• S.V. Benson, K. Beard, C.P. Behre, G.H. Biallas, J. Boyce, D. Douglas, H.F.D. Dylla, R. Evans, A.G. Grippo, J.G. Gubeli, D. Hardy, C. Hernandez-Garcia, K. Jordan, L. Merminga, G. Neil, J.P. Preble, M.D. Shinn, T. Siggins, R.L. Walker, G.P. Williams, S. Zhang
  Jefferson Lab, Newport News, Virginia
• N. Nishimori
  JAEA/FEL, Ibaraki-ken

Funding: This work supported by the Office of Naval Research, the Joint Technology Office, the Commonwealth of Virginia, the Air Force Research Laboratory, the US Army Night Vision Laboratory, and by DOE Contract DE-AC05-84ER40150.

After demonstrating 10 kW operation with 1 second pulses, the Jefferson Lab program switched to demonstrating high power operation at short wavelengths using a new 8 cm period wiggler and a THz suppression chicane. We report here on the lasing results to date using this new configuration. We have demonstrated a large reduction in THz heating on the mirrors. We have also eliminated heating in the mirror steering assemblies, making operation at high power much more stable. Finally, we have greatly reduced astigmatism in the optical cavity, allowing operation with a very short Rayleigh range. The laser has been tuned from 0.9 to 3.1 microns using the new wiggler. User experiments commenced in April of 2005 with the FEL Upgrade operating over the 1-3 micron range. We are in the process of installing a 5.5 cm permanent magnet wiggler that will give us even larger tuning range and higher power.

Corresponding author: Tel: 1-757-269-5026; fax: 1-757-269-5519; E-mail address: felman@jlab.org

• M.D. Shinn, C.P. Behre, S.V. Benson, C.W. Gould, J.G. Gubeli, D. Hardy, G. Neil, S. Zhang
  Jefferson Lab, Newport News, Virginia

Funding: This work supported by the Office of Naval Research, the Joint Technology Office, the Army Night Vision Laboratory, the Air Force Research Laboratory, the Commonwealth of Virginia, and by DOE Contract DE-AC05-84ER40150.

In order to provide widely-tunable light to our users, we used a hole outcoupler. To date, we've produced 85 W at 2.8 microns, and been able to continuously tune over a 1 micron spectral range. Besides the anticipated low outcoupling efficiency associated with this scheme, we found that we had considerable problems stabilizing the output when we operated our FEL in cw mode. We believe that this is due to the long time available for mode competition to develop. Measurements of gain, loss, and transverse mode profiles (both intracavity and output) will be compared with our models.

• G.P. Williams, S.V. Benson, G.H. Biallas, D. Douglas, J.G. Gubeli, G. Neil, M.D. Shinn, S. Zhang
  Jefferson Lab, Newport News, Virginia
• O.V. Chubar, P. D. Dumas
  SOLEIL, Gif-sur-Yvette

Funding: This work supported by the US Army Night Vision Lab, ONR, JTO, the Commonwealth of Virginia, the Air Force Research Laboratory, and DOE Contract DE-AC05-84ER40150. We thank Fred Dylla for his expert advice and encouragement.

Short bunches of electrons in the Jefferson Lab FEL emit multiparticle coherent edge radiation as they enter the dipole prior to the outcoupler mirror. This light is more collimated than synchrotron light and furthermore is modified by interference from the last chicane magnet after the high reflector. This light provides an additional heat load on the outcoupler in a wavelength range it was not designed to handle. We have performed calculations of this effect using a new extension of the Synchrotron Radiation Workshop code which, importantly, takes into account both acceleration and velocity (or Coulomb) terms of the emitted electric field. We have also measured THz properties of some of the mirrors. We show how the addition of a decompression chicane mitigates these problems.