Review of Recent Development of Photoinjectors
in Japan
Mitsuru Uesaka1^ Hokuto Iijima2\ Katsuhiro Dobashi2^ Jinfeng Yang 3\
Shuji Miyamoto4^ Akihiko Mizuno5^
^ Nuclear Engineering Research Laboratory, University of Tokyo, Tokai, Ibaraki, Japan
' National Institute for Radiological Science, 4-9-1, Anagawa, Inage, Chiba, Japan
3)
Sumitomo Heavy Industries Co,, 2-1-1, Tanido, Tanashi, Tokyo, Japan
4)
Himeji Institute of Technology, Sayo-Gun, Hyogo, Japan
5)
Japan Advanced Synchrotron Research Institute(SPring8), Sayo-Gun, Hyogo, Japan
Abstract. Systematic developments of the photoinjectors for ultrashort and high quality electron
beam works are under way in Japan. Sumitomo succeeded in transformation of the Gaussian
shape to the trapezoidal one in the temporal and transverse profiles of the drive laser and
achieved 0.9 Jtmm.mrad with 1 nC/bunch. It is the best data in the transverse aspect. Himeji Inst.
Tech. is operating very unique needle-shaped photocathode RF gun with the original Nd/Glass
laser
for
IR-FEL
and
Compton
scattering
X-rays.
U.Tokyo/JASRI(SPring8)/KEK/NIRS/BNL/etc. are developing and operating the S-band
photoinjectors with Cu, Mg and Cs2Te cathodes, and transmission-type one in near future.
Further, U.Tokyo/KEK/NIRS are designing and constructing a new X-band RF-gun/linac/laser
system to generate inverse Compton scattering hard X-rays(33-50keV) for intraveneous
angiography.
SUMITOMO HEAVY INDUSTRIES
A technique of laser pulse shaping was developed for low-emittance electron beam
generation by Sumitomo Heavy Industires [1-3]. The emittance growth due to space
charge and RF effects in the RF gun was experimentally investigated with square and
Gaussian temporal laser pulse shapes. The temporal pulse shaper was accomplished
through a technique of frequency-domain pulse shaping. The spectrum of the incident
femtosecond laser pulse was dispersed in space between a pair of diffraction gratings
separated by a pair of lenses ( Fig. 1). A computer-addressable liquid-crystal spatial
light modulator (LC-SLM) with 128 pixels was used as the phase mask. The
resolution of the phase shift on LC-SLM was near 0.01 jt. The pulse shaper was
located between the oscillator and the pulse stretcher to reduce the possibility of
damage on the optics.
The typical Gaussian and square-shaped temporal distributions of the UV laser
pulses with a pulse length of 9 ps FWHM are shown in Fig. 2. The data was measured
by an X-ray streak camera with a time resolution of 2 ps, resulting a rise time of 1.5 ps
for the square pulse shape. The pulse-to-pulse fluctuation of the shaped pulse length
was 7 %. The spatial profile of the laser beam on the cathode is shown in Fig. 3. The
beam spot size was 1.2 mm and 0.4 mm FWHM in the horizontal and vertical
CP647, Advanced Accelerator Concepts: Tenth Workshop, edited by C. E. Clayton and P. Muggli
© 2002 American Institute of Physics 0-7354-0102-0/02/$19.00
839
Photocathode
RFGun
+L_^
Electron
7
Bunch
Screen
Solenoid Magnet
Linac
Q Magnet
FIGURE 1. Experimental arrangement
(a)
X [mm]
Y [mm]
FIGURE 2. Temporal distribution of the
Gaussian (a) and square (b) laser pulse
lengthof9psFWHM.
FIGURE 3. Spatial profiles of the laser beam
in horizontal (a) and vertical (b) directions.
directions, respectively. The normalized rms horizontal emittance measured as a
function of the laser pulse length is shown in Fig. 4 for the Gaussian and square
temporal pulse shapes. The electron bunch charge was fixed at 0.6 nC and the solenoid
field was set to 1.5 kG which was optimal for compensating the space charge
emittance at 0.6 nC. The data shows that the emittance increases at shorter and longer
laser pulse length regions for both the Gaussian and square pulse shapes. This is
behaved in emittance growth due to space charge and rf effects. The normalized rms
horizontal emittance was also measured as a function of the bunch charge for the
Gaussian and square temporal pulse shapes with a pulse length of 9 ps FWHM, as
shown in Fig. 5. The measured data was fit as a function of
840
£=V(tf'02+6'2,
(1)
where a' is a fitting parameter referred to space charge force, and bf in jtmm-mrad is a
zero charge emittance. It is found that the square pulse shape reduced the space charge
force of about 50% comparing with the Gaussian pulse shape. Consequently, the
optimal normalized rms emittance of 1.2 jtmm-mrad at 1 nC was obtained by a square
temporal laser pulse shape with a pulse length of 9 ps FWHM.
