Cornell Laboratory for Accelerator-based ScienceS and Education (CLASSE)
CLASSE
Pushing beam brightness from ERL
source & photocathode R&D
Ivan Bazarov
world‘s highest brightness & current operating cw photoinjector
Cornell University
CHESS & ERL
1
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Main points
CLASSE
• Low emittance achieved
• Photocathode R&D
• Plans to improve the performance further
Cornell University
CHESS & ERL
2
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Source brightness
CLASSE
• 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
Cornell University
CHESS & ERL
3
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Ingredients for pushing the
brightness
CLASSE
• Theoretical insights
• Computer modeling
• Beam diagnostics
• Working hardware
• Right crew of people
Cornell University
CHESS & ERL
4
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Example: analytical scaling of
optics aberrations
aberrations
Solenoid geometric:
CLASSE
slice-induced
Solenoid chromatic:
RF focusing:
Cornell University
CHESS & ERL
5
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Example: high fidelity beam
dynamics modeling
• Use of experimentally
benchmarked beam dynamics
codes
CLASSE
200-CPU computer cluster dedicated
to beam dynamics simulations
• 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
Cornell University
CHESS & ERL
6
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Example: 6D beam diagnostics
key to success
CLASSE
transverse phase space (animation)
slice emittance with resolution of few 0.1ps
py
y
py
t
projected emittance
y
t
E
longitudinal
Cornell
University
CHESS & ERL
phase space
7
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Example: beam running crew
CLASSE
CHESS/ERL1
PhD students
Jared Maxson
Adam Bartnik
Cornell University
CHESS & ERL
Colwyn Gulliford
Siddharth Karkare
Florian Loehl
Heng Li
Luca Cultrera
8
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Just last month:
emittance spec achieved!
CLASSE
• Keys to the result
– Beam-based alignment (took us several months)
– Working diagnostics!!
– Fight jitters in the injector
eny(100%) = 0.4 um @ 20pC/bunch
eny(100%) = 0.8 um @ 80pC/bunch
• x2 thermal emittance! x1.3 simulated emittance
• correct scaling with bunch charge
Cornell University
CHESS & ERL
9
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Understanding beam brightness
CLASSE
• 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
ecore = 0.15 um
fcore = 73%
Cornell University
CHESS & ERL
80pC/bunch
ecore = 0.3 um
fcore = 67%
10
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Shaking old-established notions:
no Gaussians anywhere…
CLASSE
animation: scanning Spectral flux (ph/s/0.1%BW/mm2) at 50m from undulator (5GeV, 100mA, lp = 2cm)
around 1st harm. ~6keV
(zero emittance)
Cornell University
CHESS & ERL
11
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Understanding coherence: fully
consistent formalism (Wigner)
animation: scanning
around 1st harm. ~6keV
(zero emittance)
Cornell University
CHESS & ERL
CLASSE
Phase space (ph/s/0.1%BW/mm/mrad) at the undulator (5GeV, 100mA, lp = 2cm)
12
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Emittance vs. fraction for light
CLASSE
phase space
of light
emittance vs fraction (of light!)
• Final radiation phase space: convolution with electron phase space
Bazarov, Synchrotron radiation in phase space (in preparation)
Cornell University
CHESS & ERL
13
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Just how bright is Cornell
ERL injector beam?
CLASSE
ERL2
• Effective brightness (for comparison)
ERL10
• Today’s 20mA ERL injector beam is as bright as 100mA 0.6nmrad  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
Cornell University
CHESS & ERL
14
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Main points
CLASSE
• Low emittance achieved
• Photocathode R&D
• Plans to improve the performance further
Cornell University
CHESS & ERL
15
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Photocathode R&D
CLASSE
• 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
Cornell University
CHESS & ERL
16
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Photocathode research at Cornell:
building up the infrastructure
multialkali growth chamber with vacuum suitcase
MBE III-V system
Cornell University
CHESS & ERL
GaAs system with load-lock
CLASSE
in-vacuum LEED/Auger
• 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
Photocathode research: a sample
of results from the group
CLASSE
• Wide selection of photocathodes evaluated for MTE and
response time: GaAs, GaAsP, GaN, Cs3Sb, K2CsSb
time response vs theory
GaAs intrinsic emittance
10%QE in green now routine
K2CsSb MTE
GaAsP response
GaN’s MTE
Cornell University
CHESS & ERL
Just 2011 alone
APL 98 (2011) 224101
APL 98 (2011) 094104
APL 99 (2011) 152110
PSTAB (2011) submitted
PRL (2011) in preparation
18
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Photocathode research:
moving forward
CLASSE
• 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
Cornell University
CHESS & ERL
19
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Building collaboration on
photocathodes for accelerators
• Collaboration with
•
•
•
•
•
CLASSE
1st workshop
ANL
BNL
JLAB
SLAC
Berkeley
• Excitement and momentum
in the community;
• Cathode workshop at Cornell
in 2012;
• Leading the effort on creating
collaborative communitydriven Internet resource;
Cornell University
CHESS & ERL
2nd workshop
20
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CHESS/ERL1
Undergrads in cutting-edge
photocathode research
Ashwathi Iyer: in situ work function mapping
CLASSE
Rick Merluzzi: Auger/LEED characterization
Yoon Woo Hwang: 2D energy analyzer, RHEED
Cornell University
CHESS & ERL
21
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
High current operation:
photocathode performance
CLASSE
ERL2
~600 coulombs delivered (same spot)
QE before
Cornell University
CHESS & ERL
QE after
22
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Ion back-bombardment
CLASSE
cathode life-time degrading mechanisms
computed ion dose
delivered to the cathode
Cornell University
CHESS & ERL
halo, beam
losses
23
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Real-life accelerator testing for
photocathodes: high average current
CLASSE
• CsKSb cathode after delivering ~600C.
cathode after 20mA 8hour run
SEM close-up
x-ray topography showing rings of ion back-bombardment damage
Cornell University
CHESS & ERL
24
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Realistic operating conditions
CLASSE
• Good news: running 5mm off-center on the photocathode –
same emittance (20pC/bunch) due to intrinsically low
geometric aberrations in the DC gun
Cornell University
CHESS & ERL
25
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Main points
CLASSE
• Low emittance achieved
• Photocathode R&D
• Plans to improve the performance further
Cornell University
CHESS & ERL
26
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Improving brightness further
CLASSE
ERL10
• 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
Cornell University
CHESS & ERL
27
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
3D laser shaping
for space charge control
CLASSE
• Optimal 3D laser shape: practical solution 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
28
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
New DC gun
CLASSE
• Reviewed in January 2011 by
a group of external experts
• New segmented ceramic received –
fully shielded from field emission
• Construction is underway
Cornell University
CHESS & ERL
29
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Projected performance of the new
DC gun (475kV) beamline
CLASSE
• 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
Cornell University
CHESS & ERL
30
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
Outlook: defining the
state-of-the-art
CLASSE
ERL10
• 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
Cornell University
CHESS & ERL
31
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011
CLASSE
END
Cornell University
CHESS & ERL
32
I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011