University of Tel-Aviv, Faculty of Engineering, Tel-Aviv, Israel
Ariel University Center of Samaria, Faculty of Engineering, Ariel, Israel
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.
HZB, Berlin, Germany
Uppsala University, Uppsala, Sweden
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
JLab, Newport News, Virginia, USA
In principle, a ring confocal resonator allows for the use of a short Rayleigh length without the extreme sensi-tivity to mirror steering typical in a near-concentric reso-nator [1]. One possible weakness of such a resonator is that the cavity length is no longer independent of the mirror steering. This is one of the strengths of a linear resonator. In this presentation, it is shown that, in a simple 2-dimensional corner cube type ring confocal resonator, the cavity length is, in fact, not dependent on the mirror steering to first order in the mirror angles. Thus the ring-confocal resonator might be a very easy-to-operate and stable resonator for short Rayleigh range operation in FEL oscillators
[1] Stephen Benson, George Neil, Michelle Shinn, Laser and Beam Control Technologies, Santanu Basu, James Riker, Editors, Proceedings of SPIE Vol. 4632 (2002).
HUST, Wuhan, People's Republic of China
ISIR, Osaka, Japan
University of Hawaii at Manoa, Honolulu, USA
[1] M. Tecimer, PRST-AB 15, 020703 (2012).
WEA01
FEL/Duke University, Durham, North Carolina, USA
Since mid 1990s, oscillator FELs have been used to produce x-rays and gamma-rays via Compton scattering. These FELs are based on several accelerator technologies, including room-temperature and superconducting linacs and storage rings. The most successful Compton sources are operated in the gamma-ray region, beyond the reach of synchrotron radiation sources and x-ray FELs. With almost two decades of continued effort, High Intensity Gamma-ray Source (HIGS) at Duke University has been developed into a world-leading Compton gamma-ray facility for frontier research in nuclear physics and astrophysics, and applied research in national security and industry. The two outstanding features of the HIGS are (1) a wide energy range of operation from 1 to 100 MeV; and (2) an exceptionally high flux in the few MeV to 10 MeV region. At the HIGS, the further development of the oscillator FEL will lead to new gamma-ray beam capabilities. Research to develop a VUV FEL at 170 or 150 nm will allow the production of gamma-rays up to ~160 MeV. Research on the FEL beam polarization will lead to the production of gamma-ray beams with switchable helicity and rotatable linear polarization.
WEA02
ENEA C.R. Frascati, Frascati (Roma), Italy
CSU, Fort Collins, Colorado, USA
Universita' degli Studi di Milano, Milano, Italy
Mesa+, Enschede, The Netherlands
WEA03
ANL, Argonne, Ilinois, USA
SLAC, Menlo Park, California, USA
An x-ray FEL oscillator (XFELO) will produce hard x-ray pulses of ultra-fine spectral resolution (~ meV) that combines FEL brightness with storage ring stability [1]. Thus, for example, the long-standing problem of high-TC superconductivity could be solved by inelastic x-ray scattering. In addition, an x-ray spectral comb can in principle be generated, vastly expanding the reach of experimental x-ray quantum optics. The accelerator for an XFELO should optimally be of the CW superconducting type. The linac for the European XFEL can be operated in CW mode without adding more cooling capacity if the energy is lowered from 14 to 7 GeV [2]. It is also possible to drive a hard x-ray XFELO at lower than 7 GeV, if a higher harmonic is chosen as the operating wavelength [3]. We have studied XFELO for 1 Å x-rays operating at the third or fifth harmonic using the 4 GeV SCRF linac planned for LCLS-II. Assuming bunch charge=50 pC, normalized rms emittance=0.2 mm-mrad, rms energy spread=500 keV, rms bunch length=190 fs, and undulator period length=2.6 cm, the gain at 1 Å as a 5th harmonic is found to be about 40%, sufficient for lasing allowing for the various losses.
[1] K.-J. Kim, Y. Shvyd’ko, and S. Reiche, Phys. Rev. Lett. 100,244802 (2008)
[2] J.K. Sekutowicz, et al., 2013 FEL Conf.(2013)
[3] J. Dai, H. Deng, and Z. Dai, Phys. Rev. Lett. 108,034802(2012)
AES, Medford, New York, USA
CENBG, Gradignan, France
CEA, LE BARP cedex, France
Many applications of THz radiation remain impractical or impossible due to an absence of compact sources with sufficient power. A source where the interaction occurs between an annular electron beam and a cylindrical grating is capable of generating high THz power in a very compact package. The strong beam bunching generates significant power at the fundamental frequency and harmonics. A collaboration between Advanced Energy Systems and CEA/CESTA has been ongoing in performing proof-of-principle tests on cylindrical grating configurations producing millimeter wave radiation. First lasing was achieved in such a device. Further experiments performed with a 6 mm period grating produced fundamental power at 15 GHz, second harmonic power at 30 GHz and although not measured, simulations show meaningful third harmonic power at 45 GHz. Comparison with simulations shows very good agreement and high conversion efficiency. Planned experiments will increase the frequency of operation to 100 GHz and beyond. Ongoing simulations indicate excellent performance for a device operating at a fundamental frequency of 220 GHz with realistic beam parameters at 10 kV and simple extraction of the mode.