The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Beam Commiss ioning of Stretcher -Booster Ring in Tohoku Universit yFujio HINODE and the STB Group
Laboratory of Nuclear Science, Tohoku University
1-2 Mikamine, Taihaku, Sendai 982-0826, Japan
Table 1Main parameters of the STB ring.

Abstract
The 1.2 GeV Stretcher-Bo oster Ring was constructed at Laboratory of Nuclear Science (LNS) in Tohoku University. This ring has three operational modes;
the pulse stretcher mode, the booster mode and the storage mode. Beam commissioni ng of the ring has been
carried out since November, 1997. This report describes
the present status of the beam commissioning.

Circumference
Revolution Frequency
LatticeSuper-periodicity
Betatron Frequency
Moment. Camp. Factor
Chromaticity

1 Introductio nIn 1981, the first pulse stretcher ring (SSTR [1]) wasconstructed in Tohoku University and had been supplying continuous electron beams for nuclear experimentswith the beam energy of 150 MeV. To meet the requirement of the higher energy beam, the construction of the1.2 GeV Stretcher-Bo oster Ring (STB [2, 3]) was started
in 1995. Since the first beam commissioning of the STB
in November, 1997, the beam operation is continued up
to now.
IThe STB has three functions such as the pulse
stretcher, the booster and the storage ring. In the pulse
stretcher mode, the STB converts a pulsed beam accelerated by a LINAC to a continuous beam, which isutilized for coincidence experiments in nuClear physics.
In the booster mode, the beam is ramped up to 1.2 GeVwith RF acceleration in 1 second after the beam injection, and then injected to a proposed synchrotron- lightfacility. In the storage mode, the accelerated 1.2 Ge V
beam is utilized for internal target experiments and highenergy tagged photon experiments .
A. schematic layout of the accelerator in LNS isshown in Figure 1. It consists of a 300 MeV S-band
LINAC, an injection line, an 1.2 GeV STB ring andtransport lines for experiments . In the LINAC, there are
20 accelerating structures, and 5 klystrons feed power tothose structures.
The main parameters of the STB are listed in Table 1. The design detail is given by the Reference [3].
The STB ring consists of 8 bending magnets and 20
quadrupole magnets, and has four straight sections of3.125 m for injection, extraction, RF acceleration andinternal target experiments . The circumference is 49.751
m. It is desirable to have a large circumference with respect to an intense extracted beam, however, the wholesize was restricted by the limited space in the experimental hall. RF acceleration is done by a single-cell cavity
which had been originally used at Photon Factory inKEK. The RF frequency is 500.144 MHz, corresponds
to a harmonic number of 83. Vacuum chambers are
made of stainless steel to suppress eddy currents on thebooster operation. The beam monitor system consists of
5 wire monitors, 9 button-type BPMs, a DCCT and an
SR monitor port. The read-out electronics of the BPM

Circulating Beam Current
Beam Injection
Stretcher Mode
Beam Energy
Rep. Rate
Extracted Beam Current
Duty Factor
Beam Extraction
Booster-Storage Mode
Accelerated Beam Energy
Harmonic Number
RF Frequency
Radiation Loss / turnDamping Time49.7512 (m)6.026 [MHz]DBA
4Vx = 3.30
Vy = 1.20(}' = 0.03777
8v,f8o = -5.7861
8vyf8o = -4.9791
300 [mA](max)three-turn injection
300 [MeV](max)
300 (pps]
10 (pA)
90%

third-integer resonance
1.2 (CeV)
83
500.144 [M Hz]
61.2 [keV]
Tx = 8.0 [msec]
Ty = 6.5 [msec]
Tz
3.0 [msec]=uses superhetero dyne methods. The BPM can measure
the closed orbit for the storage mode, but not used forthe stretcher mode. Two wire monitors were replaced byscreen profile monitors. In addition to the above monitors, some loss monitors were placed along the beamlines. The STB control system [4] was constructed using
personal computers with commercially available application softwares. Programmab le Logic Controllers, PLCs,
are used as the device controller. In the beam injection
line, a beam collimator is installed at the just upstreamof injection septum magnets. And also, 3 beam collimators are installed in the transport line, due to eliminate
the beam halo. In both beam lines, the beam positions
and the shapes are monitored by screen profile monitors. Vacuum chambers are made of aluminum alloy,
and whose inner diameters are ¢;50 ~ 80 rnm.
2 Beam Commissio ning

Stretcher Mode
The machine time of the STB has been mainly consumed in the stretcher mode up to now. For the present,
the injected beam energy is 200 MeV so as to be relatively easy to operate the LINAC. In the stretcher mode,
the ring is operated with a repetition rate of 300 pps.
To match the injection beam parameter to those-177-

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Fig. 1 A schematic layout of the accelerator at LNS in Tohoku University.

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Path Length (m)12Fig. 3 An example of the fitted result after the injection matching.

