CURRENT STATUS OF A 1.2 GeV BOOSTER ELECTRON SYNCHROTRONAND IMPLEMENTATION FOR NUCLEAR STUDY AT TOHOKUUNIVERSITY
H. Hama, F. Hinode, O. Konno, A. Kurihara, A. Miyamoto, M. Mutoh, M. Nanao, M. Oyamada,Y. Shibasaki, K. Shinto, S. Takahashi
Laboratory of Nuclear Science, Tohoku University, 1-2-1 Mikamine, Taihaku,Sendai 982-0826, Japan
Abstract
A 1.2 GeV booster electron synchrotron (STB ring)with combined use of pulse-beam stretcher at Laboratoryof Nuclear Science (LNS) [1], Tohoku University wasconstructed for study of nuclear physics, which is alsosupposed to be an injector for a planned future 3rd
generation light source. Currently the STB ring isroutinely operated with a booster-storage mode to supply
high energy γ-rays for nuclear study produced via
Bremsstrahlung using an internal target wire. Since finite
range of a tagging counter array mounted in a bending
section, the booster has to be also operated at another flattop energy lower than 1.2 GeV to extend the γ-ray energy
toward lower side. Because characteristics of the STB ringhave not been well understood, various beam properties
such as betatron tunes and instabilities are not completely
controlled at the moment.
In order to improve
performance of the machine and to study the beam
dynamics in a circular accelerator, fundamental propertiesof the STB ring is now being measured.

1 LNS ACCELERATOR COMPLEX
An injector S-band linac consisted with 19 accelerating
structures provides multi-bunch beam with a long-pulse
duration of 1 ~ 3 µs. The linac has been operated for
multi-purpose use with high repetition rate up to 300 Hz,e.g., the lower energy beam than 50 MeV is able tobranch off and often used for radioisotope production.
Though the unloaded maximum energy is 300 MeV, anenergy of 200 MeV is normally employed for the beam
injection into the STB ring because of sufficient beam
current. The beam energy is analyzed at a dispersive
section in the transport line and selected to be withincertain width (typically ± 1.5 %). Since there is anadditional function of pulse-beam stretcher to the STB
ring, the beam extraction system is also equipped. Thequasi cw-beam extracted from the STB ring is transported
to a couple of experimental stations.

2 THE STB RING
2.1 Basic Parameters
The STB ring constructed as a multipurpose
synchrotron was commissioned in these years and has

become routinely operational recently. At the moment
there are two major tasks, i.e., a pulse-beam stretcher toproduce the quasi cw-beam at the lower energy region
from 150 to 200 MeV, and the booster-storage ring to
supply internal 1.2 GeV beam. In this issue we mainly
report on characteristics of the latter operation.
Table 1: Parameters and status of the STB ring
Lattice type
Chasman-Green
Superperiodicity
4Circumference
49.7 mMaximum energy
1.2 GeV
Injection energy
0.2 GeV (nominal)
Betatron tune
(3.24, 1.21) *)Chromaticity(~ -5.5, ~ -4.7)*)
RF frequency
500.14 MHz
RF voltage
140 kV
Harmonics
83
Natural emittance
170 nmrad (@ 1.2 GeV)
0.0378
Momentum compaction αDispersion< 10 cm*)x-y coupling coefficient
0.005*)
Beam current< 20 mA*) (@ 1.2 GeV)
Lifetime~ 10 min (@ 1.2 GeV)*)Measured value
Main parameters and current status of the STB ring issummarized in Table 1. Lattice structure of the ring is
the simplest double-bend achromat consisted of 4 cells,
which is shown in Figure 1. Three dispersion-freestraight sections are occupied by an injection septum, an
RF cavity and a wire septum for slow extraction, and an
extraction septum, respectively.
Remaining one isreserved for future project. Unfortunately there has been
no convenient room for nuclear experiments near by, thetarget wire for Bremsstrahlung γ-ray production wasobliged to be installed in a dispersive section.
At the moment, the betatron tune has been chosen tobe (3.24, 1.21) and the natural chromaticity was measured
to be (-5.5, -4.7), which is not compensated yet because
sextupoles are not equipped in the dispersive section.
Fortunately we have not observed a beam loss due to thehead-tail instability within a beam current of 20 mA. We
have been considering to install thin sextupoles for future
development of the machine with higher beam current
operation. Beam lifetime has reached approximately 10

min at ~10 mA, which is surely dominated by thevacuum pressure and has been slowly getting better.
33 02 01 000QC

Q DQ F( ν X , ν Y ) = (3.24, 1.21)

2η x (m)β x , β y (m)Q FQ DβX

ηXβY

124681 01 20

Path Length (m)Figure 1: Lattice functions of the STB ring. Closed circle
and open circle denote measured averaged horizontal andvertical betatron functions, respectively.

2.2 Beam Injection

Injection

Injection
Wire Insert

Wire Insert

Beam Current

Beam Energy

2

Temporal sequence of the booster-storage mode
operation is shown in Figure 2. Immediately after thebeam injection, the energy ramping is started. It takes
1.1 s to accelerate the beam up to 1.2 GeV from 200
MeV, which is the maximum allowable speed of thepower supply. Duration of the flat-top is on demand. In
the Bremsstrahlung γ-ray experiment, the event rate iscontrolled by the position of the target wire, and it isnormally restricted by counting rate of the tagging counter
array. Accordingly stable accelerated beam current is
crucial for such experiments. The intensity fluctuation of
the 1.2 GeV beam has been suppressed within 10 % sofar, which is certainly affected by operation of the injector
linac.

