recent lcls injector studies & plans

Feng Zhou

Thank Brachmann, Decker, Ding, Huang, Loos, Raubenheimer, Turner et al for the contributions

March 6, 2013

Recent LCLS injector studies & plans

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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

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

x~250um (Aug2, 2012)x~130um (Sep26, 2012)

• 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)

• 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

Chirp

ed

beam

Chirped beam length 11mm

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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?

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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)

12Deep Science Mining for Matter

During our taking data

More typical laser for operations

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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)

ftx

FWHMxt ,

Rms beam size at the collimator

Final bunch length after the chicane (collimator out)

Collimator gap

• 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)

Projected emittance

• 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

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