• D.J. Dunning, N. Thompson
  STFC/DL/ASTeC, Daresbury, Warrington, Cheshire
• R. Bartolini
  JAI, Oxford
• H. Geng, Z. Huang
  SLAC, Menlo Park, California
• B.W.J. McNeil
  USTRAT/SUPA, Glasgow

Optimisation studies of an HHG-seeded harmonic cascade FEL design for the UK's proposed New Light Source (NLS) facility are presented. Three separate FELs are planned to meet the requirements for continuous coverage of the photon energy range 50-1000 eV with variable polarisation, 20 fs pulse widths and good temporal coherence. The design uses an HHG seed source tuneable from 50-100 eV to provide direct FEL seeding in this range, and one or two stage harmonic cascades to reach the higher photon energies. Studies have been carried out to optimise a harmonic cascade FEL operating at 1 keV; topics investigated include modulator configuration, seed power level and effects of the HHG seed structure. FEL simulations using realistic electron beam distributions are presented and tolerance to increased emittance has been considered.

• A. Brachmann, F.-J. Decker, Y.T. Ding, D. Dowell, P. Emma, J.C. Frisch, A. Gilevich, G.R. Hays, P. Hering, Z. Huang, R.H. Iverson, H. Loos, A. Miahnahri, D. Nordlund, H.-D. Nuhn, P.A. Pianetta, J.L. Turner, J.J. Welch, W.E. White, J. Wu, D. Xiang
  SLAC, Menlo Park, California

Surface characterization of the LCLS RF gun cathode A. Brachmann On behalf of the LCLS commissioning team The first copper cathode installed in the LCLS RF gun was used during LCLS commissioning for more than a year. However, after high charge operation (~ 500 pC), the cathode showed a decline of quantum efficiency due to surface contamination caused by residual ionized gas species present in the vacuum system. We report results of SEM, XPS and XAS studies that were carried out on this cathode after it was removed from the gun. X-ray absorption and X-ray photoelectron spectroscopy reveal surface contamination by various hydrocarbon compounds. In addition we report on the performance of the second installed cathode with emphasis on the spatial distribution of electron emission.

• H. Geng, Y.T. Ding, Z. Huang
  SLAC, Menlo Park, California
• R. Bartolini
  Diamond, Oxfordshire
• D.J. Dunning, N. Thompson
  STFC/DL/ASTeC, Daresbury, Warrington, Cheshire

The crossed-planar undulator is a promising scheme for full polarization control in an x-ray FEL*. For SASE FELs, it has been shown a maximum degree of circular polarization of about 80% is achievable**. In this paper, we study the effectiveness of a cross undulator for a seeded x-ray FEL. The degree of circular polarization for both the fundamental and the harmonic radiation are considered.

* K.-J. Kim, Nucl. Instrum. Methods A445, 329 (2000).
** Y. Ding, Z. Huang, Phys. Rev. ST-AB 11, 030702 (2008).

• F.-J. Decker, R. Akre, A. Brachmann, J. Craft, Y.T. Ding, D. Dowell, P. Emma, J.C. Frisch, Z. Huang, R.H. Iverson, A. Krasnykh, H. Loos, H.-D. Nuhn, D.F. Ratner, T.J. Smith, J.L. Turner, J.J. Welch, W.E. White, J. Wu
  SLAC, Menlo Park, California

The Linac Coherent Light Source (LCLS) at SLAC is an x-ray Free Electron Laser with wavelengths of 0.15 nm to 1.5 nm. The electron beam stability is important for good lasing. While the transverse jitter of the beam is about 10-20% of the rms beam sizes, the jitter in the longitudinal phase space is a multiple of the energy spread and bunch length. At the lower energy of 4.3 GeV (corresponding to the longest wavelength of 1.5 nm) the relative energy jitter can be 0.125%, while the rms energy spread is with 0.025% five times smaller. An even bigger ratio exists for the arrival time jitter of 50 fs and the bunch duration of about 5 fs (rms) in the low charge (20 pC) operating mode. Although the impact to the experiments is reduced by providing pulse-by-pulse data of the measured energy and arrival time, it would be nice to understand and mitigate the root causes of this jitter. The thyratron of the high power supply of the RF klystrons is one of the main contributors. Another suspect is the multi-pacting in the RF loads. Phase measurements down to 0.01 degree (equals 10 fs) along the RF pulse were achieved, giving hints to the impact of the different sources.

• Y. Cai, K.L.F. Bane, K.J. Bertsche, A. Chao, R.O. Hettel, X. Huang, Z. Huang, C.-K. Ng, Y. Nosochkov, A. Novokhatski, T. Rabedeau, J.A. Safranek, G.V. Stupakov, L. Wang, M.-H. Wang, L. Xiao
  SLAC, Menlo Park, California

Over the past year, we have worked out a baseline design for PEP-X, as an ultra-low emittance storage ring that could reside in the existing 2.2-km PEP-II tunnel. The design features a hybrid lattice with double bend achromat cells in two arcs and theoretical minimum emittance cells in the remaining four arcs. Damping wigglers reduce the horizontal emittance to 86 pm-rad at zero current for a 4.5 GeV electron beam. At a design current of 1.5 A, the horizontal emittance increases, due to intra-beam scattering, to 164 pm-rad when the vertical emittance is maintained at a diffraction limited 8 pm-rad. The baseline design will produce photon beams achieving a brightness of 10²² (ph/s/mm²/mrad²/0.1% BW) at 10 keV in a 3.5-m conventional planar undulator. Our study shows that an optimized lattice has adequate dynamic aperture, while accommodating a conventional off-axis injection system. In this paper, we will present the study of the lattice properties, nonlinear dynamics, intra-beam scattering and Touschek lifetime, and collective instabilities. Finally, we discuss the possibility of partial lasing at soft X-ray wavelengths using a long undulator in a straight section.

• J.C. Frisch, C. Bostedt, J.D. Bozek, A. Brachmann, R.N. Coffee, F.-J. Decker, Y.T. Ding, D. Dowell, P. Emma, A. Gilevich, G. Haller, G.R. Hays, P. Hering, B.L. Hill, Z. Huang, R.H. Iverson, E.P. Kanter, B. Kraessig, H. Loos, A. Miahnahri, H.-D. Nuhn, A. Perazzo, M. Petree, D.F. Ratner, T.J. Smith, S.H. Southworth, J.L. Turner, J.J. Welch, W.E. White, J. Wu, L. Young
  SLAC, Menlo Park, California
• R.B. Wilcox
  LBNL, Berkeley, California

In addition to its normal operation at 250pC, the LCLS has operated with 20pC bunches delivering X-ray beams to users with energies between 800eV and 2 keV and with bunch lengths below 10 fs FWHM. A bunch arrival time monitor and timing transmission system provide users with sub 100 fs synchronization between a laser and the X-rays for pump / probe experiments. We describe the performance and operational experience of the LCLS for short bunch experiments.