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

Test of Beam Feedback-free Acceleration at TARN-ll
Shin-ichi WATANABEt, Tamaki WATANABE*, Yoshitsugu ARAKAKI*, Katsuhisa CHIDA*,
Masao TAKANAKA*and Mitsutaka KANAZAWA**.
INS, Univ. of Tokyo, Midori-cho 3-2-1, Tanashi-shi, Tokyo, 188 Japan*RIKEN, Hirosawa 2-1, Wako-shi, Saitama, 351-01 Japan**NIRS, Anagawa 4-9-1, Inage-ku, Chiba, 263 Japan

ABSTRACT

2 DIRECT DIGITAL SYNTHESIZER

The synchrotron acceleration system of the TARN-II is being upgraded toaccelerate a beam that can be captured the beam without a l'l.l/J and I':!.Rteedback. The precise control of ru:celeration frequency Frfis a part ofupgrade program. The voltage controlled oscillator (VCO) with a beam
feed back loop was replaced by the direct digital synthesizer (DDS). To
make sure the acceleration with DDS, B-Frf conversion instrument wasimproved. A ripple field of the dipole magnet was improved to makesure the generation of accurate B-dot signal An accuracy of calculated
frequency Frf depends on the ring parameters. The circumference andradius of curvature of the ring were calibrated with the calibrated injection
beam energy. The betatron tune values were chosen at stable operationpoint. The 26 MeV Alpha beam was accelerated up to 100 MeV without
the beam feedback system.

An original rf signal generator was a voltage controlled oscillator(VCO) with a phase lock loop. The VCO was replaced by the DDS(direct digital synthesizer) that is responsible for generation of precise
continuous sine wave.
The DDS is also superior to make the accurate
moving rfbucket since the VCO has a large level of spurious noise aroundthe oscillation frequency.
A previous version of DDS in place of the VCO is being used in theCERN-LEAR, GSI-ESR and NIRS-HIMAC. The application note on
DDS is appeared in the literature.[3] The DDS is one of the digital rf
components using the advanced LSI technology. We chose a board type
DDS with on-board x-tall oscillator.
The applied DDS module
comprises the AD9955 with a control data length of 32-bit.

1 INTRODUCTION
A synchrotron acceleration system comprises a beam feedback
system to hold the bunched beam in the moving rf bucket. The beam
feedback system is responsible for both dl/J and I':!.R controls with the aid ofbeam-pick up instruments.
A signal to noise ratio of the beam
instrument become the problems whenever the detectable beam current islower than the threshold level So, the beam feedback-free techniqueaiming at an acceleration of faint beam is needed when the bean1
instrument is poor.
The beam feedback-free acceleration has been studied after the rf
stacking experiment at TARN-II.[l]This technique is availabletoaccelerate a heavy ion beam such as the injected beam from the INS-SFcyclotron. The same technique is also available to accelerate a moleculebeam from the same cyclotron. To accelerate the fuint beam, present rfacceleration system of TARN-II was reviewed because the rf system wasdeveloped based on the beam feedback technique.[2]The acceleration system of TARN-II has been improved aiming at thebeam feedback-free acceleration. The rf signal source, magnet power
supply and low level rf control system were improved to accelerate thebeam without the beam feedback system. The paper describes theimproved instruments and beam acceleration test at TARN-II. The Fig.lshows block diagram of the rf acceleration system and related beam
monitor system.

Fig.l Bl!-JCk diagram of improved rf acceleration system and related
beam monitor system.
The control data is responsible for the determination of the oscillation
frequency.
A control computer generates the rf frequency data. The
rf frequency data is calculated based on the following B-Frf equation.

Frr = ch/2m?JU+<mcr/300eJl>>2J 112
where sis charge to mass ratio, qM.