QrorrfCoe![Fig. 2 An example of the parabola fitting to determine the beam parameters.
of the STB rmg, six quadrupole magnets are used in
the injection line. First, beam profiles observed by
CCD cameras are automatically processed by a personal computer in order to get horizontal and verticalbeam sizes [5]. Such beam sizes are measured withvarying the strength of an adequate quadrupole magnet, and fitted with a parabola. The emittance andTwiss parameters of the injected beam are determinedusing three sets of parabola parameters obtained withvarying three quadrupole magnets. Then the strengthof the quadrupole magnets are optimized to match theinjected beam to the ring. Figures 2 and 3 show an
example of such a measurement.
This procedure is
executed manually for the present, however, it will besophisticated so as to reach to easier operation. The
injected beam orbit is adjusted by two septum magnets
and three kicker magnets. These kicker magnets must be
also adjusted with respect to their timing taking into account of the time-of-flight of the injected beam between
them, so as to inject the beam through three turns of thering with a suitable betatron oscillation amplitude forbeam extraction. For the horizontal tune measurement,
the radiation from a wire monitor inserted in the ring
to disturb the circulating beam is detected by a beamloss monitor which consists of a plastic scintillator and aphotomultiplier. For the vertical tune measurement, thesynchrotron radiation is monitored by a photomultiplierwith a horizontal slit. In the latter case, the beam orbit
is distorted in the injection line, so that the amplitudeof the betatron oscillation is large enough. Both signals-178-

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Fig. 4 An example of the beam operation in thestretcher mode, upper: the circulating beam
current measured by DCCT (10 mA/div.),
lower: the extracte9 beam observed by a spillmonitor placed near the spectrometer.
are recorded in an oscilloscope, and then transformed
to the spectrum by a personal computer. This tentative tune monitor system is not enough to measure thetunes stably and should be improved. At the present,
the maximum circulating beam current is about 80 rnAat the injection. However, the current is usually reduced
to 20,..., 30 rnA (preliminary), by a proces to stabilize thebeam condition as described below.
,Beam extraction from the ring is accomplished by inducing a third-integer resonance in the horizontal plane.
The pulse stretcher must keep the intensity of the extracted current constant during the extraction time.
This demand is satisfied by monochromatic extraction
under 250 MeV. Achromatic extraction will be also usedover 250 MeV in the next stage. In the monochromatic
extraction, the energy spread of the injected beam ischosen to be equal to the energy loss that occurs between two successive injections, and the chromaticity ofthe ring is adjusted by a sextupole magnet so that theresonance condition occurs at the lowest energy of theinjected beam. Thus the lowest energy part of the stored
beam is extracted first, and then finally the highest partis extracted just before the next injection starts. Figure 4 shows the example of the stored current (DCCT)
and the extracted beam observed by a spill monitor
placed near the spectrometer. Extracted continuous
beam current is around 1.2 J..lA with duty factor of 70,..., 80%.
Booster Mode
Since it had been succeeded to accelerate the beam
up to 1.2 Ge V in the last year, enough machine time had
not been assigned for the booster mode by some reasons.
One reason was caused by a tuning work on the power
supply for bending magnets. In the past months, themagnetic field was ramped up with a ramping time of 6
seconds. The power supply has been recently remodeled,
so that the ramping time has become 1 second as designed. Thus the machine study with the booster mode
will be restarted soon. According to the preliminary results of the machine study, the stored beam current isabout 5 rnA after the acceleration up to 1.2 GeV.

Discussion
There have been a lot of problems in the STB operation up to now. Above all, the problems of radiation
shield and beam stabilization are serious. In the earlytime of the beam commissioning, many instruments,
such as power supplies and those controllers, monitor
control system etc., were placed near the STB ring in theexperimental hall. As the circulating beam current increased, magnet power sources and beam monitors weresometimes interrupted from a normal operation due to aheavy radiation. Although a lot of concrete shield blocks
were placed, such efforts did not bring critical effect. In·
the middle of this year, many modules of the control
system were moved out from the experimental hall, sothat the frequency of the failure in control was reduced.
However, many electronics devices are still placed in theexperimental hall, we have to cope with the radiation
problem.
Another serious problem is an unstable beam feature in the stretcher mode. At the 300 pps operation,
six kinds of pulses, which are repeated with 50 pps, areobserved in several places. For example, the DCCT signal shows a different time structure and beam current
for each of six successive pulses. Such difference is notobserved at the 50 pps operation. It seems to be causedby the LINAC. When the LINAC is carefully tuned, thedifference of six pulses becomes small, as shown in Figure 4. But this operation reduces the circulating beam
current in general. This phenomenon, the so-called hexacolor beam, is discussed in elsewhere [6].
3 Conclusion

The beam commissioning of the 1.2 Ge V StretcherBooster Ring has been carried out at LNS in Tohoku
University. Up to now, the machine time of the STB
has been mainly consumed in the stretcher mode withcontinuing some remodeling work. An extracted continuous beam current is around 1.2 J..lA with a duty factorof 70 ,..., 80 %. The beam commissioning for the booster
mode with ramping time of 1 second will be started soon.
References[1] T. Tamae et al., Nucl. Instr. and Meth. A 264,(1988)173.
[2] M. Oyamada et al., The lOth Sympo. on Accel. Sci.
and Tech., Hitachinaka, Japan, (1995)463.
[3] T. Tamae and the STB Construction Group, The
16th RCNP Osaka Int. Sympo., Osaka, Japan,(1997)145.
[4] M. Mutoh et al., Proc. in PCaPAC'99, Tsukuba,Japan, (1999).
[5] Y. Shibasaki et a!., Proc. of the 24th Linear Accel. Meeting in Japan, Sapporo, Japan, (1999)395,in Japanese.
[6] A. Miyamoto et al., in these proceedings.

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