QUASI TRAVELING WAVE SIDE COUPLE RF GUN FOR SUPERKEKB
Takuya Natsui#, Mitsuhiro Yoshida, Xiangyu Zhou, Yujiro Ogawa,High Energy Accelerator Research Organization (KEK)
1-1 Oho, Tsukuba, Ibaraki Japan
Abstract
We are developing a new RF gun for SuperKEKB.
High charge low emittance electron and positron beams
are required for SuperKEKB. We will generate 7.0 GeVelectron beam at 5 nC 20 mm-mrad by J-linac. In thislinac, a photo cathode S-band RF gun will be used as theelectron beam source. For this reason, we are developing
an advanced RF gun. We have tested a Disk and Washer(DAW) type RF gun. Additionally, another new RF gun
which has two side coupled standing wave field isdeveloped. We call it quasi traveling wave side couple RFgun. This gun has a strong focusing field at the cathodeand the acceleration field distribution also has a focusing
effect.

INTRODUCTION
The upgrade of KEKB to SuperKEKB is going on.
Since high luminosity is required in SuperKEKB,
improvement of beam emittance and charge is necessary.
Table.1 is upgrade parameter of e- and e+ beam.
Table 1charge [nC]
Emittance[mm-mrad]e- and e+ beam parameter
KEKB
SuperKEKB(e+/e-)(e+/e-)
1/1
4/5
2100 / 300
6 / 20We are developing a photo cathode S-band RF gun forhigh charge (5 nC) low emittance (20 mm-mrad) beam
generation. A thermionic cathode DC gun was used in
KEKB. However it is difficult to make a low emittancebeam with the gun. Thus RF gun must be installed
required for an electron beam source. However the
standard on-axis coupled 1.5 cell RF gun is not suitable
for this high charge beam. Because standard gun is used
up to about 1 nC by ordinary. If we obtain 5 nC in the gun,beam size will be too large. We have to consider bothbeam focus and emittance preservation. Thus it isnecessary to make a focusing field against the spacecharge in the cavities. But in this on-axis coupling cavity,
it is difficult to arrange the field freely on the axis becauseof on-axis. (Because beam hole is also coupling hole. )Thus annular coupling is acceptable.
We are testing Disk and Washer (DAW) type RF gun[1]. DAW cavity is an annular coupling cavity. Using thisgun, we evaluated the cathode of two types LaB6 or Ir5Ce.
As a result, we confirm that Ir5Ce is suitable for photo
cathode in terms of quantum efficiency and lifetime[2].
____________________________________________# takuya.natsui@kek.jp

This DAW type RF gun has strong focusing electricfield. In 5 nC beam charge, space charge effect is big
problem for beam transport in RF gun. The designed
cavity has electric field focusing. As a consequence, I
have achieved a beam generation of 4.8nC without anexternal solenoid magnetic field in this gun.
In this study, we confirmed that electric field focus(effect) technique is effective for high charge low
emittance beam generation. However, focusing is still notenough in this gun, generated beam still has divergenceangle. Since 5 nC is maximum output, this gun has no
margin. In addition, beam energy is still low (3 MeV).
Thus we have to consider the further emittancepreservation in beam transport.
We are developing a new advanced RF gun. It has new
acceleration scheme, we call it as a quasi-traveling wave.
In this method, higher accelerating field and stronger
focusing field are expected. It is very efficient
acceleration method. This quasi traveling wave cavity isrealized by using a two side couple cavities.

QUASI TRAVELING WAVE SIDE COUPLE
Annular coupled cavities as DAW or side coupledcavities are possible to make narrow acceleration gap.
The narrow gap makes the focus field. Our DAW RF gun
is using this focus field. Side coupled cavity also can bemade the narrow gap. However, these cavities have a long
drift space as Fig.1 (a) that shown normal side couple
cavities. Due to the long drift space, the DAW RF gun
generates beam with a divergence angle.

(a) Normal side coupled cavities(b) Quasi traveling wave side coupled cavities
Figure 1: Structure of the quasi traveling wave cavity

One solution is to use two standing wave cavities. If
two side coupled cavities which arranged staggered, we
obtain a double standing wave field as Fig.1 (b). These
two standing wave side coupled cavities are independent
electromagnetically. If we feed RF power with π/2 phasedifference, acceleration field is similar to traveling wave
for accelerated beam. Since two side coupled cavities are
possible to place on the same axis, a quasi-traveling wave
can be realized. Quasi-traveling wave can realize very
efficient beam acceleration and focusing.

