• W. Seidel, S. Winnerl
  FZD, Dresden

In order to decrease the average radiation power of the Rossendorf free-electron laser FELBE, as required for certain experiments (high pulse energies but moderate or low average power), the FEL repetition rate can be reduced from 13 MHz to 1 kHz. To this end, plasma switching of FEL radiation pulses was demonstrated for cw operation. The plasma switch is based on the principle of photo-induced reflectivity by an optically excited electron-hole plasma. Germanium or silicon serves as semiconductor material for the switch. The semiconductor was illuminated by a Nd:YAG laser amplifier system (1 kHz, wavelength {10}64 nm, pulse duration16 ps, 1Watt), generating an electron-hole plasma on the front surface of the semiconductor. To integrate this plasma-switch into the existing experimental set-up we build an additional by-pass to the Germanium or Silicon slab which is under Brewster’s angle. To get a high contrast in the switched beam we adjust the polarization plane of the incoming beam to the right direction by using an additional polarization rotator. We will report on first results at different wavelength. Submitted as a poster to the FEL 2010 conference.

• V. B. Asgekar
  University of Pune, Pune
• U. Lehnert, P. Michel
  FZD, Dresden

A tapered undulator experiment was carried out at the Forschungszentrum Dresden-Rossendorf (ELBE) far-infrared FEL. The main motivation was to see whether the presence of a dispersive medium due to the partially waveguided resonator has any effect on the outcome. The FEL saturated power and the wavelength shifts have been measured as a function of both positive as well as negative undulator field amplitude tapering. In contrast to the typical high-gain FELs where positive tapering (i.e. a decrease of undulator field amplitude over the beam path) proves beneficial for the output power we observe an improvement of performance at negative taper. During the same experiments we studied the characteristics of the detuning curves. The width of the curves indicates a maximum small-signal gain for zero taper while the output peak power is highest for negative taper. Whereas the saturated power output and the detuning curve characteristics agree with the known theoretical predictions, the wavelength shifts showed deviations from the expected values. Details of the experiment are presented.

• C. Evain, M.-E. Couprie
  SOLEIL, Gif-sur-Yvette
• S. Bielawski, C. Szwaj
  PhLAM/CERCLA, Villeneuve d'Ascq Cedex
• M. Hosaka
  Nagoya University, Nagoya
• M. Katoh
  UVSOR, Okazaki
• A. Mochihashi
  JASRI/SPring-8, Hyogo-ken

In FEL oscillator, a desynchronisation between the electron-bunch passage frequency and the repetition rate of the laser can lead to instability, characterised by erratic longitudinal shape of the emitted light pulses. We show that this instability can be controlled using a simple feedback system which consist in re-injecting in the cavity a part of the emitted light. Analytical, numerical and experimental studies on the UVSOR-II storage ring have been performed, and show that the energy needed to achieved the control can be extremely weak, in practical higher than the noise level[1]. We also show that another important parameter is the phase of the re-injected signal with respect to the light in the cavity. Depending of the value of this phase, we can observe a shift of the emitted light wavelength, which can go with a modulation of the laser pulse envelop. Both of this two phenomenas are quantitatively analysed.

[1] C. Evain, C. Szwaj, S. Bielawski, M. Hosaka, A. Mochihashi, M. Katoh, and M.-E. Couprie, Phys. Rev. Lett. {10}2, 134501 (2009)

• S. Krishnagopal, S.A. Samant
  BARC, Mumbai

Mode stability can restrict the tuning range of FEL oscillators. We investigate the stability of FEL oscillators as a function of wavelength as well as size of the coupling hole. We show that concentric configurations are preferred to confocal ones. We study mode-stability using multi-particle simulations, for both, symmetric as well as asymmetric modes.

• F. Wang, J. Wu
  SLAC, Menlo Park, California
• Q.K. Jia, A.L. Wu
  USTC/NSRL, Hefei, Anhui

Lots of applications with mm wave need very high power (from tens of kW to MW), such as surface processing of metals and ceramics, heating magnetically confined plasma in thermonuclear fusion reactors, isotope separation and so on. Recently developed gyrotrons can provide up to 1 MW CW mm-wave source, however there are a number of limitations, needs of supper conducting magnet, cathode lifetime degradation because of very high current, almost approaching the upper limit of their power and frequency capabilities, and so on. It is thought that the electrostatic accelerator FEL (EA-FEL) will be a promising high power IR-mm source, because of its high average power generation, high-energy conversion efficiency and high spectral purity. The property of an EA as a high quality e-beam source for a FEL is crucial for attaining high brightness spontaneous emission radiation. The unique features of EA-FELs make them naturally fitting for a variety of applications in the present and in the near future. And few high power mm-IR EA FEL facilities have been successfully built around world. Here an EA of 3 MeV with beam current of 2 A is studied for a high average power (kWs) mm-THz source

