Phase 1a Prototype as model for
injector
L0 layout + experimental plan + results to date
Ivan Bazarov
X-ray characteristics needed
• For a properly tuned undulator: x-ray phase space
is a replica from electron bunch + convolution
with the diffraction limit
• ideally, one wants the phase space to be diffraction
limited (i.e. full transverse coherence), e.g. ,rms
= l/4p, or 0.1 Å for 8 keV x-rays (Cu Ka), or
n,rms = 0.1 mm normalized at 5 GeV
June 28, 2016
Flux
ph/s/0.1%bw
Brightness
ph/s/mrad2/0.1%bw
Brilliance
ph/s/mm2/mrad2/0.1%bw
I.V. Bazarov, ERL review 03/09/07
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Injector prototype beam goals
• Demonstrate efficacy of achieving thermal
emittance at the end of the injector at a bunch
charge of 77 pC/bunch or some large fraction
thereof
• Understand the limitations in the injector (both
physics and technology) to allow for improved
design in the future
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Experimental plan: areas
I.
II.
III.
IV.
V.
VI.
June 28, 2016
Photocathode phenomena
Space charge dominated regime
Longitudinal phase space control
Emittance preservation in the merger
High average current phenomena
Achieving ultimate ‘tuned-up’ performance
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R128 vs. L0
• Simple: gun & diagnostics line
• Full phase space characterization
capability after the gun
• Temporal measurements with the
deflecting cavity
• Limited diagnostics after the
gun (before the module)
• Full interceptive diagnostics
capabilities at 5-15 MeV
June 28, 2016
• Some full beam power
diagnostics
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L0 layout: near the gun
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L0 layout: 15 MeV straight-thru
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L0 layout: merger & chicane
merger
diagnostics chicane
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Diagnostics overview
• Beam position resolution: 10 mm (spec)
• Energy spread resolution: 10–4
• Transverse beam profile resolution:
30 mm (viewscreens)
10 mm (slits)
30 mm (flying wire)
• Angular spread resolution: 10 mrad
• Pulse length (deflecting cavity&slits): 100 fs
• RF phase angle: 0.5
Ability to take phase space snapshots of the beam, both transverse
planes, and longitudinal phase space
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Emittance measurement system
• no moving parts; fast DAQ
• 10 mm precision slits
• armor slit intercepts most
of the beam
• kW beam power handling
measured phase space
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Deflecting cavity
• 100 fs time resolution
(with slits)
• Used in:
– photoemission
response meas.
– slice transverse
emittance meas.
– longitudinal phase
space mapping
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Flying wire
• 20 m/s flying carbon
wire (can go faster)
• Applicable with 0.6
MW of beam power
• Two units, one in
dispersive section to
allow studies of longrange wake fields
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THz radiation
coherently enhanced spectrum
June 28, 2016
• One of chicane dipole
magnets to be used in the
analysis of FIR radiation
spectrum
• Applicable with 0.6 MW of
beam power
• Provides the autocorrelation
of the bunch profile
• OTR foils for low beam
power measurements
I.V. Bazarov, ERL review 03/09/07
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Beam experiments
I. Photocathode phenomena
–
–
Exp1. Thermal emittance (R128) done
Exp2. Photoemission response time (R128) 2 weeks
II. Space charge regime
–
–
–
–
–
June 28, 2016
Exp3. Space charge limited extraction from the cathode
(R128) 1 week
Exp4. Effect of laser pulse shaping on emittance
compensation (R128) 2 weeks
Exp5. Phase space tomography of bunched beam (R128 &
L0) 2 weeks R128 + 2 weeks L0
Exp6. Benchmarking of space charge codes (R128 & L0)
1 week R128
Exp7. Slice emittance studies (L0) 2 weeks
I.V. Bazarov, ERL review 03/09/07
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Beam experiments
III. Longitudinal phase space control
–
–
Exp8. Ballistic bunch compression (L0) 2 weeks
Exp9. Longitudinal phase space mapping (L0)
