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

Funding: We gratefully acknowledge our funding agencies, Joint Technology Office (JTO) and the Office of Naval Research (ONR).

Photoinjector performance is a limiting factor in the continued development of high powered FELs. Presently available photocathodes have limited efficiency and short lifetime in an RF-gun environment, due to contamination or evaporation of a photosensitive surface layer. An ideal photocathode should have high efficiency at visible wavelengths, long lifetime in practical vacuum environments, and prompt emission. High efficiency cathodes typically have limited lifetime, and the needs of the photocathode are generally at odds with those of the drive laser. A potential solution is the low work function dispenser cathode, where short lifetimes are overcome by periodic in situ regeneration that restores the photosensitive surface layer, analogous to methods used in the power tube industry. This work reports on the fabrication techniques and performance of cesiated metal photocathodes and cesiated dispenser cathodes, with a focus on understanding and improving quantum efficiency and lifetime, analyzing issues of emission uniformity, and optimizing the activation procedure needed to rejuvenate the cathode. The efficiency versus coverage behavior of cesiated metals is discussed and closely matches that predicted by recent theory.

• T.V. Shaftan
  BNL, Upton, Long Island, New York

Funding: The manuscript has been authored by Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH1-886 with the U.S. Department of Energy.

One of the problems in the high-power lasers design is in outcoupling of a powerful laser beam out of a vacuum volume into atmosphere. Usually the laser device is located inside a vacuum tank. The laser radiation is transported to the outside world through the transparent vacuum window. While considered transparent, some of the light passing through the glass is absorbed and converted to heat. For most applications, these properties are academic curiosities; however, in multi-kilowatt lasers, the heat becomes significant and can lead to a failure. The absorbed power can result in thermal stress, reduction of light transmission and, consequently, window damage. Modern optical technology has developed different types of glass (Silica, BK7, diamond, etc.) that have high thermal conductivity and damage threshold. However, for kilo- and megawatt lasers the issue still remains open. In this paper we present a solution that may relieve the heat load on the output window. We discuss advantages and issues of this particular window design.

• T. Tanaka
  RIKEN Spring-8, Hyogo
• T. Bizen, D. Iwaki, X. Marechal, T. Seike, R. Tsuru
  JASRI/SPring-8, Hyogo
• T. Hara, H. Kitamura
  RIKEN Spring-8 Harima, Hyogo

In-vacuum undulators are now widely used in lots of SR facilities to provide highly-brilliant hard x-rays not only in large-scale facilities such as SPring-8, ESRF and APS, but also in medium-scale facilities with an electron energy up to 3 GeV. In addition, the SCSS (SPring-8 Compact SASE Source) project is going to adopt the in-vacuum undulator not only for reducing the electron energy to achieve angstrom X-ray FEL but also for commissioning and alignment of components in the undulator line that takes advantage of variable vacuum gap (physical aperture for the electron beam). In this talk, overview of technologies required for development of the in-vacuum undulator will be presented together with practical examples. In addition, ongoing R&Ds at SPring-8 (cryogenic undulator, in-situ field measurement system) will be described in brief.

• S.-H. Lee, S. Sasaki, I. Vasserman, D.R. Walters
  ANL, Argonne, Illinois
• D.E. Kim
  PAL, Pohang, Kyungbuk

Funding: Work supported by DOE under contract no. W-31-109-Eng-38.

A prototype vacuum chamber is being designed for use in the Linac Coherent Light Source at Stanford Linear Accelerator Center under development at the Advanced Photon Source. The chamber will be fabricated from the austenite stainless steels. In general, the magnetic properties of austenite stainless steels are affected by their compositions, processing methods and physical conditions. Austenite stainless steels are generally regarded as non-magnetic in the annealed condition and not attracted significantly by a magnet. However, cold working or welding will change their magnetic properties. This paper presents measurements use to choose a proper chamber material for LCLS undulator, to examine the fabrication processes, and to investigate the relative magnetic permeabilities of the stainless steels such as 316LN, 20Cb-3, Nitronic 33, Nitronic 40 and 310S. This paper presents the results of fabricating of 3"-long vacuum chambers along with their permeability measurements. In addition, the magnetic field variations with/without vacuum chamber under APS undulator A and numerical studies of magnetic field to the permeability of the flat/cylindrical chambers are presented.

• K.L.F. Bane, P. Emma, Z. Huang, H.-D. Nuhn, G.V. Stupakov
  SLAC, Menlo Park, California
• W.M. Fawley
  LBNL, Berkeley, California
• S. Reiche
  UCLA, Los Angeles, California

Funding: Work supported in part by the Office of Science,U.S. Dept. of Energy under Contracts DE-AC02-76F00515 and DE-AC03-76SF0098.

Energy loss due to wakefields within a long undulator, if not compensated by an appropriate tapering of the magnetic field strength, can degrade the FEL process by detuning the resonant FEL frequency. The wakefields arise from the vacuum chamber wall resistivity, its surface roughness, and abrupt changes in its aperture. For LCLS parameters, the resistive component is the most critical and depends upon the chamber wall material (e.g. Cu) and its radius. Of recent interest [1] is the so-called "AC" component of the resistive wake which can lead to strong variations on very short timescales (e.g. ~20 fs). To study the expected performance of the LCLS in the presence of these wakefields, we have made an extensive series of start-to-end SASE simulations with tracking codes PARMELA and ELEGANT, and time-dependent FEL simulation codes GENESIS1.3 and GINGER. We discuss the impact of the wakefield losses upon output energy, spectral bandwidth, and temporal envelope of the output FEL pulse, as well as the benefits of a partial compensation of the time-dependent wake losses obtained with an undulator field taper. We compare these results to those predicted analytically [2].

[1] K.Bane and G. Stupakov, SLAC PUB-10707 (2004). [2] Z. Huang and G. Stupakov, Phys. Rev. ST Accel. Beams 8, 040702 (2005).