• H. Freund
  SAIC, McLean
• M. Abo-Bakr, K. Goldammer, D. Kraemer, B.C. Kuske, A. Meseck
  BESSY GmbH, Berlin
• S. Biedron
  ANL, Argonne, Illinois

The BESSY FEL is based on a seeded cascade of High Gain Harmonic Generation (HGHG) sections followed by an amplifier to produce coherent and stable short wavelength output. Here, we report on comparative design studies carried out using the MEDUSA [1], and GENESIS [2] simulation codes. These two codes have each been used to successfully predict a variety of FEL designs and have agreed well with a number of important experiments. In addition, they were included in a comparative study of FEL simulation [3] that reported substantial agreement between the codes for the specific configurations studied. However, these codes are based on different assumptions. GENESIS treats the particle dynamics using a wiggler-averaged orbit approximation, the transverse electromagnetic field is treated using a field solver, and harmonics are not included. MEDUSA does not use the wiggler-averaged orbit approximation to treat particle dynamics, the transverse fields are treated using a Gaussian modal superposition, and harmonics are included self-consistently. Hence, the comparative study for an HGHG cascade is important. We report the results where the parameters of each stage have been optimized.

[1] H.P. Freund et al., IEEE JQE 36, 275 (2000). [2] S. Reiche, NIMA 429, 243 (1999). [3] S.G. Biedron et al., NIMA 445, 110 (2000).

• F. Ciocci, G. Dattoli, L. Giannessi
  ENEA C.R. Frascati, Frascati (Roma)
• S. Biedron, S.V. Milton
  ANL, Argonne, Illinois
• H. Freund
  SAIC, McLean

We study the spectral details of planar bi-harmonic undulators and show that they have quite interesting properties. They can indeed be exploited to enhance or suppress harmonics and can be exploited in high gain segmented FEL devices to make more efficient the mechanisms of harmonic generation. We also show that the formalism we develop can be extended to the multi-component case and can usefully be exploited to study undulator magnetization errors.

• H. Freund
  SAIC, McLean

Funding: Work supported by ONR, NAVSEA, and the JTO

Time-dependent FEL simulations use a variety of techniques. Most simulations use a slowly varying envelope approximation (SVEA). One such technique assumes that the envelope varies only in z combined with a field representation as an ensemble of discrete harmonics, which is equivalent to a time-dependent simulation [1] but is computationally prohibitive. A second technique uses an SVEA in both in z and t [2]. The particles and fields are advanced in z using the same process as in steady-state simulations and then the time derivative describing slippage is applied. This is used in wiggler-averaged codes such as GINGER [3] and GENESIS [4]. We describe the inclusion of this technique in the non-wiggler-averaged code MEDUSA [5], which is applied to amplifiers and oscillators. MEDUSA differs from GINGER and GENESIS also in the way the field is treated. GINGER and GENESIS use a field solver and must explicitly propagate the field outside the wiggler oscillators. This is computationally intensive. MEDUSA uses a Gaussian mode ensemble; hence, there is no need to propagate the fields outside the wiggler, and MEDUSA is able to simulate FEL oscillators in 3-D using relatively modest computational resources.

[1] N. Piovella, Phys. Plasmas 6, 3358 (1999). [2] R. Bonifacio et al., Phys. Rev. A 40, 4467 (1989). [3] W. Fawley, LBID-2141, CBP Tech Note-104, UC-414, 1995. [4] S. Reiche, NIMA 429, 243 (1999). [5] H.P. Freund et al., IEEE JQE 36, 275 (2000).