2 weeks
IV. Emittance preservation in the merger
–
–
June 28, 2016
Exp10. Space charge induced emittance growth in
dispersive sections (L0) 2 weeks
Exp11. CSR effect (L0) 2 weeks
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Beam experiments
V. High average current phenomena
–
–
Exp12. Ion effect (R128 & L0) 1 week R128 + 2
weeks L0
Exp13. Long range wakefield effects (L0) 1 week
VI. Achieving ultimate ‘tuned-up’ performance
–
–
June 28, 2016
Exp14. Orbit stability characterization and
feedback (L0) 2 weeks
Exp15. Exploration of ‘multi-knobs’ and online
optimization (L0) 3 weeks
I.V. Bazarov, ERL review 03/09/07
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Time need estimates
R128
L0
beam running time (everything is working the first try)
9 weeks
20 weeks
2 (reality factor)
19 weeks
June 28, 2016
40 weeks
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Physics limit of e-photoguns
Two main limiting mechanisms:
• Phase space scrambling due to nonlinear space charge
3D Gaussian initial distribution
n,x = 1.7 mm
Optimal initial distribution
n,x = 0.13 mm
• Vgun = 750 kV
• kT = 35 meV
• Optimum 3D shape
• Photocathode thermal emittance
 n ,th   x , y
kT
mc 2
Theoretical emittance min:
 n [mm - mrad ]  4 q[nC]
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Eth [eV]
Ecath[MV/m ]
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Exp1. Thermal emittance
GaAs
GaAsP
•
•
•
June 28, 2016
kT = 1218 meV at 520 nm
or 0.49 mm-mrad per 1 mm rms
GaAs still best overall perform.
I.V. Bazarov, ERL review 03/09/07
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Exp2. Cathode time response
measured temporal response
GaAs
• Measured response time from GaAs
and GaAsP at different wavelengths
• GaAs response @ 520 nm on the
order of a picosecond
• Diffusion model correctly describes
fast response and a slow tail
response to a 100 fs pulse
expected temporal profile
diffusion model: fit to data
50% emission point
800 nm: 15 ps
520 nm: 0.83 ps
50 %
18 %
• Deflecting cavity measurement of temporal profile next month
June 28, 2016
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Exp4. Laser shaping effect
• Effective means of laser shaping have been devised and tested
• Beer-can distribution is the goal for Phase1a (a better shapes exist)
laser shape: where we are today
spatial
temporal
 x = 0.84 mm,  y = 0.72 mm
gaussian
-2.5
-2
-1.5
y (mm)
-1
-0.5
0
0.5
1
1.5
2
-2
-1
0
x (mm)
1
2
3
planning to be in few weeks from now
flat-top
June 28, 2016
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First space charge running
SOL1 =
SOL2 =
3A
E-beam right after the gun (250 kV) and the solenoid
measured
simulated
cathode
uniform
gaussian
Astra Trace Space at VC1, SOL1 = SOL2 = 4A
well-defined halo
SOL1 =
SOL2 =
4.5 A
5
25
4
3
20
2
y (mm)
1
15
0
-1
10
-2
-3
5
-4
-5
viewscreen
0
-5
0
x (mm)
5
SOL1
EMS
VC1
longitudinal tail
overfocused
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particles folding-over
forms well-defined boundary
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70 pC/bunch
1
2
3
4
5
log
scale
1
5
2
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3
4
I.V. Bazarov, ERL review 03/09/07
smallest emittance
ny = 1.8 0.1 mm-mrad
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Agreement with simulations
Good agreement with Astra prediction:
77 pC/bunch: about 2 mm-mrad
data
June 28, 2016
I.V. Bazarov, ERL review 03/09/07
astra
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Exp6. Codes’ benchmarking
R128: gun & solenoid

L0: 11 MeV
• Emittance right after the gun is
within 50% of the final value
• Establish the validity of space
charge codes & high degree of
emittance compensation in R128
June 28, 2016
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Exp9. Long. phase space map.
– ensuring small energy spread, a prerequisite for
successful transport through the merger
– optimizing compression scheme
June 28, 2016
I.V. Bazarov, ERL review 03/09/07
Time
• Combination of slits & deflecting cavity to
allow detailed longitudinal phase space
mapping
• Temporal resolution 0.1 ps, energy
resolution 10–4
• Will be used in a variety of studies, e.g.
