DOE contractors' mtg, Aug 22-23

Cornell Laboratory for Accelerator-based ScienceS and Education (CLASSE) Overview of Photoinjectors for Future Light Sources Ivan Bazarov Cornell University CLASSE Main dump T-map during 35 mA high current running GaAs QE(%) map after 50 mA run damaged optics after 10s W of laser power worlds highest avg brightness & current photoinjector at Cornell Cornell University CHESS & ERL 1

I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Todays talk CLASSE Needs for linac-based light sources Different approaches, same goals Recent progress (incomplete survey) Moving beyond the state-of-the-art Cornell University CHESS & ERL 2 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 DC, RF, SRF guns CLASSE NCRF DC

LANL RF gun Cornell gun SRF Tuner cell RF / HOM ports Cathode Choke filter 3 full cells stock ELBE SRF gun plus variants Pulsed machines (FELs): NCRF a success story can always improve emittance lower machine energy CW operation: cathode fields reduced (DC 10 MV/m), NCRF ( 20 MV/m), best promise for SRF ( 30 MV/m) Main push is for increased avg current (ERLs), emittance desired several 0.1 um rms normalized range for ~100 pC

Cornell University CHESS & ERL 3 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Physics of high brightness photoguns made simple CLASSE Given a laser, photocathode cathode, and accelerating gradient max brightness is set Each electron bunch assumes a pan-cake shape near the photocathode for short (~10ps) laser pulses, max charge density determined by the electric field v dq/dA = 0 Ecath Angular spread or transverse momentum footprint is set by intrinsic momentum spread of photoelectrons leaving the photocathode (MTE = mean transverse energy), Dp ~ (mMTE)1/2

Combining these two yields the maximum (normalized) beam brightness achievable from a photoinjector defined by only two key parameters: cathode field Ecath and MTE of photoelectrons achievable brightness: Cornell University CHESS & ERL Bn f = max 0mc2 Ecath 2p MTE 4 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012

RF guns CLASSE No ceramic to worry about, no cryoplant BUT huge losses for CW operation, questionable vacuum BOEING RF gun & renewed LANL effort: it can be made to work! VHF gun (LBNL): reduce operating frequency, increase cooling area, introduce plenty of pumping slots nice solution when << GHz rep rate is acceptable Cornell University CHESS & ERL VHF LBNL gun 5 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 DC guns CLASSE

Highest average current (50mA) operation today of the 3 choices Very high voltages ( 500kV) are still difficult, despite DC guns being around for a while New generation of guns to resolve ceramic puncture problems JAEA/KEK gun with shielded ceramic Cornell University CHESS & ERL Cornell ceramic design & photo 6 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 SRF guns CLASSE A lot of R&D in the community Great promise, lots of issues Elliptical cavities and quarter wave resonator (QWR) structures

Elliptical cavities 700 MHz QWR 500 MHz (operates as a quasi-DC gap, similar to VHF NC gun) Best result so far: ELBE ~18MV/m pk with 1% Ce2Te for > 1000h 700MHz BNL/AES/JLAB Cornell University CHESS & ERL NPS & Niowave QWR gun 7 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 o r i t h m s

Example of one detailed comparison: DC vs SRF CLASSE c o m p a r e t w o t e c IVB et al., PRST-AB 14 (2011) 072001 Cornell University CHESS & ERL

8 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 DC gun geometry & field constraints Use empirical data for voltage breakdown breakdown voltage vs gap CLASSE Vary gun geometry while constraining the voltage 3 geometry parameters: gap, cathode angle & recess keep surface fields 10 MV/m Cornell University CHESS & ERL 9 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012

SRF gun geometry & field constraints 1.3 GHz 0.5-cell elliptical cavity Constrained surface fields according to TESLA spec CLASSE Equator radius used for frequency tuning 4 parameters: gap, cathode angle & recess, pipe dia Eacc 25 MV/m Epk/Eacc 2 Hpk/Eacc 4.26 mT/(MV/m) Vary beam current 0-200 mA Final bunch length 3 ps rms Cornell University CHESS & ERL

