Recent developments for the LCLS injector

Feng Zhou SLAC

Other contributors: Brachmann, Decker, Ding, Emma, Gilevich, Huang, Iverson, Loos, Raubenheimer, Turner et al

5-way meeting at SLACFebruary 4-6, 2013

Recent major LCLS injector developments

• Fantastic laser cleaning results on cathode (2011)– Greatly improved QE, and recovered emittance within 2 wks

• Simplified temporal and spatial laser distributions on cathode (2010 & 2012)– Simplified laser systems, and– Significantly reduced laser power required from amplifier, and – Improved emittance

• New uses of existing collimators (2012 & 2013)– First measured slice emittances at BC1; and plan to generate

short x-ray pulse – R&D on emittance reduction (with LANL)

LCLS laser cleaning in July 2011: QE evolution (@30⁰ laser phase)

• Original QE was only 5e-6 before any cleaning process

• QE was firstly increased by 8-10 times upon the laser cleaning

• QE was further increased by 3 times in the first 6 months following laser cleaning, and then stays at 1.1e-4 from 6th month to now, 18th months following cleaning.

QE

Gun vacuum

Laser cleaning results see PRST-AB, Oct 2012

LCLS laser cleaning: emittance evolution

Immediately laser cleaning

A few weeks later

RF conditioning may improve the electron emission profile

Simplified LCLS drive laser: temporal distribution

• LCLS design required a very uniform temporal laser distribution • Simulations shows uniform temporal has a slight better

emittance than pure Gaussian, but in the measurements we did not see obvious emittance change for 250pC.

Mar 2010No obvious emittance change observed

LCLS design (challenging) Current routine operations

Simplified LCLS drive laser: spatial distribution

• LCLS design required a very uniform spatial laser distribution• Recent simulations show spatial Gaussian-cut laser profile has

a better emittance than uniform: more linear space charge • Spatial Gaussian-cut profile has:

– Saved laser power required from laser amplifier 2-3 times– Improved emittance 30%

LCLS design (challenging)

Feb 2012

Current routine operations

Detail results see PRST-AB, September 2012

Improved emittance with spatial Gaussian-cut laser

Can we further simplify LCLS laser?(personal view only)

68MHz oscillator

Stretcher 30fs to 150ps

Regen Amplifier

Pockels cell

4-pass MPA

Compressor 150ps to 3ps

UV generator

UV telescope

IR stretcher 3ps to 8 ps

Synchronized to Main Drive Linac RF

UV transport to cathode

IR telescope & transport to laser heater

Iris to define laser size on cathode

760nm, 8-10ps, 1mJ253nm,

3ps, 2mJ253nm, 3ps,

<100uJ

760nm 30fs• Develop loadlock for new cathode with only QE>0.5%

– Not worry about QE any more

• Amplifier can be totally removed– System simplifier and more reliable & better stability – Temporal/spatial much cleaner: better for emittance/BI

4mJ

30mJ

Measured slice emittances at BC1 with collimator

• In Nov/December 2012 we developed a new technique to measure slice emittances at LCLS BCs using existing collimators in the middle of BCs:– Measure slice emittance

both x & y planes, and – check slice emittance

preservations through BC1/BC2

Chirp

ed

beam

Linac off-crest

Chirped beam length 11mm

0.7mm

LCLS slice emittance at BC1 (taken on 12/19/2012)

• Collimator wakefield effects are negligible (confirmed by Bane) • Measured slice x-emittance at BC1 (0.5-0.6m) is larger than

injector <0.4m. More MDs are coming in next a few wks

R&D on emittance reduction using collimator (with LANL)

• Collimator placed in non-dispersion area: to collimate beam size

• 40% emittance reduction is expected with the collimator

• Collimator wakefield is the major concern

40% emittance reduction

BC1 flat collimator: observed wake effect for smaller gaps

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

Analytical shows circular collimator has much lower wake effect than flat one (K. Bane)

Wake simulations for flat and circular are underway (Z. Li & L. Xiao)

Summary

• Laser cleaning on LCLS cathode was very successful. However, to make it as a robust cleaning method for XFEL, we need more cleaning tests in RF gun test facilities.

• New uses of the traditional collimators: – Slice beam diagnostics, and short x-ray pulse generation– Emittance reduction (design/construct/test low wakefield

collimator)

• Simplified LCLS drive laser systems to have simple temporal & spatial laser distributions; e-beam performance is improved as well:– May further simplify laser system and significantly improve

e-beam performances

backup

LCLS Cu cathode history

• 1st cathode (2006 - July 2008): original QE was <1e-5, and increased to 3e-5 after cleaning process.

• 2nd cathode (July 2008 - May 2011): QE was 5e-5 but decayed to a half in 7-10 days at 120 Hz:– Had to frequently move laser to next spots – Took much tuning time and also not every spot on the cathode

can deliver desired small emittance beam

• 3rd cathode (May 2011 - present): original QE was <1e-5, which was not sufficient for users’ operations: – Applied laser cleaning– Laser cleaning does work well so far: QE is 2-3 times last cathode

and has no decay so far (~18 months).

Laser cleanings on LCLS cathode in July 2011

• 1st area at 3.5mm X-offset (trial test on 6/15/2011)

• 2nd area (A) at cathode center cleaned on 07/04/2011 has been used for routine operations for 18 months (QE~1.1e-4)

• 3rd area (B) at 2.5mm Y-offset cleaned on 07/26/2011: QE ~1.3e-4

7 months later following laser cleaning

• For the laser-cleaned area B not used for routine operations, QE was also increased from 6e-5 to 1.3e-4 following the laser cleaning.

• But the QE is not changed at the non-laser-cleaned areas

5 of14

QE imaging for half laser in cleaned and the other half in un-cleaned areas

17 months later following laser cleaning

6 of14

LCLS thermal emittance • LCLS QE is changed by a factor of 2.5 after laser

cleaning, but:– Thermal emittance is not changed, and– Corresponding projected emittance at 150/250pC is not

changed

• These contaminations changing QE seem not modify the copper work function.

• LCLS injector emittance is mostly limited by the copper cathode thermal emittance.

Laser cleaning results see PRST-AB, Oct 2012

LCLS drive laser

68MHz oscillator

Stretcher 30fs to 150ps

Regen Amplifier

Pockels cell

4-pass MPA

Compressor 150ps to 3ps

UV generator

UV telescope

IR stretcher 3ps to 8 ps

Synchronized to Main Drive Linac RF

UV transport to cathode

IR telescope & transport to laser heater

Iris to define laser size on cathode

760nm, 8-10ps, 1mJ

253nm, 3ps, 2mJ

253nm, 3ps, <100uJ

760nm~30fs

• Need sufficient laser energy on cathode• Need proper spatial/temporal distributions on cathode• Need reliable/stable laser

LCLS laser pulse length: slice emittance (250pC)

Full open255pC0.46um

0.75mm gap250pC0.54um

0.3mm gap155pC0.49um

0.2mm gap105pC0.34um

0.17mm gap80pC0.24um

Data taken on 09/26/2012