• M. Ferianis
  ELETTRA, Basovizza, Trieste
• J.M. Byrd, J.W.  Staples, R.B. Wilcox
  LBNL, Berkeley, California
• J. Chen, F.O. Ilday, F.X. Kaertner, J. Kim
  MIT, Cambridge, Massachusetts
• A. Winter
  Uni HH, Hamburg

Fermi is the fourth generation light source that is currently being designed at ELETTRA, in the frame of a collaboration that includes LBNL and MIT. The timing system will play a crucial role in achieving the expected performance of this and other Linac based FELs due to the sub-ps electron bunch length and the expanded use of fs-lasers as key components in future light sources. Furthermore, the requirements of the timing system are also tightly linked to the applications of the generated ultrafast x-ray pulses. In this paper we present the requirements for the FERMI timing system, which will be based on optical timing distribution concepts, currently seen to be the only technique to enable an RMS jitter at the 10fs level. The timing system, intended for a user facility that is operated on a 24-h, 7-d basis, must operate stable and reliable. The fundamental components of the system are analyzed, such as the optical reference oscillator, the fiber optic stabilized links and the local optical to electrical (O/E) converters, needed for the RF plant synchronization. Furthermore, solutions for the synchronization of the diagnostic tools for the Linac as well as user related synchronization issues are presented and discussed.

• W.H. Hwang, J. Choi, Y.J. Han, J.Y. Huang, H.-G. Kim, W.W. Lee
  PAL, Pohang, Kyungbuk

Funding: Work supported by MOST and POSCO.

In PALXFEL [1], the specification of the beam energy spread and rf phase is tighter than PLS Linac. We examined the rf performance in the present PLS 2.5GeV Linac. The beam energy is changed by cooling temperature, air condition, and modulator high voltage jitter. The main factor to change the beam energy is the rf phase drift by environmental conditions. We measured rf phase drift according to the variation of environmental condition and cooling temperature. We reduced the beam energy drift and the rf phase drift in long-term by improvement of cooling and air conditioning control system. Also, rf phase compensation system is needed for stable beam quality. This paper describes the microwave system for the PALXFEL the rf phase measurement and phase compensation system.

[1] Pohang Accelerator Laboratory, POSTECH Pohang 790-784, Korea

• Y.-C. Huang, Y.-Y. Lin
  NTHU, Hsinchu

Funding: Center for Advanced Information System and Electronics Research(CAISER)

A Smith-Purcell radiator produces transversely asymmetric radiation modes due to the arrangement of a grating on one side of the electron beam. This asymmetric output could limit the usefulness of such a device in the THz spectrum where diffraction of waves is severe. It is possible to produce symmetric radiation from a double-grating waveguide driven by an electron beam traversing the waveguide gap. We derive a theory that describes the modes and small signal gain of this novel grating-waveguide free-electron laser. Our theory shows that extremely high laser gain is obtained when the electron beam is phase matched to the middle or edge of the radiation bands where the radiation modes have zero group velocity. In our calculation we obtained 66dB/mm gain at 298 µm for a 5 mA, 30keV driving beam in a grating waveguide with a 50-micron, 40% duty-cycle grating period, a 60-micron groove depth, and a 150 micron waveguide gap. This extremely high gain indicates that this novel device establishes resonance without resonator mirrors in a one-dimensional photonic-crystal lattice or from distributed feedbacks in the grating pairs. Experimental progress will be reported in the conference.

• S. Zhang, D. Hardy, G. Neil, M.D. Shinn
  Jefferson Lab, Newport News, Virginia

Funding: This work supported by the Office of Naval Research, the Joint Technology Office, the Commonwealth of Virginia, the Air Force Research Laboratory, and by DOE Contract DE-AC05-84ER40150.

We report the characterization and performance of a diode-pumped, high-power, picosecond laser system designed for high-current photo-cathode accelerator injector at repetition rates of both 75MHz and 750MHz. Our characterization includes measurement of the system's amplitude stability, beam quality, pulsewidth, and phase noise for both frequencies.

• K. Hayakawa, Y. Hayakawa, K. Nakao, K. Nogami, T. Sakai, I. Sato, T. Tanaka
  LEBRA, Funabashi

Funding: "Academic Frontier" Project for Private Universities: matching fund subsidy from MEXT (Ministry of Educatin, Culture, Sports, Science and Technology), 2000-2004

In general, maximum gain and maximum power of a free-electron laser (FEL) oscillator are not simultaneously satisfied at an identical length of the optical resonator. Use of a short bunch electron beam, therefore, can cause a large fluctuation of gain and saturated power of the FEL due to only a small change in the resonator length. If the length of the resonator can be adjusted at the middle in the macropulse duration of the electron beam, both maximizing conditions will be satisfied simultaneously, which will result in a large FEL output power compared with a normal operation. Since it is difficult to change the length of the resonator during the macro pulse, modulation of the bunch interval has been attempted for the LEBRA FEL system by modulating the phase of the accelerating rf of the electron linac, which has an equivalent effect to change of the resonator length. The modulation of the rf phase has resulted in intensification of the output energy per macro pulse by approximately twice compared with that in normal operation, which is consistent with numerical simulation.

