Proceedings of the 20th Linear Accelerator Meeting in Japan (September 6—8,1995, Osaka, Japan)- 15

DESIGN OF THE BUNCHING SYSTEM OF THE KEKB LINAC
J-Y. Choi, A. Enomoto, S. Ohsawa, Y. Ogawa, T. Kamitani, I. Sato
KEK, National Laboratory for High Energy Physics
1-1 Oho, Tsukuba, Ibaraki, 305 JapanA B STRA CT
As the pre-injector for the KEK B-factory, the injec­
tor of the PF-2.5 GeV Linac is being upgraded so as
to include subharmonic bunchers. Its main purpose
is to produce single bunches of high intensity of morethan 10 nC. The design of the bunching system wasperformed using PARMELA. The design results haveshown that the newly designed bunching system canproduce single bunches of more than 15 nC within 10ps with satisfactorily low emittance growth. *IN T R O D U C T IO N
The injector linac for the KEK B-factory(KEKB)
is reauired to accelerate a single bunch electron beamof more than 10 nC to the positron target to obtainrequired positron intensity[1]. Considering a possi­
ble charge loss due to wake field at regular accelerat­
ing sections, we make it the goal to produce a single
bunch of more than 15 nU in the bunching system.
Thus far, the bunching system including elec­tron gun has been constantly upgraded for KEKB
project[2, 4]. Preliminary studies have been madeof the bunching and acceleration of high intensity
single bunch electron beams[3, 4]. From simulation
studies and extensive acceleration studies performed
using one subharmonic buncher of 470 MHz, it has
been clear that for charges more than 10 nC, single
bunch beams are difficult to obtain without satellite
bunches.
Double subharmonic buncher system was adopted
with one additional subharmonic buncher with lower
RF frequency. The frequencies of the subharmonicbunchers(SHBl, SHB2) have been selected in connec­
tion with the frequencies of the linac and the KEKB
ring, 114.24 MHz, and 571.2 MHz[3], 25th and 5thsubharmonics of S-band, respectively. In this paper,
the results of the design of the bunching system in­cluding subharmonic bunchers with PARMELA aredescribed.

B U N C H IN G SYSTEM AND B U N C H IN G
BEHAVIOR

In this section, the optimum simulation result withPARMELA will be described with a brief account of
the entire bunching system. The calculations withPARMELA were made from SHB1 for simplicity.
The debunching by space charge force in drift spacefrom the gun to the subharmonic bunchers is negli­
gibly small in our gun energy, 200 keV\ and as will
be seen, in the next section, emittance growth in thisregion can be nearly prevented by a suitable focusing
magnetic field.
The new bunching system including subharmonicbunchers is shown in F ig .1 . The components fol­lowing the first prebuncher (PB1) has been alreadyupgraded[2, 4].
The prebunchers are of traveling wave type(27r/3mode,/? = 0.7), and consist of seven and fivecavities, including RF couplers, respectively. Thebuncher consists of the capture section of six cavi­ties, where the phase velocity varies from 0.7c to thevelocity of light, followed by 29 cavities with the ve­locity of light. Concerning buncher, the electric field
will be strengthened from 15 MV/m to 20 MV/m,
to improve ounching efficiency and to mitigate wakefield effects in the buncher.
As mentioned above, it is our goal in the bunching
section to produce a single bunch electron beam morethan 15 nC at the exit of the buncher. Taking accountof the loss in the bunching process, the total charge ofthe pulse from the electron gun was chosen to be 20nC. It is important to provide the beam with a linear
energy modulation at SHB1, since this will make easythe‘ bunching at SHB2. The initial electron beampulse used in the simulations is 2 n s ,10 A. Our gun
can produce more than 10 A.
The maximum energy modulation by SHB1 is cho­sen to be 40 kV. The modulation voltage is selectedso as to keep the peak power low for two reasons. Thefirst one comes from power source; the adoption of asolid state power amplifier for 114.24 MHz is desir­able for simplicity of manufacturing and operation.
For this purpose, the lower peak power is favorable.
The other one is directly related to the bunching ef­
ficiency. For the electron gun for high intensity elec­

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Figure 1 : Bunching system of KEKB linac. The positions of SHB1, SHB2, PB1, and PB2 are shown with respect
to the starting point of calculation. ML:Magnetic lens, WCM:Wall current monitor, FC:Focusing coil CM:CoreMonitor, PRMrProfile monitor, STC:Steering coil

co2i0<7f5(e21At the entrance of the buncher, the bunch length
was 70o of S-band. The beam was bunched to 25° m
the first six cavities. A further bunching continued inthe regular cavities. The various characteristics of thebunch at the exit of the buncher are shown in Fig. 2.
The total transmission rate is 99 percent. Eighty
two percent of the initial charge is bunched into 10(D1The optimum voltage which SHB2 should provide
for the compressed beam was 80 kV. With this mod­ulation, the beam was compressed to about 200 de­grees at the entrance of PB1, located at ol cm down­wards from the SHB2 gap of length 3 cm. The electricfield in PB1 is 0.35 MV/m for the optimum bunch­ing. The drift space between PB1 and PB2 is 15.9cm, which had been fixed from the configuration of
the RF waveguide and focusing coils. The optimum
longitudinal focal point was about 10 cm after PB1,and as a matter of fact, with above distance, debunchmg has occurred. A compression of a factorof three was achieved by PB1 and PB2. The second
prebuncher and buncher are mechanically joined tominimize drift space between them, whereas they areseparated in electromagnetically.

