Review of experimental results on photoemission electron sources
Ph. Piot, DESY Hamburg
Introduction
RF-guns
DC-guns
Conclusions
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Ph. Piot, DESY
1
Introduction
•Application of high-brightness photo-injectors:
- high energy linear colliders (needs flat beam ey/ex <<1)
- radiation sources (FELs, linac-based SR)
- X-rays production (XTR, Thomson)
- plasma-based electron sources-drivers,…
•Many accelerator test facilities in operation based on photo-injectors:
- dedicated to beam physics (BNL, UCLA, DESY-Z, NERL...)
- drive user-facility (Jlab, DESY-HH,…)
•Figure-of-merits: emittance (FELs requires e<l) , peak current,
average current (photon flux), local energy spread, bunch length (e.g.
for probing ultra-fast phenomena)…
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Photo-emission from metals and semi-conductors
semi-conductor
Material QE
Range
(%)
Metal
0.020.06
CsK2Sb ~5
Cs2Te ~5
LaB6
~0.1
GaAs
~10
(Cs)
metal
l(nm) Lifetime Required
260
Vacuum
(T)
Months 1E-7
527
260
355
527
Months
Months
Days
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1E-10
1E-9
1E-7
1E-11
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(from Spicer et al. SLAC-PUB 6306)
Few words on Lasers
•For metal, typical laser energy required: 5-500 mJ/pulse
•For semi-conductor: 0.5 mJ/pulse
•Metallic cathodes are bad candidates for high-average power
machine [one might need an FEL-based photo-cathode laser to
have 100 W level in the UV. E.g. see Zholents’s talk at BNL
PERL workshop 01/2001]
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Emittance and Brightness
Phase-space emittance (Liouvilian invariant)
1
e
me c 2
x
2
px
2
 xpx
2
(what codes give)
Trace-space emittance (experimentally measurable)
e TS   || ||
x2
x2  xx
2
Normalized brightness
B 
I
4
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
2I
beam current
e xe y
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Thermal Emittance
Electrons are emitted with a kinetic energy Ek
r Ek laser spot assumed uniform
e th 
2 me c 2 with radius r
   , or EG  E A
Ek  h      RF ERF sin  RF
1.2
0.8
1
0.75
eN(mm-mrad)
(mm-mrad)
eN
Example of measurement for Cu-cathode (Courtesy of W. Graves)
0.8
0.6
0.65
0.6
0.55
0.4
0.5
0.2
0
0
0.7
0.45
0.2
0.4
0.6
0.8
1
Horizontal RMS laser size (mm)
Linear fit gives Ek=0.43 eV
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0.4
1.2
0
10
20
30
40
50
60
RF phase (degrees)
70
80
Nonlinear fit gives rf=3.1+/-0.5,
cu=4.73+/-0.04 eV, and Ek=0.40 eV
Ph. Piot, DESY
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Thermal Emittance (CNT’D)
To date no thermal emittance measurement for Cs2Te cathodes has been
performed [plan at INFN Milano are underway]
Several groups have measured thermal emittance of GaAs:
* Duhnam et al., on the Illinois/CEBAF polarized beam (PAC1993) at
room temperature
* Orlov et al., at Heidelberg (Appl. Phys. Lett. 78: 2171 (2001)) at 70 K
The measurements indicate that a reduction of the cathode temperature
results in a lower transverse kT for the emitted e-. This is particular to NEA
cathodes where electrons from thermalized population can escape.
The price to pay is the long emission time of 10-20ps
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Generic photo-injectors
Split injectors
gun
• 1-1/2, 2-1/2 cell cavity with
high E-field
• booster section downstream
of the gun
• E.g. BNL-gun, FNAL,
AWA, DESY,…
booster
Integrated injectors
• typically 10-1/2 cell cavity
with moderate E-field
• long solenoid lens
• E.g. AFEL, PEGASSUS
gun
• DC column with HV 500
kV and higher achieved
• Solenoids + rf-buncher
• Booster section
• E.g. IR-Demo
DC-gun
gun
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booster
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Frequency Scaling of photo-injectors
PARAMETER
Cavity
dimension
Accelerating
field
Peak current
Bunch charge
Bunch energy
Bunch
emittance
Bunch
brightness
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SCALING
-1
•If the operating parameters
are scaled following the
Table, one would expect:
Brightness~ 2
1
0
-1
0

•this assumes: E-field~ 1
•Naively scaling the present
BNL gun (120 MV/m) e.g. to
17 GHz would imply:
E-field~ 720 MV/m!!!