• M.B. Danailov
  ELETTRA, Basovizza, Trieste
• F.O. Ilday, F.X. Kaertner
  MIT, Cambridge, Massachusetts

Laser systems will undoubtedly be one of the key factors determining the performance of VUV and X-ray FELs. In particular, harmonic generation scheme based FELs require at least three mutually synchronized solid-state laser systems: photoinjector laser, seeding laser, end station lasers. In addition, a laser heater is also included in recent FEL designs. It is therefore very important to consider the possibility of integrating these systems to a maximum possible degree. In this paper we consider a promising approach to the integration of the above specified laser systems for the FERMI@ Elettra FEL, based on the distribution of a fiber laser generated seed signal at 1550 nm. This signal, after further amplification and frequency doubling, is used as a seed for Ti:Sapphire amplifiers at the different locations. The paper presents a general layout of the system, the main pulse parameters (i.e. pulse energy and duration) needed in different parts of the system and discusses possible technical solutions

• C.A. Brau, H.L. Andrews, C.H. Boulware, J.D. Jarvis
  Vanderbilt University, Nashville, Tennessee

Funding: Medical Free Electron Laser Program of the Department of Defense under grant number F49620-01-1-0429.

In a SP-FEL, the electrons interact with an evanescent mode of the grating whose frequency is below the lowest frequency for SP radiation [1] and which travels along the grating with no losses except from dissipation. At low electron energy, the group velocity is negative and the SP-FEL operates on an absolute instability; no optical resonator is required. Due to the finite conductivity of the grating surface, dissipative losses attenuate the evanescent wave [2]. Computations for a lamellar grating show that attenuation is important at frequencies above 1 THz, and dominates when the group velocity is small. Due to the interaction with the evanescent wave, the electrons are bunched at the evanescent wave frequency. The superradiant emission from periodic bunches is characterized by spectral and angular narrowing at harmonics of the bunching frequency. Experiments are in progress to demonstrate these effects using a 40-keV electron beam photoemitted from a needle cathode in 5-ns pulses. The grating is 15 mm long, with a 250-micron period. We expect lasing at a wavelength near 1 mm, which will allow us to observe superradiant emission near 330 microns (third harmonic of the evanescent wave) on the second order of the SP radiation.

[1] H. L. Andrews and C. A. Brau, Phys. Rev. ST-AB 7, 070701 (2004). [2] H. L. Andrews, et al., Phys. Rev. ST-AB (in press).

• A. Winter, H. Schlarb
  DESY, Hamburg
• dc. Cheever, J. Chen, F.O. Ilday, F.X. Kaertner, J. Kim, D. Wang, T. Zwart
  MIT, Middleton, Massachusetts
• P. Schmuser
  Uni HH, Hamburg

X-ray pulses with a pulse duration of down to 30 fs FWHM or even sub-fs are desired for various experiments planned at next generation free electron lasers, such as the European XFEL. A synchronization of the probe system in the experimental area to the x-ray pulses with stability on the order of the pulse width is highly desirable for these experiments. This requirement translates to distributing an ultra-stable timing signal to various subsystems of the machine and the experimental area to provide synchronization at the fs level over distances of up to several kilometers. A few years ago, a timing and synchronization system providing stability to the fs level was unthinkable. Recent advances in the field of ultra-short pulse lasers have made optical synchronization systems with such a precision feasible. This talk will focus on an optical approach using a train of ultra-short pulses distributed through optical fiber links. The timing information is contained in the precise repetition rate. First results of such a system operating in an accelerator environment will be reported.

• J.W.  Staples, R.B. Wilcox
  LBNL, Berkeley, California

Funding: This work supported by the US Department of Energy under contract No. DE-AC03-76SF00098

Stabilization of the transit time through a glass fiber using an optical heterodyne technique promises to provide jitter reduction down to the few femtosecond level using inexpensive commodity hardware. An acousto-optical frequency shifter provides the optical frequency offset that is used to downconvert phase shifts at optical frequency to equivalent phase shifts at radio frequency which are used to close a phase-lock loop driving a piezoelectric phase shifter. Using the stabilized fiber transmission medium, two spectral lines of a mode locked laser lock two low-power CW lasers which are transmitted to a receiver which phase locks the same spectral lines of a second mode-locked laser to the first. The optical transmission system operates at low power and is linear, providing excellent signal-to-noise ratio and allows many signals to be transmitted without mutual interference. Experimental results will be presented.

• I. Will
  MBI, Berlin
• S. Düsterer, J. Feldhaus, E. Plönjes, H. Redlin
  DESY, Hamburg

Funding: This work was funded by the European Commission under Contract no. HPRI-CT-1999-50009

The VUV-FEL at DESY provides ultra-short pulses with pulse durations below 50 fs. To explore a wider field of time resolved experiments a complex laser system has been installed delivering 150 fs pulses at a wavelength of 800 nm with 50 μJ pulse energy at 1MHz repetition rate during the FEL burst (of 800 μs). In order to perform two color pump-probe experiments the laser has to be synchronized to the FEL. To ensure precise and reliable synchronized operation of the laser, various diagnostic experiments have been developed. Concepts as well as first results of the synchronization will be shown.