HIGH CHARGE LOW EMITTANCE RF GUN FOR SUPERKEKB
Takuya Natsui #, Mitsuhiro Yoshida, Xiangyu Zhou, Yujiro Ogawa
High Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba, Ibaraki 305-0801 Japan
Abstract
We are developing a new RF gun for SuperKEKB. We
are upgrading KEKB to SuperKEKB now. High charge
low emittance electron and positron beams are required
for SuperKEKB. We will generate 7.0 GeV electron beamat 5 nC 20 mm-mrad by J-linac. In this linac, a photo
cathode S-band RF gun will be used as the electron beamsource. For this reason, we are developing an advanced
RF gun. Now, we are testing a Disk and Washer (DAW)
type RF gun. Its photo cathode material is LaB 6 or Ir5Ce.
Normally, LaB6 or Ir5Ce are used as a thermionic cathode,but they are suitable for long-life photo cathodeoperation. This gun has a strong focusing field at thecathode and the acceleration field distribution also has afocusing effect. We obtained 3.0 nC beam charge with theRF gun.

INTRODUCTION
We are currently upgrading whole system of acceleratorfor SuperKEKB. Since high luminosity is required inSuperKEKB, improvement of beam emittance and charge
is necessary. Table.1 is upgrade parameter of e- and e+beam.
Table 1：e- and e+ beam parameter
Charge [nC]
Emittance [mm-mrad]
e1 to 5300 to 20e+
1 to 42100 to 6

DAW TYPE RF GUN
Cavity design
The idea of DAW type accelerating cavity is older
method of accelerator. However, DAW type cavity wasoff from practical use, since it has complicate structure.
Fortunately, the machining and calculation technicalimprovement of these days makes it possible to develop aDAW type accelerating cavity. We adopt DAW type RF
gun to achieve high charge low emittance beamgeneration.
The RF gun has four cavities. These cavities havestrong coupling value. It has cathode rod, 3 disks andcoaxial coupler as shown Fig.2. The cathode diameter is 6mm. The accelerating field distribution is generated asshown Fig.3 which is resonant mode calculation by using
SUPERFISH. Fig.4 is calculation result of electric field
on axis. This field distribution makes Alternative Phase
Focus (APF). Beam is focused by this APF effect.

We are developing a RF gun for high charge (5 nC) lowemittance (20 mm-mrad) beam generation. This RF gun isnot typical 1.6 cell RF gun but Disk and Washer (DAW)
type RF gun. This gun has strong focusing RF electricfield to cancel space charge effect from 5 nC high beamcharge. Fig.1 is a schematic view of KEK accelerator
system. The gun will be installed at start point of J-linac.
First, we construct a test beam line at middle of J-linac (32 section). This beam line is constructed on the angle.
J-linacee+PF
2.5~3.0 GeVHER
7 GeVFigure 2: Cut view of DAW type RF gun

SuperKEKB
LER
4 GeVAR
6.5 GeVFigure 1: schematic view of KEK accelerator system___________________________________________#takuya.natsui@kek.jp

Figure 3: 2D resonant mode calculation (E-field)

Additionally, Ir5Ce has work function that is lower than
LaB6 and longer lifetime. There are few actual
performances of Ir5Ce. We are testing these cathodes.

RF GUN BEAM LINE

Figure 4: E-field distribution on-axis

Beam tracking simulation
Beam tracking simulation was run with General
Particle Tracer (BPT). Emission beam parameter is 30 psbunch length and 6 mm diameter. The simulation result isshown as Fig.5 and Fig.6. Beam energy is 3.4 MeV. Fig.5
shows that the beam focused at nearby cathode. This gun
has very thin beam pipe, however 5 nC high charge beam
was transmitted without external magnet. Emittance is 6mm-mrad in this simulation. Target value of end of linac
is 20 mm-mrad. Thus less than 10 mm-mrad of beamemittance is required in the RF gun. This simulation result
shown that this gun has sufficient performance.