-1

(Rosenzweig and Colby PAC95
Also L C.-L. Lin et al., PAC95)
2
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MIT 17 GHz gun
Mission: Advanced
ultra-bright
accelerator developments
1/ has commissioned a 1.5 cell gun
2/ work on a 2.4 cell gun (>2 MeV)
Achived values
Frequency
Charge/bunch
17 GHz
0.1nC
Field on cathode
200 MV/m
RF pulse length
50 ns to 1 ms
Input power
4 MW coupled in
Laser radius
0.5 mm
Laser length
1 ps
Beam energy
1.05 MeV
(PR. ST. AB vol. 4:083501 (2001))
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MIT 17 GHz gun
•Measured emittance at 50 pC to be
1mm-mrad at the gun exit
•Brightness=80 A/(mm-mrad)2
•It will be boosted to ~800 A/(mmmrad)2 after emittance compensation
•Emittance compensation presently
non effective (velocity spread) need
to increase the beam energy at gun
exit (will use a 2.4 cell gun)
(PR. ST. AB vol. 4:083501 (2001))
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3 GHz CLIC drive beam photo-injector
Operational since 1996. About 1000h of running
each year since, mainly for CLIC 30 GHz power production
Goal / achieved
Total charge in 48 bunch train
(334 ps bunch spacing)
Beam current
640 nC / 750 nC
43 A / 50 A
Highest charge in single bunch operation
20 nC / 100 nC
Bunch length (fwhm)
10 ps / 10 ps
ex (rms, normalized)
50 mm / 100mm
e y (rms, normalized)
100 mm / 100mm
Field on cathode
100 MV/m / 105 MV/m
RF pulse length
2 ms / 2 ms
Rep. rate
10 Hz / 10 Hz
Input power
18 MW/ 20 MW
Quantum efficiency (typical)
Hours of operation per cathode
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1.5 % / 4%
>40 h / >250 h
Ph. Piot, DESY
(Courtesy of H Braun)
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BNL/UCLA/SLAC gun
•Popular design, used at BNL (ATF & SDL),
SLAC (GTF), ANL (LEUTL), Tokai (NERL),…
•Since its first design the gun has undergone
improvements; latest foreseen are: a mode-lock
system and a split symmetric RF input coupler
LCLS Goal / achieved
Charge/bunch
1 nC / 1 nC
Field on cathode
140 MV/m / 120 MV/m
RF pulse length
3 ms / 3 ms
Rep. rate
Input power
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120 Hz / 10 Hz
14 MW/ -
Ph. Piot, DESY
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Recent results from ATF, BNL
•Beam based alignment of quad to
center beam in the TWS
•Optimized optics (with a high-) to
overcome problems inherent to the
screen resolution
•Measured beam emittance using the
multi-monitor technique
•Obtained: e=0.8 mm-mrad for
Q=0.5nC and I=200 A
Example of fitted envelope at 70 MeV
centered beam
mis-steered beam
30 um wire focused spot
ICFA/BD Sardinia July 2002
(Courtesy of V. Yakimenko)
Ph. Piot, DESY
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Recent results from ATF, BNL
(extracted from ATF News Letter 03/2002)
60
90
80
50
70
100
•As predicted by simulation, uniform
beam gives the best emittance
•Emittance doubles for the 50 %
modulation case
ICFA/BD Sardinia July 2002
Ph. Piot, DESY
•Measurement of impact of
transverse non-uniformity on
emittance
•Used a mask
•Q=0.5 nC (kept constant)
•Emittance for uniform beam is
about 1.5 mm-mrad
•Long. Length is 3 ps FWHM
100 %
90 %
60 %
50 %
15
Recent results from SDL, BNL
undulators
linac zero-phased
75 MeV
dumpmeasurements
Slice emittance
dump
linac
75 MeV
5 MeV
y
•Parametric study of emittance (projected +
t
slice) vs various parameters
•Preliminary data indicate brightness
(Courtesy of W. Graves et al.)
improves as charge is decreased
10 pC
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Ph. Piot, DESY
200 pC
16
Recent results from SDL, BNL
Observation of sub-picosecond compression by velocity bunching
•Used the TWS tank downstream of the
rf-gun as a buncher (operated far offcrest) [see M. Ferrario’s talk]
•Measurement were performed using
both frequency- and time-domain
technique
(Piot’s talk in Working Group I)
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Recent results GTF, SLAC
•Parametric study of emittance
versus bunch charge
•Achieved LCLS project
parameters (1.5 mm-mrad for
I~100 A)
Longitudinal Distribution After Gun
•Reconstructed the longitudinal phase
space from a set of energy profile
measurement.
