• M. Boscolo, M. Ferrario, V. Fusco, B. Spataro, C. Vaccarezza
  INFN/LNF, Frascati (Roma)
• L. Giannessi, M. Quattromini, C. Ronsivalle
  ENEA C.R. Frascati, Frascati (Roma)
• M.  Migliorati, L. Palumbo
  Rome University La Sapienza, Roma
• L. Serafini
  INFN-Milano, Milano

The first phase of the SPARX project is essentially an R&D activity focused on developing techniques and critical components for future X-ray FEL facilities. The SPARXINO test facility will generate ultra-high peak brightness electron beams at 1 GeV, thanks to the upgrade of the existing Frascati 800 MeV linac. This facility will allow driving a single pass FEL experiment in the range of 3-5 nm, both in SASE and SEEDED FEL configurations. A peculiarity of this linac design is the choice of integrating a rectilinear RF compressor in the early stage of the acceleration, producing a 300-500 A beam, with a magnetic chicane afterwards, for a further compression up to 1 kA. In this paper we discuss the dynamics of the beam, which is in the space charge dominated regime throughout almost all the linac. Start to end simulations and preliminary stability studies taking into account some significant parameter fluctuations are also reported.

• C. Limborg-Deprey
  SLAC, Menlo Park, California

Funding: SLAC is operated by Stanford University for the Department of Energy under contract number DE-AC02-76SF00515

If the laser pulse driving photoinjectors could be arbitrarily shaped, the emittance growth induced by space charge effects could be totally compensated for. In particular, for normal conducting RF guns the photo-electron distribution should approach a 3D-ellipsoidal shape. The emittance at the end of the injector would reduce to the combination of cathode emittance and RF emittance. We explore how the emittance and the brightness can be optimized for normal conducting photocathode RF gun depending on the peak current requirements. Techniques available to produce those ideal laser pulse shapes are also discussed.

• J.R. Roensch, J. Rossbach
  Uni HH, Hamburg
• K. Abrahamyan, G. Asova, J.W. Baehr, G. Dimitrov, H.-J. Grabosch, J.H. Han, S. Khodyachykh, M. Krasilnikov, S. Liu, V. Miltchev, A. Oppelt, B. Petrosyan, S. Riemann, L. Staykov, F. Stephan
  DESY Zeuthen, Zeuthen
• M.V. Hartrott, D. Lipka
  BESSY GmbH, Berlin

Funding: This work has partly been supported by the European Community, contract numbers RII3-CT-2004-506008 and 011935, and by the 'Impuls- und Vernetzungsfonds' of the Helmholtz Association, contract number VH-FZ-005.

The main goal of the Photo Injector Test facility at DESY Zeuthen (PITZ) is to test and to optimize photo injectors for Free-Electron Lasers (FELs). The demands on such a photo injector are small transverse emittances, short bunches and a high bunch charge. A FEL is driven by an accelerator which consists of a rf gun followed by an acceleration section and a magnetic bunch compressor. For the effective bunch compression detailed studies of the longitudinal phase space have to be performed. The correlation between the positions of the particles in the bunch and their longitudinal momenta has to be understood and the non-linearities of the longitudinal phase space have to be analysed. A special apparatus for longitudinal phase space tomography at 5 MeV using a dipole, a Cherenkov radiator, an optical transmission line and a streak camera was developed. Results of longitudinal phase space measurements are presented and compared with simulations.

• D.R. Gillingham, T. M. Antonsen
  IREAP, College Park, Maryland

Funding: Work supported by the Office of Naval Research and the Joint Technology Office.

High power Free Electron Laser (FEL) designs require high-brightness beams. These beams may suffer beam quality degradation during transport through bending sections from effects that were previously insignificant at low bunch charges. Potential mechanisms include microbunching from longitudinal space-charge and transverse emittance dilution from coherent synchrotron radiation. The effects are not well-understood in the transient state, in the presence of conducting boundaries or when these effects operate together and in conjunction with the beam dynamics. A simulation method applicable under the conditions of a MW-class average power FEL driver has been developed that accounts for radiation, space-charge and boundary conditions in a self-consistent manner. This simulation may be useful in evaluating design concepts under consideration including chicane bunch compressors and energy recovery bending arcs.

• J.R. Harris
  University of Maryland, College Park, Maryland
• J.G. Neumann
  IREAP, College Park, Maryland
• P.G. O'Shea
  University Maryland, College Park, Maryland

Funding: Department of Energy, Office of Naval Research, Army Research Laboratory, and Directed Energy Professional Society

All beams are dominated by space charge forces when first created. After a beam is accelerated, space charge directly plays a less important role. However, at low energy space charge will drive changes in the beam which will become "frozen in" as the beam is accelerated, and may have adverse consequences even at high energy. In this paper, we report on the generation and evolution of modulated beams in the University of Maryland Electron Ring, a low energy (10 keV), high current (100 mA) electron recirculator for the study of beams in the extreme space charge dominated regime. Such intense, modulated beams have application to future high power FELs and novel light sources.

• J. Wu, P. Emma, Z. Huang
  SLAC, Menlo Park, California

Funding: Work supported by the U.S. Department of Energy under Contract No. DE-AC02-76SF00515.

Longitudinal space charge (LSC) force can be a main effect driving the microbunching instability in the linac for an x-ray free-electron laser (FEL). In this paper, the LSC-induced beam modulation is studied using an integral equation approach that takes into account the transverse (radial) variation of LSC field. Changes of beam energy and the transverse beam size can be also incorporated. We discuss the validity of this approach and compare it with other analyses as well as numerical simulations. We apply this approach to study the LSC effect in the LCLS accelerator

• 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).