1. APEX at LBNL as a Photocathode Test Facility
Fernando Sannibale
for the APEX Team
Photocathode Physics for Photoinjectors (P3) Workshop
Cornell University, October 19, 2012

2. THE NGLS PROPOSAL AT LBNL
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
Array of (ultimately 10) configurable FEL
100 kHz/FEL CW pulse rate, capability of 1 FEL of MHz+ rate.
Independent control of the pulse characteristics.
Each FEL configured for experimental requirements;
Beam transport and switching
~ 2.4 GeV CW
superconducting linac
Injector
Laser heater
Bunch compressors
Low-emittance,
MHz bunch rate photo-gun
≤ 0.3 nC
≤1 mm-mrad

3. THE NGLS R&D ACTIVITIES
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
Beam manipulation and conditioning
Beam distribution and individual beamline tuning
FEL Physics,
Undulators
High-brightness,
1 MHz rep-rate electron gun
Laser systems,
timing & synchronization
High-brightness,
1 MHz rep-rate electron gun
Many R&D areas to be pursued.

4. REQUIREMENTS FOR THE NGLS INJECTOR
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
In a FEL the ultimate electron beam quality is defined at the gun/injector
To achieve the NGLS goals, the electron source should simultaneously allow for:
repetition rates up to ~ 1 MHz
charge per bunch from few tens of pC to ~ 0.3 nC,
sub 10-7 (low charge) to 10-6 m normalized beam emittance,
beam energy at the gun exit greater than ~ 500 keV (space charge),
electric field at the cathode greater than ~ 10 MV/m (space charge limit),
bunch length control from tens of fs to tens of ps for handling space charge effects, and for allowing the different modes of operation,
compatibility with significant magnetic fields in the cathode and gun regions (mainly for emittance compensation)
control dark current within acceptable values
Torr operation vacuum pressure (high QE photo-cathodes),
“easy” installation and conditioning of different kind of cathodes,
high reliability compatible with the operation of a user facility.

5. Peak wall power density
THE LBNL VHF RF GUN
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
The Berkeley normal-conducting scheme satisfies all the LBNL FEL requirements simultaneously.
Frequency
186 MHz
Operation mode CW
Gap voltage
750 kV
Field at the cathode
19.47 MV/m
Q0 (ideal copper)
30887
Shunt impedance
6.5 MW
RF Q0
87.5 kW
Stored energy
2.3 J
Peak surface field
24.1 MV/m
Peak wall power density
25.0 W/cm2
Accelerating gap
4 cm
Diameter/Length
69.4/35.0 cm
Operating pressure
~ Torr
J. Staples, F. Sannibale, S. Virostek, CBP Tech Note 366, Oct. 2006
K. Baptiste, et al, NIM A 599, 9 (2009)
At the VHF frequency, the cavity structure is large enough to withstand the heat load and operate in CW mode at the required gradients.
Also, the long lRF allows for large apertures and thus for high vacuum conductivity.
Based on mature and reliable normal-conducting RF and mechanical technologies.
186 MHz compatible with 1.3 and 1.5 GHz super-conducting linac technologies.

6. APEX: THE ADVANCED PHOTO-INJECTOR EXPERIMENT
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
15 m
3 m
Located in the Beam Test Facility (BTF) at the ALS.
Main Goals:
Demonstrate the high-brightness high-repetition rate capability of an injector based on the LBNL VHF Gun.
Cathode physics: selection of the best cathode for NGLS.
Additional Goals:
Diagnostics: demonstrate diagnostics systems for NGLS.
Dark current characterization, removal techniques. …
Low energy collimation schemes.
New undulator technology test, electron diffraction, … (funds?).

7. APEX: PHASES 0 & I Phase 0 Phase I Phase 0 scope:
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
Phase 0 scope:
Demonstration of the RF performance at full repetition rate.
Vacuum performance demonstration.
Dark current characterization.
High QE cathode physics
(QE and lifetime measurements)
Funded.
Under commissioning
Phase 0
Phase I
Phase I scope:
(Phase 0 + extended diagnostics)
High QE cathode physics
(Intrinsic emittance measurements)
Diagnostics systems tests.
Low energy beam characterization
In operation in summer 2013

8. (*BTF shielding limited) If funding profile confirmed
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
APEX: PHASE II
Phase II scope:
Demonstration of the brightness performance at ~ 30 MeV at low repetition rate*
(*BTF shielding limited)
If funding profile confirmed
operation in mid 2014

9. APEX REQUIRED PERFORMANCE TABLE
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
Comparison between APEX performance and what is required for NGLS in terms of main parameters
@ FELs

