XFEL Oscillator in ERLs R. Hajima Japan Atomic Energy Agency March 4, 2010.

X-ray FEL Oscillator Typical electron beam parameters for XFELO K-J. Kim et al., PRL (2008), PRST-AB (2009) Typical electron beam parameters for XFELO Energy, charge, emittance, repetition are similar to ERL beams. R. Hajima

ERL Injector Performance design example Ultimate Injector 8 pC, 0.1 mm-mrad will be feasibly obtained. 500-kV DC gun, 3-D pulse shaping, and 10-MeV SCA. 80 pC, 0.1 mm-mrad will be obtained. 750-kV DC gun, 3-D pulse shaping, and 10-MeV SCA. R. Hajima et al. Compact ERL CDR (2008) I.V. Bazarov et al., PRST-AB (2005) R. Hajima 3 3

Integration of XFELO and ERL Beam dump ERL Gun FEL Gun Location Operation mode straight section of the loop additional branch independent concurrent From the ERL side, XFELO adds a new feature with minor modification ERL and XFELO provides complimentary X-rays Injector share an ERL injector use a FEL injector R. Hajima 4 4

Growth of emittance and energy spread in a loop E=5 GeV, r=25m, “half arc” = TBA x 15 coherent SR for 20pC/2ps tuning of cell-to-cell phase advance  negligible emittance growth incoherent SR FEL gain is preserved energy spread after FEL lasing Energy recovery is preserved R. Hajima 5 5

XFELO lasing at 5-GeV? 0.1nm XFELO with a 5-mm gap Halbach-type undulator 7 GeV, lw=1.88cm, gap=5mm, aw=1  l=0.1nm 5 GeV, lw=1.43cm, gap=5mm, aw=0.59  l=0.1nm assuming same peak current and same mode area, 1-D gain for 5 GeV beam is “0.65 x 1-D gain for 7 GeV” further gain reduction due to the emittance effect. R. Hajima 6

XFELO with 5 and 7-GeV ERLs 0.3 1Å X-FELO analytical estimation of small-signal FEL gain Energy 5 GeV 7 GeV charge 20 pC  st 2 ps sE/E 1e-4 aw 0.59 1.0 lu 1.43 cm 1.88 cm Nu 3000 b*=ZR 10 m en 0.1 mm-mrad gain 14 % 22 % 0.25 7 GeV 0.2 small-signal FEL gain 0.15 0.1 5 GeV 0.05 0.1 0.15 0.2 0.25 0.3 normalized emittance (mm-mrad) The above calculations are based on a Halbach-type undulator. DELTA undulator gives 1.4 times larger FEL gain. R. Hajima

Velocity bunching in an ERL main linac Injector SCA #1 SCA #2 SCA #3 Velocity bunching for a SASE-FEL injector Velocity bunching for an ERL light source Velocity bunching for an X-FELO (1) no additional component is required (2) only 2-3% SCAs are used for the velocity bunching (3) residual energy spread is smaller than magnetic compression (4) moderate emittance growth for low bunch charge L. Serafini and M. Ferrario, AIP-Porc. (2001) H. Iijima, R. Hajima, NIM-A557 (2006). R. Hajima, N. Nishimori, FEL-2009 R. Hajima

Gain reduction by bandwidth mismatch K-J. Kim et al., PRL 100, 244802 (2008). growth rate of the m-th mode gain loss cavity length detuning bandwidth mismatch bandwidth of the Bragg mirrors = 12 meV 12 meV In the following calculations, we choose reflectivity and phase shift for a cavity round trip R. Hajima

Example of the velocity bunching st E Energy (MeV) bunch length (ps) z (m) velocity bunching 1 2 3 4 5 6 7 8 9 10 15 20 25 30 injector merger SCA #1 SCA #2 PARMELA simulation bunch charge q = 7.7 pC velocity bunching bunching in 8 cavities injection 10.9 MeV, 1.3 ps, -85 deg. gradient Eacc = 8.5 MV/m 4 cavities 4 cavities emittance growth by chromatic aberration 0.2 0.4 0.6 0.8 1 1.2 5 10 15 20 25 30 2 ex sx 1.6 ey sy beam size (mm) 1.2 norm. emittance (mm-mrad) 0.8 0.4 R. Hajima 5 10 10 15 20 25 30 z (m) z (m)

