• C. Pellegrini
  UCLA, Los Angeles, California

Funding: Work funded by the US Department of Energy, grant 4-444025-PG-57689

We study the use of Radio Frequency electromagnetic waves as undulators for short wavelength FELs and undulator radiation sources. Magnetostatic undulators have a gap much smaller than the period, limiting how short a period we can use. The relation between period and gap can be overcome using electromagnetic waves to produce the force wiggling the electrons. The wave frequency is chosen to optimize the system performance. In the case of centimeters or mm waves a waveguide is used to propagate the field over a long distance. We call an undulator based on a waveguide a TWU. In this paper we show that a TWU using X-band RF is a practical and convenient device for short wavelength FELs, and to produce sub-nanometer undulator radiation circularly or linearly polarized.The recent development of high power X-band microwave sources make it possible today to build TWUs of practical interest. In this paper we will discuss the characteristic of the TWU, how to control the effects of RF power losses in the waveguide walls, and how to optimize a TWU and the associated electron transport system for use in a synchrotron radiation source or FEL.

• 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.

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

In a recent paper [1] we have described a novel variational formulation for the propagation and generation of radiation in wave-guides. The formulation is based on the representation of all the involved quantities in the frequency domain and the decomposition of field and currents in terms of the wave-guide transversal Eigen function. In this work we present the utilization of this formalism to the derivation of a numerical scheme that is used to study the build up of radiation in free electron lasers in the linear approximation.

[1] Asher Yahalom, Yosef Pinhasi & Yuri Lurie "Spectral and Variational Principles of Electromagnetic Field Excitation in Wave Guides" submitted to Physics Letters A (2004).

• U. Lehnert, P. Michel, W. Seidel, D. Stehr, J. Teichert, D. Wohlfarth, R. Wuensch
  FZR, Dresden

First lasing of the mid-infrared free-electron laser at ELBE was achieved on May 7, 2004. Since then stable lasing has been achieved in the IR range from 4 to 22~μm using electron beam energies from 15 to 35~MeV. At all wavelengths below 20~μm a cw optical power higher than 1~W can be produced with an electron beam of 50~pC bunch charge or less. The optical pulse width at its minimum (2.2~ps measured at 17~μm) resembles the typical electron bunch length of 2~ps without bunch compression but can be increased by detuning the optical cavity. The optical bandwidth was in all cases close to the fourier limit.

• T. Kanai, K. Awazu, S. Yoshihashi-Suzuki
  Osaka University, Suita

Free Electron Laser (FEL) at Osaka University can be continuously varied in the range of 5.0-20.0 μm. A FEL has a double pulse structure. The structure consists of a train of macropulses of the pulse width 15 μs, and each macropulse contains a train of 330 micropulses of the pulse width 5 ps. The tunability and short pulse afford new medical applications such as investigation of protein dynamics and ablation of soft tissues. Precise control of micropulse train is very important for medical applications using FEL because macropulse with long pulse duration sometimes leads to undesirable thermal effects. FEL pulse control system using an acousto optic modulators (AOM) was developed in order to investigate of non-thermal effect between the FEL and tissue. This system provide a very good efficiency (~60 %) and a fast switching speed (>200 ns). A phosphorylated protein was irradiated with FEL that controlled the pulse. These result confirmed that the thermal effect is controlled by pulse duration. This system will be expected as a novel tool for investigation of interaction between the FEL and normal tissue.

• D. Li
  ILT, Suita, Osaka

The conventional cavity for a free-electron laser (FEL) oscillator forms Gaussian optical beam, transversely spreading along the interaction region. The transverse divergence will induce reduction of the FEL gain by three aspects: degenerating filling factor, causing diffraction loss and limiting the effective interaction distance. Bessel optical beam has been experimentally demonstrated diffraction-free characteristic, providing a possibility of improvement of FEL gain. In this paper, we present a conceptual design of a Bessel beam cavity for the free-electron laser. This cavity generates nondiffracting optical beam in the wiggler, which can improve the filling factor, decrease the diffraction loss and elongate the effective interaction distance.