• J.T. Donohue
  CENBG, Gradignan
• J. Gardelle, P. Modin
  CESTA, Le Barp

At FEL 2009, we presented experimental results on coherent Smith-Purcell obtained at CESTA in the microwave frequency domain * . Those results strongly supported the two-dimensional theory proposed by Andrews and Brau some years ago ** , and were consistent with simulations performed with the PIC code "MAGIC". That experiment used a large current, 200 A, for a grating of width 10 cm. In a follow-up experiment, emittance slits were used to reduce the current to as low as 2 A, with a quite thin, flat, and wide beam. The gain as a function of current and also of vertical beam position was measured in detail. In particular, the start current for our set-up was found. In parallel, 2-D simulations of the experiment with "MAGIC" were extensively compared with the experimental results. Very good agreement between simulations and experiment is obtained. This lends confidence that simulations of a scaled-down version of our experiment will be a reliable guide for Terahertz frequency coherent Smith Purcell experiments. Such simulations suggest that radiation in the range 100-200GHz should be feasible.

* J. T. Donohue, J. Gardelle, L. Courtois and P Modin, Proceeedings of FEL 2009.
** H. L. Andrews and C. A. Brau, Phys. Rev. ST Accel. Beams 7, 070701 (2004).

• J.D. Jarvis, C.A. Brau, J.L. Davidson, B.L. Ivanov, J.L. Kohler
  Vanderbilt University, Nashville, TN
• H.L. Andrews
  LANL, Los Alamos, New Mexico

Recently, the theory of the Smith-Purcell free-electron laser has been confirmed by the experiments of Andrews, et al. [1], and of Gardelle, et al. [2] In addition, high-brightness cathodes have been developed using field-emission from arrays of diamond pyramids [3]. By combining these developments we have designed an ultracompact (“shirt-pocket”) free-electron laser and we have begun constructing the device. The electron beam comprises an array of 2-micron diamond-pyramid field emitters that overfills an einzel lens 200-microns wide and 1-mm long, fabricated using ps-laser machining. The beam is accelerated to 10 keV and focused in the short dimension over a lamellar metal grating with a period of 150 microns and a length of 10 mm. The predicted start current at a wavelength of {10}84 microns is 11 mA, which corresponds to 9 A/cm² at the cathode, before focusing. We have tested cathodes at 30 A/cm² and 600 mA total current; higher current density should be possible.

[1] Andrews, et al, JAP {10}5, 024904 (2009)
[2] Andrews, et al, PRST-AB 12, 080703 (2009)
[3] Gardelle, et al, PRST-AB 12, 110701 (2009)
[4] Jarvis, et al, JVSTB 27, 2264 (2009)
[5] Jarvis, Thesis, 2009

• W.M. Fawley
  LBNL, Berkeley, California
• K.-J. Kim, R.R. Lindberg, Yu. Shvyd'ko
  ANL, Argonne

The oscillator package within the GINGER FEL simulation code has now been extended to include angle-dependent reflectivity properties of Bragg crystals. Previously, the package was modified to include frequency-dependent reflectivity in order to model x-ray FEL oscillators[*] from start up from shot noise to saturation. We will present a summary of the algorithms used for modeling the crystal reflectivity and radiation propagation outside the undulator, discussing various numerical issues relevant to the domain of high Fresnel number and efficient Hankel transforms. We give some sample XFEL-O simulation results obtained with the angle-dependent reflectivity model, with particular attention directed to the longitudinal and transverse coherence of the radiation output.

[*] R.R. Lindberg et al., submitted to PRST-AB, 2010.

• A. Malkin, N.S. Ginzburg, N.Yu. Peskov, A. Sergeev, V.Yu. Zaslavsky
  IAP/RAS, Nizhny Novgorod

Periodical Bragg structures can be considered as an effective way of controlling the electromagnetic energy fluxes and provision of spatial coherence of radiation in the electron devices with oversized interaction space. Advance of FEL with 2D distributed feedback [*] into the terahertz waveband can be achieved basing on a two-mirror hybrid scheme in which a new modification of Bragg reflector exploiting the coupling between the two counter-propagating waves and a cutoff mode is used as an upstream mirror. This reflector provides effective mode selection over the "narrow" transverse coordinate directed between the plates forming planar waveguide. Synchronization of radiation from a sheet electron beam over the "wide" coordinate can be obtained by 2D Bragg structures providing 2D distributed feedback used as a downstream mirror. Both upstream and downstream Bragg reflectors are compatible with intense beam transport. Thus the advantage of suggested scheme against traditional THz band FEL [**] is the possibility of realization of long-pulse (microsecond) generation regimes with high (mulimegawatt) output power level.

