Rainbow Curves
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Transcript Rainbow Curves
Overview of advanced cathodes
for High Brightness Beams
L. Cultrera
(INFN-LNF)
Workshop on the physics and application of high brightness electron beams
Maui, Hawaii
November 16-19, 2009
Cathodes
Photocathodes
Metals
Coated
Thermionic
CNT
CeB6
Semiconductors
Spindt
Pure
Field Emitter
PEA
Si nano
NEA
BaO
Cu
Cu-CsBr
Mg
Cu-MgF2
Pb
Cu-Cs
Y
W-Cs
K2CsSb
Cs2Te
Cs:GaAs
Cs:GaN
Cs:GaAsP
SEY Diamond Window
NOT EXAUSTIVE LIST!
Brigthness of an electron beam
Bn n ,x n , y
2I
• Brightness is limited by:
– thermal emittance
• Emission size
• Transverse momentum distribution
– achievable current
• Photocathode => QE and laser damage
• Field emitter => arcing and Joule effect
• Thermoionic => Electric field
Metallic photocathodes
• Prompt time response makes them suitable for:
– Very short electron bunches
– Temporal pulse shaped profile
• Relative insensitivity to contamination:
– Preparation
– Handling
– Vacuum requirements
– Operational lifetime
QE and thermal emittance
• “QE is a measure of the longitudinal
momentum distribution”
• “Thermal emission is by definition a
measure of the transverse momentum of
the electrons that are emitted”
• “Longitudinal momentum determine which
electrons are emitted and thus which
electrons’ transverse momentum contribute
to the emittance”
D.H. Dowell and J.F. Schmerge, Phys. Rev. ST Accel. Beams, 12, 074201 (2009)
QE and thermal emittance
1 R
QE
8 E
1
2
eff
opt
ee
th x
eff
F
eff
v eff
3m c2
Generally speaking to get higher QE from metallic photocathode
it should be accepted that this will give higher thermal emittance.
D.H. Dowell and J.F. Schmerge, Phys. Rev. ST Accel. Beams, 12, 074201 (2009)
Metallic photocathode: lifetime
Background Pressures
w/o RF ~ 5x10-10 mbar
with RF ~ 2x10-9 mbar
Cu
Y
Despite their claimed contamination
insensivity even in UHV (10-9 mbar
range) low work function metals as
Mg, Y but also the most and inert
Cu
may
suffer
from
the
contamination due to chemical
species present in residual gases
(H2, CO, CO2, H2O).
Mg
Improving lifetime
• Wide band gap thin film coatings are giving
interesting and in some way unexpected results
Accurate choice of the coating material to be transparent
to the laser wavelength. The thickness may be designed to
create an antireflecting coating at the cathode surface.
Metals with CsBr coating
• Transmission @ 257 nm
• Photoemission arises from
intraband states
• Electrons injected through
the metal insulator junction
sustain the emission
• Reflection @ 257 nm
• 50 times higher QE
• No strong degradation of
QE due to air exposre
Z. Liu et al., Appl. Phys. Lett., 89, 111114 (2006)
J. Maldonado et al., Phys. Rev. ST Accel. Beams, 11, 060702 (2008)
Metals with MgF2 coating
• Higher shottky
effect due to the
electric field
inside MgF2?
• Can we expect
lower dark
current due to
higher width of
the barrier?
• Short response time
P. Musumeci, This Workshop
SPARC and SPARX photocathode R&D
• Cu and Mg needs UV photons (266 nm, 3rd
harmonic Ti:Sa) while Y should emits electrons
in visible range (400 nm, 2nd harmonic Ti:Sa)
Yttrium emission curve @ 406 nm after laser cleaning
QE ~ 3.4x10-5
1 nC should require about 100 mJ laser pulses @ 406 nm
QE
Yttrium Lifetime
Pbg=10-9 mbar
E = 1.7 MV/m
= 406 nm
Time (s)
May be improved with MgF2 coating?
Lead photocathode in SRF gun
J. Smedley, T. Rao and J Sekutowicz, Phys. Rev. ST Accel. Beams, 11, 13502 (2008)
Cesium Telluride
• Operated in user and test facility (Pitz, FLASH, CLIC…)
• Still suffer of short lifetime
Coating with 2 nm CsBr significantly decreased the
quantum efficiency, without improving the lifetime. Other
groups claimed that QE is reduced to 7% after application
of the coating and keep stable for at least 2 months
E. Chevallay et al., Proceedings of LINAC 2000
L. Monaco et al., Proceedings of PAC07
D. C. Nguyen et al. , Proceedings of FEL 98
KCsSb with protective coating
• Loosing efficiency by means of protective
layer increases the stability versus reactive
gases
E. Shefer et al, Nucl. Instr. Met. A, 433, 502 (1999)
GaAs with cesiated surface
•Negative affinity achieved by “yo-yo”
techinque;
•Thermal emittance values measured are
very good even with IR or VIS radiation;
•This cathode type suffers from ion
backstream that limits the lifetime to few
hours;
•Achieved high QE (~15%), low thermal
emittance (0.2 mm mrad for 1mm );
•Response time < 1 ps
I. Bazarov et al., Phys. Rev. ST Accel. Beams, 11, 040702 (2008)
Strained GaAs-GaAsP
for polarized electron beam
Strained QW structure removes
degeneracy between light and
heavy holes.
