Photo injector studies at CERN

Photo injector studies at CERN
M. Petrarca
CERN, November, 2010
Outline:
CTF3 facility overview
CTF3 photo injector laser
PHIN photo injector
Photoemission laboratory overview
SPARC FEL photo injector
Laser pulse shaping
Beam dynamic result
CTF3 facility overview:
CTF3: CLIC Test Facility
is a facility at CERN to study the feasibility of the key CLIC-technological
challenges for the CLIC two-beam scheme
Combiner Ring: x4
Delay Loop:2
Drive Beam Injector
thermionic gun
Drive Beam Accelerator
DL
CR
F D
F
D
F
D
F
F
D
D
F
D
F
D
F
F
D
DF
D
F
F
DFD
DFD
FD
DFD
DFD
D F D
DF
D
FD
F
D
F
CLEX: CLIC experimental area
DC DRIVE BEAM
CLEX: CLIC experimental area
1.5GHz PROBE BEAM
12 GHz DRIVE BEAM
CTF3 electron sources:
Drive beam base line: thermionic gun, produces a continuous e-beam
which is opportunely modulated to obtain the bunched structure
Drive Beam Injector
thermionic gun
DL
CR
F D
F
D
F
D
F
F
D
D
F
D
F
D
F
F
D
DF
D
F
F
DFD
DFD
DFD
DFD
FD
DF
D
FD
D F D
F
D
F
Probe beam: photo injector “CALIFES”
accelerator
•Complex bunching system required after the thermionic gun
•~7% satellite presence
acceptable in CTF3 un-acceptable for CLIC
CTF3 electron sources:
Drive beam photo injector option, advantages:
good handle of the initial condition transverse-longitudinal profile and
time distribution of the generated electron bunch
Photo injector advantages:
No bunching system;
Single pulse mode
No satellites
Polarized electron generation
Smaller emittance
DL
CR
F D
F
D
F
D
F
F
D
D
F
D
F
D
F
F
D
DF
D
F
PHIN
F
DFD
DFD
FD
DFD
DFD
D F D
DF
D
FD
F
D
F
“PHIN” photo injector
Studies the feasibility to replace the Drive Beam thermionic injector simplifying
the drive beam generation and improving the drive beam quality
PHIN is a Joint Research Activity within the Coodinated Accelerator Research in Europe (CARE) project,
devoted to improve the performance of the electron sources for future colliders
CTF3 photo injector laser:
CTF3 photo injector laser:
M. Petrarca, M. Martyanov, M. Divall, G.Luchinin
accepted to IEEE J. Quantum. Elec.
Micro pulses at 1.5 GHz
~666ps
~8ps
Seed beam: 1.5GHz repetition rate (~666ps) ;
10 W avg power 7 nJ energy
3KW peak power
AMP1 diode pump (90A)
Nd:Ylf 3 pass
amplification stages:
(1-50)Hz, 3KW, 2mJ
8.5KW peak power
AMP2 diode pump (90A)
Nd:Ylf 2 pass
amplification stages:
(1-50)Hz, 8.5KW, 6mJ
1.8J
400ms
Gating system:
1ns -> 10ms
Dt
Dt
SHG+FHG
IR ->Green->UV
2
IR
Dt
4
Green
UV
CALIFES (85m away)
(1->140) ns
PHIN (11m away)
>1200ns
Laser overview: amplifiers
AMP1
AMP2
AMP1 pumping diode:
AMP2 pumping diode:
3KW peak power each
3.6 KW peak power each
(square pulse 400ms long)
(square pulse 250ms long)
15 KW total power
18 KW total power
Rod : 8cm long; 7mm diameter
Rod 12 cm long; 1 cm diameter
Laser overview: amplifiers
Harmonics
+ pulse picker
A
M
P
2
Pockels
Cell
A
M
P
1
Osc +
Preamp
Laser overview: optical
Pockels cell  down to 150ns
(4ns rise & fall time)
Pockels
cell
Harmonics
+ pulse
picker
A
M
P
2
A
M
P
1
Osc +
Preamp
Pulse picker:
From 140 ns to <1ns for just
one micro pulse selection
Laser overview: frequency conversion stages
1047nm
523nm
262 nm
Second Harmonic
Fourth Harmonic
RMS stability
(macro pulse)
0.23%
0.8%
Energy/pulse [μJ]
5.37
