, Institute of Molecules and Materials, Nijmegen K. Dunkel, C. Piel ACCEL, Bergisch Gladbach U. Lehnert, P. Michel, R. Wuensch FZD, Dresden C. A.J. van der Geer Pulsar Physics, Eindhoven A. F.G. van der Meer FOM Rijnhuizen, Nieuwegein P. J.M. van der Slot Twente University, Laser Physics and Non-Linear Optics Group, Enschede W. J. van der Zande Institute for Molecules and Materials, Radboud University Nijmegen, Nijmegen
	In 2006, the Radboud University in Nijmegen received funding via the Netherlands NWO-BIG program to realize a THz laser system and a 45 T hybrid magnet system. The specifications of the THz FEL system are geared towards material science at high (30-45 T) magnetic fields (saturation spectroscopy and pulse-echo experiments), and applications e.g. in the field of biomolecular spectroscopy. A study performed during the last year demonstrated the feasibility of a THz FEL that will cover the 100-1500 micron spectral range and that operates in either a “spectroscopic mode” providing 100 Watt bandwidth limited pulses of several microsecond (spectral resolution better than 100000/1) or pump-probe pulsed mode providing macropulses with 3 GHz. micropulses. Technical challenges are in the 3 GHz operation of the source, and the narrowband operation. The latter will be obtained by filtering a single mode out of the frequency comb, realized by ensuring full coherence between the micropulses. Coherence is imposed by the stability of the electron beamμpulses (“spontaneous” coherence) or by the use of an intra-cavity (Fox-Smith) interferometer. We will present details of the chosen design.
	Slides
TUPPH060	Three Years of CW-Operation at FELBE - Experiences and Applications	382
	W. Seidel, E. Cizmar, O. Drachenko, M. Helm, M. Justus, U. Lehnert, P. Michel, M. Ozerov, H. Schneider, R. Schurig, D. Stehr, M. Wagner, S. Winnerl, D. Wohlfarth, S. Zvyagin FZD, Dresden L. Eng, S. C. Kehr Dresden University of Technology, Institute of Applied Photophysics, Dresden
	This paper reviews the basic properties of the infrared free-electron laser FELBE at the Forschungszentrum Dresden-Rossendorf. A few highlight experiments using the cw-operation are discussed. Driven by a superconducting linear accelerator, FELBE continuously generates infrared pulses with a repetition rate of 13 MHz. In addition, operation in a macropulse modus (pulse duration >100 μs, repetition rate ≤ 25 Hz) is possible. At present FELBE delivers microJ pulses with typical duration of about 0.9-30 ps in the wavelength range 4-230 micrometer. Furthermore we give an outlook on the experiments will use the beam of FELBE in the High Magnetic Field Laboratory Dresden (HLD). The HLD will provide pulsed magnetic fields up to 60 T. It operates as a user facility since 2007.
THAAU02	Initial Commissioning Experience with the Superconducting RF Photoinjector at ELBE	467
	J. Teichert, A. Arnold, H. Buettig, D. Janssen, M. Justus, U. Lehnert, P. Michel, P. Murcek, A. Schamlott, Ch. Schneider, R. Schurig, F. Staufenbiel, R. Xiang FZD, Dresden T. Kamps, J. Rudolph, M. Schenk BESSY GmbH, Berlin G. Klemz, I. Will MBI, Berlin
	A radio frequency photo injector with a superconducting acceleration cavity (SRF gun) for installation at the Radiation Source ELBE was developed within a collaboration of BESSY, DESY, FZD, and MBI. The aim of the project is to improve the electron beam quality and to extend the parameter range of the ELBE accelerator. Especially the bunch charge will be increased up to 1 nC and the transverse emittance will be reduced to 1 - 3 mm mrad. At present, the thermionic injector at ELBE delivers bunches of 77 pC at about 8 mm mrad. Furthermore, the SRF gun together with its diagnostic beam line is an excellent test bench for extended studies and improvements of this new and promising injector type. The gun cryostat, the electron diagnostic beamline, and the driver laser with optical beamline were installed in summer and fall 2007. In November the first beam was produced. It will be reported on the experience gained at the first phase of commissioning. Results of rf and beam parameter measurements with Cu and Cs2Te photo cathodes will be presented.
	Slides
FRAAU03	Investigation and Improvement of Beam Stability at the ELBE FELs	543
	P. Michel, M. Justus, U. Lehnert, P. Michel, D. Proehl, R. Schurig, W. Seidel, J. Teichert FZD, Dresden
	At the radiation source ELBE in the Forschungszentrum Dresden-Rossendorf two free electron lasers (4-20 μm and 20-230 μm) are in routine user operation for a wide range of IR experiments for some years. The lasers are driven by a superconducting RF Linac which permits the generation of a cw-beam with high average beam power. For many experiments the frequency and power stability of the laser beam is of outstanding importance. Therefore studies on fluctuations and drifts in different time scales (from μs to hours) were accomplished and possible causes for these instabilities were investigated. To improve the long and short term stability we developed and implemented active feed back controls for electron energy and thus laser wavelength and out-coupled IR-beam power at ELBE.