0
2
4
6
8
10
12
14
16
0.5
1
Electron charge [nObunch]
Luerpuke length |ps]
FIGURE 4. The emittance versus laser
pulse length at 0.6 nC for the Gaussian
(triangle) and square (dot) pulse shapes.
FIGURE 5. The emittance versus bunch
charge for the Gaussian (triangle) and
square (dot) pulse shapes at a pulse
length of 9 ps FWHM.
PHOTO-NEEDLE RF GUN
A laser-excited RF-gun using a tungsten needle photocathode (Fig.6a) was
proposed. Design studies of gun performance with numerical calculations and a
preliminary experiment of a needle-RF-gun were performed. A tip radius of a needle a
low energy-spread and a relatively low field-emission current (Fig.6b). The results of
design studies for our 15MeV linac (LEENA) RF-gun indicate that the optimum tip
radius is 8.8 \Ji m, and a peak photocurrent of 92 A with a pulse width of ~ 6 ps and an
energy-spread of ~ 1 % can be obtained using a mode-locked Nd:YLF laser which has
a wavelength of 351 nm, a micropulse duration of 10 ps, a peak intensity of 330
MW/cm2, and an initial phase of 20 deg. In this design calculation, an enhanced
quantum efficiency of photo emission due to the high electric field [4] is assumed.
ONE CELL S-BAND RF GUN AT SPRINGS
In the SPring-8, a photo-cathode RF gun with a pill-box type single cell cavity (Sband) are studied. The cavity is shown in bottom right of the Fig. 9. A cavity wall
made of OFHC cupper is used as a photocathode. There are two rf ports in order to
avoid unsymmetrical filed distribution. The reasons of choice of the single cell cavity
841
macropulse current
time-averaged
photo-emission
RF input
field-emission
mode-locked
>ulsed UV laser
usec
needle ,
photo- /tip radius
cathode/ ~10 ^m
micropulse current
-175 psec
SCL current
\
irrent
RF-gui
cavity
:F tuner
field-emitted
current
(b)
20
90
180
laser injection phase [deg]
FIGURE 6. Schematic view and scenario of photo-current emission, (a) Schematic of a laserexcited needle-RF-gun system, (b) Field emission compornrnt in macropulse current and in
micropulse current.
200
J- 0
10
15
20
25
30
20
35
40
60
Initial phase [deg]
z-axis [mm]
80
100
FIGURE 8. Calculated peak photo-current
at the cavity exit
FIGURE 7. The calculated electric field
distribution and the field-strength on the
z-axis without a needle.
are as follows. Firstly, simple structure of the cavity is easily compared with
simulation. Secondly, the Q value can be designed low and a filling time becomes
short, so that high field on the cathode surface is easily achieved. The maximum field
on the cathode is 175 MV/m when the beam energy at the exit of the cavity is 4.1
MeV.
The schematic drawing of the experiment setup is shown in up left of the Fig.9. The
emittance can be measured by two slits scanning each other. Then x-xf data set of
beam current can be obtained by a Faraday-cup in the straight section. From these data,
each expected value of<^ 2 >, <x'2 > and <*•*'> can be calculated. Therefore, the
842
To
dummy
To
Todummy
dummy
load
load
load
coil
| Solenoid
Solenoid
coil
Solenoid
coil
RF
RF
Cavity
Cavity
Faraday cup
cup
Faraday
Faraday cup
Screen monitor
Screen monitor
Bending
magnet
Bending
Bendingmagnet
magnet
Wall-current
^Wal
-current monitor
Wall-current
monitor
X-V
slit
X-Y
X-Yslit
slit
Cu
cathode
Cucathode
cathode
UV-Laser
path
UV-Laser
UV-Laserpath
path
2424degrees
degrees
laser
laser port
port
totodummy
dummy
load
to
dummyload
load
rfrf
rf
FIGURE
cavity
and
an
experimental
FIGURE
setup
FIGURE9.9.9.AAAcavity
cavityand
andan
anexperimental
experimentalsetup
setup
Gaussian Beam
Flat Beam
FIGURE 10. The laser homogenizer
FIGURE10.
