Part1: what have we learned from LCLS-I injector?

• Drive laser systems• Cathode• Gun & its accessories• Injector beamline and operational issues

Thanks laser group, injector commissioning/operation team, particularly Dowell, Emma, Brachmann, etc

Feng Zhou, LCLS-II accelerator design meeting, October 12, 2011

Laser-related experience from LCLS-I

• Uniform temporal profile is NOT necessary to get high quality beam, which simplifies laser system: single Gaussian with 2-4ps FWHM is good

• Does spatial have to be very uniform? – Had some studies but need more experiments to make solid

conclusion • Variable wavelength (PSI): it allows to reduce thermal

emittance by moving wavelength to 260-270nm if QE is in very good shape

• 100J is sufficient for 250pC if QE is ~4e-5 but we suggest to have more headroom (200J) in case we are in bad situation of poor QE

Emittance vs. laser pulse length(250pC)

• SC is better with longer pulse length but time-dependent RF emittance ( ) is worse

• Slice emittance is better with longer pulse length (RF kick has less impact on slice emittance)

2z

Spatial Gaussian-cut (250pC)• Simulations and theory show that certain spatial Gaussian-cut

beam has better emittance than the one with uniform.• LCLS MD result shows that with the Gaussian-cut

– Slice emittance is 0.37m vs. 0.34m with uniform. The expected emittance improvement is probably washed out by non-smooth laser (Thales) and asymmetrical shoulders-cut.

– Laser transmission through the iris is double of the uniform case• Plan more MD with Coherent laser

Regular uniform

Gaussian-cut

LCLS measurements

Variable wavelength (PSI results)

Hauri, et al., PRL 104, 234802 (2010)

Laser experience (con’t)

• Coherent has narrower BW and quasi-Fourier limit temporal profile; it turns out 2ps for coherent vs. 400fs for Thales after compression. With Coherent: – Less stretching with the compressor to reach 3-4ps – UV tripler works better: smooth beam profile– After October downtime, we are planning to switch to Coherent – LCLS-II plans to use Coherent

• What about our LCLS-II laser baseline: 2-pulse vs. 1-pulse? Need clear specifications: – What is charge/bunch in 2-pulse mode operation?– Should leave space to accommodate long delay line – Phase I R&D may study multi-bunch mode

Cathode – big concern although it is tiny

• The cathode material physics is our weakest understanding; LCLS has used 3 cathodes:– 1st cathode (2007 - July 2008): QE was low but ok for

commissioning – 2nd cathode (July 2008 - May 2011): QE was 4~5e-5 but

quickly decayed to 2~3e-5 in ~7days at 120Hz and thus had to frequently move to next location took machine time and also NOT every location had good e-beam

– 3rd cathode (May 2011 - present): original QE was only <1e-5, which can not be used for users’ operation; • Fortunately laser cleaning does work and works very well so far

but still some concerns for future application to LCLS-II

Laser cleaning observations • During operations after cleaning, QE is not decayed

but keeps gradually increased from 5.e-5 to 8.5e-5 from July 4 to October 4 2011: – Caused by continuous vacuum improvement (?)

• Need a few days to condition the cleaned surface to achieve small emittance after cleaning; it is probably caused by following two factors: – Regular laser operations to smoothen out cleaned surface (?) – RF/vacuum conditioning (?)

• E-beam performance can be improved greatly at the laser-cleaned spot even when it is in idle: – Immediately after cleaning: x/y=0.74/0.55m (150pC), and QE=5e-5.– After 6 wks RF/vacuum conditioning only: x/y=0.54/0.48m and mid

slice 0.4m (250pC) and QE=6e-5.

QE and emittance evolutions after laser cleaning

3 months QE/gun vacuum 1 month emittance

Center area (x=0,y=0)

off-center (X=0, y=-2.5mm) Idle spot

For current routine operation

LCLS cathode alignment• Offset beam experiences RF kick,

which impacts projected emittance but less on slice emittance (250pC)

• Laser location on cathode w.r.t solenoid center within 2-mm looks OK (vs. 100s m @LCLS original requirement)

On LCLS thermal emittance

• It was stated that the LCLS measured thermal emittance is 2x of theoretical expectation … – And then hope to get x2 thermal emittance improvement

• Personally I am cautious for the above statement:– The Dowell’s model (0.6 m/mm) vs. measured (0.9

m/mm): already closes each other – According to the model: th~f(wf)~f(QEmeas) , the thermal

emittance should increase if QE increases. But the LCLS reality may be NOT the case:• Measured LCLS data• Is the measured QE purely determined by work function? • The model probably does not include all realistic effects

