Development of an electron gun for the ERL light source in Japan

1
Development of an electron gun for the ERL light
source in Japan
ERL07 WG1 Session 1 DC Guns

• N. Nishimori, R. Hajima, H. Iijima, R. Nagai
• Japan Atomic Energy Agency (JAEA)
• ERL Development Group
• T. Nishitani
• RIKEN

May 22, 2007.
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JAEA,KEK,ISSP Collaboration for an ERL Light
Source
As a first step of the collaboration, we started
to develop key technologies for the ERL, and are
planning to construct together an ERL test
facility at KEK site.
Tentative parameters of the ERL Test facility
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An EUV ERL SASE FEL for semiconductor lithography
580MeV energy recovery loop
dump (10MeV, 120kW)
SCA 1
Bunch compressor
undulator
EUV FEL
SCA 2
RF
80m
Single pass SASE FEL
10MeV Injector
Acceleration
R. Hajima et al., EUV source workshop, Barcelona
Spain October 2006.
Deceleration
beam parameters
FEL parameters
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Outline

• status of 250 kV 50 mA gun development
• simulations of beam envelopes and planned
  diagnostics
• a photocathode test bench
• summary

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Development of a 250 kV-50 mA DC gun
A DC gun is under development.
high voltage test without beam loading
main chamber
load-lock chamber
preparation chamber
H. Iijima et al., Proc. Acc. Meeting in Japan
(2006)
solenoid
beam diagnosis
e-beam
R. Nagai et al., Proc. Acc. Meeting in Japan
(2006)
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Gun chambers
.
e-beam
load-lock system
load-lock chamber
preparation chamber
main chamber
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Gun vacuum
.
vacuum 3 x 10-10 Pa (design value)
all the chambers are made of Titanium.
Ion 500l/s NEG 2000l/s
Ion 200l/s NEG 2000l/s
e-beam
Turbo 300l/s Cryo 1700l/s
Ion 500l/s NEG 2000l/s
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Gun electrode
all the electrodes are made of Titanium.
40 mm
40 f
cathode holder
1 mm
e-beam
cathode
laser
120 f
8 f
anode
cathode holder
anode
180 f
Anodization has not been applied.
The electric field is 5 MV/m.
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Design of emittance compensation solenoid
solenoid magnet
minimum emittance and drift length
77pC ? en 0.59 mm-mrad
2.5
1.4
1.2
mrad
2.0
main coil
m
1.0
?
mm
1.5
0.8
drift length
0.6
1.0
emittance
0.4
0.5
0.2
Bucking coil to compensate Bz at the cathode
surface
0.0
0.0
220
230
240
250
260
270
280
solenoid field
Gauss
emittance of a 250-keV electron bunch (PARMELA)
0.59 mm-mrad for 77 pC 0.11 mm-mrad for 7.7 pC
(initially, Gaussian in longitudinal and uniform
in transverse)
z (cm)
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Simulated transversal envelope for 250keV 77pC
beam
emittance
1 mm mrad
1 m
2 m
sz 60 ps
Beam size
1 mm
slit scan tool
solenoid
cathode
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Space charge and emittance dominant beamlet
rms beam envelope equation for a relativistic
beam in a drift space
en2
I
sx

g2b2sx3
g3b3I0(sxsy)
where I is the peak beam current, I0ec/re
is the characteristic current en is the
normalized rms emittance
Id2
2
250keV (g 1.49, b0.74) 77pC, 25ps (2s),
3A en0.3 mm-mrad I017,000 A
(re2.82x10-15m) d20mm Rb0.33
Rb
gbI0 en2
3p
25 mm slit is already available at SPring8
Compact SASE Source.
K. Togawa et al., PRST AB 10, 020703 (2007).
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Bunch length measurement using a kicker cavity
500MHz TM010 cavity shunt impedance 2.5MW
transit time factor 0.93 3.7kW RF power is
needed
250keV
20keV
-20keV
100ps
8kW 500MHz RF power supply
500MHz kicker cavity
f 72
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emittance of NEA photocathode
CB
emittance is a function of laser wavelength
and laser spot size.
100keV-10mA
vacuum
B.M. Dunham et al., PAC-1995, 1030.
VB
120keV-10nA
0.1mm-mrad is available at small current
Longer wavelength is better for low emittance
emittance vs laser wavelength
N. Yamamoto et al. Proc. Acc. Meeting in Japan
(2006)
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a photocathode test bench at JAEA
cathode holder
Optimization of cathode material for the better
QE and life.
Cs holder
heater
cathode holder
UHV chamber and laser
photocathode prepared at Nagoya Univ(1).
bulk-GaAs, bulk-AlxGa1-xAs
x0.17, 0.28
(1) Venture Business Laboratory, M. Tabuchi, Y.
Takeda et al.
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Performance of bulk-GaAs and bulk-AlGaAs
Quantum Efficiency
Cathode Lifetime
gun voltage 200V, current 100nA, photon energy
band gap 0.2eV

• Al0.28Ga0.72As
• GaAs

twice Q.E.
gt10-times longer life

• Al0.28Ga0.72As
• GaAs

Bulk-AlGaAs shows higher QE than bulk-GaAs
Bulk-AlGaAs shows longer life than bulk-GaAs
Larger band gap and higher JDOS ? more
efficient excitation of electrons
electron affinity GaAs 4.1 eV
Al0.28Ga0.72As 3.8 eV
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How to design a long-life cathode
vacuum
without Cs
with Cs
effect of Cs
?eff
?eff
fD
?
vacuum
CB
CB
fBB
Eg
Eg
activation of NEA
VB
VB
effective electron affinity
effective electron affinity
damage on the Cs layer ? a rise of vacuum
potential
cathode material with smaller c is preferable
for keeping the NEA state (ceff lt0)
c 4.1 eV (GaAs), c 3.8 eV (Al0.28Ga0.72As)
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band gap energy of AlxGa1-xAs
Band gap is variable from 1.42 to 2.17 eV.
Egap
QE
direct transition
Yb 2w
photon energy
indirect transition
NOPA
thermal emittance (thin cathode)
Tisap.
fabricated at JAEA/Nagoya-U.
ref GaAs and Related Materials Sadao Adachi,
World Scientific (1994)
same for a thick cathode ?
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Summary

• We have initiated RDs for a photocathode DC gun.
• a 250kV-50mA DC gun is under development.
• normalized emittance is expected to be
  0.6mm-mrad for 77pC, 0.1mm-mrad for 7.7pC at
  250-keV.
• the first beam from the gun will be this summer.
• QE and life have been measured for bulk-GaAs and
  bulk-AlGaAs.
• Bulk-AlGaAs shows better QE and life as predicted
  by semiconductor theory. (QE 2x, life 10x )