10.The
Thelaser
laserhomogenizer
homogenizer
FIGURE
normalized rms. emittance value can be obtained from the formula,
normalized rms.
rms. 2emittance value
value can
can be
be obtained
obtained from
from the
the formula,
formula,
normalized
=< Y >< j3 > V < x2 ><emittance
x' > - < x •
e =< g >< b > <2 x 2 ><¢2x¢2 > - < x ⋅¢x¢ 2>.2 .
e =< The
g >< laser
b > <system
x >< x consists
> - < x ⋅ of
x >a Ti-Sapphier laser oscillator, an amplifier with YAG
Thelaser
lasersystem
systemconsists
consistsofofa aTi-Sapphier
Ti-Sapphier laseroscillator,
oscillator, an amplifier
amplifier with YAG
YAG
The
pumping laser and a third harmonics generator.laser
Bad laser beaman
spatial profilewith
was one
pumping
laser
and
a
third
harmonics
generator.
Bad
laser
beam
spatial
profile
was
one
pumping
laserproblem
and a third
harmonics
laser
spatial
was one
of a serious
of the
SPring-8 generator.
system. AsBad
shown
in beam
Fig. 10,
a laserprofile
homogenizer,
of
a
serious
problem
of
the
SPring-8
system.
As
shown
in
Fig.10,
a
laser
homogenizer,
of a serious problem of the SPring-8 system. As shown in Fig.10, a laser homogenizer,
843
which consists of microlenz array, can improve the spatial profile. The emittance was
reduced by using this system.
The measured horizontal emittance values are plotted in the Fig.l 1, as a function of
beam charge. The data consists of three series that operation dates were different. Each
three series has a high reproducible. The minimum emittance is 2.3 paimmmrad with
charge of 0.2 nC, energy of 3.1 MeV. Also a fully-3D simulation code has been
developed in the SPring-8, good agreement with the measured value is shown in the
figure, in the region of less than 0.8 nC/bunch.
FIGURE 11. Emittance value as a function of beam charge
MG PHOTOINJECTOR BY U.TOKYO/SPRING8/KEK/ETC
Aiming to generate an electron bunch with tens nC, a Mg photocathode RF gun
had been constructed and installed to an S-band. Especially precise machining and
diamond polishing in order to decrease a dark current manufactured the RF gun. The
dark current after 22 days aging term was 600 pC/pulse for RF power of 6.6 MW.
Quantum efficiency of the Mg cathode was measured to be 1.3 x 10"4 before the laser
cleaning. Figure 12 shows a result of the generated charge as a function of the laser
energy. The maximum charge 4 nC/bunch was obtained for 210 \Ji J. Horizontal and
Vertical emittance are measured to be 80 paimm-mrad and 40 paimm-mrad for the
charge of 2 nC/bunch and beam energy of 22 MeV. Compressed bunch duration was
measured to be 0.7 ps (FWHM). Figure 13 shows typical bunch duration of the streak
camera image. To decrease timing drift and jitter between pump-beam and probe-laser,
fluctuation of temperature in the linac building was held on ±0.5 °C. The timing jitter
after improvement of air condition is being measured now. In future the laser cleaning
will be performed to realize dense bunch generation. In addition, radiation chemistry
experiment starts, using the RF gun.
844
|
/!'
3E= 1.3X 10-4|....._.
4
3.5
3
1
i
'........i.........r*ii
1
-----------J----------J----------J-------4- -----
r™-~~
r--- -----------t-----------p--------t^:-----l------
! V \
1.5
i
4444---------
L........
r-
\
...;X4---4---4----i---(.. . . . . j. . . . . .j. . . . . .j. . . . . .j. . . . . .
4
............J.............L............L.
n
Laser Energy [joj]
20ps
FIGURE 12.
