1. Design of an ECHO-seeded FEL at nm wavelength
D. Xiang
March

2. Introductions
External seeding to improve temporal coherence of FEL output
Success of ECHO relies on preservation of long-term memory of phase space correlations
ECHO-7 to verify basic physics
Before applying the ECHO technique to a user facility at ultrahigh harmonic number (100~200), it should be tested at a moderate harmonic number (25~50) to prove technologies and identify possible risks.
SLAC provides an excellent base and offer much of the required infrastructure for the R&D
Topic is to design a ~2 GeV ECHO-seeded FEL at SLAC

3. Time tables
, Fix basic beam parameters (energy, current, energy spread) and laser parameters
, Design of the photoinjector, linac, bunch compressors, undulators
, S2E simulation from cathode to undulator
, Time-dependent simulation for the ECHO-seeded FEL at nm wavelength
Deliverables: a design report should be documented for SLAC-PUB.

4. Tentative parameters
Seed laser wavelength: 258 nm (tripled from a Ti:Sapphire laser, the same as LCLS cathode laser)
Target FEL wavelength: nm (25th) / 5.16 nm (50th)
Undulator period: 3 cm (LCLS-I)
Beam energy: 1.64 GeV
If we start from a 774 nm seed laser, the harmonic number is 75th and 150th accordingly

5. Open questions Coherent radiation, or FEL lasing?
How many BCs? At what energy?
20 pC, 250 pC, 1 nC?
Laser heater?
Where to put the ECHO beam line?
Compatible to FAULT?
High harmonic number or short wavelength?