10. APEX COLLABORATIONS “Do not reinvent the wheel!
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
“Do not reinvent the wheel!
Use the good stuff that already exists!”
LLNL/UCB. Photocathode laser.
ANL-AWA. Linac accelerating sections.
Cornell University. Beam diagnostics and RF components.
INFN-Milano LASA. Cs2Te cathodes.
BNL. Photocathodes. …
Also help from: SLAC (LCLS, SSRL), DESY-PITZ, …
STTR with QPeak

11. APEX PRESENT ACHIEVEMENTS
Gun and Phase 0 beamline built
and installed.
Gun conditioned at full RF power.
Dark current characterized:
< ~ 1 mA (acceptable level)
Very promising vacuum
performance from initial tests.
First photo-emitted electron beam with low QE cathode generated.
Nominal photo-emitted electron
beam energy demonstrated.
Gun technology fully demonstrated!
F. Sannibale, et al., PRST-AB 15, (2012)
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012

12. VACUUM LOAD LOCK SYSTEM
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
Installation completed.
Baking to start.
Vacuum “suitcase” compatible with airplane transportation (NEG pump)
Collaboration with INFN – LASA
Adapted version of the INFN/PITZ/FLASH load-lock system
Ready for “serious” photocathode tests

13. PHOTO-CATHODES FOR APEX
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
PEA Semiconductor: Cesium Telluride Cs2Te (In collaboration with INFN-LASA)
- <~ps pulse capability
- relatively robust and un-reactive (operates at ~ 10-9 Torr)
- successfully tested in NC RF and SRF guns
- high QE > 1%
- photo-emits in the UV (~250 nm) (3rd/4th harm. conversion from IR)
- for 1 MHz reprate, 1 nC, ~ 10 W 1060nm required
First 3 cathodes successfully developed at INFN/LASA and delivered to LBNL.
PEA Semiconductor: Alkali Antimonides CsK2Sb, (developed at LBNL)
- <~ps pulse capability
- reactive; requires ~ Torr pressure
- high QE > 1%
- requires visible light (eg. 2nd harm. Nd:YVO4 = 532nm)
- for nC, 1 MHz reprate, ~ 1 W of IR required
LBNL
measurements
Cathodes development at LBNL (H. Padmore’s group) in collaboration with BNL.
Promising lifetime and intrinsic emittance results (Cornell and LBNL).
Preparation& transfer chamber in fabrication (ready ~end 2012).
Collaboration with BNL for diamond amplifier/photocathode

14. Measurements by D. Sertore
Cs2Te FROM INFN-LASA COLLABORATION
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
409.1: Cs2Te, Te = 10nm, deposited November 17, 2010
date
239nm
245nm
297nm
334nm
365nm
405nm
436nm
18-Nov-10
14.7%
11.1% 4%
7.3e-1%
1.1e-1%
1.8e-3%
2.1e-4%
4-Apr-11
14.9%
11.2%
4.2%
7.6e-1%
9.2e-2%
9.6e-4%
3.9e-5%
9-Sep-11
n.a.
10.3%
4.4%
7e-1%
9.9e-2%
1e-3%
3.3e-5%
11-Oct-11
10.5%
Measurements by D. Sertore

15. QE CHECK OF THE INFN-LASA Cs2Te CATHODES
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
The Cs2Te quantum efficiency was re-measured in the INFN vacuum “suitcase”
The average current collected by the anode was measured as function of the bias applied (from 0 to 500V) between the cathode-anode gap of ~ 3 mm.
265 nm, <1ps, 500 μm, laser pulse at 1 MHz repetition rate was used for the measurements, with 9 μW average power.
QE~13%
October 10, 2012
Measurements on cathode # 409.1, deposited on November 17, 2010 and kept in vacuum suitcase
@ ~ Torr
D. Filippetto
Almost two years in the vacuum suitcase with no apparent QE degradation!

16. PHOTO-CATHODE LASER
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
LASER 1: 1 MHz reprate Yb fiber laser from a LLNL/UCB/LBNL collaboration.
Present: 0.7 W at ~1052 nm.
Margin for improvement.
~240 mW
@ 526 nm
(> 1.5 nC, 1 MHz CsK2Sb)
~85 mW
@ 263 nm
(~ 300 pC, 1 MHz Cs2Te)
D. Filippetto, J. Feng
210 nm (5th harmonic) coming soon!
LASER 2: Commercial 1 MHz 2 W Calmar 1052 nm tested & available.