12. The
The charge
charge as
as aa function
function of the
FIGURE
laser energy.
energy.
laser
40ps
50ps
FIGURE 13. Streak image
image of
of the
the Cherenkov
Cherenkov
light.
RF GUNS
GUNS FOR
FOR COMPACT HARD X-RAY SOURCE
RF
SOURCE FOR
FOR
ANGIOGRAPHY
Hard X-rays
X-rays of
of 10~50keV
10~50keV are now very useful
Hard
useful for medical
medical science,
science, biology,
biology,
material science
science etc. For
etc. For example, Dynamic Intravenous Coronary
material
Coronary Arteriography
Arteriography
(IVCAG) by
by aa high
high quality
quality monochromatic
monochromatic hard
(IVCAG)
hard X-ray
X-ray via
via Synchrotron
Synchrotron Radiation
Radiation
(SR) isis proposed
proposed and
and tested
tested in
in some
some institutes.
institutes. Most
SR sources
(SR)
Most of
of SR
sources are
are too
too large
large to
to
apply
spread
use
of
IVCAG.
Then,
we
are
going
to
develop
a
compact
hard
apply spread use of IVCAG. Then, we are going to develop a compact hard X-ray
X-ray
(10-50 keV)
keV) source
source based
based on
on Laser-electron
Laser-electron collision
collision using
(10~50
using by
by X-band
X-band (11.525GHz)
(11.525GHz)
linac
system
for
dynamic
IVCAG.
The
X-band
linac
is
introduced
linac system for dynamic IVCAG. The X-band linac is introduced to
to realize
realize very
very
compact
system.
compact system.
Compact hard
hard X-ray
X-ray source
source based
based on
on X-band
X-band linac
linac that
Compact
that we
we propose
propose is
is shown
shown in
in
Figure
14.
Multi-bunch
beam
generated
by
thermionic-cathode
RF-gun
is
accelerated
Figure 14. Multi-bunch beam generated by thermionic-cathode RF-gun is accelerated
by X-band
X-band accelerating
accelerating structure.
structure. The
The beam
beam is
is bent
by
bent and
and focuses
focuses at
at the
the collision
collision point.
point.
We
are
going
to
design
a
thermionic-cathode
RF-gun
and
Photo-cathode
We are going to design a thermionic-cathode RF-gun and Photo-cathode RF-gun.
RF-gun.
We have
have performed
performed aa fundamental
fundamental design
design for
for the
We
the X-band
X-band photo-cathode
photo-cathode RF-gun
RF-gun using
using
the
PARMALA
code.
Numerical
analysis
of
beam
transport
for
whole
the PARMALA code. Numerical analysis of beam transport for whole system
system
including photo-cathode
photo-cathode X-band
X-band RF-gun
RF-gun and
and X-band
including
X-band accelerating
accelerating structure
structure is
is already
already
presented[5].
Beam
parameters
at
the
collision
point
(C.P.)
are
shown
in
Table
presented[5]. Beam parameters at the collision point (C.P.) are shown in Table 1.
1.
845
Laser Dump
Bending Magnet
Laser
Laser Dump
Dump
Alpha
AlphaMagnet
Magnet
Q-Magnet
Multi-bunch Electron Beam % ^Bending
Magnet
Bending
Magnet
X-band Accelerating Structure
Q-Magnet
Q-Magnet
Alpha Magnet
X-band
X-band Accelerating
Accelerating Structure
Structure
X-band
X-band
Power
Source
Power Source
h he r r
i t c t cs s e
w wLi a L a
- S SG
Q QYA- A G
Y
d: :
N Nd
Thermionic cathode
X-band RF-gun
Thermionic
cathode
Thermionic
cathode
X-band
X-bandRF-gun
RF-gun
Timing
Timing
System
System
Collision Point
Collision
Point
Beam
Dump
Collision
Point
Beam Dump
BeamDump
Mirror
Mirror
Lens
Laser light^ N£ Lens
(1064light
nm)
Laser
Hard X-ray(33keV)
(1064
nm)
(1064 nm) Hard X-ray(33keV)
, x^
Hard X-ray(33keV)
FIGURE 14. Schematic illustration of Compact Hard X-ray source based on termionic-cathode X-band
FIGURE
14.Schematic
Schematic
illustration
ofand
Compact
HardNd:YAG
RF-gun,
X-band
accelerating
structure
Q-switch
laser.based on termionic-cathode
FIGURE
14.
illustration
of
Compact
Hard
X-ray source
termionic-cathode X-band
X-band
RF-gun,X-band
X-bandaccelerating
accelerating structure
structure and
and Q-switch
Q-switch Nd:YAG
Nd: YAG laser.
