JAERI-Conf 95-021

PB09
A BEAM SPILL CONTROL SYSTEM AT HIMAC
N. Araki, K. Noda, E. Takada, K. Sato•, A. Itano••, M. Kanazawa, M. Kumada,
Y. Yamamoto•••, S. Sato, M. Torikoshi, Y. Sato, S. Yamada and H. Ogawa

Division ofAcclerator Physics & Engineering, National Institute ofRadiological Sciences, Chiba, Chiba 263, japan*Research Center for Nuclear Physics, Osaka University, Jbaraki, Osaka 567, japan**Public Health & Environment Department, Hyogo Prefectural Government, Kobe, Hyogo 650, japan***Electronics & Control System Department, Hitachi Zosen Corporation, Osaka, Osaka 554, japan

Abstract
A beam spill control system has been designed andinstalled in order to improve a spill structure of extracted
beams from synchrotron at HIMAC. The system concept
is to optimize a current setting pattern for correction
quadrupole magnets (QDS), by utilizing an iterative
control based on information of a spill structure from thebeam ripple monitor.

1. Introduction
A high accuracy for the dose distribution isrequired in heavy ion therapy because of its sharp
localization and high RBE (relative biologicaleffectiveness). It is thus necessary to obtain a uniform
dose with sharp boundary in the lateral direction, and tocontrol precisely the total dose per patient. An irradiation
system at HIMAC adopts a beam wobbling method alongwith a scatterer to obtain such uniformity within ± 2%[1). However, when a frequency of ripple in the extracted
beam spill is close to the driving frequency of wobbling
magnets (57Hz), the dose uniformity can be lost due to abeat phenomenon.
In a resonant slow extraction at HIMAC, a main
ripple of beam spill is caused by a current ripple
generated in the power supply of main focusing
quadrupole magnets (OF), which is 100Hz. It ismonitored with a thin scintillator at extraction beam line.
A signal of harmonics synchronizing with the PLL(phase locked loop) was fed forward to the active filter ofOF power supply, which was effective to reduce ripple[2). This observation suggests a possibility of ripple
reduction by quadrupole components. A beam spill
control system has, therefore, been developed in order toreduce a beam ripple and to secure m1iformity by shaping
a spill envelope. A set of ODS, originally installed fortune-shifter, is utilized in the system. This paper reports
a design consideration and preliminary experimental
results concerning the beam spill control system.

reduction is harmonics of power supply driving
frequency. Superpositioned harmonics forms starting
point of ODS output current pattern. However, there
must be other sources of ripple and also fluctuation in
AC line, for example, affect actual ripple output. An
adjustment is necessitated for current setting to reflect
these effects. It means that our system should have afunction of feed forward setting and feed back from ripple
signal. Iterative control of current pattern by beam signal
would provide long term operability against various
fluctuation conditions.

2-2. Optimization Algarithm
The important aspect of the system is analgorithm to optimize a current pattern for QDS
automatically by utilizing the iterative control [3) basedon information of a spill structure from the beam ripple
monitor.
Assumptions on the algorithm is summarized asfollows: (1) transfer function concerning the parametersof digital filter and a power supply are known; (2) theextracted beam spill is reproduced at each operation cycleof synchrotron while the beam intensity is fluctuating;(3) gain of the iterative control can be optimized in orderto make the control system stable.
The optimization for the current pattern is carriedout separately at low (for shaping a spill envelope) andhigh (for correcting a beam ripple) frequency regimes, asshown in fig.l. At a low frequency regime, an error
between the spill signal after the low-pass filter and acurrent pattern is fed back to ODS after averaging. At
high frequency regime, the spill signal after the band-passfilter with phase compensation is fed forward to ODS.
Each filter is employed a FIR (finite impulse response)
digital filter because of considering stable. The
optimization is completed when a root mean square of theerror between the spill signal and the pattern becomes
smaller than a specified value. Parameters of filters at
both frequency regimes as well as the feed-back and feedforward gain are designed to be adjustable.

2-3. Structure of hardware

2. Design Consideration

2-1. Framewark
As mentioned above, basic component of ripple

A beam spill controller and ODS magnet power
supply have been developed to realize the systemalgorithm. The system structure is shown in fig.2.