2.5
Beam size [μm]
418 X 372
370 X 224
1.3%
Response to long train
_ electron beam
_ laser beam
0.87
1.3 μs
Conversion
efficiency
47%
35% ADP
26% KDP
Crystal
KTP 11mm
ADP 20mm @23.60C
KDP 10mm
1.3 μs
beam’s center
1047nm
beam’s edge
523nm
262 nm
Phase coding:
Electron time distribution required
for the Delay Loop
~333ps
Laser time distribution required at
cathode
Laser
~999ps
A fiber based electro-optical interferometer “Phase Coding” is required to modify the laser oscillator time distribution
Principle of operation demonstrated at the Univ. of Milan INFN on different laser, final demonstration and improvement
of the system on going at CERN
82%
Losses in %
Oscillator
320mW
waveplate 20%
EO modulator
11%
2% Fiber splitter
coupler
EO modulator
82%
20%
333ps
delay
Variable
attenuator
20%
T=9%
11%
Booster
amplifier
300mW
Fiber
outcoupler
•Planned commissioning
on PHIN Feb/2011
PHIN photo injector
PHIN photo injector
PHIN is a Joint Research Activity within the Coordinated Accelerator Research in Europe (CARE) project,
devoted to improve the performance of the electron sources for future colliders
The photo injector test bench has been developed under this activity
therefore it is now called as PHIN photo injector
Parameter
Specification
Charge per Bunch (nC)
2.33
Charge per Train (nC)
4446
Train Length (ns)
1273
Current (A)
3.5
Normalized Emittance (mm mrad)
<25
Energy Spread (%)
<1
Energy (MeV)
5.5
UV Laser Pulse Energy (nJ)
370
Charge Stability (%)
<0.25 rms
Cathode
Cs2Te
Quantum Efficiency (%)
3
RF Gradient (MV/m)
85
RF Frequency (GHz)
2.99855
Micro pulse Repetition Rate (GHz)
1.5
Macro pulse Repetition Rate (Hz)
1-50
PHIN photo injector layout…beginning 2008!
Beam diagnostic:
Cathode load-lock to
install the Cs2Te
cathode under good
vacuum ~10-11mbar
OTR screen, Multi slit mask
for emittance, Spectrometers
2+1/2 cell
3GHz cavity
Propagation direction
PHIN photo injector layout….since end 2008
Solenoid Coils: 0.3 T
PHIN results: beam size along the pulse train
Main studies: beam size, energy, energy spread and emittance stability along the 1.3ms pulse
train of the beam
-long pulse train
...
gate duration
e-beam
FCT
Previous results – O. Mete et al, CLIC Note 809
17
PHIN results: beam size along the pulse train
-long pulse train
...
gate duration
Statistical Deviation
Constant Gate Duration
PHIN results: emittance along the pulse train: 1.2ms
19
PHIN results: beam loading effect
Rf power
in the gun
Faraday cup
reflected
power
Without beam
With beam
PHIN results: current stability
Best current stability probably thanks to saturation
effects from the cathode
Current stability when no saturation effect from the cathode is correlated to laser intensity fluctuation ~1.2%
Major sources of instability :
Replace with Solid
metal mirror
Laser environment to be
improve: reduced air flow
(partially accomplished),
temperature stabilization
Cs2Te photocathode response
‣ Micro pulse energy is constant for each curve
‣ Macro pulse length is increasing for each data point
‣ Integrated charge increases linearly with the macro pulse length
‣ No effect from the macro pulse on the charge extraction process
‣ Fixed macro pulse length
‣ Extracted charge saturates with the increasing micro pulse energy.