The RF gun is installed at middle of linac beam line.
Thus we designed an injection beam line on the angle.
This beam line has the gun, solenoid, chicane, 1 m
traveling wave accelerating structure, doublet and bendmagnet as shown fig.7. The output beam from gun isdefocusing beam. A solenoid is needed nearby RF gun.
Next component is chicane. It can perform bunch
compression. Since this is optional, first beam study is
without bunch compression. After chicane, beam isaccelerated to 10 MeV with 1 m S-band traveling wave
accelerating structure. Because 3.4 MeV beam is notsuitable to transport. The angle is 26.56 degree(arctan1/2). Next of bend magnet is an existing beam line.
This gun will be used as PF injection electron source after
high charge beam study.
The RF gun photo is shown as Fig.8. Fig.9 is photo of
constructed injection beam line. In this line, there is twowaveguides. These waveguides lead to different klystrons.
26.56 deg

80

Gun80

80

11050100

250180300

50

210030030Solenoid

ACC
170 mT

ChicaneBend 202.2 mTmagnet

Figure 7: Injection beam lineFigure 5: Beam size in the RF gun

Figure 8: Photo of RF gun
Figure 6: Beam emittance in the RF gun

Photo cathodeWe select LaB6 or Ir5Ce as photo cathode. Normally,
LaB6 is used as thermionic cathode. However work
function of LaB6 is lower than metal. Thus quantum
efficiency (QE) is suitable and it can be used as photo
cathode with high power laser system. An advance pointof LaB6 is long life time. For SuperKEKB, stable beam
generation is necessary. Therefore solid type long lifetime
photo cathode is suitable.
Figure 9: Photo of Injection beam line

BEAM STUDYCONCLUSION

We were starting beam study from September 2011.
First, we use LaB6 cathode. We achieve 1.2 nC beam
charge with LaB6. We performed a laser cleaning with532 nm laser. However, QE of LaB6 cathode is notenough. It was under 1.0e-5. Laser cleaning was effective.
However the QE is reduced immediately. Furthermore wefaced breakdown problem. When we input over 0.5 mJ
laser pulse, breakdown rate at cathode was too high tocontinue beam study.
The cathode was replaced to new Ir5Ce cathode asshown Fig.10. In this cathode, breakdown problem camegood.

We are developing a photo cathode RF gun for
SuperKEKB. Final goal of electron beam parameters is 5nC 20 mm-mrad. The RF gun has DAW type cavities.
Since 5 nC beam has strong space charge effect, we
designed the gun which has a strong focusing RF electricfield. As a result of simulation of this gun, 5 nC 6 mmmrad beam generation was achieved.
We tested LaB6 and Ir5Ce as photo cathode. Finally, wechoose Ir5Ce cathode. Its QE is 4e-5. Laser system wasconstructed for photo cathode. Its pulse length is 30 psec
and pulse energy is 1.5 mJ at 5pps.
We achieve a 3.0 nC beam generation with RF gun.
Beam study is continued. We aim increase a beam charge
and measurement of beam emittance.

REFERENCES

Figure 10: Ir5Ce cathodeWe were testing a two way laser injection angle which
is 0 degree injection and 60 degree injection. The angled
injection was expected to improve QE by schottky effect.
As a result of RF gun beam measurement, 60 degree
injection is effective for Q.E. increase. The measured QEof this cathode is 4e-5 without laser cleaning. We
achieved 3.0 nC beam generation from the gun. This
beam can be transported to end of J-linac. Fig.11 is BPM
measurement data of beam transport. Beam energy fromRF gun is 3.4 MeV with 4.0 MW RF at 2 pps. 1.5 mJ
laser pulse energy is generate at 266 nm wave length. On
the other hand, the energy is 0.5 mJ at cathode. There isbig power loss at laser transportation. It should beimproved.

Figure 11: Operation log of 3.0 nC beam generation[1] Takuya Natsui, Mitsuhiro Yoshida, Xiangyu Zhou, Yujiro
Ogawa, “DEVELOPMENT OF HIGH CHARGE LOW
EMITTANCE DAW TYPE RF GUN FOR SUPERKEKB”
Proceedings of the 8th Annual Meeting of Particle
Accelerator Society of Japan 2010[2] Mitsuhiro Yoshida et al., “SuperKEKB Injector

Upgrade for High Charge and Low EmittanceElectron Beam” TUPPD035, these proceedings.