pSASE: concept and implementation at
LCLS-II
D. Xiang, Y. Ding and Z. Huang, SLAC
H. Deng, SINAP
1/30/2013
Thanks to many discussions with S. Reiche, H.-D. Nuhn, ……
More information in PRST-AB 16, 010703 (2013)
Introduction
 SASE: Self-Amplified Spontaneous Emission
In FELs, radiation overtakes e-beam by one radiation
wavelength  per undulator period
 Slippage length: N ≈ 3000 ×0.15 nm ≈ 1.5 fs
N
N
N
Radiation fields with distance larger than N
evolve independently and therefore are uncorrelated in phase
FEL power
FEL spectrum
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Purified SASE (pSASE) FEL
SASE: temporal profile and spectrum
 Cooperation length: lc =/4πρ (slippage length within one gain length)
lb
c/2πlc
seed
c/lb
2πlc
FEL power
FEL spectrum
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Purified SASE (pSASE) FEL
Improving temporal coherence
 Frequency doubler/tripler (SASE HGHG)
 = 45 nm
 = 9 nm
Feldhaus et al., NIM A, 2004; Xing et al., NIM A, 2011
SASE
SEHG
 iSASE
Bandwidth
reduction factor
Thomson & McNeil, PRL 2008; Dunning, Thomson & McNeil,
IPAC10; Pellegrini, Wu et al., 2012
 Harmonic lasing
Need many mini-chicanes
with moderate B-field
Bandwidth reduction
factor ~ harmonic number
McNeil et al, PRL 2006; Schneidmiller & Yurkov, PRST-AB,
2012; Schneidmiller & Yurkov, DESY 12-173, 2012
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Need many phase shifters to
suppress lasing at longer
wavelengths
Purified SASE (pSASE) FEL
Harmonic interaction
K=1
K=3
K = 10
Ratio of power gain length
at the n-th harmonic to that
at the fundamental:
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Purified SASE (pSASE) FEL
pSASE
6 GeV
U13 … U18
U10 U11 U12
K=6.3,λ1 =4.2 nm λ1 =0.6 nm
U1 U2 U3 … … U7 U8 U9
K = 2, λ1 =0.6 nm
Amplify seed
radiation through
harmonic interaction
Standard SASE
length ~ 10 gain length
SASE with a
purified seed
Purified seed
Increase slippage
further amplified
length by n
to saturation
Provide ~100 MW radiation (seed)
while keeping energy spread
growth small
Seed is amplified
(~500 MW) and
purified
Fundamental radiation starts from shot
noise; suppression not needed
6
Enhanced
spectral
brightness
Purified SASE (pSASE) FEL
pSASE @ LCLS-II
6 GeV
U13 … U18
U10 U11 U12
K=6.3,λ1 =4.2 nm λ1 =0.6 nm
U1 U2 U3 … … U7 U8 U9
K = 2, λ1 =0.6 nm
At the exit of
U9
At the exit of
U10
At the exit of
U11
At the exit of
U12
2πlc≈ 4~ 5 fs
2πlc≈1 fs
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Purified SASE (pSASE) FEL
pSASE @ LCLS-II
6 GeV
U13 … U18
U10 U11 U12
K=6.3,λ1 =4.2 nm λ1 =0.6 nm
U1 U2 U3 … … U7 U8 U9
K = 2, λ1 =0.6 nm
At the exit of U18
SASE
pSASE
SASE
pSASE
Average over 10 shots
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Purified SASE (pSASE) FEL
pSASE
Self-seeding with central wavelength
depending on beam energy
Beam energy jitter leads
to fluctuations in FEL
central wavelength
FEL power is not sensitive
to beam energy jitter
iSASE with continuous phase shifter
Shift radiation field by (n-1)λ after each undulator period (FEL
power grows in the phase shifter too)
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Purified SASE (pSASE) FEL
Summary and outlook
 Easy way to reduce FEL bandwidth (PS/mini-chicane not needed) at
ANY wavelength
 pSASE FEL power not sensitive to beam energy jitter
 Suited for FELs (LCLS-II, EXFEL…) that use variable gap undulators
*LCLS-II CDR
 Use 17th harmonic interaction (K=10) to enhance the LCLS-II FEL
spectral brightness by >10 at NO additional cost
 Promising alternative to self-seeding for high rep-rate FELs (NGLS)
where the heat might damage the grating required in self-seeding
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Purified SASE (pSASE) FEL