*Ginzburg N.S., Peskov N.Yu., Sergeev A.S. // Optics Сommun. 1994. V.112. P.151.
**G.R.Neil, C.L.Bohn, S.V.Benson, et al. // Phys. Rev. Lett.2000. V.84. P.662.

• V. Litvinenko, I. Ben-Zvi, Y. Hao, C.C. Kao, D. Kayran, J.B. Murphy, V. Ptitsyn, D. Trbojevic, N. Tsoupas
  BNL, Upton, Long Island, New York

BNL plan to build 5-to-30 GeV energy recovery linac for its future electron-ion collider, eRHIC. In past few months the laboratory turned its attention to FEL potential of this unique machine, which was initially assessed in our early paper [1]. In this talk we present current vision of a possible FEL farm and narrow-band FEL-oscillators driven by this accelerator.

[1] Potential Use of eRHIC's ERL for FELs and Light Sources, V.N. Litvinenko, I. Ben-Zvi, Proceedings of FEL'2004 http://jacow.org/f04/papers/WEBOS04/WEBOS04. PDF

• H. Ogawa, N. Sei, K. Yamada
  AIST, Tsukuba, Ibaraki

Inverse Compton X-rays were obtained during lasing of the NIJI-IV free electron laser (FEL) in the infrared range. The position of the Compton collisions between electron bunches and laser pulses inside the FEL oscillator strongly influenced the energy spectrum of the Compton X-rays. Collisions outside the undulator magnetic field led to a sharp and clear Compton edge, while collisions inside the undulator magnetic field made it quite obscure. The position of collisions can be determined by changing the bunch-filling pattern on the 16 RF successive buckets. In this experiment two or three bunches selectively remained in suitable RF buckets. The maximum X-ray energies were 0.7-2.1 MeV for the laser wavelengths of 2.6 μm - 0.88 μm with a fixed e-beam energy of 310 MeV. Relative energy width of the 1.2 MeV Compton X-rays was observed to be 11 % with a lead collimator of 10 mm in diameter. The maximum X-ray yield was of the order of 10⁶ photons/sec in a three-bunch operation mode. The energy spectra and X-ray yields were investigated under various Compton collision conditions. The results will be discussed in the conference.

• Y. Socol
  Falcon Analytics, Netanya
• G.N. Kulipanov, O.A. Shevchenko, N. Vinokurov
  BINP SB RAS, Novosibirsk
• A.N. Matveenko
  HZB, Berlin

Lithography over the last years has been actively used to produce more compact and powerful computers. The dimensions of the microchips still require shorter wavelengths of light to enhance future ‘nano’ scale production. It is envisaged that 193 nm lithography is beginning to reach its limit. Extreme Ultraviolet (EUV) lithography of 13.5 nm wavelength could provide a solution for the next step of miniaturization, however presently no light source exists with sufficient average power. We report here results of a study, showing the feasibility of a FEL EUV source driven by a multi-turn superconducting energy-recovery linac (ERL). The proposed 40x20 m² facility will be located underground for radiation safety purposes. With MW-scale consumption from the power grid it is estimated to provide 5 kW of average EUV power. We elaborate in some detail the SASE option, which is presently technically feasible, however regenerative-amplifier option should be also kept in mind. The proposed design is based on a short-period (2-3 cm) undulator. The corresponding electron beam energy is about 0.6-0.8 GeV. The proposed accelerator consists of photoinjector, booster, and a multi-turn ERL.

• R. Hajima, R. Nagai, N. Nishimori, M. Sawamura
  JAEA/ERL, Ibaraki
• T. Hayakawa, M. Seya, T. Shizuma
  JAEA, Ibaraki-ken

Hybrid K-edge densitometer (HKED) is used for concentration measurement of U, Pu and minor actinides in liquid solution samples. In the HKED, the concentration of the most-abundant element is determined by K-edge densitometer and concentrations of other elements are derived from XRF signals. We propose a multi-turn small-size energy-recovery linac (ERL) to produce laser-Compton scattered X-rays for the HKED. The X-rays with good monochromaticity and energy tunability allow measurement of actinides with much better resolution than the existing HKED systems based on X-ray tubes. The ERL energy is 85 MeV to produce 130-keV X-rays. In the present design, we adopt a racetrack configuration, in which electrons are accelerated six times by L-band superconducting linac and decelerated six times for the energy recovery. Design and expected performance of the ERL-HKED are presented.