•For Q=200 pC, FWHM d=8%, FWHM
t=3 ps (initial laser FWHM=4.3 ps)
Energy (MeV)
0.2
0
0.2
2
0
Time (ps)
(Courtesy of J. Schmerge)
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2.856 GHz PWT gun (under commissioning at UCLA)
•Integrated injector
installed at the PEGASUS
facility, UCLA
•Exit energy ~20 MeV
•E-field 40-60 MV/m
•Charge 1 nC
•Input power 20 MW
Proposed to be used to produced polarized electron beam using
GaAs (which requires 1E-11 T vacuum) because of the better
vacuum conductance compared to usual cavity-based photo-injector
[Clendenin et al. SLAC-PUB-8971]
(Telfer et al., PAC 2001)
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FNPL(FNAL) & TTF injector II (DESY)
typical parameters for TTF 1-FEL:
repetition rate:
pulse train length:
bunch frequency:
bunch charge:
bunch length (rms):
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[see also S. Schreiber’s talk]
1 Hz
norm. emit., x,y: 3-4 µm ( @ 1nC)
1-800 µs
dpp:
0.13 % rms ( @ 17 MeV )
1-2.25 MHz
injection energy: 17 MeV
1-3 nC
~3 mm ( 1 nC,
(Schreiber et al. EPAC2002)
after booster )
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Ph. Piot, DESY
Results at TTF Injector 2 (1nC setup)
Emittance measurements
sol. 1/2
emit. x
emit. y
4.19  0.13
4.58  0.15
220 / 104
3.02  0.17
3.47  0.12
240 / 104
4.08  0.57
4.52  0.47
200 /
104
Bunch length measurement (streak cam.)
(Schreiber et al. PAC2001)
(Honkaavara et al. PAC2001)
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VUV-FEL driven TTF injector
 Primary electron bunches (charge 3nC) are produced by laser-driven rf gun
 During single pass of the undulator primary bunch produces powerful VUV radiation (l=95 nm)
 Radiation is reflected by plane SiC mirror and is directed back to the photocathode of rf gun
 Electron bunch produced by SASE radiation (charge up to 0.5 nC) is accelerated
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Ph. Piot, DESY
(Faatz et al. FEL2002)
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Results at FNPL, FNAL
Transverse Emittance Studies
•Systematic optimization of the rf-gun
parameters (solenoids, laser radius) for
various charges
•Estimate of brightness indicates it
improves with decreasing charge
Production of Flat beams
•Used the inverse
Derbenev transform
to convert a
magnetized round
beam in a flat beam
[see S. Lydia’s talk]
•High ratio of
ex/ey~50
demonstrated
(Courtesy of J.-P Carneiro)
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DESY 1.3 GHz gun
•Second generation of gun for TTF
user facility
•Fully symmetrized cavity using a
coaxial input-coupler
•Test facility at DESY-Z just
commissioned
•Cs2Te thermal emittance
measurement are foreseen
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Ph. Piot, DESY
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LANL AFEL Facility
Mission: Advanced free-electron
laser experiment at Los Alamos.
The gun has driven a IR SASE-FEL
•1.3 GHz, 10+1/2 cells
•E-field=20 MV/m
•Typical charge 1 to 4 nC
•Exit energy 15-20 MeV
(from Nguyen’s talk at PERL
workshop BNL, Jan 2001)
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•Macropulse current up to 400 mA
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Results, LANL AFEL
•Measure slice emittance
using a combined
quadrupole scan with a
streak camera
•Measured slice emittance
of 1.6 mm-mrad at 1nC
•PARMELA predicts 0.6
mm-mrad (without thermal
emittance)
(from S. Gierman’s Thesis -- UCSD)
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SRF gun (DROSSEL collaboration)
First phase: proof-of-principle: observe
photo-emission of a cathode in a
superconducting rf-cavity
Later: built a “real” gun that could be
used for CW operation of the ELBE
free-electron laser based at
Forschungszentrum Rossendorf
•frequency=1.3 GHz
•Number of cell~ 0.5
•Half-cell is a TESLA cavity shape
with a shallow cone
•Use a Cs2Te
•No solenoid => focusing provided
by rf (conic-shaped back plate)
•First photo-electrons observed last
March
(Courtesy of P. Janssen et al.)