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Beam Spill Data -+-+~''F!Bgain il
Previous
Pattern

QDS~TMagnets

The High Frequency regime.

Fig. 1 Each algorithm enclosed by broken lines indicates the low and high frequency regimes.

QDS power supply is designed to have bipolar
outputs, so that QF current pattern can be the same withor without the beam spill controller system. QDS powersupply is operated according to the current patten that isproduced based on the beam wavefonn.
In the control device of QDS system, VME
computers are used to carry out the real-time control.
VME computers consists of processors, memories andI/O's in the unit, which use a digital signal processor(DSP). CPU is MVME147S, which is used for theoperating and ftle management, DSP is DSP8031 for
ftltering and other pattern control function, memory is
HIMV210, ND converter is DSP8112, D/A converter isDSP8124 and clock generator is DSP8240 for making
sampling clock of 6kHz. The system realizes fast I/0
handling by using a "mtt Link" bus independently. Thesystem has been designed to be maintained by E.W.S.,
including file management [4]. The sampling cycle ofbeam wavefonn is 1200Hz. That of the current pattern toQDS power supply is 6kHz, which was chosen to reduce

Clock(1200Hz),-------------------------------------------------T

The Beam Stop Signal(from Irradiation System)
Fig. 2. Schematic diagram of spill control system and HSWG.

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In addition, ODS magnets and power supply areused to determine the total dose for treatment. They candirectly move a horizontal tune from the resonance. Whenthe "beam stop" signal comes from the irradiation
system, forcing in ODS power supply without software
handling directly realizes stopping of a beam extraction
faster than few ms.

3-2. Computing system with VME
Encouraged by the experimental result of HSWG,
the beam spill control system was tested where HSWG
was removed from the system. The upper and lower limitof the band-pass filter were set to 150Hz and 100Hz. The
dimensions of FIR filter are 81. The relation between thegain of the beam control system and the 100Hz
component at the beam spill is shown in fig.4.
An effect in correcting the beam ripple of 100Hz
has been uridoubtedly recognized. However, it has not yet
been satisfactory. Possible explanations are as follows:(1) the parameters of the band-pass filter seem to beinappropriate; (2) time delay due to the. power supply andthe load must be taken account; (3) the. sampling cycle ofthe beam waveform seems to be too low. Next
experiments will be carried out to shape the spillenvelope at the low frequency.

3. Experimental result and discussions
3-1. HSWG withfeedfarward
The experiment was carried out first by using only
the "Harmonics Superposition Waveform Generator"
called "Ripplebasher" [5] for the feed forward to ODS
power supply. It is shown in the part enclosed by brokenlines in fig. 2.
Fig.3 shows beam spills and their FFf analysis in
the operation with and without HSWG, under thecondition of 3 x 108pps C6+ beam with an energy of350MeV/u. The amplitude and phase of HSWG wereadjusted to reduce the frequency component of 100Hz at
the beam ripple, respectively. The effect for ripple
reduction was -14dB at 100Hz.

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11 01000...E 90>E 80.

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00.02

0.04
gain

0.06

0.08

0.1

Fig. 4. Relation between the beam ripple of 100Hz and F/F
gain (high frequency regime). Points at gain 0 (e) represent
the ripple before the correction, while the others (0) showcorrection effect the system.

Acknowledgment
The authors would like to thank the crew of Ace.
Eng. Corp. and to Dr. K. Kawachi and the members of
the Division of Accelerator Physics and Engineering at
NIRS for helpful discussions.
(a)

(b)

References.

Fig.3 Comparison between without (a) and with HSWG (b).
beam spills and their FFf analysis are shown in the upperand lower photographs. Arrows points out a component of100Hz.

Harmonics of 50Hz component in the case ofsynchronizing with the PLL was decreased by somedegree compared with that in the asynchronizing case.

[1] Y.Hirao et al., HIMAC report, HIMAC-001.
[2] K.Sato et al., Nucl. Phys., A558, 229(1995).
[3]T.Inoue et al., IEEEtrans, Nucl. Sci., NS-26,3322(1979).
[4] E.Takada et al., The Hitachi Zosen tech.rev., vol.53,
No.4, 1992, pp317-323.
[5] M.Kumada et al., Proc. 4th EPAC., London, 1994,
pp.2338-2340.

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