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The rl acceleration voltage, ~¢8 , is given by,

3 RJPPLE REDUCTION OF DIPOLE MAGNET CURRENTVsin¢8 =27rpRxlBflt

where dOOtis Bdot clock rate (Gauss/sec).
A current pattern of the lattice magnet was a trapezoid wave-form.
A bottom of t1Je current pattern was subjected to accumulate a beam at
the injection energy. A digital memory module (DMM) is subjected tostore the data of rf frequency from the capture to the top of thesynchrotron acceleration. The data length of DMM is 16 bit. So, thefrequency resolution is Frj2 16 per bit however the inherent frequency
resolution of DDS is 232. The oscillation ~uency Frfinax at the full bit
data is designed at 5.368709 MHz. So, the frequency step corresponds
81.921HVbit. The spurious noise and 2nd hannonics around theoscillation frequency lMHz are -67 dB and -46 dB, resiJectively. The
measurement result shows that the phase jump due to a change of control
data of DDS is smoothly continued. This means a lack of moving rf
bucket is not appeared. The change of oscillation frequency take 5 nsafter a data-write command is received.
The B-dot clock signal is subjected to read the frequency data stored in
the DMM. The resolution of the B-dot clock signal is responsible fordetermination of the step-height of frequency data The B-dot clock
generator has a resolution of 1.0, 0.5 and 0.25 Gauss/pulse.
Fig. 2shows an equivalent circuit (upper) of B-dot clock generator and their
response (lower) calculated by MA'ILAB.
The lower shows thatramped B signal is converted to the B-dot count number with a time-lag.

The B-dot signal was measured by a search coil located in the gap ofdipole magnet. The search coil is sensitive to the noise since the dEAlt of
the noise is higher than the ramping speed of excitation field. A ripple
component in the magnetic field is to be set as low as possible from theview point of accurate frequency control The power supply of a latticemagnet is responsible for reducing a ripple component of the magnetic
field. A lap of tum-on of the 12 phases thyristor rectifier generates aspike noise on the output of the power supply. This spike noise becomes
the origin of common mode current in the lattice magnet strings. Acommon mode filter (CMF) was inStalled between the power supply andthe dipole magnet strings. The magnet coil circuit is a series of passive
elements where the inductance, resistance and capacitance are included.
The detailed circuit of CMF is as shown in Fig.3.
The Fig. 3 shows acommon ground which means the earth ground. The common-mode
currents are transmitted in the series of magnet coil circuits and arepartially terminated with the common ground through the parallel
impedance of the lattice magnet. The CMF is subjecteji to block thecommon mode current pass through the magnet coil circuit. A Cut-off
frequency was chosen at near the twelve times the 50Hz. Damping
resistor of the CMF was tuned so as to avoid the self oscillation of thepower supply. Finally, the insertion effect of CMF was evaluated withthePSpice.
Power In...

..8!:Power Out (P)::J....,c0

Power Out (N)Fig.3 Equivalent circuit of CMF.

c::J00The CMF allows the flow of normal excitation current with a small
ohmic loss. The CMF acts as a normal mode filter because the mutualRampinc timeFig.2 Circuit model and signal response of B-dot clock generator.
A B-dot clock counter accumulates a count number of the B-dot
clock signal When the accumulated count number coincides with apreset number, a reset signal is generated aiming at clear the DMM. The
preset number means an expected magnetic field at the flat-top of thedipole magnet current. The readout sequence of acceleration frequency
data is stopped then. As shown in Fig.2, the delay of the count number
result in the error of readout frequency data
So, the error due to thisreason should be corrected. The correction of this error was carried out
by means of correction of the frequency data of DMM per readout clock.

coupling constant is 0.5. Due to a limited budged, iristalJed CMF is 1/5
power model of full specifications. The measured specifications of CMF
are as follows: Self-inductance = 1.23mH(U) or 1.22mH(D); Ohmicloss= 2.155 rnn.
The old DCCT was replaced by the HOLEC-DCCT. The oldDCCT has a strong 50Hz noise. The new DCCT is responsible for
reducing a 50Hz or higher ripple-currents in the excitation current.

4 BEAM ACCELERATION TFST
An injection energy of Alfa beam was set at 26MeV. A momentum
spread and beam current at the exit of SF-cyclotron were set at 0.1% and
0.8 ~ respectively. A transmission efficiency of beam transport line isabout 40 % at the injection point of TARN-11.[4] The extracted beamenergy from the SF- cyclotron was tuned to match the magnetic rigidity of-102-

