Working Group II

Linac-Based, High-Gain FELs

1. To provide a useful guide to the future light source community on possible designs of linac-based high-gain FELs.
2. To establish possible design trade-offs.
3. To underscore the significance of particular technologies or problems and highlight areas where additional work has to be done.

The wavelength range of interest is from the VUV to hard x-rays (1 Angstrom to 2000 Angstroms).

To focus the activities of the Linac-Based High-Gain FELs Working Group, we will work on producing two main figures as a graphical output.

1. Gain length as a function of wavelength.
2. Saturated power as a function of wavelength.

• Normalized rms emittance at the output of the gun (1 and 0.5 mm-mrad, possibly other values).
• Linac technology (Superconducting L-Band and normal-conducting S-Band).
• Compression.
• Trajectory wander in the wiggler.

An accelerator model will be provided which will:

• Include relevant wake field formulary / estimates for energy spread and emittance growth as a function of the beam parameters and hardware configuration in the acceleration.
• Calculate emittance growth in the compression stages. The number of compression stages may be between zero and three, depending on the wavelength. A few possible energies will be tried for compression.
• Provide estimate for the matching error at the input to the wiggler.
• Provide estimates to the wake fields in the wiggler given its gap.

• Emittance (which may change along the wiggler due to wiggler wake fields)
• Energy spread (which may change along the wiggler due to wiggler wake fields)
• Wiggler strength
• Trajectory wander
• Energy
• Current

In addition, the report will include Invited Papers from group members that should serve as a detailed design guide for Linac-Based High-Gain FELs. The information I would like to have in the report will be a self-contained and convenient guide to:

• Emittance from the gun as a function of various parameters with other parameters optimized. As fixed parameters take frequency, cathode technology, and emittance compensation technology. As variable parameters take charge, peak field. As optimized parameters take pulse length, emittance compensation solenoid field, cathode spot size and phase.
• Wake fields in the linac as a function of aperture, cell length, charge, charge, pulse length, gradient, survey and BPM error etc.
• Wake fields in the wiggler as a function of gap, survey and BPM error, smoothness, wall resistivity etc.
• Compression: How many compressors, their optimal location and compression value. Effects of CSR wake.
• A discussion of how to treat wake fields non-local emittance blow-up in the FEL performance.
• A guide to convert wiggler dipole RMS error and BPM errors to trajectory RMS error.
• A guide to the calculation of FEL gain and saturated power at various wavelengths given electron beam parameters, wiggler gap, field and trajectory RMS error, etc. using quick (Mathematica or Spread Sheet) recipe.
• FEL and SR light quality, attainment and property of short pulses, seeding/monochromatization and related subjects.

Group members making presentations are requested to bring color copies and, if possible, electronic files of their presentation material for inclusion in the workshop proceedings.

• State-of-the-art wiggler strength as a function of period and gap.
• State-of-the-art strong focussing strength as a function of period and gap.
• Rms dipole error as a function of period and gap.
• Best achievable alignment precision (long range)
• Period of BPM/corrector stations.
• Deviation of the mean of the peak field values along the whole wiggler.

From Working Group VII:

• Best achievable BPM resolution.
• Alignment precision.

• In what ways may ring-based sources complement linac-based sources and vice versa?

• How fast can the electron energy of a LINAC be changed (on a continuous scale) for tuning the photon energy of a SASE FEL? What is the duty cycle of a LINAC in a multi-user facility if the electron energy has to be modulated in order to fit the user needs at several different undulators?
• Tolerable trajectory wander and tolerable phase errors in long undulators.
• Are there important tolerances that have not yet been addressed like second field integrals for quadrupole or sextupole errors?

• Input on "conventional" linacs and on high power FELs.

• Desired e-beam and optical beam measurements for commissioning and operation not only for the desired 4th-generation device but for those that will be required along the way to develop technology to a level sufficient to build such a light source.