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 nm if QE is in very good shape 100  J is sufficient for 250pC if QE is ~4e-5 but we suggest to have more headroom (200  J) 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)

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 0.37  m vs  m 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, (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: – 1 st cathode ( July 2008): QE was low but ok for commissioning – 2 nd cathode (July 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 – 3 rd cathode (May 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 : – 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 =0.74/0.55  m (150pC), and QE=5e-5. – After 6 wks RF/vacuum conditioning only:  x /  y =0.54/0.48  m and mid slice 0.4  m (250pC) and QE=6e-5.

QE and emittance evolutions after laser cleaning 3 months QE/gun vacuum1 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  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(QE meas ), 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 (1-2 years period)?

YearASTA (led by Jongewaard) LCLS-II injector phase ILCLS-II injector change required 2012 & 2013 Cu cathode Mg cathode Cu(111)/70  port Cu/Cs-Br films Laser cleaning H-cleaning Under constructionN/A 2014New materials More cathode tests Study Cu for nominal config. Spatial Gaussian-cut vs. uniform Variable gun gradient Variable UV wavelength Systematic studies of laser cleaning H-cleaning No 2015Continue cathode studies Low charge (1-10pC) Mg cathode* Cu/Cs-Br films* Cu(111) & 70  port* Velocity bunching in the gun & linac Blow-out regime (few 100fs) Multi-bunch (equal & unequal charge/bunch) Trickle heating No Yes (laser) Yes (solenoids) 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) 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, (2010)