BEAM TEST OF INITIAL BEAM LOADING COMPENSATION FOR
ELECTRON LINACS
M. Satoh†, T. Matsumoto, T. Shidara, S. Fukuda, H. Kobayashi and Y. Kamiya, AcceleratorLaboratory, KEK, Ibaraki 305-0801, Japan
N. Nakamura, Synchrotron Radiation Laboratory, The Institute for Solid State Physics,
The University of Tokyo, Chiba 277-8581, JapanT. Koseki, Beam Physics and Engineering Laboratory, The Institute of Physical and Chemical
Research (RIKEN), Saitama 351-0198, Japan
T. Sakai, K. Yokoyama, Y. Hayakawa, K. Hayakawa, T. Tanaka and I. Sato, Laboratory forElectron Beam Research and Application, Atomic Energy Research Institute, Nihon University,Chiba 274-8501, JapanS. Miura, Mitsubishi Heavy Industries, LTD., Hiroshima 729-0393, JapanAbstract
For high-intensity electron linacs, one of the noble
schemes to obtain a sufficient intensity is to accelerate amulti-bunch beam in an RF pulse generated by a klystron.
It is a very useful technique for FEL linacs and/or toincrease the injection rate for storage rings. Furthermore,it can realize a high-energy transfer efficiency of the RFpower to the beam. In multi-bunch operation, however, italso has a troublesome defect called “Initial-BeamLoading” effect, which causes a large energy spread along
the bunch train within the beam pulse. The initial-beamloading effect can be considered to be the sum of thesingle-bunch beam loadings. Since it may cause serious
beam loss in linacs and beam lines to follow, it should becorrected to a tolerable level by using the some suitable
method. We have developed an initial-beam-loading
compensation system, which can modulate the phase andthe amplitude of a low-level RF signal [1,2]. A beam testusing this system was carried out at the 125-MeV electron
linac of Laboratory for Electron Beam Research andApplication (LEBRA) of Nihon University. The resultshows that our system well corrects the energy spread dueto the initial-beam-loading effect. In this paper, we reporton the results of beam tests.

1 INTRODUCTION
In high-intensity electron linacs, a long bunch train isaccelerated on each RF pulse. In such a case, the negative
longitudinal wake potential increases along the bunchtrain during one filling time of the accelerating structure,and it makes an energy spread within the beam pulse.
Because heavy loading gives rise to a large energy spreadthat may cause serious beam loss, it should becompensated by some correction method. Initial-beam___________________________________________†masanori.satoh@kek.jp

loading effect can be compensated to a certain extent by
adjusting the beam injection timing and/or ECS.
However, these methods are not very effective for a large
energy spread amounting to several tens of percent.
We adopted the ∆Φ-A method (the amplitude
modulation or ∆T method), which can completely correct
the initial-beam-loading effect on each accelerating
structure [3,4,5]. In this scheme, pre-filling of thestructure with RF before beam injection can be done sothat the energy gain of each bunch during the transientperiod is equal to that of each bunch of steady state. The
characteristic of our method is that the RF amplitude andthe phase are simultaneously modulated at a low power
level by using the response curve of the amplification
system: the solid-state amplifier and klystron. A merit ofour system is that it can be easily installed in and removed
from the RF amplification system owing to its simple
setup (Fig. 1).
∆Φ−Α modulator
M.O.
2856MHz

klystrone- beam
Accelerating
structure

Figure 1: Schematic drawing of the low-level RF ∆ΦA modulation method.

2 COMPENSATION SYSTEM
Figures 2 and 3 show a block diagram and aphotograph of the initial-beam-loading compensation
system, respectively. The compensation system consists

of a ∆Φ-A modulation system and an I/Q(Inphase/Quadrature) detector.
The ∆Φ-A modulator we developed consists of two fastvariable phase shifters and two 3-dB power dividers. A
hybrid-coupled type using a varactor-diode was adopted
for the fast phase shifter. Two 100-MHz programmablearbitrary waveform generators (Pragmatic Instruments,Inc. 2416A) with a control voltage from 0-V to 10-V are
used for controlling two phase shifters, each of which has
a 12-bit resolution and a rising time of less than 5-nsec.
A 100-MHz pulse generator is used for the common timebase of two arbitrary waveform generators in order toavoid any relative jitter, which may deteriorate theaccuracy of the output RF control in both amplitude andphase.
The modulator can achieve a maximum
attenuation of more than 20-dB, and can transform theinput CW signal into a pulse.
The I/Q detector as a fast phase detector we developed
consists of two DBMs, two low-pass filters, a power
divider and a 3-dB quadrature hybrid, which divides the∆Φ-A modulator

M.O.
2856MHz

Var.att.

divider

900W
Solid-state
amplifierPV-3030
Klystron No.2
18 MW

Var.att.

Bethe hole coupler

Var.AMP.

Att.
-60dB

REF. in〜14dBmCnt.V1

FG

RF in
6dBm

Cnt.V2AcosΦ

FGA sinΦ

IQ Detector

LPF
Att.
-10dB

Pulse generator Clock Base
100MHzOscilloscope

Att.
-20dB

divider-3dB

GPIBRF Detector

BPF
Att.
-20dB

PC

Figure 2: Block diagram of the initial-beam-loading
compensation system.

variable RF amplifier
FGRF signal into two equal outputs in amplitude with a 90degree phase difference between them. In principal, it
can simultaneously detect the amplitude and phase of theRF signal. In this beam test, an ordinary RF detector wasused for amplitude measurements.
This compensation system has already been tested atthe klystron test bench of KEK-LINAC without a beam,
and its result showed a good performance of ourcompensation system.