RF-gun,
Beam energy
|Beam
Beamenergy
energy
Charge/bunch
|Charge/bunch
(^ge/himch
Bunch
length (FWHM)
Bunch
length (FWHM)
Beam size(rms)(x,y)
|Beam
Beamsize(rms)(x,y)
size(rms)(x,y)
Beam emittance(x,y)
|Beam
emittance(x,y)
Beam emittance(x,y)
Momentum
spread
|Momentum
Momentum spread
spread
56 MeV
156
56 MeV
900MeV
pC
1900
900 pC
19 ps
|l9ps
19
77,ps
77 mm
177,
77, 77
77 mhn
p mm mrad
6.3, 6.2 m
16.3,
6.3, 6.2
6.2 p^mm
mrad
mm mrad
0.03
0.03
0.03
TABLE 1. Beam parameters at the collision point[5]
TABLE 1.
1. Beam
Beam parameters
parameters at
at the
the collision
collision point[5]
TABLE
point[5]
10 20 30 40 50 60 70 80 90 100
X-ray energy [keV]
X-ray energy [keY]
FIGURE15.
15. Energyspectrum
spectrumand
and energy (left
(left figure
figure )) vs.
scattering
angle
X-ray
FIGURE
vs. scattering
scattering angle
angle(right
(rightfigure)
figure)of
X-rayin
FIGURE
15. Energy
Energy spectrum
and energy
energy
(left figure
) vs.
figure)
ofof of
X-ray
inin
singlebunch(20pC/bunch)
bunch(20pC/bunch)
collision
with Q-switch
Q-switch
Nd:YAG-laser(2J/pulse)
.(right
Scattering
angle
0 0rad
single
collision
with
Nd:YAG-laser(2J/pulse)
.
Scattering
angle
of
rad
single
bunch(20pC/bunch)
collision with Q-switch Nd:YAG-laser(2J/pulse) . Scattering angle of 0 rad
direction
of electronbeam.
beam.
isisisdirection
directionof
ofelectron
electron beam.
To concentrateon
onR&D
R&D of
of the accelerator,
accelerator, we
To
we use
use existing
existinglaser
lasersystem
systemfor
forlaserlaserTo concentrate
concentrate
onTo
R&D
of the
the
accelerator,
we
use
existing
laser
system
for
laserelectron
collision.
realize
simple
and
compact
system,
we
apply
a
Q-switch
electron
collision.
To
realize
simple
and
compact
system,
we
apply
a
Q-switch
electron
To realize
simple
and compact
system, we
apply a Q-switch
Nd:YAGcollision.
laser with
with
intensity
2J/pulse,
repetition
lOpps,
Nd:YAG
laser
intensity
2J/pulse,
repetition rate
rate 10pps,
10pps, pulse
pulse length
length
Nd:YAG
laser
with
intensity
2J/pulse,
repetition
rate
pulse
length
lOns(FWHM),
and
wavelength
1064nm,
which
is
commercial
product.
10ns(FWHM),
and
wavelength
1064nm,
which
is
commercial
product.
10ns(FWHM), and wavelength 1064nm, which is commercial product.
846
We choose
choose aa very
very simple
simple system
system by
by focussing
focussing on
on only
only averaged
averaged X-ray
X-ray flux.
flux. We
We
We
construct the
the system
system with
with the
the thermionic-cathode
thermionic-cathode RF-gun
RF-gun (20
(20 pC/bunch,
pC/bunch, ~10
~1044
construct
bunches/RF-pulse, 10pmm
mrad) and
and Q-switch
Q-switch Nd:YAG
Nd:YAG laser
laser (2J/pulse,
(2J/pulse, pulse
pulse length
length
bunches/RF-pulse,
lOjtmm mrad)
10ns
in FWHM,
FWHM, repetition
repetition rate
rate lOpps).
10pps).
