Design of an ECHO-seeded FEL at nm wavelength D. Xiang April-22-2011

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

Tentative parameters Seed laser wavelength: 258 nm (tripled from a Ti:Sapphire laser, the same as LCLS cathode laser) Target FEL wavelength: 10.32 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

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 ?

Time tables 05-2011, Fix FEL parameters (1nm?), basic beam parameters (energy, current, energy spread) and laser parameters 07-2011, Study the feasibility 10-2011, Design of the photoinjector, linac, bunch compressors, undulators 2-2012, S2E simulation from cathode to undulator 05-2012, Time-dependent simulation for the ECHO-seeded FEL at nm wavelength Deliverables: a design report should be documented for SLAC-PUB.