THOC :: FEL Theory

Date/Time: 25-Aug-05 :: 15:45—17:30
Chair: L. Giannessi, ENEA C.R. Frascati, Frascati (Roma)

Paper Title Page
THOC001 Recent Progress in High-Gain FEL Theory 656
 
  • Z. Huang
    SLAC, Menlo Park, California
 
 

Funding: Work supported by US Department of Energy contract DE-AC02-76SF00515.

High-gain free electron lasers (FEL) are being developed as extremely bright x-ray sources of a next-generation radiation facility. In this paper, we review the basic theory and the recent progress in understanding the startup, the exponential growth and the saturation of the high-gain process, emphasizing the self-amplified spontaneous emission (SASE). We will also discuss how the FEL performance may be affected by various errors and wakefield effects in the undulator.

In memory of the late scientist Ming Xie.

 
   
THOC002 Quantum Theory of SASE-FEL with Propagation Effects 664
 
  • R. Bonifacio, R. Bonifacio
    Universidade Federal de Alagoas, Maceio
  • N. Piovella
    Universita' degli Studi di Milano, MILANO
  • G.R.M. Robb
    Strathclyde University, Glasgow
 
 

We present a proof of principle of the novel regime of quantum SASE with propagation effects. Using a self-consistent system of Schrodinger-Maxwell equations, we show that the dynamics of the system is determined by a properly defined "quantum FEL-parameter", ρ', which rules the number of photons emitted per electron, as well as the electron recoil in units of ћk. In the limit ρ'>>1 the quantum model reproduces the classical SASE regime with random spiking behavior and broad spectrum. In this limit we show that the equation for the Wigner function reduces to the classical Vlasov equation. In the opposite limit, ρ'<1, we demonstrate "quantum purification" of SASE: the classical spiking behavior disappears and the power spectrum becomes very narrow so that the temporal coherence of the SASE spectrum is dramatically improved. Photon statistics, electron-photon entangled states, minimum uncertainty states and quantum limitations on bunching and energy spread will be discussed.

 
   
THOC003 Schemes of Superradiant Emission from Electron Beams and "Spin-Flip Emission of Radiation" 668
 
  • A. Gover
    University of Tel-Aviv, Faculty of Engineering, Tel-Aviv
 
 

A unified analysis for Superradiant emission from bunched electron beams in various kinds of radiation scheme is presented. Radiation schemes that can be described by the formulation include Pre-bunched FEL (PB-FEL), Coherent Synchrotron Radiation (CSR), Smith-Purcell Radiation, Cerenkov-Radiation, Transition-Radiation and more. The theory is based on mode excitation formulation - either discrete or continuous (the latter - in open structures). The discrete mode formulation permits simple evaluation of the spatially coherent power and spectral power of the source. These figures of merit of the radiation source are useful for characterizing and comparing the performance of different radiation schemes. When the bunched electron beam emits superradiantly, these parameters scale like the square of the number of electrons, orders of magnitude more than spontaneous emission. The formulation applies to emission from single electron bunches, periodically bunched beams, or emission from a finite number of bunches in a macro-pulse. We have recently employed the formulation to calculate a ne kind of coherent radiation from electron beam: enhanced Electron Spin Resonance Emission from a polarized electron beam. Estimates of the characteristics and possible applications of this effect will be presented.

 
   
THOC004 Effect of Losses on the Gain and Start Current in Smith-Purcell Free-Electron Lasers 672
 
  • 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).

 
   
THOC005 High Gain FEL Oscillators: Inclusion of Pulse Propagation Effects
 
  • G. Dattoli, A. Renieri
    ENEA C.R. Frascati, Frascati (Roma)
  • P.L. Ottaviani, S. Pagnutti
    ENEA-Bologna, Bologna
 
 

The theory of pulse propagation of low gain Free Electron Laser oscillators is extended to the high gain regime. The physics underlying the relevant phenomenology is discussed and it is shown that the system dynamics, from small signal to saturation, can be reproduced using simple scaling formulae. The inclusion of non linear harmonic generation is considered too, the performances of a source operating in the extreme UV is considered and compared with those operating in the SASE configuration.