By appropriate tuning of the
exciting wavelength up to 90%
polarization of electron beam can
be achieved
As for GaAs and GaN the
negative affinity is achieved by
cesiation of the surface. Ion
backstream to the cathode
damage Cs layers.
Mo Spindt Field Emitter Array
15 mm
40 000 pyramidal molybdenum tips form a 1 mm diameter
active emitting area on the silicon substrate.
Achieving 2.5 mA per emitter will give 100 mA for the array.
E. Kirk et al, Eighth IEEE International Vacuum Electronics Conference (2007)
Mo Spindt Field Emitter Array
50000 conical molybdenum tips form
a 1 mm diameter active emitting area
on the silicon substrate.
Achieved 2 mA with DC pulses of 100
ns and 40 kV.
Beamlet single slit
Beamlets pinhole mask
S. C. Leemann, A. Streun, and A. F. Wrulich, Phys. Rev. ST Accel. Beams, 10, 071302 (2007)
Carbon nanotubes
0.25 mm2
N. de Jonge, Y. Lamy, K. Schoots and T. H. Oosterkamp, Nature, 420, 393 (2002)
W.I. Milne et al., J. Vac. Sci. Technol. B, 24, 345 (2006)
Photo field assisted cathode
L. Hudansky et al., Nanotechonology, 19, 105201 (2008)
R. Ganter et al., Phys. Rev. Letter, 100, 064801 (2008)
Dispenser cathodes
•
•
•
Is well know that cesiated surfaces present very low work function
values suitable to generate photoelectron with high QE values at
reasonable wavelengths
Cesium desorption or surface contamination by residual gases
contribute to QE degradation.
By supplying fresh Cs to surface in situ is possible to restore QE
K.L. Jensen et al., J. Appl. Phys., 102, 074902 (2007)
N.A. Moody et al., Appl. Phys. Lett., 90, 114108 (2007)
Dispenser rejuvenation
External Cs
source on
Silver
illuminated
with 375 nm
laser
Subsurface Cs source on W cathode
“Two modes of cathode operation
were demonstrated: periodic and
continuous rejuvenation.
The 1/e effective (continuous duty
cycle) lifetime in this mode was an
astounding 47 days.”
J. Montgomery et al., Proc of AAC 13° Workshop, 599 (2009)
N.A. Moody et al., Proc. FEL 2006, 748 (2008)
Dispenser controlled porosity
E.J. Montgomery et al., Journal of Directed Energy, 3, 66 (2008)
Thermionic single crystal
• Extremely reliable
Materials
Workfunction
(eV)
Current density
(A/cm2)
CeB6
2.4
73.1
LaB6
2.6
20.1
• Thermal emittance is on the
same order of magnidude of
photocathodes
• Is there any reason to try to
get pulsed instead of CW
emission?
SCSS gun CeB6 cathode
T. Shintake, EPAC 2006
http://www.a-p-tech.com/
Thermionic optically switched
MAX-lab linac injector
has been equipped
with a Ti:Sa laser to
operate the BaO
thermionic cathode as
photocathode.
When used as
photocathode BaO
temperature is lowered
from 1100 to 700 °C
S. Thorin et al. ,Nucl. Instr. Meth. A, 606, 291 (2009)
Diamond Window Amplifier
NEA surface to generate cold electron beam
Secondary electron to get high cuurent beam
Transparent
Conductor
(Sapphire w/ITO
or thin metal )
Laser
Diamond
Primary Secondary
electrons electrons
Thin Metal
Layer
(10-30 nm)
Hydrogen
Termination
Primary Secondary
electrons electrons
Photocathode
(K2CsSb)
3 – 10 kV
Diamond
(30 μm)
T. Rao et al., Adv. Acc. Concept,11th Workshop (2004)
Diamond Window Amplifier
Capsule mounting
has been
engineered;
Gain higher than
200 have been
measured;
Electron beam
amplified (gain 40)
transported in
vacuum outside
the diamond and
observed on a
scintillator screen
J. Smedley , Energy Recovery Linac Workshop 2009
Conclusion
• Still a lot of work to do:
– Lifetime and stability of emission
• Photocathodes
• Field emitter
– New ideas in making hybrids emitter
combining
advantages
of
different
emission mechanisms
Thank you for the attention!