3 RESULT OF BEAM TEST
A beam test of the initial-beam-loading compensation
was carried out at LEBRA of Nihon University. The
LEBRA facility has both a free electron laser (FEL) and aparametric X-ray (PXR) generation system based on theS-band (2856-MHz) 125-MeV electron linac.
Theelectron linac consists of two klystrons, a DC gun, a prebuncher, a buncher and three 4-m accelerating structures,
each of which consists of 110 cells. Each klystron unit
has an IΦA system and a 900-W solid-state amplifier(Class-C). The experimental setup of the beam test is
shown in Fig. 4.
For energy compensation of two accelerating structures(Acc2 and Acc3), a ∆Φ-A modulation system is installed
upstream of solid-stale amplifier No.2 driving klystron
No.2, instead of the high-speed IΦA unit. Therefore, inthis beam test, the energy spread caused by Acc1 is notcompensated, and we aim to reduce the energy spreadcaused by only Acc2 and Acc3. The main parameters ofthe beam test are given in Table 1.
Figure 5 shows the measured and calculated beamenergy spread before and after compensation. It showsthat the energy spread is reduced from 24% to 6% by ourcompensation system. The reason for the energy spreadremaining after compensation is that the initial-beamloading effect is not compensated at Acc1.

4 SUMMARY & FUTURE PLAN

I/Q detector∆Φ-A modulator

pulse generator
for the common time base of two FGs

Figure 3: Photograph of the initialbeam-loading compensation system.

The ∆Φ-A modulation system and the I/Q detector
have
been
developedfor
initial-beam-loading
compensation. The first beam energy measurement wascarried out using this compensation system.
When the same ∆Φ-A modulation system is also
installed in klystron system No.1 and a more highperformance preamplifier, for example a Class-AB
amplifier, is used for driving the klystron, the energy
spread can be reduced to almost zero. Therefore, theresult of this study shows for the first time that the ∆Φ-A
modulation scheme of the low-power RF is a powerfultool for initial-beam-loading compensation.
To perform pulse-to-pulse feedback, a fast feedback
control system based on a VMEbus is being developedand tested. Its photograph is shown in Fig. 6. Its
performance and a detailed system description will bepresented elsewhere.

CM: Core Monitor
Beam dump

BM 1CM 5BM 4Acc 3Acc 2CM 4Acc 1CM 3CM 2 CM 1B PB G14MWSlit
BM 5CM 6FEL(Undulator)

30dB
BM 29MW

PXR

Klystron 29MW
3dB
18MW

Beam dump

20MW
Klystron 1

Low-speed φ shifter
CM 7BM 65.5dB

RF Amp.

RF Amp.

4kW

3.6MW
G:DC Gun
PB:PreBuncher
B:Buncher[ Injector ]High-speed IΦΑ 1BM 3
Beam dump

Oscillator 2856MHz
Oscillator 28.56MHz∆Φ-A modulation system

Figure 4: Experimental setup for the ∆Φ-A initial-beam-loading compensation system.
Table 1: Main parameter of the beam test.
Beam energy gain of steady-statewith compensation
89 MeV
without compensation
100 MeV
Beam pulse width
2.3 µsec
Average beam current
at the DC gun
180 mA
at the accelerating structure (No.1)
140 mA
at the accelerating structure
100 mA(No.2 and No.3)
Klystron output power
Klystron No.1 (without compensation system) 20 MW
Klystron No.2 (with compensation system)
18 MW

Figure 5: Measured beam energy before and after
initial-beam-loading compensation.

5 REFERENCES

Beam energy spread ∆E/E [%]26
Before compensation (measurement)
24
Before compensation (calculation)
22
After compensation (measurement)
20
After compensation (calculation)
18
16
141210864
20-2
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2t [µsec]

Figure 6: Photograph of a new ∆Φ-A control system underdevelopment.

[1] M. Satoh et al, “COMPENSATION OF INITIAL
BEAM LOADING FOR ELECTRON LINACS”,
Proceedings of the Seventh European Particle
Accelerator Conference, Vienna, pp. 2462-2464(2000).
[2] M. Satoh, K. Iida, K. Harada, H. Takaki, T. Koseki, N.
Nakamura, T. Sakai, K. Ishiwata, K. Yokoyama, Y.
Hayakawa, K. Hayakawa, T. Tanaka, I. Sato, S.
Miura, M. Matsumoto, T. Shidara, S. Fukuda, H.
Kobayashi and Y. Kamiya, First Beam Test of ∆Φ-A
Initial Beam Loading Compensation for ElectronLinacs", Proceedings of 2001 Particle Accelerator
Conference, pp. 3806-3808 (2001).
[3] K. A. Thompson and R. D. Ruth, “Simulation andCompensation of Multibunch Energy Variation inNLC”, Proceedings of the 1993 Particle Accelerator
Conference, p.3693 (1993).
[4] I. V. SYRACHV and T. HIGO, “NumericalInvestigationofTransient
BeamLoading
Compensation in JLC X-band Main Linac”, KEK
Report 96-8 A (1996).
[5] S. Kashiwagi et al., “Beam Loading Compensation
using Phase to Amplitude Modulation Method inATF”, ATF INTERNAL REPORT 98-27 (1998).