Ce:YAG at the end of C2
Energy
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Exp11. CSR in the merger
Dx,n,CSR  0.25 mm
elegant
• EMS systems placed before
and after the merger to isolate
the CSR emittance growth
• Phase space dilution studies as
a function of varying charge
and bunch length
• Longer term possibilities –
smaller bends, shielded
chamber
three 15-deg dipole merger
CSR emittance growth [mm-mrad]
0.25
0.2
0.15
0.1
0.05
0
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
bunch length after the injector [mm]
June 28, 2016
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Exp12. Ions
• Initial calculations show that running 100 mA CW will cause
problems with safe beam dump operation
• Full beam neutralization over 4 s at 10–9 Torr
• Possible approaches:
– develop the average-current dependant optics to account for the full beam
neutralization and slowly ramp up the current (test in R128)
– introduce the ion gap, e.g. 6 ms every 60 ms (test in R128)
– the ion gap will cause large RF transients, it won’t work in L0
 Energy stored in the gun: 15.6 J 1% transient over 1.5 ms
 Energy stored in a cavity: 0.5-5 J 1% transient over 0.1 ms
– introduce clearing electrodes (non-trivial changes to the beamline, would
rather avoid)
June 28, 2016
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DC beam in R128 (250 kV)
gun through the dump
Nominal size at the
dump 4 = 20 cm
zoomed in
Full neutralization assumed
• Ions ‘helping’ to have a small beam
• 250 kV 25 kW over 4 cm diameter is probably safe on the dump
• 0.6 MW will not be so forgiving!
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L0 dump
• Two extremely short focal-length quads near the dump blow up the
beam by a factor of more than a hundred
• Even with the raster, the spot size cannot be less than 8 cm rms at
the dump plane. Ions will throw a monkey wrench into the optical
setting.
• The optics will have to incorporate the ions to avoid the dump
failure mechanism
• Challenge: we are essentially blind at 0.6 MW near the dump as far
as the beam profile is concerned.
June 28, 2016
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Exp15. Multi-knobs & tune-up
•
Virtual injector allows absolute control of parameters, real system with a
dozen of sensitive parameters will not
Pulse duration rms
Spot size rms
Charge
Solenoid1 Bmax
Solenoid2 Bmax
Cavity1 phase
Cavity2 phase
Cavity3-5 phase
Buncher Emax
Cavity1 Emax
Cavity2 Emax
Cavity3-5 Emax
Q1_grad
Q2_grad
Q3_grad
Q4_grad
June 28, 2016
21.5  1.4 ps
0.640  0.057 mm
80  5.8 pC
0.491  0.010 kG
0.532  0.010 kG
-41.6  1.7 deg
-31.9  2.0 deg
-25.7  2.0 deg
1.73  0.04 MV/m
15.4  0.3 MV/m
26.0  0.5 MV/m
27.0  0.5 MV/m
-0.124  0.002 T/m
0.184  0.002 T/m
0.023  0.002 T/m
-0.100  0.002 T/m
100 random seeds (outliers removed)
ave(x) = 1.04 mm ave(y) = 0.95 mm
std(x) = 0.52 mm std(y) = 0.62 mm
I.V. Bazarov, ERL review 03/09/07
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Possible strategies
• Should develop ad-hoc means to tune-up the nonlinear system for
optimal performance
• ‘Manual’ optimization using a calculated Hessian matrix of the
beam emittance from the space charge codes:
C
H ij 
pi p j
2
• Use SVD of the Hessian to form ‘multi-knobs’ that correspond to
top few eigenvalues
• Other potentials: use online direct search method (e.g. simplex) or a
stochastic search (e.g. genetic algorithms). Analog computer
evaluations will be limited to a few hundred at most.
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Summary
• Experimental plan outlined, both R128 and L0 parts
are essential
• Move to L0 once n  0.5 mm-mrad demonstrated at
the nominal bunch charge (77 pC) from the gun;
premature move is advised-against
• There are things we know we don’t know (e.g. ions),
and there are things we don’t know we don’t know.
We are concentrating on the former.
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