• R.T. Jongma, A.C.N. Engels, R.W. Lof, F.J.P. Wijnen, G.F.A.J. Wulterkens, V. Zhaunerchyk, P.A.W. van Dael, A.J.A. van Roij, A.P. van Vliet, W.J. van der Zande
  Radboud University, Nijmegen
• K. Dunkel, C. Piel
  RI Research Instruments GmbH, Bergisch Gladbach
• U. Lehnert, P. Michel, W. Seidel, R. Wünsch
  FZD, Dresden
• A.F.G. van der Meer
  FOM Rijnhuizen, Nieuwegein

The Radboud University in Nijmegen received funding to realize a narrow-band THz laser system and a 45 T hybrid magnet system. Based on results of predesign studies [1], all review committees agreed to continue the project and enter the realization phase. In this paper we present the technical solutions for realization of the main system components. We present the details of the RI Research Instruments GmbH (a former ACCEL Instruments GmbH activity) LINAC system. Operation of the full system (including the electron source) at 3 GHz is desirable and deemed feasible after first experimental studies. As the Nijmegen FEL will operate at wavelength up to 1.5 mm, the cavity will be fully waveguided, complicating the incorporation of an intra-cavity Fox-Smith interferometer required to induce coherence between micropulses and a Michelson interferometer as most ideal outcoupler. The optical distribution system comprises 150 m of vacuum tubing with 25 cm effective diameter (planar and refocusing) mirrors. A robust yet cost efficient realization taking boundary conditions on optical beam parameters at diagnostics station and user stations into account is foreseen.

[1] Design of the Nijmegen high-resolution THz-FEL, R.T. Jongma_, et al. Proc. FEL-08.

• V. Zhaunerchyk, R.T. Jongma, W.J. van der Zande
  Radboud University, Nijmegen

The THz region of the spectrum does not know versatile powerful light sources. FEL technology clearly is well suited for the generation of THz radiation. The Nijmegen THz-FEL oscillator is scheduled to be in operation early 2011 and will produce THz light in the 0.1 mm (3 THz)- 1.5 mm (0.2 THz) spectral range. Electron bunches will be generated by an RF LINAC at a rate of 3 GHz such that 150 optical pulses will propagate simultaneously through a 7.5 m long optical cavity. A relatively short length of the electron bunches and the high Q-factor of the RF LINAC will give rise to strong coherent spontaneous emission. Allμpulses will tend to be coherent. Our aim is to induce full inter-pulse coherence by means of an intra-cavity Fox-Smith interferometer. The present study is devoted to the investigation into competition between spontaneous and induced inter-pulse coherence which is foreseen to be relevant for the most ambitious aim of the Nijmegen FEL project: generation of single mode THz radiation with pulse lengths of 7 microseconds, based on a RF type electron accelerator.