Cornell Laboratory for Accelerator-based ScienceS and Education (CLASSE)

1. Pushing beam brightness from ERL source & photocathode R&D Cornell Laboratory for Accelerator-based ScienceS and Education (CLASSE) Ivan Bazarov world‘s highest brightness & current operating cw photoinjector

2. Low emittance achieved Photocathode R&D Plans to improve the performance further Main points

3. Source brightness • Key parameter for ERL • Improvements in the source immediately translate to a better performance of the entire facility • Source development at Cornell • 5-15 MeV injector • New 500kV DC gun + beamline DC gun dev lab 5-15 MeV injector

4. Ingredients for pushing the brightness • Theoretical insights • Computer modeling • Beam diagnostics • Working hardware • Right crew of people

5. Example: analytical scaling of optics aberrations slice-induced aberrations Solenoid geometric: Solenoid chromatic: RF focusing:

6. Example: high fidelity beam dynamics modeling 200-CPU computer cluster dedicated to beam dynamics simulations • Use of experimentally benchmarked beam dynamics codes • Pioneered the use of evolutionary multi-objective algorithms for flexible (nonlinear) beam optics design • Cornell approach is now used in a number of labs world-wide

7. Example: 6D beam diagnosticskey to success transverse phase space (animation) slice emittance with resolution of few 0.1ps py y py t projected emittance y t E longitudinal phase space

8. CHESS/ERL1 Example: beam running crew PhD students Jared MaxsonColwynGullifordSiddharthKarkareHeng Li Adam Bartnik FlorianLoehl Luca Cultrera

9. Just last month:emittance spec achieved! • Keys to the result • Beam-based alignment (took us several months) • Working diagnostics!! • Fight jitters in the injector • x2 thermal emittance! x1.3 simulated emittance • correct scaling with bunch charge eny(100%) = 0.4 um @ 20pC/bunch eny(100%) = 0.8 um @ 80pC/bunch

10. Understanding beam brightness • Single RMS emittance definition is inadequate for linacs • Beams are not Gaussian • Various groups report 95% emittance or 90% emittance (or don’t specify what exactly they report) • The right approach • Measure the entire phase space, then obtain emittance of the beam vs. fraction (0 to 100%) 80pC/bunch ecore = 0.3 um fcore = 67% 20pC/bunch ecore = 0.15 um fcore = 73%

11. Shaking old-established notions: no Gaussians anywhere… animation: scanning around 1st harm. ~6keV (zero emittance) Spectral flux (ph/s/0.1%BW/mm2) at 50m from undulator (5GeV, 100mA, lp = 2cm)

12. Understanding coherence: fully consistent formalism (Wigner) animation: scanning around 1st harm. ~6keV (zero emittance) Phase space (ph/s/0.1%BW/mm/mrad) at the undulator (5GeV, 100mA, lp = 2cm)

13. Emittance vs. fraction for light • Final radiation phase space: convolution with electron phase space emittance vs fraction (of light!) phase space of light Bazarov, Synchrotron radiation in phase space (in preparation)

14. ERL10 ERL2 Just how bright is Cornell ERL injector beam? • Effective brightness (for comparison) • Today’s 20mA ERL injector beam is as bright as 100mA 0.6nm-rad  0.006nm-rad storage ring Gaussian beam! • Parity in transverse brightness with the very best storage rings is already achieved; the result can only improve

15. Low emittance achieved Photocathode R&D Plans to improve the performance further Main points

16. Photocathode R&D • Brightness is ultimately set by the photocathode intrinsic properties • Cornell has emerged as a leader in photocathode R&D for accelerator applications • Expertise now covers a range of NEA materials, alkali-based: next is ‘material engineering’ of photocathodes for accelerators x-ray topography map showing ion damage profile for a cathode with 1000 coulombs 1/e lifetime

17. Photocathode research at Cornell: building up the infrastructure multialkali growth chamber with vacuum suitcase GaAs system with load-lock in-vacuum LEED/Auger • Also on campus AFM, EDX, SEM, STM, SIMS, ARPES • + CHESS (XRF, x-ray topography, EXAFS, and much more) MBE III-V system

18. Wide selection of photocathodes evaluated for MTE and response time: GaAs, GaAsP, GaN, Cs3Sb, K2CsSb Photocathode research: a sample of results from the group time response vs theory GaAs intrinsic emittance 10%QE in green now routine K2CsSb MTE Just 2011 alone APL 98 (2011) 224101 APL 98 (2011) 094104 APL 99 (2011) 152110 PSTAB (2011) submitted PRL (2011) in preparation GaAsP response GaN’s MTE

19. Photocathode research:moving forward • Ultra-cold electrons are in the view; • About to grow our first MBE III-V with tuned parameters; • Impact way beyond accelerators; Longitudinal Energy unique 2D energy analyzer for photoelectrons ps-resolution of response time structural studies of the bandgap structure Transverse Energy

20. Building collaboration on photocathodes for accelerators 1st workshop • Collaboration with • ANL • BNL • JLAB • SLAC • Berkeley • Excitement and momentum in the community; • Cathode workshop at Cornell in 2012; • Leading the effort on creating collaborative community- driven Internet resource; 2nd workshop

21. CHESS/ERL1 Undergrads in cutting-edge photocathode research Rick Merluzzi: Auger/LEED characterization Ashwathi Iyer: in situ work function mapping Yoon Woo Hwang: 2D energy analyzer, RHEED

22. ERL2 High current operation: photocathode performance ~600 coulombs delivered (same spot) QE before QE after

23. Ion back-bombardment cathode life-time degrading mechanisms halo, beam losses computed ion dose delivered to the cathode

24. Real-life accelerator testing for photocathodes: high average current • CsKSb cathode after delivering ~600C. SEM close-up cathode after 20mA 8hour run x-ray topography showing rings of ion back-bombardment damage

25. Realistic operating conditions • Good news: running 5mm off-center on the photocathode – same emittance (20pC/bunch) due to intrinsically low geometric aberrations in the DC gun

26. Low emittance achieved Photocathode R&D Plans to improve the performance further Main points

27. ERL10 Improving brightness further • Already close to the fundamental emittance limit (0.5 mm-mrad for given photocathode, cathode electric field) • Detailed model/measurement verification is underway • Merger characterization – promising simulations at ~12MeV show minimal emittance degradation • Improvements to laser 3D shaping, element stability Examples of optimal laser shapes

28. Optimal 3D laser shape: practical solution identified >50% of light gets through, emittance (sims) ~20% higher than the optimal 3D laser shapingfor space charge control temporal profile temporal – birefringent crystal pulse stacking transverse – truncated Gaussian transverse profile PRSTAB 11 (2008) 040702 Appl. Opt. 46 (2011) 8488

29. Reviewed in January 2011 by a group of external experts New segmented ceramic received – fully shielded from field emission Construction is underway New DC gun

30. Cornell’s construction of the MK II DC Gun: Adjustable cathode/anode gap “dial-in” a field! Segmented insulator: pushing the voltage limits. Full 6D phase space characterization out of the gun Projected performance of the new DC gun (475kV) beamline

31. Cornell ERL photoinjector project has achieved beam brightness that exceeds that of the best storage rings; Diverse and aggressive program to improve beam brightness and injector performance further; Detailed model/measurement benchmarking (6D phase space) Improving laser shape, element stability, diagnostics New gun & photocathode materials ERL10 Outlook: defining the state-of-the-art

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