APEX
description, status and plans
John Corlett for the APEX team
Lawrence Berkeley National Laboratory
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Motivation
Coherent X-rays with high repetition rate, unprecedented average brightness, and
ultrafast pulses
Tomorrow’s x-ray
laser sources
≲millijoule
~ microseconds
Intense pulses at
high rep rate
~ attoseconds to femtoseconds
Approach
High average power electron beam distributed to an array of FELs from high reprate injector and CW SCRF linac
Beam spreader
High-brightness,
CW superconducting linac,
high rep-rate gun laser heater, bunch compressor
and injector
Array of independent FELs
X-ray beamlines and endstations
Injector design goals – APEX gun
• Repetition rate 1 MHz
• Charge per bunch from ~10 pC to ~1 nC
• Emittance <10-6 mm-mrad (normalized)
• Electric field at the cathode ≥~10 MV/m
(space charge emission limit)
• Beam energy at the gun exit ≥~500 keV
(space charge control)
• Bunch length ~100 fs to ~10 ps for handling
space charge effects, and for allowing
different modes of operation
• Compatible with magnetic field control
within the gun (emittance exchange and
compensation)
• 10-11 Torr vacuum capability (cathode
lifetime)
• Accommodates a variety of cathode
materials
• High reliability for user operations
The gun is the most challenging component
LBNL approach uses a CW VHF cavity
APEX gun
VHF cavity operates in CW mode
• Low power density on cavity walls
• High conductance vacuum slots
• High gradient at cathode
Frequency
187 MHz
Operation mode
CW
Gap voltage
750 kV
Field at the cathode
19 MV/m
Q0
30887
Shunt impedance
6.5 MW
RF Power
90 kW
Stored energy
2.3 J
Peak surface field
24 MV/m
Peak wall power density
25 W/cm2
Accelerating gap
4 cm
Diameter/Length
70/35 cm
Operating pressure
< 10-11 Torr
Buncher
Injector design and beam dynamics –
multiobjective genetic optimization
300 pC
RMS bunch length (m)
Gun
Projected normalized emittance (m-rad, 100%)
APEX stages
Phase I:
Beam
characterization
at gun energy
(750 keV)
Phase-II:
Beam characterization at 15–30 MeV
•6-D brightness measurements
Phase 0:
Gun and
photocathod
e tests
Diagnostics systems in
collaboration with Cornell
CLASSSE
Accelerating cavities in
collaboration with ANL AWA
• Planning for final installation in 2013
APEX stages – 0, I
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)
Under commissioning
Phase 0
Phase I scope:
(Phase 0 + extended diagnostics)
• High QE cathode physics
(Intrinsic emittance measurements)
• Diagnostics systems tests.
• Low energy beam characterization
Planned operation in fall 2012
Phase I
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APEX stages – II
Phase II scope:
• Demonstration of the
brightness performance at ~ 30
MeV at low repetition rate*
(*Shielding limited)
APEX Phase II
layout
Planning for
operation in late 2013
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APEX Description
Status & Plans
(J. Corlett)
Gun cold-test and installation
Successful low power RF test
Tuning mechanism principle tested
1.2 10-9 Torr achieved with 1 (out of 20) NEG pump and no bake
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APEX gun: high-brightness MHz
electron source
• APEX cavity is successfully RF conditioned
APEX in the Beam Test Facility
APEX Activity and Plans
Yb fiber photocathode drive laser
F. Sannibale
• 1 MHz reprate Yb fiber laser
• LLNL/UCB/LBNL collaboration
Photocathode materials R&D
K2CsSb:
6% QE at 532 nm
0.36 microns / mm rms en
>> 1 week lifetime
4
x 10
3.5
100
3
200
2.5
300
400
2
500
1.5
600
700
1
800
0.5
900
1000
High QE
Good
lifetime at
10-9 mBar
200
400
600
800
1000
0
Low transverse
momentum
APEX Description
Status & Plans
(J. Corlett)
Photocathodes
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 or 4th harm. conversion from IR)
- for 1 MHz reprate, 1 nC, ~ 10 W 1060nm required
FLASH
INFN-LASA
Cathodes successfully developed at INFN/LASA and delivered to LBNL
PEA Semiconductor: Alkali Antimonides CsK2Sb, (developed at LBNL)
- <~ps pulse capability
- reactive; requires ~ 10-10 Torr pressure
- high QE > 1%
- requires green/blue light (eg. 2nd harm. Nd:YVO4 = 532nm)
- for nC, 1 MHz reprate, ~ 1 W of IR required
Cathodes under development at LBNL (H. Padmore’s group)
Promising lifetime and intrinsic emittance results (Cornell and LBNL)
Transfer chamber to VHF gun in preparation
Collaboration with BNL for diamond amplifier testing
15
Status
The
ofAPEX
the APEX
Project
Project
at LBNL
F.(J.Sannibale
Corlett)
Cathode mount & vacuum load-lock
Adapted version of the
INFN/PITZ/DESY
load-lock system
Modified version of the FLASH
plug for reduced field emission.
