i.v. bazarov, beam brightness and cathode r&d, nsf annual review, oct 25, 2011 classe cornell...
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Cornell Laboratory for Accelerator-based ScienceS and Education (CLASSE) Cornell Laboratory for Accelerator-based ScienceS and Education (CLASSE)
Pushing beam brightness from ERL source & photocathode R&D
world‘s highest brightness & current operating cw photoinjector
Ivan Bazarov
1
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Main points
• Low emittance achieved
• Photocathode R&D
• Plans to improve the performance further
2
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
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
5-15 MeV injector
DC gun dev lab
3
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Ingredients for pushing the brightness
• Theoretical insights
• Computer modeling
• Beam diagnostics
• Working hardware
• Right crew of people
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Example: analytical scaling of optics aberrations
aberrations slice-induced
Solenoid chromatic:
Solenoid geometric:
RF focusing:
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
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
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Example: 6D beam diagnosticskey to success
y
py
t
y
py
t
E
transverse phase space (animation)
slice emittance with resolution of few 0.1ps
projected emittance
longitudinal phase space 7
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
PhD students
Jared Maxson Colwyn Gulliford Siddharth Karkare Heng Li
Example: beam running crew
Luca CultreraAdam Bartnik Florian Loehl
CHESS/ERL1
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
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
ny(100%) = 0.4 um @ 20pC/bunch ny(100%) = 0.8 um @ 80pC/bunch
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
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%)
20pC/bunch core = 0.15 um fcore = 73%
80pC/bunch core = 0.3 um fcore = 67%
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
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, p = 2cm)
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Understanding coherence: fully consistent formalism (Wigner)
Phase space (ph/s/0.1%BW/mm/mrad) at the undulator (5GeV, 100mA, p = 2cm)
animation: scanning around 1st harm. ~6keV
(zero emittance)
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Emittance vs. fraction for light
phase space of light
emittance vs fraction (of light!)
Bazarov, Synchrotron radiation in phase space (in preparation)
• Final radiation phase space: convolution with electron phase space
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
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
ERL10
ERL2
14
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Main points
• Low emittance achieved
• Photocathode R&D
• Plans to improve the performance further
15
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
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
16
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Photocathode research at Cornell: building up the infrastructure
multialkali growth chamber with vacuum suitcase GaAs system with load-lock in-vacuum LEED/Auger
MBE III-V system
• Also on campus AFM, EDX, SEM, STM, SIMS, ARPES
• + CHESS (XRF, x-ray topography, EXAFS, and much more)
17
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Photocathode research: a sample of results from the group
• Wide selection of photocathodes evaluated for MTE and response time: GaAs, GaAsP, GaN, Cs3Sb, K2CsSb
10%QE in green now routineGaAs intrinsic emittance
time response vs theory
GaN’s MTE
GaAsP response
K2CsSb MTE
Just 2011 aloneAPL 98 (2011) 224101APL 98 (2011) 094104APL 99 (2011) 152110PSTAB (2011) submittedPRL (2011) in preparation
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
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;
Transverse Energy
Long
itudi
nal E
nerg
y
unique 2D energy analyzer for photoelectrons
structural studies of the bandgap structure
ps-resolution of response time
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Building collaboration on photocathodes for accelerators
• 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;
1st workshop
2nd workshop
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Undergrads in cutting-edge photocathode research
Ashwathi Iyer: in situ work function mapping
Rick Merluzzi: Auger/LEED characterization
Yoon Woo Hwang: 2D energy analyzer, RHEED
CHESS/ERL1
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
High current operation: photocathode performance
~600 coulombs delivered (same spot)
QE before QE after
ERL2
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Ion back-bombardment
halo, beam losses
cathode life-time degrading mechanismscomputed ion dose
delivered to the cathode
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Real-life accelerator testing for photocathodes: high average current
• CsKSb cathode after delivering ~600C.
cathode after 20mA 8hour run SEM close-up
x-ray topography showing rings of ion back-bombardment damage
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
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
25
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Main points
• Low emittance achieved
• Photocathode R&D
• Plans to improve the performance further
26
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
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 stabilityExamples of optimal laser
shapes
ERL10
27
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
3D laser shapingfor space charge control
• Optimal 3D laser shape: practical solution identified
• >50% of light gets through, emittance (sims) ~20% higher than the optimal
temporal – birefringent crystal pulse stacking
transverse – truncated Gaussian
temporal profile
transverse profile
PRSTAB 11 (2008) 040702Appl. Opt. 46 (2011) 8488
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
New DC gun
• Reviewed in January 2011 by a group of external experts
• New segmented ceramic received – fully shielded from field emission
• Construction is underway
29
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Projected performance of the new DC gun (475kV) beamline
• 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
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I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
Outlook: defining the state-of-the-art
• 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
31
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSECLASSE
Cornell UniversityCHESS & ERL
END
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