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

2. Introductions External seeding to improve FEL temporal coherence
Echo seeding is a future option for LCLS-II and NGLS
Basic physics can be verified in ECHO-7 experiment
1 nm is the goal (feedback from LCLS SAC members)
Before applying the ECHO technique to a user facility at ultrahigh harmonic number (100~200), it should be tested at an intermediate 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. 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); variable gap?
Beam energy: ~ 1.64 GeV (need higher energy to reach 1 nm)
If we start from a 774 nm seed laser, the harmonic number is 75th and 150th accordingly

4. Open questions Cost limit ? 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 ?
What is the best way to reach 1 nm ?

5. Time tables
, Fix FEL parameters (1nm?), basic beam parameters (energy, current, energy spread) and laser parameters
, Study the feasibility
, Design of the photoinjector, linac, bunch compressors, undulators
2-2012, 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.