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

The Bp of extracted beam energy was measured by theTARN-II.
analyzer magnet BA- J where is the one of the beam transport magnets.
The slits before and behind the BA-1 were set at 5 mm to center the beam.
The beam orbit of the circulating beam was measured by the Mmonitor. The Fig. I shows measurement system The sum of M signal
The
was down converted to an intermediate frequency 455kHz.
455kHz.
themeasuretomodezero-span
atset
wasspectrum analyzer
The closed orbit is centered to the mechanical center of M monitor in thering. A COD of the ring was corrected within +/-0.5 mm.[5]The average vacuum pressure of the ring was lxi0-10 mBar. The
betatron tune values of the ring were set at 'Ux=l.6088 and uy=l.7427,

The linear region was 3.5 seconds. The time square region near the flatbottom was 0.3 seconds. The time square region near the flat-top was0.3 seconds. The injx;tion and top energy fields were set at 1.8152 andThe rf frequencies at the injx;tion and top
3.5020KG, respectively.

energies of the beam were set at 0.90303 and 1.6963MHz, respectively.
The tracking error of ramped excitation current of the lattice magnet

familywassetwithin w-3.

A life time of the injx;ted beam was 20 seconds. Therespectively.
stored beam current was measured with a beam DCCT. The measured
beam intensity was I0~ ·It was concluded that a circumference of thering (2rrR) was 77.712m. The radius of curvature (p) of the ring wasalso concluded at 4.03275±0.00248m These values were taken into
account the calculation of the acceleration frequency, Frf.

Fig.5 TheM (upper) and the intensity (lower) of the accelerated beam.
CONCLUSIONFig.4 B-dot signal and accelerated beam signal
The acceleration test was carried out after the measurement ofproperty of the injx;ted beam. The B-dot clock signal was set at 0.5
Gauss/pulse. In the Fig.4, upper and lower show the B-dot signal andA rising period of B-dot signal
the beam intensity, respectively.
comprises the time square process of the magnetic field ramping. The
flat top of the magnetic field is identified as a flat region of the B-dot signal
where the beam signal is disappeared. The top of magnetic field
From the experiments, wecoincides with a beam energy of 100 MeV.
can say that the acceleration efficiency was 12%. The Fig.5 shows
accelerated beam intensity and M signal The upper shows beam
position signal from the M monitor. It is recognized that the position of
the accelerated beam is slightly moved to the outer side of the vacuum
The following descriptions are brief note on experimentalchamber.
notifications.
First, the beam was captured after the multi-tum beam injx;tion at theAs shown in Fig. 1, aflat bottom of the trapezoid magnetic field.
phase-lock system of the ferrite bias power supply was tuned so as to
track the rf signal from the DDS. The measurement error of B-dot signal
generator brings the fatal error of acceleration frequency. The fineA realcorrection of the acceleration frequency was then required.
time spectrum analyzer as shown in Fig.l was subjx;ted to measure thetracked beam signal or higher harmonics during the beam handling process.
The typical operation parameters were as follows: The ramping time of
the magnetic field comprises a linear and both the time square regions.

An acceleration efficiency depends on not only the rfbucket-height but
also the B-dot clock resolution The rf bucket height was designed tocover the 100% of injx;ted beam. The acceleration efficiency was nearly
12% when the B-dot clock resolution was 0.5 Gauss/pulse. It is
expected that more precise B-dot clock resolution promise the higher
acceleration efficiency. The fine tuning of the rf frequency was requiredat the beginning of acceleration So, the error of B-Frfconverter shouldbe improved since present instrument includes time lag component for theripple reduction.
ACKNOWLEDGMENT
The first· author express his thanks to Prof. Sato, and Mr.Sueno,
KEK for their encouragement to improve the power supply. The author
also express his thanks to Dr. Goto, RIKEN for his support. Theauthors thanks Mr. Ono, INS and SF-Cyclotron crew for their operationof SF C)\:lotron
REFERENCES[1] S.Watanabe et al. INS-Rep.-1169, Oct. 1996.
[2] T.Katayarna et al. NI&M in Phys. Res. A 336 (1993) 391-400.
[3] G.C.Schneider PSIRF/Note95-16 , CERN-PS Division[4] S.Watanabe et al. Proc. ofC)\:lotrons and Their App!K;ations,Vancouver1992.
[5] T.Watanabeetal. NI&MinPhys. Res. A381 (1996) 194-208.

t CNS, School of Science, univ. of Tokyo, Tanashi, Tokyo 188.
:j: KEK(Tanashi) , Tanashi, Tokyo 188.

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