Most solutions Epk 50 MV/m 10 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Beamline parameters DC Gun SRF Gun SRF Linac Solenoids Buncher The SRF beamline is simplified: NC buncher cavity is ineffective at high beam energy Only one solenoid included 11 Emittance Performance DC Gun

SRF Gun 12 A closer look at 80pC case SRF Gun X2-3 larger peak emittance! but final is very close DC Gun 13 Comparison results recap The two technologies did not show much difference in the final 100% rms emittance in these simulations despite > x3 larger field at the cathode for SRF case (the beam core must be brighter for SRF) DC gun case requires more cancellations to get to small emittances at the end how well can it be done in real life? Space charge energy chirp after the gun:

leaves a nasty chromatic aberration through the solenoids! Far more prominent in DC case. Must anti-chirp with buncher! Perhaps cathodes (MTE) are more important than the field (beyond a certain point)! Recent alignment & emittance run @ Cornel ERL injector: Measured: 1.3-1.4 x model (so far) 14 p u s h Photoemission source development @ Cornell p h o t o

i n j e c t o r s t a CLASSE ~3m Cornell University CHESS & ERL 15 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 e

n t o f Cornell ERL photoinjector highlights CLASSE 5 2 m A f r o m a p h o t

University oCornell CHESS & ERL 16 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 *it happens CLASSE spring11: opened leak in the beam dump at 25mA Designed for 600 kW average power Now 80 thermocouples monitor the repaired dump temperature over its entire surface Dramatic accelerator physics drilled a hole in the dump (1 Al) with electron beam! Raster/quad system wired/set incorrectly

Cornell University CHESS & ERL 17 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 BOEING gun tribute CLASSE Cornell photoinjector: 52 mA (Feb 9, 2012) New current record is 52 mA (Feb 9, 2012) at Cornell using GaAs!! beats Dave Dowells 32 mA record of 20 years! Cornell University CHESS & ERL 18 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Pushing for high current CLASSE

Key developments: Expertise in several different photocathodes (both NEA and antimonides) Improvements to the laser (higher power) Feedback system on the laser Minimization of RF trips (mainly couplers) Minimizing radiation losses Feb 9, 2012 first 50mA!! GaAs Laser intensity feedback system (developed by F. Loehl) Cornell University CHESS & ERL 19 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012

High current operation (offset CsKSb gives excellent lifetime) CLASSE ~600 Coulombs delivered (same spot) active area QE before QE after 6AM: sleepy operator Cornell University CHESS & ERL L. Cultrera, et al., PRST-AB 14 (2011) 12010120 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Real-life accelerator testing for photocathodes: high average current

CLASSE Main message: moving off-axis gives many kiloCoulombs 1/e lifetime from K2CsSb or Cs3Sb (same spot) Now understand that pits in EC are the result of machine trips cathode after 20mA 8hour run SEM close-up X-ray topography showing rings of ion back-bombardment damage Cornell University CHESS & ERL 21 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Laser Laseroff-center Off-center CLASSE

Good news: running 5 mm off-center on the photocathode gives the same emittance (20pC/bunch) due to intrinsically low geometric aberrations in the DC gun This is very important, as we know that we cannot run with the laser at the center of the cathode due to cathode damage issues. Cornell University CHESS & ERL 22 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 6D beam diagnostics: key to low emittance CLASSE transverse phase space (animation) slice emittance with resolution of few 0.1ps

So far the smallest emittance 0.7 mm-mrad at 80 pC/bunch (rms, 100%) py py y t projected emittance y t E longitudinal Cornell University CHESS & ERL phase space