17. 3rd GENERATION CATHODE PLUG (PUCK)
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
2nd generation: modified version of the INFN/FLASH plug for reducing field emission (used by FNAL and others).
Problems with the insertion/extraction operation damaging the RF spring pushed us to modify the cathode plug
Smaller spring “invitation” angle: 20o instead than 30o
Slightly smaller diameter (-230 mm) at the RF spring contact
Gold plated stainless steel RF spring with no discontinuity
Any photocathode deposited on a same geometry plug
(usually in molybdenum) can be potentially tested at the VHF gun

18. Thanks to DOE-BES for supporting the program.
A TEAM WORK
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
K. Baptiste, M. Chin, J. Corlett, C. Cork, S. De Santis, L. Doolittle, J. Doyle, D. Filippetto, G. Huang, H. Huang, T. Kramasz, S. Kwiatkowski, V. Moroz, W. E. Norum, H. Padmore, C. Papadopoulos, G. Portmann, H. Qian, F. Sannibale, J. Staples, T. Vecchione, M. Vinco, W. Wan, R. Wells, M. Zolotorev, … .
Thanks to DOE-BES for supporting the program.

19. Backup Slides
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012

20. DRAFT Schedule. Dependent on actual funding profile
APEX TENTATIVE SCHEDULE
2012
I II III IV
2013
2014
Phase 0. Installation completion
Phase 0. Gun commissioning
Phase 0. Initial diamond tests
Phase 0. Load lock completion
All Phases. Cs2Te & CsK2Sb & C
Phase I. Fabrication
Phase I. Installation
Phase I. Low energy beam test
Phase II. Procurements
Phase II. Installation
Phase II. High energy beam test
DRAFT Schedule. Dependent on actual funding profile
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012

21. Beamline installation completed.
PHASE 0 BEAMLINE INSTALLED
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
Beamline installation completed.

22. DARK CURRENT
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
Measurement performed in CW mode by a coaxial Faraday cup right downstream the beam pipe exit.
New measurements with the Faraday cup at the end of Phase 0 beamline indicated less than a mA of dark current at the nominal operation field (collimated out by the vacuum chamber).
March 22, 2012
The dark current image on a YAG screen reveals several “butterflies” indicating the presence of “hot” tips on the temporary “dummy” plug.
No evidence of field emission in the plug/gun contact area.
Measured values in line with data from others guns in operation.
Expected to decrease with installation of real cathodes with better polished plugs.

23. Baking the gun with no water cooling (!) and 2-4 kW average RF power.
“RF BAKING”
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
Baking the gun with no water cooling (!) and 2-4 kW average RF power. H2 He O HO
HO2 N N2 O2 Ar
CO2 C

24. Only 1 NEG out of 20 activated after the 2.5 days RF baking.
“BASE VACUUM PRESSURE”
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
Only 1 NEG out of 20 activated after the 2.5 days RF baking.
The 400 l/s ion pump on.
Very promising!

25. APEX BEAM DIAGNOSTICS Phase 0: cathode physics
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
ICT
Steering
Coil
Light
Box
Profile
Monitor
Cornell design
Phase 0: cathode physics
Phase I: 6D phase space characterization at the gun energy.
Phase II: 6D phase space characterization at full injector energy
D. Filippetto

26. RF SYSTEMS
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
Single* RF source: 1.3 GHz, 25 MW pulsed at 10 Hz with 10 ms pulse.
(*real estate and funding limited)
RF Distribution System
Deflecting cavity:
Modified Cornell Type
Buncher cavity:
LBNL design
LINAC: 3 accel. sections.
AWA/ANL type
S. Kwiatkowski

27. Tuning mechanism principle tested.
GUN FABRICATION COMPLETED (~90% at LBNL Shops)
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
R. Wells, R. Kraft
Successful low power RF test
Tuning mechanism principle tested.

28. PHOTO-EMITTED BEAM CURRENT
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
Photoemitted average current measured by a lockin amplifier with the MHz laser trigger as reference and the Faraday cup signal in input.
Laser ON
Laser OFF
Laser average power at the cathode: ~ 30 mW at ~ 266 nm.
Cathode: Molybdenum plug, QE ~ 10-6
Expected charge per bunch : ~ 6 fC
Expected average current at 1 MHz: ~ 6 nA (dark current ~ 100 nA)
Measured ~ 10 nA consistently with expectations!

29. E = 745 keV sE = 41 keV ENERGY MEASUREMENT OF PHOTEMITTED BEAM
F. Sannibale, APEX as Photocathode Test Facility - P3 Workshop - Cornell Univ., October 19, 2012
Nominal RF Power. Solenoid 1 ON to focus beam on screen.
Horizontal Corrector 2 current scanned for the measurement.
The fit slope is proportional to the particle momentum
E = 745 keV
sE = 41 keV
Expected value at nominal power: 750 keV