‣ Peak QE18%
‣ Laser : sigma=300mm
‣ Fixed macro pulse length
‣ Extracted charge saturates with the increasing micro pulse energy.
‣ Peak QE8%
‣ Peak QE 5%
‣ Laser : sigma=1mm
22
PHIN: conclusion and perspective
Parameter
Specification
Achieved
Charge per Bunch (nC)
2.33
4.4
Charge per Train (nC)
4446
>4446
Train Length (ns)
1273
1300
Current (A)
3.5
3.5
Normalized Emittance (mm mrad)
<25
14
Energy Spread (%)
<1
0.7
Energy (MeV)
5.5
5.5
UV Laser Pulse Energy (nJ)
370
>370
Charge Stability (%)
<0.25 rms
1.2
Cathode
Cs2Te
Quantum Efficiency (%)
3
18 (peak)
RF Gradient (MV/m)
85
85
RF Frequency (GHz)
2.99855
2.99855
Micropulse Repetition Rate (GHz)
1.5
1.5
Macropulse Repetition Rate (Hz)
1-5
1-5
Charge stability laser stabilization feedback to be implemented, laser environment to be improved
Phase coding system to be implemented and tested
Cathode life time  to be more investigated on long term stable machine operational condition, higher macropulse r.r
Extracted Charge  to be investigated for bigger laser beam size, different RF gradients
Vacuum Response to longer macropulse train and consequent cathode response
Vacuum studies and improvement toward CLIC; we have observed a Vacuum increased from 10 -9mbar to 10-8mbar
when macropulse length increase from 200ns to 1200ns
Photoemission laboratory:
a dedicated facility to produce and study different cathodes
•DC gun (8MV/m)
• Fix electrode gap:1cm
•Electrode: Ti
•1x10-11<p<1x10-10 mbar
• Typical laser spot size 4mm
UHV (6 x10-12mbar) transport carrier
to transport good quality cathode to different facilities
Photoemission laboratory overview
Ionic pump
Nd:Yag
Electrode (Ti)
Anode: Stainless steel
Sublimation pump
HV
OFHC: Oxigen free high conductibility copper
The photoemission laboratory preparation chamber for
cathode production: Cs2Te ..
UV laser beam
Shutter
RF
oven
Te thickness
measurement
Photocathode
plug
Cs thickness
measurement
Electron collect.
electrode
Cs & Te
Evaporators
Important Goals
NOW:
IR (1047nm)
Green (523.5nm)
UV (262nm)
Intensity Stability
0.3%
0.8%
1.3%
IR->Green efficiency:
47%
Green ->UV efficiency:
25%
It is generally true that by using a cathode with a similar QE but on longer l:
1) more energy will be available: ex. at l=Green ~75% more energy available from the
laser to match cathode lifetime and to achieve more charge/pulse
2) better “a priori” (no stabilization feedback) stability
3) less losses from transport line
4) great simplification of the laser chain; very important for the CLIC requirements:
( CLIC drive beam: 8.4nC/pulse, 500MHz, 140ms train length,50-100Hz )
Cathodes: Cs3Sb, CsK2Sb, GaAs, …….
Vacuum improvement in RF gun strongly required
CLIC main beam
polarized electrons
Parameters
CLIC (3 TeV)
Electrons/microbunch
0.6x1010
Charge / microbunch
1 nC
Number of microbunches
312
Total charge per pulse
1.9x1012
Width of Microbunch
~ 0.1 ns
Time between microbunches
0.5002 ns
Width of Macropulse
156 ns
Macropulse repetition rate
50 Hz
Charge per macropulse
300 nC
Average current from gun
15 mA
Average current in
macropulse
Peak current of microbunch
Current density (1 cm radius)
Polarization
1.9
9.6 A
3 A/cm2
>80%
SLAC scheme as baseline:
DC gun generating a macropulse.