10ns in
We assume
assume head-on
head-on collision.
collision. This
This system
system generates
generates X-rays
X-rays with
with l.VxlO
1.7x1077
We
88
photons/pulse (l.VxlO
(1.7x10 photons/s)
photons/s) that
that is
is sum
sum of
of each
each bunch.
bunch.
photons/pulse
Gun type
type
Gun
Thermionic
Thermionic
-cathode
-cathode
Electroon beam
beam
Electroon
20pC/bunch
20pC^unch
4
10
bunches/pulse
10 bunches/pulse
Laser
Laser
Q-switch Nd:YAG
Nd:YAG
Q-switch
2J/pulse, 10ns,
10ns, lOpps
10pps
2J/pulse,
X-rayyield
yield (photons)
(photons)
X-ray
8
77
1.7 10
/pulse (1.7x108/s)
/s)
1.7xl0
/pulse(1.7xl0
( 430nW)
(430nW)
Photo-cathode
Photo
-cathode
500pC/bunch
500pC/bunch
20TW Ti-Sapphire
Ti-Sapphire
20TW
1J/pulse, 50fs,
50fs, lOpps
10pps
U/pulse,
1.6 107 /pulse(1.6xl0
/pulse (1.6x108 /s)
/s)
1.6Xl0
Photo-cathode
Photo
-cathode
500pC/bunch
500pC/bunch
Photo-cathode
Photo
-cathode
(Multi-bunch)
(Multi-bunch)
500pC/bunch
?0pp^/bunch
20bunches/pulse
20bunches/pulse
Nd:Glass
2.1 108 /pulse
/pulse
Nd:Glass
2.1Xl0
10J/pulse, 10ps,«lpps
10ps, <<1pps
lOJ/pulse,
6.3 N ）/pulse
115nJ/bunch+Super-cavity
5nJ/bunch+Super-cavity （
f3x7V)/pulse
(15 N )nJ/bunch, 7ps
7ps
(15XAOnJ/bunch,
7
8
8
TABLE 2.
2. X-ray
X-ray yields
yields of
of various
various laser
laser system
system
TABLE
Energy distribution
distribution and
and angler
angler distribution
distribution of
of generated
generated X-ray
X-ray isis shown
shown in
in Figure
Figure
Energy
15.
spectrum calculated
calculated by
by Klein-Nishina's
Klein-Nishina’s formula
formula and
and
15. Solid
Solid line
line indicates
indicates spectrum
Luminosity calculation, and Histogram
Histogram shows
shows the
the result
result of
of beambeam- beam
beam interaction
interaction
Monte-Carlo simulation code
code CAIN.
CAIN. X-ray
X-ray energy
energy reached
reached to
to 57keV
57keV at
at beam
beam energy
energy
56MeV.
Dynamic image
of coronary artery
<3m
X-band Klystron Gun >C-baiid acceleratmg structure
2D X-ray detector
<5m
FIGURE
FIGURE 16.
16. Final
Final target
target of
of this
this work.
work.
847
X-ray yields of various laser system is summarized in Table 2. The system with
Thermionic-cathode and Q-switch laser is not only very simple and compact but also
can generate high flux X-ray with intensity 108photons/s. To achieve 10nphotons/s
required for dynamic IVCAG, we use technique of circulation of laser light, which
enhance luminosity 10 times. Laser power and repetition rate must be reached to
lOJ/pulse and 50pps.
Final target of this study is the integrated system for dynamic IVCAG shown in
Figure 16. This system has X-band RF-source and moving arm including X-band linac,
Q-switch Laser system and X-ray detector. We can perform dynamic IVCAG easily
and can get clear dynamic image of coronary artery with less distress for patient.
REFERENCES
1. J. Yang et al., J, Appl Phys. 92, 1608 (2002).
2. J. Yang et al., Proceedings of EPAC2002, 3-7 Jun. 2002, Paris,France.
3. J. Yang et al., NucL Instrum. Meth. A, to be published.
4. T. Inoue, S.Miyamoto, S.Amano, M.Yatsuzuka, T.Mochizuki, submitted to Jpn, J, Appl Phys. "Enhanced
Quantumefficiency of Photocathode under High Electric Field",
5. H.Iijima, et al., Proceedings of EPAC2002, 3-7 Jun. 2002, Paris,France.
6. K. Dobashi, et al., Proceedings of EPAC2002, 3-7 Jun. 2002, Paris,France.
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