The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Development of Multi-bunch Beam Energy Compensation Method*Shigeru KAS~Iw_AC:H ,*Hitoshi HAYANO , *Timo KORHONEN, *Kiyoshi KUBO, *Takashi NAITO,
Katsunobu OIDE, Setshi TAKEDA, *Junji URAKAWA, **Shougo NAKAMURA and ***Fumihiko TAMURAThe Graduate University for Advanced Studies
1-1 Oho, Tsukuba-shi, lbaraki, 305, Japan*High Energy Accelerator Research Organization
1-1 Oho, Tsukuba-shi, lbaraki,305, Japan**Department of ~hysic.s, Faculty of Engineering, Yokohama National University79-2 Tokiwadai, Hodogaya-ku, Yokohama-shi, Kanagawa, Japan***University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo, JapanAbstract

A method to compensate for beam loading effects in a
multi-bunch beam is under development at Accelerator Test
Facility (ATF) in KEK. In this paper we describe the rf highpower test for LlT energy compensation by using the SLED
cavities. In this AT (early injection and amplitude
modulation) energy compensation method, the input
wavefo~ into accelerating structure is changed bycontrollmg the rf phase and combining the rf-power fromtwo ~ystrons with a 3 dB hybrid combiner to compensate
multi-bunch beam energy for various beam currents. In thistest, .an arbitrary waveform was generated by changing ther~tatl~g speed of the each klystron phase into the opposite
d1rect10n and the beam test will be done soon.
1Introduction

For future linear colliders, one of essential techniqueto get a sufficient luminosity is the ability to accelerate
multi-bunch beam with small bunch spacing. In the linac of
the KEK JLC design [1], the bunch train which is 85
bunches of 7.2xl09 electrons/bunch and 1.4 ns bunch
spacing. Beam loading voltage generates a large energy
spread along the bunch train. In the S-band linac, as thepulse length of a multi-bunch beam is shorter than thefilling time of accelerating structures, the energy gain of
successive bunches drops by approximately linear function
due to a transient beam loading in the accelerating
structu~es. If this energy spread is not properly compensated,the ermttance will grow by dispersion and chromatic effectin linac.
In this case it is necessary to compensate transient
beam loading by some method. In linear collider thevariation of bunch spacing is not acceptable, so theconventional energy compensation system (ECS) with four
dipole magnets is not applicable. The ilF energy
compensation system (ilF ECS) is tested at ATF which cancompensate the multi-bunch beam energy by keeping thebunch separation [2]. This LlF energy compensation method
is to OOd two (or more) accelerating structures running at
slightly higher and lower than the fundamental accelerating
frequency and with roughly in 90 degree out of phase fromthe acceleration. On the other hand, the use of il T

compensation method is also considerable in linear collider,this Ll T method is to inject a beam before an rf pulse has
filled in an accelerating structure. In the low frequency (Shand) linac, average accelerating gradient and requirement oflength of linac for the two compensating methods areroughly the same. The LlF compensation method has a highflexibility for bunch population changes, and it is easier totune. However, since AF compensation is not local
correction, the alignment tolerance of the quadrupole magnet
will be tighter than the AT compensation method. In
addition, the AT method in different unit of accelerator isindependent.
2 AT Beam Loading Compensation
The first order compensation of the beam loading canbe done by injecting the beam before the rf pulse has filled
the accelerating structure. The way that the AT
compensation principle works is shown in Fig. 1, in which
the voltage V(t) produced by a square rf pulse is plotted as afunction of time for a traveling-wave structure. The beamloading voltage Vbz(t) is also plotted as a function of time.
The resultant sum of V(t) and Vbl(t) is shown as the dashedline. The optimum injection time is determined by the V(t)and Vbl(t) slope. If we use this simple early injection
method, the beam current at which the energy compensation
can be comfortably performed is limited to some range, a00
acceleration efficiency will be poor. We apply thus theamplitude modulation on the input rf pulse for the pulse
compression. Therefore in case of using the SLED system,
we can obtain the desirable slope of unloaded voltage V(t)by changing input rf waveform for SLED cavities.
However, it is not a good idea to directly modulate theamplitude of the driving rf power to klystron. For a stable
operation, a klystoron usually needs to be used in thesaturation mode. Thus, modulating the klystron drive rf
phase would be a better method. To modulate the amplitudeof rf pulse for the SLED cavities at constant phase, twoklystrons are needed They run in saturation, keeping theinput rf level constant. Then, we control their phases a00
combine the rf power from two klystrons by using a 3 c:B
hybrid combiner. Fig. 2 shows a scheme in which the rf
phases of two klystrons are rotated into opposite directions
relative to each other. The sum of two vectors is delivered to-191-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

the SLED cavities. The phase modulation of the twoklystrons effectively realizes amplitude modulation using
this method. (PM-to-AM method)

3.1 Experimental Setup

We preliminary tested PM-to-AM modulation using
two-klystoron combination at ATF linac, Fig. 4 shows thehigh power rf test setup. It consists of an 85 MW klystron(TOSHIBA 3712), the dual-iris S-band SLED cavities, 3 <Bhybrid combiner, high power waveguides, rf loads andvacuum pump system. In this rf test, rf power is measured
by using -70 dB Bethe-Hole coupler at each klystron out,
between 3 dB hybrid combiner and SLED cavity and theentrance of an accelerating structure.
The 2856 MHz reference signal was generated bymultiplying the output signal of a 1428 MHz CW
synthesized generator by 2. The 2856 MHz phase shifter(No. 1) tune rf phase to the beam. And by using Delay &Pulse Modulator, it is modulated into a short pulse with 4.5
JlS width and the rf timing to klystron modulator maintrigger is adjusted. The phase shifter (No. 2) is used to rotate
the drive phase. A control pulse of the phase shifter is
ge_nerated by combining two output pulse with 1.0 JlS
width of pulse generators (HP 8112A), and change a leadingedge (LEE) of the other pulse generator as shown in Fig. 4 .

v

Fig. 1 Principle of the .1T beam-loading compensation.