Phase (deg)s lov ^ kd io .o^At 200 cm from SHB1 gap, the beam enters SHB2
in a little more broad range than ±110° of RF wave ofSHB2, and the beam at this point has a longitudinalcharge distribution of nearly triangular shape. Fur­thermore, the modulated energies from SHB1 vanlsnes owing to the repulsive force of space charges
as the beam bunches, and at the gap of SHB2, theenergies of the particles as a whole converge to 200keV.

degrees of the final bunch. Fifty one percent of theinitial charge is contained in 4 degrees at FWHM.
About four percent of the initial charge was formed
into satellite bunches.

(.
dmv)Itron beams, there is a tendency to raise gun voltage
to alleviate the space charge effects in the low en­ergy regions like the bunching section. In bunchingwith these electron guns, a high energy modulation
results in a velocity asymmetry among the head andtail parts of the beam pulse because of the relativisticeffects. Therefore, we lowered the energy modulation
at SHB1 to the limit that the satellite bunches might
not occur in the subsequent bunching.

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Figure 2: Bunch characteristics at the exit of thebuncher. (a) longitudinal bunch shape (b) energyspreadT R A N SV ER SE D YNAM ICS
Emittances of high intensity beams can increase
largely in the bunching section by space charge ef­
fects. Though the emittance value requirements for
the KEKB linac have been specified at the end of thelinac (1000 7r.mm. mrad) and none for the bunching{p j e.e e.i o s uwmO O U B U I E

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To suppress emittance growth further, a stronger
focusing field is necessary. By raising the magnetic
field by 20% at the buncher, the emittance was low­ered below 50 7r*mm* mrad, and it is shown in Fig. 4.
In this case, the emittance value reduced by half at
the cost of the longitudinal bunching efficiency. We
could, however, still have a good bunching; 50%, 74%,
and 86% of the initial charge was contained in 4 ° ,10。，
and 15° around the peak, respectively. The variationof the horizontal radius in this case is shown in Fig. 5
It follows from the above result that we can get ansufficiently low emittance beam for KEKB at the exit

20 0300400

Z (c m )Figure 5: The variation of the radius with bunchingin case stronger focusing coils.
of buncher with a little change of focusing coils.

SUM M ARYooo oFigure 3: The variation of the emittance and the dis­tribution of magnetic field using the present focusingcoil.

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Figure 4: The variation of the emittance and thedistribution of magnetic field using stronger focusingcoils.

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Ji( )Xsystem, it is desirable to minimize emittance growthat the bunching section.
The transverse focusing in the bunching system
is provided by Helmholtz coils arranged along thebunching section with an interval of 165 mm. For
optimum bunching with low emittance, the magnetic
field was adjusted. In case the magnetic field is toohigh in drift space, the beam begins to scallop, thatis, its radius changes in a violent and periodic fash­ion, resulting in particles gaining excessive transverse
momentum at the waist of the scallop, and leading toemittance growth in drift space. Therefore, we kept
the field low so as to maintain constant radius in thedrift space to SHB2. The emittance increased fairly
after SHB2, where a strong bunching starts. Figure 3
shows the variation of the normalized rms emittancein this case. The bunching condition is the same asthat of the optimum bunching in the previous sec­tion. The initial emittance value from the electron
gun is about 7 7r-mm-mrad, the value calculated withEGUN. It can be seen from the Fig. 3 that emittance grows mainly in the prebunchers and buncher.
The maximum focusing field of the present buncheing
system is 1000 Gauss. The value is a little too weakat the buncher, and the radius diverged there. The
initial radius, 2 mm, increased to 4 mm in the firstpart of the buncher. The final emittance is about 100
7r-mm* mrad, which is satisfactorily low.

We designed the bunching section for KEKB using
the simulation code PARMELA. We conclude thatsingle bunches with the desired qualities for KEKB
can be produced by adding two subharmonic bunchers to the present bunching system. The emittancecan be kept quite low below requirements.

R eferences[1 ]A. Enomoto et al., Proc. 1994 H
184

Linac Conf.

[2] S. Ohsawa et al., Proc. 1993 Particle Accelerator
Conf. 3087
[3] S. Ohsawa et.al., Proc. 1994 H

Linac C o n f. 193[4] S. Ohsawa et al., these proceedings