QE=5e-5

QE=8.5e-5

QE=6.6e-5

QE=8.5e-5

What we learned from cathode• Current SLAC technique is not able to reliably provide cathode with

nominal QE, 4~5e-5. • Laser cleaning works very well so far but need more understanding

– It needs 1-2wks conditioning time to achieve good emittance– Do not fully understand why QE improves rather than decay: it is due to

continuous vacuum improvement?– Is it reproducible for both QE and emittance evolutions

• 1-2mm cathode offset looks OK to have a good beam• LCLS-II injector phase I R&D (cathode part):

– To better understand laser cleaning : we may make solid conclusion after a few data points

– New metal cathodes test given successful tests at ASTA • Tiny cathode may kill us if we do not pay attention!!

– Good QE and long lifetime at 120Hz (more serious at potential 360Hz) are essentially needed.

LCLS gun & accessories

Dowell, FLS 2010

LCLS-II gun duplicates LCLS-I except the replacement of dual-RF window to single RF window assembly (Jongewaard)

Injector beamline changes

• A few beamline components are deleted and some added (PE’s list + Axel + Tor, etc);

• These changes are already integrated into LCLS-II design.

Availability issues in injector operations

• Laser lost RF lock• L0A & L0B: design 0.1% amplitude and 0.1 but does often

cycle during operations• Laser (2%/0.4) and gun (0.1%/0.1) jittering or cycling• Need laser/RF experts to address these issues

LCLS injector modeling

• It looks the model (ImpactT) has reasonably good predictions to the measurements

• Has setup modeling using realistic laser beam (thanks Yuantao for the great help)

• Ready for any kind of LCLS injector simulations

• The proposed programs are greatly benefited from:– PE’s list– Discussions with Brachmann, Emma, Huang,

Jongewaard, Pellegrini, and Raubenheimer

Part2: R&D Programs at LCLS-II Injector Phase I

LCLS-II injector phase I • Phase I beamline

– Gun to BPM10 (just downstream of OTR2), which is same as phase II, temporary spectrometer (vert.) and a dump (new placement).

– Installation of LH system might be delayed depending on time/budget• Available beam time at phase I (restricted by PPS):

– Phase I of LCLS-II injector can be operated when FACET is operated since dump and some elements are in linac tunnel, and

– Lockup from CID to sector-20 when FACET is downtime (LCLS-I still on)– Scheduled to be available in 2014-2015 (1-2 years period)?

Year ASTA(led by Jongewaard)

LCLS-II injector phase I LCLS-II injector change required

2012 & 2013

Cu cathodeMg cathodeCu(111)/70 port Cu/Cs-Br filmsLaser cleaning H-cleaning

Under construction N/A

2014 New materialsMore cathode tests

Study Cu for nominal config.Spatial Gaussian-cut vs. uniformVariable gun gradientVariable UV wavelengthSystematic studies of laser cleaningH-cleaning

NoNoNoNoNoNo

2015 Continue cathode studies

Low charge (1-10pC)Mg cathode*Cu/Cs-Br films*Cu(111) & 70 port*Velocity bunching in the gun & linacBlow-out regime (few 100fs)Multi-bunch (equal & unequal charge/bunch) Trickle heating

NoNoNo

Yes (laser)Yes (solenoids)

Yes (laser)Yes (laser)

No (LH available)

*Assume successful tests at ASTA

Cu/CsBr films

Maldonado, Stanford university

• ASTA needs to answer before applying into LCLS-II injector R&D:– Can CsBr survive in the transfer from test chamber to gun?– Can CsBr survive in the RF environment?

• What about thermal emittance?

Cu(111) & 70 injection

• Cu(111) surface state emission channel gives a larger QE enhancement over normal emission in comparison with Cu(100) – To be careful: this is relative value!!

• What about QE of Cu(100) vs. Cu(111) at normal incident injection? wf (100)=4.59eV vs. wf(111)=4.94eV, with same photon energy: – QE(111) < QE(100) although th(111) < th(100) – never free lunch!– QE(100) and QE(111) may be close if 70 injection. – But it is worth to test.

W. Wan, et al LBL results

NIM A 507 (2003) 310-313

Velocity bunching

Blow out regime

• Taking advantage of the ultrafast (~100fs) of the laser as it expands longitudinally under the strong space-charge forces, and eventually evolves into ellipsoidal e-beam distributions.

• The space charge forces are linear after expansion but a temporal asymmetry is observed

• Comparison with ~ps laser

Trickle heating

• Trickling heating may be minimized by changing optics of the LH – need studies

Huang, et al., PRST-AB 13, 020703 (2010)