...···

3 High Power rf Test for PM-to-AM./.........

SLED lnpul pbMe

Fig. 2 Principle of phase modulation to amplitude modulation(PM-to-AM method)

2.1 Calculation of Energy Gain

The early injection and PM-to-AM r energy
compensation method can be applied to reduce· the multibunch energy spread to a value even below that of the single
bunch energy spread. Fig. 3 shows the calculation result for
optimum compensation of the transient beam loading with
SLED system, the multi-bunch energy spread is compressed
less than 0.1 %. If an accelerating structure with short filling
time is used, the slope of the energy galn curve will be
more flexible and rf power is saved than the case of theATF structure (830 ns).

'-------1!8100

L3Fig. 4 High power rf test setup for PM-to~AM using twoklystron at ATF Linac

15

3.2 Experimental Results and Consideration

Emilled Field from SLED

Tlme[as]Fig. 3 Energy gain and output waveform of SLED (Each klystron
output power : 30MW, the filling time of accelerating
structure : 830 ns)

To combine the same rf power from each klystron,
we measured rf power of each klystron at output of at 3 <Bhybrid combiner and adjusted each klystron voltage. In thisif test the output power of each klystron was about 20 MW.
Fig. 5 shows the obtained SLED output pulse. In the rightside of Fig. 5 LEE of the pulse generator was set to 500 ns,
however, it had actuary about 600 ns phase slope which iscame from the slow response of the phase shifters.
The phase shifter in this rf test for PM-to-AM
modulation is the reflection-type phase shifter using varactor
diode. This type of varactor diode phase shifter is basically-192-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

analog device in which the. varactors function as variable
reactance elements. This variable reactance is achieved
through voltage-tuned capacitance of the diode under reversebias condition, therefore each ·of the rotating speed isdifferent when the rf phases are rotated into opposite
direction. Now we are planning to use the phase shifter
which is 1/Q phase shifter to vary the drive phase (Fig. 6).

modulation method. At down stream of pre-injector theenergy distribution in the bunch train is measured by using
analyzer magnet and BPM. To achieve the loading
compensation, the rotating speed of rf phase was configured
based on the measured beam current and accelerator gradient
which is calculated from monitored rf power. The timing ofrf pulse relative to the beam in each klystoron is adjusted. tominimize the energy variation along the bunch train.

II

Vertical Chk:ue\II. \\-10 Beam rrom preotajedor

I\Fig. 5 rf waveform from SLED
(right : LEE=5.5 [ns] left: LEE= 500 [ns])3 dB hybridI\-s\\ II-IS~~,Q modul.wrlmodua.tor

Fig. 6 The reflection-type phase shifter using varactor diode(left) and UQ phase shifter (right).

Fig. 7 Layout of beam line to measure the energy variation
along the bunch train (up), and optics of beam line (down)

4 Beam Test of AT Energy Compensation
To verify that the AT beam loading compensation
scheme works as predicted, we are planning beam test at thefirst regular section of ATF injector linac. ATF linac
accelerates a multi-bunch beam that consists of 20 microbunches with 2.8 ns spacing. Fig. 7 shows the layout andoptics of the bean line to measure the energy variation along
the bunch train.
For the energy measurement, the vertical chicane ismade at down stream of an accelerating structure which isfilled amplitude modulated if power. This chicane contains a
stripline-type beam position monitor (BPM) which ismounted at off-axis beam line to measure the beam positionin linear sensitive region. At the this BPM, the verticaldispersion function (TJy) is about 20 mm. The multi-bunch
beam signal from this BPM was measured by using fastsample hold (SIH) circuit and the digital oscilloscope of 5.0
GHz sample and changing the gate timing with 2.8 ns stepfor fast SIH. It is assumed that the position resolution of
BPM is 11 !liD, this value is achieved with 2.8 ns bunch
spacing in ATF pre-injector linac [3] and correspond to
0.055% energy spread in the bunch train. If the beam.
position of many pulses is measured, statistically,resolution of the measurement will be improved.
On the other hand, in this beam test we will use thesimple· AT (only early injection) energy compensation
technique to verify clearly the effect of PM-to-AM

Conclusions
From this rf high power test, we need to develop thephase shifter which is good time response. To be easy thephase control and beam operation, it is necessary to develop
the phase and energy feed-back system.

Acknowledgment
The authors would like to acknowledge the continuous
encouragement of Professors Y. Kimura, M. Kihara and K.
Takata. The rf test could not be performed without the helpof Mrs. S. Morita, T. Matsui, A. Miyamoto and K. Wachiof E-CUBE corporation. We wish to thanks these people for
their help. We would like to express our thanks to all themember of the ATF group for their useful discussion.

References[1] JLC Design Study Group, "JLC Design Study Report," KFK
Report 97-1 A (1997)[2] S. Kashiwagi et al., "Preliminary Test of +-t1F Energy
Compensation System". Proc. of the 18th Int. Linac Conf.,
Geneva, p.848 (1996)[3] H. Hayano, "Development of Multi-bunch Beam Position
Monitor". Proc. of the 20th Linear Accelerator Meeting, p.242(1995)-193-