• Y. Pinhasi, Yu. Lurie, G.A. Pinhasi, A. Yahalom
  CJS, Ariel

Microwave tubes and free-electron lasers are based on distributed interaction between electromagnetic radiation and gain media. When such devices are operating in an amplifier configuration, a forward wave is amplified while propagating in a polarized medium, in a stimulated emission process. Formulating a coupled mode theory for excitation of both forward and backward waves in a distributed gain medium, we have identified in previous works [1] conditions leading to efficient excitation of backward wave without any mechanism of feedback or resonator assembly. The induced polarization is given in terms of an electronic susceptibility tensor, resulting in a coupling coefficient betweens the waves. In this work we extend our previous results in two directions: 1. We discuss the case of a complex coupling coefficient between the backward and forward waves and extend our previous results with respect to a real coupling coefficient, thus the present work discusses a more general and realistic case. 2. We discuss the solution of the same problem relaxing the "high gain" assumption. This leads to a more complex set of third order differential equations.

[1] "Backward Wave Excitation and Generation of Oscillations in Distributed Gain Media and Free-Electron Lasers in the Absence Of Feedback" the 26th International FEL Conference, Trieste, Italy.

• D. Kipnis, E. Dyunin, A. Gover
  University of Tel-Aviv, Faculty of Engineering, Tel-Aviv

Smith-Purcell radiation is the emission of electromagnetic radiation by an electron beam passing next to an optical grating. Recently measurement of relatively intense power of such radiation was observed in the THz-regime [1]. To explain the high intensity and the super-linear dependence on current beyond a threshold it was suggested that the radiating device operated in the high gain regime, amplifying spontaneous emission (ASE) [1,2]. We contest this interpretation and suggest an alternative mechanism. According to our interpretation the device operates as a distributed feedback (DFB) laser oscillator, in which a forward going surface wave, excited by the beam on the grating surface, is coupled to a backward going surface wave by a second order Bragg reflection process. This feedback process produces a saturated oscillator. We present theoretical analysis of the proposed process, which fits the reported experimental results, and enables better design of the radiation device, operating as a Smith-Purcell DFB laser.

[1] A.Bakhtyari, J.E.Walsh, J.H.Brownell, Phys.Rev. ·10⁶⁵ 006503 (2002). [2] H.L. Andrews, C.A. Brau, Phys.Rev. ST-AB 7, 070701 (2004).

• S. Bielawski, C. Bruni, C. Szwaj
  PhLAM/CERCLA, Villeneuve d'Ascq Cedex
• M.-E. Couprie, D. Garzella
  CEA/DSM/DRECAM/SPAM, Gif-sur-Yvette
• G. De Ninno, B. Diviacco, M. Trovo
  ELETTRA, Basovizza, Trieste
• D. Fanelli
  Universita di Firenze, Florence
• M. Hosaka, M. Katoh, A. Mochihashi
  UVSOR, Okazaki
• G.L. Orlandi
  ENEA C.R. Frascati, Frascati (Roma)
• Y. Takashima
  Nagoya University Graduate School of Engineering, Nagoya

Dynamical instabilities lead to unwanted full-scale power oscillations in many classical lasers and FEL oscillators. For a long time, applications requiring stable operation were typically performed by working outside the problematic parameter regions. A breakthrough occurred in the nineties [1], when emphasis was made on the practical importance of unstable states (stationary or periodic) that coexist with unwanted oscillatory states. Indeed, although not observable in usual experiments, unstable states can be stabilized, using a feedback control involving arbitrarily small perturbations of a parameter. This observation stimulated a set of works leading to successful suppression of dynamical instabilities (initially chaos) in lasers, sometimes with surprisingly simple feedback devices [2]. We will review a set of key results, including in particular the recent works on the stabilization of mode-locked lasers, and of the super-ACO, ELETTRA and UVSOR FELs [3].

[1] Ott et al. Phys. Rev. Lett., 64, 1196 (1990). [2] Bielawski et al. Phys. Rev. A 47, 327 (1993). [3] Bielawski et al. Phys. Rev. E. 69, 045502 (2004), De Ninno & Fanelli, Phys. Rev. Lett. 92, 094801 (2004), Bruni et al., proc. EPAC 2004.

• E. Allaria, G. De Ninno
  ELETTRA, Basovizza, Trieste
• A. Antoniazzi, D. Fanelli
  Universita di Firenze, Florence
• F.T. Arecchi
  UNIFI, Sesto Fiorentino (FI)
• R. Meucci
  INOA, Firenze

We numerically investigate the effect of a delayed control method on the stabilization of the dynamics of the Elettra storage-ring free-electron laser in Trieste (Italy). Simulations give evidence of a significant reduction of the typical large oscillations of the laser intensity. Results are compared with numerical and experimental data obtained with a derivative feedback. The possibility of an experimental implementation of the proposed method is also discussed.

• Y. Kim, K. Floettmann, S. Schreiber
  DESY, Hamburg
• D. Son
  CHEP, Daegu

Funding: for the TTF2 VUV-FEL Team

By the help of nonlinearity in the longitudinal phase space, the VUV-FEL at the TESLA Test Facility phase 2 (TTF2) is under operating in the femtosecond (fs) FEL mode which generates coherent and ultra-bright SASE source with photon pulse duration time of around 20 fs (FWHM) and wavelength of around 32 nm. For the fs FEL mode operation, bunch length of electron beams should be compressed by two bunch compressors to have a leading spike in the longitudinal beam density distribution or peak current. The required peak current at the spike is higher than about 1.0 kA, and the spike length is shorter than around 200 fs (FWHM). In this paper, we describe our commissioning experiences to optimize two TTF2 bunch compressors for the fs FEL mode operation.