The II th Symposium on Accelerator Science and Technology, Harima Science Garden City, I997

Phase stabilization of injection beam to RIKEN Ring Cyclotron
Misaki KOBAYASHI, Akira YONEDA, Masayuki KASE, Masanori KIDERA, Akira GOTO and Yasushige YANOThe Institute of Physical and Chemical Research ( RIKEN ) , Hirosawa 2-1, Wako-shi, Saitama, 351-01 JAPAN

Abstract
When RIKEN Ring Cyclotron (RRC) is coupled with its
one of injectors, a heavy-ion linac (RILAC), the beamtransmission rate through RRC is sometimes limited by thephase instabilities of the injection beam from RILAC.
Recently we tried to reduce the phase instabilities by aclosed loop system and to improve the efficiency of thebeam transmission through RRC.

1 Introduction
The RRC has two injectors, the RILAC and the AVF
cyclotron. The RILAC has six rf cavities which arefrequency tunable between 17 and 45 MHz. In the case ofthe RILAC injection, the beam transmission rate throughthe RRC has been 60 to 95%, depending on the rf
conditions of the RILAC.
From a signal by a capacitive pick-up probe installed in abeam line between the RILAC and the RRC, a instability ofa beam phase is sometimes observed and it has several tens
deg. in rf. The instability which has a frequency range of 10- 100 Hz is caused by the fluctuation of the rf voltages ofsome cavities of the RILAC. The beam energy at the exit of
the RILAC is affected by this fluctuation and the beamphase spreads after drifting along the long beam line (60m).
It sometimes becomes beyond the phase acceptance of there-buncher which is installed on a half way of the beam line.
Naturally, some particles in the beam overflow the phase
acceptance of the RRC and the beam transmission
efficiency through the RRC becomes worse.
At present, it is needed to increase the beam intensity
beyond 1pf.LA to meet requirements of a nuclear physics
experiment on a super-heavy element research. Moreover,in the future, much more beam intensities will be required
as an injector of 'RI beam factory' [ 1]. In thesecircumstances, the beam transmission rate must beimproved as high as possible.
The beam phase fluctuations are tried to suppress byadjusting a phase of the rf voltage of the final cavity of theRILAC correspondingly, and as the result, it was observed
the improvement of the beam transmission rate through theRRC. Details are mentioned in the following sections.

can be easily observed by using an oscilloscope after proper
amplification. A zero-cross point of the wave-shaped signal
means the time when a charge-center of a beam bunch
passes through the electrode. It is observed that the signal
from the probe installed in the beam line from the RILAC tothe RRC has some fluctuations of this zero-cross point on atime axis.
In order to convert the fluctuation into simple electricsignals, the measurement system of a beam phase is used asshown in Fig.1. The system extracts a second-harmoniccomponent from the signal and converts it into a signal withan intermediate frequency of 455 kHz. Fig.2 shows thedetails of the phasemeter. By the phasemeter, two kinds ofoutputs are obtained; Acoscj> and Asincj>. 'A' is corresponding
to a beam intensity and 'cj>' is the beam phase relative to rf.
The system has a response of 1 kHz. These two signals aregot through an interface-board of DIM [2] at every 2 ms.
The data are stored in an 8 kB memory in DIM. These data
are calculated and converted into two parameters; A and cj>
when they are transferred to a host computer.
The result is shown in Fig.3. The 2000 measurement
points (A, <j>) are plotted on the graph indicating the twodimensional distribution. The beam at a moment isexpressed by one plot, and a distance between the data point
and the origin (A) is corresponding to a relative beam
intensity. The angle between a and ~ is the beam phaserelation to rf (<j>).
In the figure, it can be said that the beam intensity isCAPACITIVEPICK UPBEAM LINE

2 Beam phase measuremen t
A beam phase is measured by using a capacitive pick-up
probe with a ring-shape electrode whose length anddiameter of aperture are 40 mm and 40 mm, respectively. A
bi-polar signal induced on the electrode by a beam bunch

Fig.l The measurement system of the beam phase-531-

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

BEAM PULSE
SIGNAL .....-fllliHI>-Icnfo, On)[o=l11045MHz

Nr,-r,
fd=45SkHzNEW SIGNAL FEEDBACK LINE

SYNTHESIZEDSIGNAL GENERA TOR

MHzFig.2 The block diagram of the phase meter

Fig. 4 The signal feedback system of the beam phase to therf tank of the RILAC. The surrounded part by the dot
l:ine is the new additional feedback line.

