Femtosecond Pulse Generation at LCLS using a SlottedSpoiler and Possible Testing at DUVFEL Paul Emma

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Femtosecond Pulse Generation at LCLS using a
Slotted-Spoiler and Possible Testing at
DUV-FELPaul Emma,K. Bane, M. Cornacchia, Z.
Huang, G. Stupakov, D. Walz, SLACH. Schlarb,
DESYFeb. 19, 2004

• Basic scheme as applied to LCLS
• Parameters for DUV-FEL testing

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Transverse Emittance is Critical for X-ray FEL
lt 1 mm at 1 Å, 15 GeV
eN 1.2 mm
P P0
eN 2.0 mm
P P0/100
Can we spoil most of the e- bunch and leave 1-fs
to lase?
courtesy S. Reiche
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0.1 mm (300 fs) rms
Easy access to time coordinate along bunch
50 mm
x, horizontal pos. (mm)
2.6 mm rms
z, longitudinal position (mm)
LCLS BC2 bunch compressor chicane (similar in
other machines)
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Add thin slotted foil in center of chicane
BEFORE FOIL
1-mm emittance
AFTER FOIL
5-mm emittance
1-mm emittance
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Difficult to verify cold, unspoiled beam core
without FEL
LCLS
spoiled beam
unspoiled beam
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Choose slot for shortest e- pulse, while
retaining full peak current
peak current reduced
2Dx 50 mm
peak current preserved
2Dx 250 mm
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Dx
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Track 200k macro-particles through entire LCLS up
to 14.3 GeV
200 fs
DE/E
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gey ? 6.7 mm
gey ? 1.0 mm
gex ? 6.7 mm
gex ? 1.0 mm
PRIOR TO FOIL
JUST AFTER FOIL
includes thin foil effect
Coulomb scattering in thin-foil Elegant (M.
Borland)
Simple formula not accurate for thin foils
15-mm thick Be foil Z 4, A ? 9.01, r ? 1.85
g/cm3 X0 ? 35 cm, qrms ? 12 mrad
20 over-estimate for x/X0 ? 4?10-5
?DE/E0? ? 2.5?10-5 (est.) sE/E0 ? 2.5?10-5 (est.)
Cross-checked by G. Stupakov using
reference W.R. Leo, "Techniques for Nuclear and
Particle Physics Experiments", Springer-Verlag,
Berlin, 1994.
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Transition Radiation Wakefield of Foil (g gtgt 1)
Ê
t
K.L.F. Bane and G. Stupakov, SLAC-PUB-9726, June
2003.
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Foil-wakefields at 4.54 GeV with ?125-mm slot
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Pulse Length Control With Stepping Motor
Tapered slot allows pulse length control
Double slit to generate 2 precisely timed pulses
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DUV-FEL 2-m beta function at center of chicane
bx ? 2 m
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DUV-FEL
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DUV-FEL
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Difficult to verify cold, unspoiled beam core
without FEL
DUV-FEL
spoiled beam
unspoiled beam
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DUV-FEL
220 fs FWHM
FEL gain-narrowing may reduce this 220-fs e-
pulse to a 100-fs photon pulse
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DUV-FEL
no foil
with foil
Coulomb scattering in thin-foil Elegant (M.
Borland)
15-mm thick Be foil Z 4, A ? 9.01, r ? 1.85
g/cm3 X0 ? 35 cm, qrms ? 570 mrad
tails
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Transition Radiation Wakefields not a problem in
DUV-FEL
(K. Bane code)
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Horizontal position and angle distribution with
and without foil at end of chicane
x-distribution
108 mm
x'-distribution
388 mm
250 mrad
555 mrad
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What we need to test scheme in DUV-FEL

• 15-mm thick Be foil 2 cm square with tapered
  vertical slot of full width varying from 0.4 mm
  to 2 mm
• Slider for remote control of insertion depth
  (maybe use existing POP-0 slider)
• bx ? 2 m at foil in chicane (close to B3 if
  possible)
• Emittance measurement on POP-11, 12, 13 (or
  similar)
• FEL lasing (SASE or seeded) with slippage length
  lt 200 fs
• Measure HGHG spectral width (100 fs FWHM ?)

2.3 nm
Thanks to Xijie for invitation to attend