23 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Sept 2011: initial emittance spec achieved! CLASSE Keys to the result Beam-based alignment (took us a couple of months) Working diagnostics Fight jitters in the injector ny(100%) = 0.4 um @ 20pC/bunch Cornell University CHESS & ERL ny(100%) = 0.8 um @ 80pC/bunch 24 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Some proselytizing: which

emittance is right to quote CLASSE Single RMS emittance definition is inadequate for linacs Beams are not Gaussian Various groups report 95% emittance or 90% emittance (or dont specify what exactly they report) The right approach Measure the entire phase space, then obtain emittance of the beam vs. fraction (0 to 100%) 20pC/bunch core = 0.15 um fcore = 73% Cornell University CHESS & ERL 80pC/bunch core = 0.3 um fcore = 67%

25 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Single rms emittance is inadequate for comparisons CLASSE Better to quote 3 numbers 100% rms emittance (or 95% or 90%) core emittance (essentially peak brightness) core fraction Cornell University CHESS & ERL 26 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Emittance vs. fraction for light Wigner distribution = phase space density phase space of undulator radiation

CLASSE emittance vs. fraction of light There are fewer Gaussians around than one might think More about it in my afternoon talk in joined SR&ERL WG Cornell University CHESS & ERL IVB, arXiV 1112.4047 (2011)27 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Measured beam brightness so far CLASSE Effective brightness (for comparison) E.g. demonstrated at 20mA ERL injector beam, if accelerated to 5GeV, is as bright as 100mA 50 50 pm-rad Gaussian beam! The result can only improve!

Cornell University CHESS & ERL 28 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 e N e w Lasers & cathodes CLASSE g u n s = l o a

d l o c k c Cornell University CHESS & ERL 29 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Lasers CLASSE Z. Zhou et al., Opt. Express 20 (2012) 4850 Plenty of laser power when coupled with good cathodes 1.3GHz laser at 65W Next steps:

better 3D shaping engineering and integration into the machine via stabilization loops (all degrees of freedom) Practical shaping techniques Temporal stacking (uniform) Transverse clipping (truncated Gaussian) better than beer-can; only 20% emittance increase compared to highly optimized shapes Blowout regime if Ecath is high enough Cornell University CHESS & ERL 30 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Building collaboration on photocathodes for accelerators

CLASSE 1st workshop Collaboration with ANL, BNL, JLAB Cornell, SLAC Berkeley, more Excitement and momentum in the community; Cathode workshops at BNL in 2010; in Europe 2011; nd coming up at Cornell in 22012 workshop http://www.bnl.gov/pppworkshop/ http://photocathodes2011.eurofel.eu http://www.lepp.cornell.edu/Events/Photocathode2012 Cornell University CHESS & ERL

31 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 h o t o i n j e c t o r Conclusions CLASSE R & D

D i v i d Cornell University CHESS & ERL 32 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Acknowledgements (for the Cornell team) CLASSE Photoinjector team: John Barley, Adam Bartnik, Joe Conway, Luca Cultrera, John Dobbins, Bruce Dunham, Colwyn Gulliford, Siddharth Karkare, Xianhong Liu, Yulin Li, Heng Li, Florian Loehl, Roger Kaplan, Val Kostroun, Tobey Moore, Vadim Vescherevich, Peter Quigley, John

Reilly, Karl Smolenski, Charlie Strohman, Zhi Zhou, and more. Main support NSF DMR-0807731 for ERL R&D also DOE DE-SC0003965 CAREER grant Cornell University CHESS & ERL 33 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 CLASSE END Cornell University CHESS & ERL 34 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012 Optimal Gun Geometry: DC Gun: cm, V=470 kV

Pushed for max field over focusing. Cathode recess unimportant. SRF gun: cm cm, cm and cathode recess seemed unimportant. 35 Beamline Specifics 36 3D laser shaping for space charge control CLASSE Optimal 3D laser shape: practical solutions identified temporal birefringent crystal pulse stacking transverse truncated Gaussian

>50% of light gets through, emittance (sims) ~20% higher than the optimal temporal profile transverse profile PRSTAB 11 (2008) 040702 Appl. Opt. 46 (2011) 8488 Cornell University CHESS & ERL 37 I.V. Bazarov, Overview of Photoinjectors for Future Light Sources, March 6, 2012

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