Not tested yet!
Flexibility for testing different photocathode materials
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APEX
APEX
Activity
Description
and Plans
Status & Plans
(J. Corlett)
VHF amplifier
• The 120 kW CW 187 MHz RF amplifier required to operate the VHF gun
has been manufactured by ETM Electromatic
• Acceptance test completed at LBNL, fully operational
17
APEX
APEX
Activity
Description
and Plans
Status & Plans
(J. Corlett)
APEX phase-II RF distribution
Single RF source: 1.3 GHz, 25 MW pulsed at 10 Hz with 10 ms pulse
RF Distribution System
Buncher cavity:
LBNL design
LINAC: 3 accel. sections. Deflecting cavity:
Modified Cornell Type
AWA/ANL type
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APEX
APEX
Activity
Description
and Plans
Status & Plans
(J. Corlett)
Coaxial feed multipacting cure
Alternative approach
Minimal length, NTi coated and water cooled.
The solenoid cure worked!
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APEX
APEX
Activity
Description
and Plans
Status & Plans
(J. Corlett)
RF conditioning completed
Gun RF conditioning started on November 7, 2011
CW
Pulsed
At Dec. 15, 2011 integrated < 120 hours of conditioning.
December 15: CW mode at nominal power ran for 12,5 hours without faults
Results reconfirmed the day after (> 24 hours no fault)
Power stability ~2x10-3
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APEX Description
Status & Plans
(J. Corlett)
Dark current
Measurements performed in CW mode on a coaxial Faraday cup right
downstream the beam pipe exit
Dark current follows Fowler-Nordheim dependence on the E field
At the nominal field (19.5 MV/m) the present value is ~ 8 mA
Expected to decrease when the beamline is installed and vacuum bake
completed
21
APEX Description
Status & Plans
(J. Corlett)
Vacuum performance
Measurements performed with a Dycor 2000 RGA system
1 NEG pump out of 20 activated, 1 ion pump, 2 turbo pumps; no bake
H2 He
C
N O HO HO2
C
N2
O2
Ar
CO2
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APEX Description
Status & Plans
(J. Corlett)
Thermal effects on frequency
We have two “regimes”: cold cavity in the first ~ 30 min where the copper is
going to temperature; warm when the copper temperature stabilizes
TS2
TS1
TC2
TC
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Summary – APEX R&D program
• The APEX injector is being commissioned
• Designed for high-brightness electron beam at MHz repetition rate
• VHF cavity
• Flexibility in photocathode materials
• Progress:
• VHF cavity is RF conditioned
• Initial characterization of dark current and tuning
• Cathodes under development
• Plans:
• Photoemission experiments to start soon
• Phases I and II add diagnostics, bunching, acceleration
• Plan to complete construction and begin final beam
characterization in 2013
A team effort
F. Sannibale, B. Bailey, K. Baptiste, J. Byrd, M. Chin, D.
Colomb, J. Corlett, C. Cork, S. De Santis, S. Dimaggio, L.
Doolittle, J. Doyle, J. Feng, D. Filippetto, G. Huang, H. Huang,
T. Kramasz, S. Kwiatkowski, R. Lellinger, W. E. Norum, H.
Padmore, C. Papadopoulos, G. Penn, C. Pogue, G. Portmann,
J. Qiang, D. Garcia Quintas, J. Staples, T. Vecchione, M.
Venturini, M. Vinco, W. Wan, R. Wells, M. Zolotorev, F. Zucca,
…
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