• T. Watanabe, D.F.L. Liu, J.B. Murphy, J. Rose, T.V. Shaftan, T. Tsang, X.J. Wang, L.-H. Yu
  BNL, Upton, Long Island, New York
• P. Sprangle
  NRL, Washington, DC

Funding: Work supported by the Office of Naval Research.

A laser seeded near IR FEL amplifier experiment was initiated at the BNL SDL [1] to explore various schemes of FEL efficiency improvement and generation of short Rayleigh length (SRL) FEL output. The FEL achieved first SASE lasing at 0.8 μm on May 6, 2005. The experimental characterization of the laser seeded FEL output power, spectrum and transverse mode structure evolution will be presented.

[1] A. Doyuran et al., PRSTAB, Vol. 7, 050701 (2004).

• T.V. Shaftan, S. Krinsky, D.F.L. Liu, J.B. Murphy, J. Rose, X.J. Wang, T. Watanabe, L.-H. Yu
  BNL, Upton, Long Island, New York
• H. Loos
  SLAC, Menlo Park, California

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.

We discuss analysis of an experiment with High-Gain Harmonic Generation Free Electron Laser at BNL (DUV FEL). The tunability concept [1] of a seeded FEL with a fixed seed wavelength has been verified experimentally. During the experiment we recorded about 200 radiation spectra corresponding to different energy chirps in the electron beam. We have analyzed this set of spectral data to obtain properties of HGHG radiation. Correlations and trends in the radiation spectrum at 266 nm have been observed and studied.

[1] T. Shaftan and L.H. Yu, Phys. Rev. E 71, 046501 (2005)

• 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.

• L.-H. Yu, D.F.L. Liu, J.B. Murphy, J. Rose, T.V. Shaftan, X.J. Wang, T. Watanabe
  BNL, Upton, Long Island, New York

An important requirement for a high average power laser system is a manageable power density on the first optical element. One possibility to achieve this is a single pass amplifier which generates a short Rayleigh range (SRL) light beam. We present design parameters and calculated performances for several SRL configurations. These include a simulation of the optically guided (pinched) MW class FEL [1], the scalloped beam FEL amplifier [2] and high gain TOK amplifiers we propose to explore at our SDL facility.

[1] P. Sprangle et al. , IEEE J. of Quantum Electronics, Vol. 40, No. 12, 1739, 2004 [2] D. Nguyen, private communication, 2005.

• T. Watanabe, D.F.L. Liu, J.B. Murphy, I.P. Pinayev, J. Rose, T.V. Shaftan, J. Skaritka, T. Tanabe, T. Tsang, X.J. Wang, L.-H. Yu
  BNL, Upton, Long Island, New York
• S. Reiche
  UCLA, Los Angeles, California
• P. Sprangle
  NRL, Washington, DC

The design of a Short Rayleigh Length (SRL) FEL amplifier based on the strong focusing VISA undulator [1] is presented in this study. The SRL FEL amplifier will be operating in the IR (0.8 - 1 μm), and consists of a two-meter VISA undulator with a peak seed laser power of about 1 kW. The FEL power and transverse mode evolution along the undulator were investigated using the three-dimensional numerical code GENESIS1.3. The evolution of the FEL output from the undulator exit to the first downstream optics is also studied. The possibility of using the proposed amplifier for a two-stage cascaded HGHG FEL [2] at the BNL SDL is also explored. The design parameters and the numerical results will be presented.

[1] R. Carr et al., PRSTAB, Vol. 4, 122402 (2001). [2] J. Wuard and L.H. Yu, NIMA 475, 104 (2001).

• T. Watanabe, D.F.L. Liu, J.B. Murphy, J. Rose, T.V. Shaftan, Y. Shen, T. Tsang, X.J. Wang, L.-H. Yu
  BNL, Upton, Long Island, New York
• L. Giannessi, S. Spampinati
  ENEA C.R. Frascati, Frascati (Roma)
• P. Musumeci
  Universita di Roma I La Sapienza, Roma
• S. Reiche
  UCLA, Los Angeles, California

Funding: Work suppoted by the Brookhaven National Lab and Office of Naval Research

The SDL facility at BNL[1] is an excellent platform to explore some of the recent ideas related to superradiance in a seeded single pass FEL. At the SDL facility there is an operating FEL with a Ti:Sapphire seed laser and a high brightness e-beam with an energy up to 250 MeV. Seeding may be realized with pulses shorter than the e-beam bunch length to induce the superradiant regime. A status report concerning this experiment will be presented.

[1] A. Doyuran et al., PRSTAB, Vol. 7, 050701 (2004).