Figure 2: First cavity shape and searched parameters

2D CAVITY DESIGNThe first cavity of RF gun is most important for beam
quality. Since beam energy of cathode cell is still low,
space charge affects beam size and emittance. First cavity
should be designed to have strong focus field. However
nonlinear component of the strong focus field causes
emittance growth. In addition, we must avoid the electricfield concentration at the cavity surface.
In the 2D cathode cell cavity design, a lot of parameters
shown Fig.2 were searched. The parameters were
optimized through the beam tracing simulation in thecavity design. Thus we need complicated procedure fordesigning. We automatized this procedure by using
downhill simplex method. The emittance, beam size andenergy spread were minimized under the conditions thatwere 100% transmission at 5 nC and surface electric fieldstrength of less than 120 MV/m. SUPREFISH and GPT(General Particle Tracer) calculation code were used forthis calculation.
Fig.3 is whole cavities structure design and electricfield (SUPERFISH result). This cavity shape is obtainedby using automatic downhill simplex method calculationin the condition with the maximum field strength of lessthan 100 MV/m in the main cavity. This RF gun has totalof seven acceleration cavities. These are divided into two
standing wave structure of 3 and 4 side coupled cavities
respectively. There are no couplings to next cavity on theaxis.
Figure 5 shows the beam tracking simulation for 5 nC
beam charge result; emittance is 5.5 mm-mrad: beam sizeis 0.4 mm (standard deviation) at exit of RF gun (z = 250mm). In the Fig.4, we can find that the beam size
becomes gradually smaller in the RF gun. This is caused
by not additional magnetic field but the focusing electricfield of RF gun. Beam energy will be 11.5 MeV with 20
MW RF input. The energy spread is 0.6 %. These results
satisfy the requirement in our application.
In addition, we confirm that this gun can generate 10nC beam generation by calculation; emittance is 10 mmmrad; beam size is 1.2 mm; energy spread is 1 %. It isenough margins.

Figure 3: Designed RF gun cavities (SUPERFISH
calculation result)

Figure 4: Beam tracking simulation result

3D CAVITY DESIGN
We used CST MICROWAVE STUDIO for 3D cavity
design. Figure 5 is the calculation result of the regular cellof a side coupled cavity. The acceleration mode andcoupling mode are adjusted to be same frequency.
Coupling value k is 3 %. This gun has two standing wave
cavities; we designed two types coupler as shown Fig.6.

(a) Accelerating made(b) Coupling mode
Figure 5: regular cell cavity calculation result

Figure 6: Two type couplers and calculation result
Figure 7 shows the whole cavity shape. The side couple
cavities of the two standing wave cavities are mounted as
90 degrees in the azimuthal angle. It has two ports for RF
feed. We use 90 degree hybrid for RF feed. We
manufactured a compact 90 degrees hybrid. It will bemounted RF gun directly. We already finishedmechanical design as shown in Fig.8.

type RF gun and quasi-traveling wave RF gun wasdeveloped for SuperKEKB.
A DAW type RF gun was tested. In the study of DAW
type RF gun, we achieved 4.8 nC beam generation. We
confirmed electronic field focusing technique in thecavity. However DAW type RF gun is still not enough to
our SuperKEKB operation.
Thus the new quasi-traveling wave side coupled RF
gun has developed. The quasi-traveling wave is a new
acceleration scheme. We have to use two standing wave
cavities for the quasi-traveling wave. Two side coupledcavities on same axis can realize it. It is suitable for thehigh charge and low emittance beam generation. Its
mechanical design is already finished. Fig.8 is photo of
manufactured cavity cell. This RF gun is ready to in readyto brazing.The quasi-traveling wave RF gun will be tested
soon.

Figure 8: Photo of manufactured cavity cell

REFERENCES[1] Takuya Natsui et al., TUPPD057, IPAC12, Louisiana, USA,2012.
[2] Daisuke Satoh et al., “Development of Better Quantum

Figure 7: Whole cavity shape

Efficiency and Long Lifetime IrCe Photocathode forHigh Charge electron RF Gun” MOPFI023, IPAC13,
Shanghai, China, 2013
[3] Xiangyu Zhou et al., “Ytterbium Laser Development of
DAW RF Gun for SuperKEKB” WEPME018, IPAC13,

Shanghai, China, 2013

Figure 8: mechanical design

CONCLUSIONHigh charge and low emittance electron and positron
beam are required for SuperKEKB injection. An RF gun
will be used for electron beam source. Required electron
beam parameter is 5 nC and 20 mm-mrad. Thus we aredeveloping a new photo cathode S-band RF gun for highcharge and low emittance beam generation. The annular
coupled cavity is suitable for this beam generation. DAW