STOPn
611 ----+

1J1I----..
NO--- ---- __ L __j__Acos<J>
Fig.3 Observed beam phase at RILAC. The A<l> in FWHM is
28.4deg. The beam.condition is 7.5 MeV/u I29Xe at
f=l8.8 MHz. The beam energy at the probe is
0.5 MeV/u.
relatively stable, but the beam phase changes widely. Thespread of the beam phase, A<j>, was 28.4 deg in FWHM.
The probe is set in 1Om downstream from the exit of theRILAC. This phase spread is mainly due to the energy
fluctuation of the beam of the RILAC. After drift along thebeam line, the energy fluctuation is converted into the phaseone. Since there is still 20 m drift distance until there-buncher, this phase spread is too large to ensure that all
beam is accepted by it.

3 The signal feedback system
The phase and the energy of the beam are given .asfunctions of the rf amplitudes and phases of every cavities
in use. Among them, the energy depends smoothly on therf phase of the final cavity in use. In other words, theFig.5 The algorithm for the calculation to revise thebeam phase.

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

energy of the beam from the RILAC can be tuned
intentionally by adjusting the rf phase of the final cavity[3].
The fluctuation of the beam energy at the exit of the RILAC
might be suppressed by adjusting the rf phase of the final
cavity corresponding to the beam phase at the probelocation.
According to this expectation, a new feedback system
was made as shown in Fig.4. A local computer with theADC & DAC board was prepared, because the systemneeds a time response around 1 kHz, . After the two signals
from the phasemeter are got into the ADC, a phase control
signal is calculated by the program in the PC, and set on theDAC immediatly. The relation function for the output
signal is given by4 Conclusion
From the result, it is clear that this new feedback system
works effectively. A loss of the beam in the RRC can. bealmost nothing by this method. Next we are to change thecalculation program to set the optimization. of theparameters automatically in order to optimize the feedback
system.
References[1] Y.Yano et al., "RII<EN RI Beam Factory Project",in this proceedings.
[2] T.Wada et al., Sci. Papers I.P.C.R., 79 (1985) 28

Vout=Vo+( cj>-cj>o )*G,
[3] M.Kase et al., Sci. Papers I.P.C.R., 79 (1985) 45-50
where G and Vo are the gain parameter and the pedestal
voltage of the system, respectively. Vo is a constant of+5.0V normally. cj>o is a center of the fluctuating phase. Thecalculation done in the PC follows the algorithm shown inFig.5. The system gets the signals of Asincj> and Acoscj> andcalculates A, cj>, and Vout. After that, it sets Vout on theDAC when A is larger than Ao. Ao is a threshold of thebeam intensity. The circulation period is around 100 J.l.s.
The output signal from the DAC is fed to the phaseshifter which is connected with the master oscillator signal
line of the phase controller with a fast responce of 100kHz.
The data included as shown in Fig.6. It also consisted of2000 measurement points. The Acj> for this data is 16.5 deg
in FWHM, so that the data with almost half straggling value
could be taken by using the system. Further, by the use ofthe system, the improvement of the transmission rate of theRRC was observed.

.1 ...

l:'t-t .

~ "'" _:2_7t_-+--0+-_--+-+---_ -_-I!~ ~- ---t--+---+---+--t------ J-r----r--ri
1I,I- __ .!..~. ~---L~~·_j__-~-~j__________l____L_ _Acos<!>
Fig.6 Observed beam phase at RILAC with using the new
feedback system. The Acj> in FWHM is 16.5 deg.

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