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SRF gun (DROSSEL collaboration)
(Courtesy of P. Janssen et al.)
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SRF gun (DROSSEL collaboration)
700
14
SRF-Gun, 19.03.02
Laser 50 Hz, 4 ms, 2.4 mV diode, -1 kV DC
12
600
500
Icath
8
400
energy
300
6
energy / keV
current / µA
10
200
4
Idump
2
100
0
0
20
40
60
80
100
120
140
160
0
180
laser phase / deg
(Courtesy of P. Janssen et al.)
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The APLE BOEING (decommissioned)
•0.433 GHz, 2 cells
Bucking coil
•E-field=25 MV/m
•Typical charge 1 to 5 nC
•Exit energy ~2 MeV
•Laser: 53 ps (FWHM), 5 mm radius
•K2CsSb cathode
coil
•duty cycle: 25%
•Macropulse frequency: 30 Hz
•Macropulse length: 8.3 ms
•Micropulse frequency: 27 MHz
(Courtesy of D. Dowell)
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Recent results, ELSA-2 Bruyeres-le-chatel
120
60 ps (mars 2001)
ATRAP 60 ps
60 ps (avr 2002)
100
electron bunch (ps)
•0.144 GHz, 2 cells
•E-field=25 MV/m
•Typical charge 1 to 10 nC
•Exit energy ~2.6 MeV
•Laser: 60 ps (FWHM), 4 mm radius
60
6
emittance e n,rms (µm)
80
laser pulse
exp. data
5
40
4
corrected
0
2
4
6
8
10
12
14
Bunch charge (nC)
3
•Macropulse frequency: 10 Hz
•Macropulse length: 150 ms
•Micropulse frequency: 14.4 MHz
2
1
0
0
2
4
6
8
10
12
(Courtesy of Ph. Guimbal)
Charge (nC)
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DC-GUN, JLab IR-Demo
insulating
ceramic
•DC gun with GaAs photo-cathode
•Buncher needed despite the 20 ps laser
•In the Ir-Demo gun is coupled to a ¼
photo-cathode cryounit (2 CEBAF-type 5-cell SRF
anode
cavities at 10 and 9 MV/m)
•advantage: ran CW at 75 MHz (1/80th
of 1497 MHz)
solenoid
•Recently developed laser (M. Poekler
PAC 2001) allows CW ope. @ 1.5GHz
laser
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(D. Engwall et al. PAC1997, Ph. Piot et al. EPAC1998)
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DC-gun, JLab IR-Demo
•High voltage operation of DC-gun
limiter by field-emission
•Collaboration Jlab + College of
William & Mary: study reduction of
field-emission by Nitrogen ions
implantation on the electrodes
•Experiment performed in a test
chamber demonstrate the benefits of
ion implantation: up to 25 MV/m
DC-field could be achieved with less
than 40 pA “dark” current.
(C.K. Sinclair et al. PAC2001)
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Comparison of Peak brightness
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Conclusions
•ATF at BNL has set new record in brightness
•Both BNL-type and DESY-type gun have driven short wavelength
single-pass FELs to saturation (LEUTL, TTF-1).
•ELSA-2 at Bruyeres-le-Chatel has demonstrated the targeted emittance
number of 1 mm-mrad at 1 nC (to the expense of bunch length)
•Presently achieved performances with a DC gun are comparable to rfgun running with high duty cycle (in term of brightness).
- better candidate to drive high photon-flux based on ERL?
- largest average brightness
- and E-field of 25 MV/m have been achieved in experiment
•Many other developments I have not addressed (hybrid DC/RF guns,
hybrid plasma/photo-emission guns, needle cathodes, etc…)
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Grazie Mile!
# Thanks to all the individual aforementioned for their
contributions
# To M. Ferrario, K. Floettmann , W. Graves , P. Hartmann,
C. Sinclair for discussions
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