Recent LCLS injector studies
& plans
Feng Zhou
Thank Brachmann, Decker, Ding, Huang, Loos, Raubenheimer, Turner et al
for the contributions
March 6, 2013
Contents
• Collimator as an aperture to reduce emittance
• New applications of a collimator located in BCs:
- For slice emittance measurements
- Trace back OTR2 emittance issue
- Shorter x-ray FEL production (planning)
• Thoughts of future R&D programs at ASTA and LCLS
injector
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Collimator used to reduce emittance
• Placed collimator in a
non-dispersion area:
to collimate  beam size
• Expected 40% emittance
reduction with one circular
collimator
• Collimator wakefield is of
the major concern
40% reduction
Observed wake effect for a smaller gap
• Analytical shows circular
collimator has much
lower wake effect than
the flat one
(Bane/Stupakov)
• Wake simulation for flat
and circular collimator is
underway (Li/Xiao)
x~250um (Aug2, 2012)
x~130um (Sep26, 2012)
• Design/fabricate a
circular collimator and
test it at the LCLS
injector CY14 IF
simulations confirm the
circular one to be good.
Collimator used to measure slice emittance
Chirped beam length 11mm
BC1 emittance vs. collimator gap
• Evaluate side effects: space
charge (x/y), collimator
wakefield (x, y?), CSR and
dispersion (x)
• Model/data show negligible
collimator wake effect (Bane)
• Y-data/model show negligible
space charge effect
• X-dispersion slightly leaks out
• Can y-emittance represent
BC1 slice emittance?
6
Dependence of X-band cavity offset?
• Slice emittance is independence of X-band cavity
transverse offset – easier to understand.
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BC1 slice x/y emittances (150pC)
• BC1 slice y-emittance is ~0.45um with routine laser profile.
• BC1 slice x-emittance is slightly larger due to the leaked-out
dispersion
• How to compare with LCLS injector OTR2 emittance?
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OTR2/WS2 emittance comparison (150pC)
• During routine operation, what is the LCLS injector
realistic emittance?
• OTR2 vs. WS2, LH chicane ON vs. OFF
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Area counts comparison (OTR2/WS2)
• Obviously, notable lights from the LH chicane are added
to the OTR2 profile, which make OTR2 emittance data
deviate from the reality.
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OTR2 slice emittance comparisons
• OTR2 slice emittance with chicane-off is similar to BC1
slice emittance – data from routine operation (150pC)
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During our taking
data
More typical laser
for operations
Deep Science Mining for Matter
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Discussions on OTR2/BC1 emittances
• OTR2 emittance with LH chicane-on under-estimates the
injector emittance:
- Additional lights from the LH chicane modify the OTR2 beam profiles
- OTR2 with LH chicane-on may not provide absolute emittance value,
but the emittance trend
• Bunch length with LH chicane-on is 10% shorter than
chicane-off – probably the additional lights impact the data.
• With a routine laser profile, slice emittance at OTR2/WS2
and BC1 is ~0.45um for 150pC;
- Plan to measure 250pC.
• BC1 collimator used to measure slice emittance has been
validated – the technique is reliable to measure slice beam.
• BC2 slice emittance measurements (planning)
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Shorter x-ray FEL production (planning)
Collimator gap
 t FWHM 
x
x
Final bunch length
after the chicane
(collimator out)
 t, f
Rms beam size at
the collimator
• To compare with slotted foil (BC2): better or cleaner FEL
background? Plan to do MD.
• Lanfa helps us to check simulations
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Possible to reduce LCLS emittance
• Lengthen laser pulse to reduce
space charge and use smaller
laser size: improved slice
emittance but projected emittance
may be slightly worse (RF
emittance).
• September 2011 data: different
laser pulse length (2.4-4.4ps) but
with same laser size on the
cathode (250pC)
September 2011 data
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Further test at LCLS injector with longer
laser pulse (CY14)
• Simulations show that slice emittance is reduced by 30%
with 7ps laser pulse with laser size of 0.8mm, against
current 3ps with optimum 1mm of laser size (250pC) – may
be tested CY14
Projected
emittance
simulations
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Thoughts of ASTA R&D programs
1. Robust laser cleaning (very near-term, CY13)
2. Upgrade RF gun & laser for 360Hz operations
- Address gun heating issues for 360Hz
- Upgrade laser for 360Hz operation, and MOD/klystron
3. Develop a load-lock system and higher QE (>5%)
cathodes (e.g., Cs2Te):
- May reduce thermal emittance to zero
-
Operate cathode with 360Hz and/or multi-bunch operations
Significantly simplify laser systems (remove 4-pass MPA)
Greatly improve laser profiles thus the emittance and µBI
Great benefit for FACET-II 3-5nC RF gun operation
They are fully aligned with future LCLS demands
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