• K. Jensen
  NRL, Washington, DC
• A. Balter
  ,
• D.W. Feldman, N.A. Moody, P.G. O'Shea
  IREAP, College Park, Maryland

Funding: We gratefully acknowledge funding provided by the Joint Technology Office and the Office of Naval Research.

Photocathodes for FELs and accelerators will benefit from rugged and self-rejuvenating photocathodes with high QE at the longest possible wavelength. The needs of a high power FEL are not met at present by existing photocathode-drive laser combinations: requirements generally necessitate barrier-lowering coatings which are degraded by operation. We seek to develop a controlled porosity dispenser cathode, and shall report on our coordinated experimental and theoretical studies. Our models account for field, thermal, and surface effects of cesium monolayers on photoemission, and compare well with concurrent experiments examining the QE, patchiness, and evolution of the coatings. Field enhancement, thermal variation of specific heat and electron relaxation rates and their relation to high laser intensity and/or short pulse-to-pulse separation, variations in work function effects due to coating non-uniformity, and the dependence on the wavelength of the incident light are included. The status of methods by which the theory can be extended to semiconductor photocathodes and efforts to provide emission models to beam simulation codes is also treated.

• O. Williams, A. Doyuran, R.J. England, C. Joshi, J. Lim, J.B. Rosenzweig, S. Tochitsky, G. Travish
  UCLA, Los Angeles, California

An Inverse Compton Scattering (ICS) experiment investigating the polarized harmonic production in the nonlinear regime has begun which will utilize the existing terawatt CO2 laser system and 15 MeV photoinjector in the Neptune Laboratory at UCLA. A major motivation for a source of high brightness polarized x-rays is the production of polarized positrons for use in future linear collider experiments. Analytical calculations have been performed to predict the angular and frequency spectrums for various polarizations and different scattering angles. Currently, the experiment is running and we report the set-up and initial results. The advantages and limitations of using a high laser vector potential, ao, in an ICS-based polarized positron source are expected to be revealed with further measurement of the harmonic spectrum and angular characteristics.

• B.W. Adams
  ANL, Argonne, Illinois

Funding: U.S. Department of Energy, Office of Basic Energy Sciences under contract W-31-109-ENG-38

A way is proposed to obtain intense infrared/visible light from an electron bunch in an x-ray free-electron laser in femtosecond synchronism with the x-rays themselves. It combines the recently proposed technique of emittance slicing in a free-electron laser with transition undulator radiation (TUR). The part of the electron bunch that is left unspoiled in the emittance slicing process is the source of both coherent x-rays and of coherent TUR at near-infrared wavelengths. An extension of the concept also exploits the fact that the electrons that participate in the free-electron lasing process lose a significant part of their energy.

• S.-H. Park, Y. Cha, Y.U. Jeong, B.C. Lee, K. Lee
  KAERI, Daejon
• S.V. Miginsky
  BINP SB RAS, Novosibirsk

The KAERI SC RF linac with one 352 MHz cryomodule is routinely operating at 10 MeV. The maximum accelerating gradient achieved so far is about 7.7 MV/m and is expected to increase up to 9 MV/m, if thermal loss and/or vibration instability is sufficiently suppressed. As a next step, we plan to generate Compton X-rays using external lasers at the straight section, just after the SC linac. This beamline will be relocated to downstream next to undulator beamline for a FEL, when the recirculating beamline is built. In this presentation, we estimate the parameters of Compton X-rays at a given system and suggest the new scheme to increase the flux, or to generate fs X-ray pulses using electron beams with a few tens ps pulse duration, using an intense ultra-short laser. We discussed a coherent condition for Relativistic Nonlinear Thomson Scattered (RNTS) radiation (or Nonlinear Compton Scattered radiation).

• K. Lee, Y. Cha, Y.U. Jeong, B.C. Lee, S.-H. Park
  KAERI, Daejon

In contrast to some recent experimental results, which state that the Nonlinear Thomson Scattered (NTS) radiation is incoherent, a coherent condition under which the scattered radiation of an incident laser pulse by a bunch of electrons can be coherently superposed has been investigated. The Coherent Relativistic Nonlinear Thomson Scattered (C-RNTS) radiation makes it possible utilizing the ultra-short pulse nature of NTS radiation with a bunch of electrons, such as plasma or electron beams. A numerical simulation shows that a 25 attosecond X-ray pulse can be generated by irradiating an ultra-intense laser pulse of 4x10(19) W/cm2 on an ultra-thin solid target of 50 nm thickness, which is commercially available. The coherent condition can be easily extended to an electron beam from accelerators. Different from the solid target, much narrower electron beam is required for the generation of an attosecond pulse. Instead, this condition could be applied for the generation of intense Compton scattered X-rays with a modulated electron beam.

• F. Banfi, G. Ferrini, P.G. Galimberti, C. Giannetti, S. Pagliara, F. Parmigiani, E. Pedersoli
  Universita Cattolica-Brescia, Brescia
• J.N. Corlett, S.M. Lidia
  LBNL, Berkeley, California
• B. Ressel
  ELETTRA, Basovizza, Trieste

Funding: U.S. DOE, Office of Science, under Contract No. DE-AC03-76SF00098

A Monte Carlo study of electron transport in Cs2Te films is performed to investigate the transverse emittance epsilon at the cathode surface. We find the photoemitted electron angular distribution and explain the physical mechanism involved in the process, a mechanism hindered by the statistical nature of the Monte Carlo method. The effects of electron-phonon scattering are discussed. The transverse emittance is calculated for different radiation wavelengths and a laser spot size of 1.5*10(-3) m. For a laser radiation at 265 nm we find epsilon = 0.56 mm-mrad. The dependence of epsilon and the quantum yield on the electron affinity Ea is also investigated. The data shows the importance of aging/contamination on the material.

• G.L. Orlandi
  ENEA C.R. Frascati, Frascati (Roma)

In the electromagnetic radiative phenomena originated by relativistic charged beams, angular distortions as well as variations of the photon flux are commonly observed as a function of the ratio between the beam transverse size and the observed wavelength, even at a wavelength shorter than the longitudinal bunch length. In the framework of a single particle theory of the transition radiation, diffractive alterations of the spectrum due, for instance, to the finite size of the radiator screen are already known. For relativistic three-dimensional charged beams, it could be interesting to check if the transition radiation emission undergoes modifications depending on the finite value of the beam transverse size with respect to the observed wavelength. Taking into account the beam diagnostics applications of the transition radiation in a linear accelerator, such an experimental check can offer promising perspectives. The theoretical background and physical basis of the spatial coherence effects affecting the spectral distribution of the transition radiation intensity in conditions of temporal incoherence will be presented. The main beam diagnostics applications will be also contoured.