RF bunching system creates the bunched structure
Charge production has been demonstrated
Bunching system has been simulated at a first stage
showing a satellite presence
Zohu et al, SLACP-PUB-13780/CLIC-note-813
SLAC-PUB-13514
Option:
2GHz laser.
Oscillator exists nowadays.
Amplifier feasible, R&D required
FHWM <100ps to get reasonable bunch spacing at 2GHz
Charge production:
1nC at 2GHz rep rate and such FWHM to be demonstrated
(Peak Current with 100ps = 10A)
Acknowledgment
Photo injector:
Eric Chevallay
Anne Dabrowski
Marta Divall
Steffen Doebert
Nathalie Lebas
Thibaut Lefevre
Alessandro Masi and his team
Louis Rinolfi
….
….
Valentine Fedosseev (CERN, EN/STI/LP section leader)
Roberto Losito (CERN, EN/STI group leader)
Collaboration:
Guy Cheymol, M. Gilbert
I. Boscolo INFN-Milano
V. Lozhkarev
G. Luchinin
M. Martyanov
E. A. Khazanov
CEA-Saclay
(Institute of Applied Physics (IAP), Nizhny Novgorod, Russia)
(head of the department of the IAP, Russian Academy of Sciences)
Acknowledgements
• We acknowledge the support of the European
Community-Research Infrastructure Activity under
the FP6 “Structuring the European Research Area”
programme (CARE, contract number RII3-CT-2003506395).
Brightness of electron beam:
I
peak current
B 2 

emittance2
Natural shape of laser
pulses
minimized by triggering the photoemission
using flat top-pulses
Ti:Sa laser chain must
provide pulses with a
flat-top shape
at 266nm
Target shape of laser
pulses
Laser:
•The laser time structure is transferred into the electron time structure:
good handle of the initial condition “transverse and longitudinal profile” and
of the time distribution of the generated electron bunch
Two pulse shaping techniques
studied and developed at LNF on the SPARC laser system
2004-2007 LNF-INFN (Frascati):
SPARC Laser
at LNF-INFN (Frascati)
Dazzler
(acousto-optic modulator)
First technique based on (1/2):
•the possibility to transfer the spectral profile in the temporal domain
In this configuration the target pulse can be produced by obtaining a flat top spectrum
First technique based on (2/2):
•possibility to create a UV flat-top spectrum,
how does the harmonic crystal behaves:
•Convolution product just (Transform Limited pulse) between two
rectangular shape IR spectra yields a triangle spectrum in the SH
•Introducing a quadratic phase factor in the IR the convolution product
yields a more rectangular shape in the SH
Starting from a T.L. rectangular IR spectrum and introducing a small chirp a
rectangular UV spectrum is achieved
Increasing chirp
(ps^2)
First technique results:
Rise and fall time ~ 2.6 ps --> need to be faster
..............Second technique
Second technique: UV pulse shaper
Performed directly in the UV:
A sharp cut of the spectral tails induces overshoots in the
UV temporal profile of the pulse, which could be used to
compensate the curvature of the initial time profile.
asymmetric 4f-stretcher
system
1) The beam is sent onto a diffraction grating 4350 lines/mm which spatially
disperse the optical frequency
2) these are then collected and focused by a lens f~50cm at the focal plane (Fourier Plane)
full correlation between wavelengths and transverse positions is established.
Here we can make any amplitude modulation of the spectrum
3) The beam is then re collimated by a second lens and sent to the 2nd grating
(h=0 no chirp; h<0 chirp+; h>0 chirp-)
4) By the retro-reflector mirror the beam is sent back
Second technique results:
Direct shaping in the UV by cutting the spectral tails of a quasi gaussian UV
spectrum
Acting directly on the UV pulses this second technique is easier to perform
Experimental observation:
double emittance minimum behavior