3 TUNE MEASUREMENTS BY USING A
STRIP-LINE
3.1 Measurement System

The beam current from the linac beam is expected to be~100 mA, and the long-pulse width of 1 µs is usuallyemployed for the beam injection into the ring. However,
because of the wakefield in the accelerating structure, theenergy distribution in one beam-pulse is considerably
broad, so that acceptable pulse width for the STB ring is
supposed to be ~200 ns that is almost equivalent to therevolution time of the ring (166 ns). Indeed, the ring
current at the injection is momentarily ~80 mA. Becauseof finite energy acceptance of the RF bucket (±1.2 % for
200 MeV) and the multi-bunch linac beam, the captured
beam current is usually 20 ~ 30 mA.
Emittance of the linac beam in both planes has beenmeasured to be about 300 nmrad, which seems to be
sufficiently small to achieve better injection efficiency.
Using 6 quadrupoles in the transport line, Twiss
parameters, dispersion function and its slope at theinjection point have been matched with those of the ring.
Although certainty of the matching is not obvious, atpresent we have presumed the injection efficiency isgoverned by the longitudinal dynamics than the transverseone.

02.3 Operation Scheme

468Time (s)101214

Figure 2: Temporal sequence for the experiment with thebooster-storage mode of the STB ring.

A strip-line type electrode installed in a straight sectionof the STB ring was designed so as to that intrinsic
impedance of each strip-line should be 50 Ω . Tune
measurements are performed by which two beam signals
from strip-lines of diagonal position are fed into a 180˚hybrid mixer and analyzed by a real time spectrum
analyzer. Betatron oscillations are excited by applying
external RF dipole field to another pair of diagonal striplines. White noise at a certain frequency range can beused as an RF knock-out field. Accordingly betatrontunes in both plane are able to be measured
simultaneously.

3.2 Stability of Excitation Current
In a preliminary measurement after installing thesystem we have observed considerable periodic tune
fluctuation as shown in Figure 3.
Because thechromaticity is not corrected at all, the tune signals
contain the synchrotron sidebands including many
harmonics. In the measurement of Figure 3, the RF
voltage was much reduced to make the betatron sidebands
clear. It is obvious that the tunes, particularly the verticalone, beat with 50 Hz repetition rate that corresponds tothe electricity line frequency. As the lattice functions
shown in Figure 1, among 3 families of quadrupole, QF
and QD dominate the horizontal and the vertical betatronfunctions, respectively, meanwhile QC controls thedispersion function. The vertical tune shift which must
be mostly arisen by instability of QD magnetic field
implies a current ripple having relative amplitude of ~103. Although the horizontal tune shift seems to be smaller
and different pattern from the vertical one, it may also
indicates somewhat the current instability for QF. We aregoing to investigate source of the current ripple in thepower supplies, which would be a serious problem toimprove the machine performance and precise machine
control for the study of the beam physics as well.

Figure 3: Tune fluctuation in a storage mode at theinjection energy of 200 MeV. Betatron oscillations wereexcited by an external RF white noise at a frequency rangeof 1 ~ 2 MHz.

Large tune shift is still seen after the ramping started.
In addition it was found out that crossing of the tunes was
also occurred. Specific feature is that the vertical tune isincreased immediately and then decreased, meanwhile thehorizontal one is getting decreased. Accordingly the tunes
are crossing each other after a couple of ms. We havenoticed a considerable beam loss occurred at 2 ~ 3 ms after
the ramping started. This is certainly due to the difference
resonance, in which the large horizontal betatron
amplitude brought by the beam injection moves to thevertical betatron motion. An average horizontal betatron
amplitude produced the beam injection is expected to be J
= 7.2 mm mrad and the damping time is much longer (~2sec). The vertical displacement at the location of QD
would reach ~20 mm that is equivalent to the vacuum
chamber height, if the horizontal amplitude is fully
converted to the vertical one. A half of the beam
intensity is possibly lost due to the difference resonance.
As showing in Figure 5, the coupled motion at thedifference resonance can be clearly seen. From theminimum distance of the tunes, the coupling coefficientis deduced to be 0.005, which is rather small value. We
presume this is due to small number of the quadrupoles
and no sextupoles.

3.3 Tune Shift in Energy Ramping
At the present, we have employed a simple pattern for
excitation of the magnetic fields in the energy ramping.
All magnet currents are linearly increased, but theexcitation slope of the quadrupoles are bit steeper thanthat of the dipoles because the beam loss occurred justafter the ramping started when the quadrupoles wereexcited with same slope as the dipoles, which wasprobably caused by touching integer resonance. Figure 4
shows the tune shift in an early stage of the energy
ramping.

Figure 5: Observed coupled motion of the horizontal andthe vertical betatron oscillations at νx – νy = 2.

4 SUMMARYThe STB ring is still commissioning stage. However
characteristics of the ring has gradually become clear.
Along with improvement of the stability of the power
supplies, investigation of the tune shift in the energy
ramping may be crucial for progress of the machine
performance. Study of variation of actual magnetic field
including effect of Eddy current in the ramping is underway.

REFERENCES

Figure 4: Tune shift in early stage of the energy ramping.
The ramping is started at 10 ms after the beam injection.

[1] F. Hinode, et. al., “Beam Commissioning ofStretcher-Booster Ring in Tohoku University”, Proc.
12th Symp. on Accelerator Science and Technology,
RIKEN, Wako, Japan, September 1999, p177-179.