TUP065
Radboud University, Nijmegen, The Netherlands
ISIR, Osaka, Japan
University of Gothenburg, Gothenburg, Sweden
Uppsala University, Uppsala, Sweden
The (0.2 – 3) THz free-electron laser FLARE is equipped with a waveguide extending over the full cavity length. Therefore, the tuning gaps observed in the long-wavelength range of FELIX, FELBE and CLIO, which were attributed to mode-conversion at the waveguide free-space transitions, are avoided. Unfortunately, an even more severe scanning problem is observed and continuous tuning of the photon energy is up to this moment impossible. The origin of this problem is not yet understood and experiments to gain insight into the problem are ongoing. We have investigated the (coherent) spontaneous emission as a function of wavelength, the gain build-up in the vicinity of tuning gaps, and the operation at a micro-pulse repetition frequency at which only a single photon bunch circulates in the cavity. The latter is explored to investigate if the low-frequency mode (the slow wave) that can also build up in a wave-guided cavity and travels at lower group velocity than the electron bunches, interferes with the efficient power build-up of the desired high-frequency mode in the trailing bunches. Status and results of the experiments will be discussed.
TUP067
University of Gothenburg, Gothenburg, Sweden
Radboud University, Nijmegen, The Netherlands
The Terahertz (THz) frequency range is highly relevant in many applications ranging from medicine to security and communication. Among different available THz sources, free electron lasers (FELs) are the most powerful and versatile sources that provide tunable light in the whole THz region. THz FELs usually operate as oscillators and employ a waveguide to suppress diffraction losses. When a waveguide covers only a part of the optical cavity, substantial drops of the output power at certain wavelengths are observed *. The THz FEL FLARE operating in the wavelength range of 0.1-1.5 mm comprises a waveguide which covers the whole cavity length**. Surprisingly, the spectral gaps are still observed. To get insight into origin of the gaps, we perform numerical simulations taking into account both lower and higher resonant frequency branches, as well as interaction between 150 THz pulses that simultaneously propagate through the FLARE cavity. Simulations predict that the lower frequency branch can hamper amplification of the other branch and, thus, can lead to the spectral gaps.
* R. Prazerez et al. Phys. Rev. ST Accel. Beams 12, 010701 (2009)
** R. Chulkov et al. Multi-Mode Dynamics in a Short-Pulse THz FEL. Phys. Rev. ST Accel. Beams, to be published in 2014