|Narrow Linewidth, Chirp-Control and Radiation Extraction Optimization in an Electrostatic Accelerator FEL Oscillator
|In recent years the electrostatic accelerator FEL based in Ariel has undergone many upgrades. By varying the accelerating potential the resonator allows lasing between 95-110 GHz. It is now possible to remotely control the output reflectivity of the resonator and thereby vary both the power built up in the resonator and that emitted. This has allowed fine control over the power for different user experiments. A voltage ramping device has been installed at the resonator/wiggler to correct drops in voltage which occur due to electrons striking the walls of the beam line. This has allowed stable pulses of just over 50 μs with a chirp rate of ~80 kHz/μs.
|Scanning Problems of FLARE, a THz-FEL Waveguide
Funding: FLARE is part of the NCAS project funded through the “Big Facilities” programme of the Netherlands Organisation for Scientific Research (NWO).
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.
|Poster TUP065 [4.287 MB]
|Facility for Coherent THz and FIR Radiation
|Linac based THz sources are increasingly becoming the method of choice for a variety of research fields, justifying the increasing demand for high repetition rate THz FEL facilities world wide. In particular, pump and probe experiments with THz and IR radiation are of major interest for the user community. In this paper, we propose a facility which accommodates an SRF-linac driven cw THz-FEL in combination with an IR undulator which utilizes the microbunched beam. The layout permits almost perfect synchronization between pump and probe pulse as well as nearly independently tunable THz and IR radiation.
|Poster TUP066 [1.655 MB]
|Influence of the Lower Frequency Branch on the Performance of a Waveguided THz FEL
Funding: We would like to acknowledge the financial support from Swedish Research Council and Swedish FEL Center.
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
|Poster TUP067 [1.457 MB]
|Cavity Length Change vs. Mirror Steering in a Ring Confocal Resonator
Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-84-ER40150, the Office of Naval Research, and the Joint Technology Office.
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 . 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
 Stephen Benson, George Neil, Michelle Shinn, Laser and Beam Control Technologies, Santanu Basu, James Riker, Editors, Proceedings of SPIE Vol. 4632 (2002).
|Numerical Calculation of Diffraction Loss for Characterisation of a Partial Waveguide FEL Resonator
|Waveguide is widely used in long wavelength Free-Electron Lasers to reduce diffraction losses. In this paper the amplitude and phase transverse distribution of light emission produced in a partial-waveguide FEL resonator is calculated by Fresnel principle. To acquire high power out-coupled and optimize resonator structure of HUST THz-FEL, the characterisation of reflecting mirror is discussed to reduce diffraction loss.
|High Power Operation of the THz FEL at ISIR, Osaka University
|The THz FEL at Osaka University is based on the L-band linac that provides a multi-bunch electron beam with an 8 us duration in the energy range from 12.5 to 20 MeV. Although the RF frequency of the linac is 1.3 GHz, the bunch intervals are expanded to 9.2 ns for the FEL using a sub-harmonic buncher system that operates at 108 MHz, to enhance the bunch charge to 1 nC/bunch. The FEL covers the wavelength range from 30 to 150 um, and maximum energies of the macropulse and the micropulse are 3.7 mJ and 11 uJ, respectively, at ~70 um measured at an experimental station. To enhance the FEL power further, the electron beam current cannot be increased simply because the beam loading in the acceleration tube is too high. To solve this problem, we have developed a 27 MHz grid pulser for the thermionic electron gun that makes the bunch intervals 4 times longer and increases charge of the bunch 4 times higher whereas the beam loading is the same as that in the 108 MHz mode. In this new operation mode, where a single FEL pulse lases in the cavity, we have succeeded in obtaining the micropulse energy exceeding 100 uJ at a wavelength of 68 um.
|High Power Coupled FEL Oscillators for the Generation of High Repetition Rate Ultrashort Mid-IR Pulses
100-200 MeV range ERL-FELs generating few cycle short, high intensity mid-IR pulses with tens of MHz repetition rates might become attractive tools in various strong field applications. In a recent study  a mode locked coupled FEL oscillator scheme has been presented to produce multi-mJ level, ultra-short (<10 cycles) pulses tunable within the entire IR region. In this work an improved coupled FEL oscillator scheme is described. The coupled system operates unidirectionally (feedback in the reverse direction less than 10-8 level). The various operational regimes of the system are discussed. Some of the conclusions stated in  have been revised.
 M. Tecimer, PRST-AB 15, 020703 (2012).
|Progress using an FEL Oscillator for Inverse Compton Scattering
Funding: Work supported by U.S. Grant: DE-FG02-97ER41033.
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.
|Slides WEA01 [23.535 MB]
|Free Electron Laser Oscillator: Short Pulses, Mode Locking, Harmonic Generation and Tapering
|In Free Electron Laser oscillators the growth of the intracavity laser power determines the most interesting aspects of the system dynamics. In the case of short pulses operation the system undergoes genuine mode-locking mechanisms, which provide a wealth of interesting phenomena associated with the possibility of generating very short pulses. We explore the mechanisms of superradiance in FEL operating in the over-saturated regime and analyze the emerging short pulse structures and the relevant physical meaning. We also explore the pulse shape of the higher order harmonics generated in this regime and the possibility of modelling the pulse width and power by suitable combination of cavity length control and of undulator tapering.
|Slides WEA02 [13.588 MB]
|Higher Harmonic XFELO with the Planned 4 GeV LCLS II SCRF Linac
Funding: This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences under contract No. DE-AC02-06CH11357 (ANL) and DE-AC02-76SF00515 (SLAC)
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 . 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 . 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 . 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.
 K.-J. Kim, Y. Shvyd’ko, and S. Reiche, Phys. Rev. Lett. 100,244802 (2008)
 J.K. Sekutowicz, et al., 2013 FEL Conf.(2013)
 J. Dai, H. Deng, and Z. Dai, Phys. Rev. Lett. 108,034802(2012)
|Slides WEA03 [2.703 MB]
|First Lasing from a High Power Cylindrical Grating Smith-Purcell Device
Funding: Work supported by ONR under Contract No. N00014-10-C-0191 and N62909-13-1-N62.
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.
|Slides WEA04 [2.344 MB]