Optimum design of the velocity bunching injector merger SCA #1 SCA #2 bunch charge q = 7.7 pC velocity bunching bunching in 6 cav. + on-crest 2 cav. injection 10.9 MeV, 1.3 ps, -90 deg. gradient Eacc = 8.5 MV/m at the SCA#2 exit E = 27.7 MeV, st = 380 fs, sE = 250 keV ex = 0.16 mm-mrad, ey = 0.13 mm-mrad 10 30 9 25 8 st 7 20 6 E bunch length (ps) 5 Energy (MeV) 15 4 3 velocity bunching 10 2 5 1 5 10 15 20 25 30 z (m) 2 0.2 0.4 0.6 0.8 1 5 10 15 20 25 30 ex ey z (m) norm. emittance (mm-mrad) 1.6 sx 1.2 sy beam size (mm) 0.8 0.4 5 10 15 20 25 30 R. Hajima 11 z (m)

Phase plot at the SCA #2 exit R. Hajima

Enhancement of the FEL gain by velocity bunching 0.8 for 5-GeV, 1-Å X-FELO 0.7 0.6 with velocity bunching 0.5 7.7 pC, 0.38 ps, sE/E=5x10-5 small-signal FEL gain 0.4 0.3 0.2 without velocity bunching 20 pC, 2 ps, sE/E=10-4 0.1 0.1 0.15 0.2 0.25 0.3 normalized emittance (mm-mrad) Significant enhancement of the FEL gain by velocity bunching. Gain~40% is possible even with emittance growth during the bunching. R. Hajima

Simulation of XFELO (5 GeV with velocity bunching) saturation After the saturation: pulse duration t=1.2 ps (FWHM) photons/pulse (intra cavity) Np = 2x1010 photons/pulse (extracted) Np = 7x108 R. Hajima

2-Loop Design of 5-GeV ERL HOM-damped cavity high threshold current of HOM BBU allows 2-loop configuration. TESLA　(HOM:5×2) ERL　(HOM:6×2) R. Hajima

Possible Scheme for Combining ERL and XFELO 5 GeV ERL for SR use : accelerate 2 times 7.5 GeV recirculating linac for XFELO : accelerate 3 times Optional We can switch two operation modes by introducing an orbit bump having lrf/2= 11.5 cm. S. Sakanaka, talk at PF-ISAC (2010) R. Hajima

Growth of emittance and e-spread for 3-pass 7.5-GeV 1st-loop: E=2.5 GeV, r=8.66m, 2x14-cell FODO 2nd-loop: E=5 GeV, r=25m, TBAx30-cell Den DsE 1st loop (2.5 GeV) 8pC/400fs 20pC/2ps incoherent SR 0.029 mm-mrad 34 keV 34 keV coherent SR assumed to be compensated 37 keV 11 keV 2nd loop (5 GeV) 0.027 mm-mrad 130 keV 130 keV 53 keV 16 keV acceptable for FEL DsE/E = 2 x 10-5 R. Hajima 17 17

Stability of SRF A/A< 2·10-5 P < 0.01 deg Cornell LLRF System Demonstrated: Exceptional field stability at QL = 106 to 108 Lorentz-force compensation and fast field ramp up Piezo microphonics compensation with ~20 Hz bandwidth A/A< 2·10-5 P < 0.01 deg M. Liepe, ERL-09 energy stability << FEL gain band width This requirement is fulfilled by current LLRF technology. R. Hajima 18 18

Conclusions Hard X-ray ERL can accommodate XFELO. we can extend the frontier of X-ray beam parameters 0.1nm-XFELO is feasibly realized at 5-GeV ERL with velocity bunching 7.5-GeV beam from a 2-loop 5-GeV ERL an ERL injector is shared, no major modification is needed XFELO can be installed either at a loop or a branch. however, beam loss in a long narrow duct might be a problem for a XFELO in a loop In the Japanese collaboration, XFELO is considered as a part of 5-GeV hard X-ray ERL. R. Hajima 19 19