Proceedings of IBIC2012, Tsukuba, Japan

MOPA37RELIABLE BEAM-INTENSITY CONTROL TECHNIQUE
AT THE HIMAC SYNCHROTRON
K. Mizushima#, T. Furukawa, T. Shirai, S. Sato, Y. Iwata, K. Katagiri, Y. Hara, K. Noda,
National Institute of Radiological Sciences, Chiba, Japan
Abstract
Short extraction called preliminary extraction has been
added before irradiation in order to avoid an overshoot ofthe beam spill. A fast beam shutter was developed andinstalled in the extraction line to prevent the beam
delivering to the patient during preliminary extraction.
The fast shutter enables us to switch from preliminary
extraction to irradiation in 100 ms, and the reliability ofthe beam-intensity control system was drastically
improved by the preliminary extraction technique.

INTRODUCTION_________________________________________#mizshima@nirs.go.jp

Figure 1: Typical overshoot of the beam spill at thestart of extraction.
Accordingly, the required solution is a repeatabilityindependent way, such as the one which removes theuncontrollable spilled particles selectively.
We have added short extraction, called preliminaryextraction (pre-extraction), as a technique for avoiding thespill overshoot. Before irradiation, the particles causing
the spill overshoot are removed unfailingly from thesynchrotron by pre-extraction. It is necessary to preventthe beam delivering to the patient during pre-extraction.
In addition, for reducing the dead time, it is desirable thatthe switching between pre-extraction and irradiation
performs in a short time. Therefore, a fast beam shutter
was developed and installed in the HIMAC extractionline.
The spill overshoot was prevented in the experiment at
the HIMAC, and the reliability of the beam-intensity
control system was drastically improved by the preextraction technique. In this paper, we describe the preextraction system including the developed fast shutter andthe experimental results.

PRE-EXTRACTION SYSTEM
System Layout
The pre-extraction system has been incorporated into abeam-intensity control system of the HIMAC
synchrotron. A block diagram of the beam-intensity
control system is shown in Fig. 2. An RF-knockout
controller works as the generator and feedback amplifierof the low-level RF voltage. The reference and actual
values of the beam intensity are provided from anirradiation control unit to this controller. The actual value
is measured by an ionization chamber.
The pre-extraction control unit is a Programmable
Logic Controller. This unit carries out the control of theISBN 978-3-95450-119-9

Feedbacks and Beam Stability

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○In raster scan irradiation for charged particle cancer
therapy, an upper limit of the beam intensity depends onthe magnetic scan speed. The beam intensities beyond thelimit cause extra dose deposition, which must be avoided
for reducing the damage to normal tissues. Some therapy
facilities, therefore, employ the feedback control systemof the extracted beam intensity [1], and National Instituteof Radiological Sciences (NIRS) in Japan is also one ofthem.
NIRS has carried out carbon-ion therapy with beamscanning [2] since May 2011. The carbon-ion beam isslowly extracted from the Heavy Ion Medical Accelerator
in Chiba (HIMAC) synchrotron using the third-orderresonance with the RF-knockout method [3]. The
intensity of the extracted beam is kept constant bycontrolling the amplitude of the transverse RF-field withthe feedback system [4]. In the beam-off period, thetransverse RF-field is turned off, and the betatron tune is
moved away from the resonance by exciting fast
quadrupole magnets. Their magnets are also turned off at
the start of extraction. However, then some particles
having large betatron amplitude go out of the stableregion at a stroke, and they are observed as a spillovershoot. At the HIMAC synchrotron, it was often
induced by a slight variation of the beam emittance inoperation cycles. A typical figure of the spill overshoot isshown in Fig. 1. The high overshoot of the beam spill iscapable of bringing dose hot spot inside the target
volume, because the tolerable beam-intensity in scanning
irradiation is low.
It is impossible to avoid the spill overshoot with the
feedback control of the transverse RF-field amplitude,
because the spill overshoot occurs even if the transverse
RF-field remains turned off. Although the fundamental
solution of this problem is the improvement of theoperation repeatability, it is difficult to maintain the beamemittance with high accuracy on a long-term basis.

MOPA37

Proceedings of IBIC2012, Tsukuba, Japanc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○beam shutter and the pre-extraction gate while receiving agate signal, which indicates permission for extraction,from the HIMAC timing system. It also sends anirradiation enable signal to the irradiation control unit.
A schematic timing chart of the pre-extraction control
is shown in Fig. 3. The beam is accelerated to the flattop
energy, and the pre-extraction is performed with the beam
shutter closed. The pre-extraction time is set to 200-400
ms. The beam shutter is opened as soon as the preextraction is finished. While driving the shutter, the beam
extraction is paused. The irradiation to the patient isstarted when the beam shutter is completely opened. Theirradiation is also turned off at the end of the flattop, andthe beam shutter is closed.

closed period. Using the measured value with theionization chamber, the beam intensity is controlled by
the RF-knockout controller even during pre-extraction.
The driving shaft is hollow, and the cables for the signal
readout and high-voltage supply pass through the hollow
shaft. The shutter plate and rotary table also have athrough hole to connect the cables to the ionizationchamber. The inside of the ionization chamber and hollow
shaft is filled with air. The internal air is confined within
the ionization chamber by closing an air valve, which isequipped on the middle of the shaft.
The driving time, which means all time spent on theopen or close controls of the fast beam shutter, isestimated at about 100 ms from the specifications of therotary actuator. The beam shutter enables us to switchfrom pre-extraction to irradiation with this driving time. It
was installed in the HIMAC extraction line, as shown inFig. 5.

Figure 2: Block diagram of the beam-intensity control
system at HIMAC.
Figure 4: Schematic drawing of the fast rotary beam
shutter.

Figure 3: Schematic timing chart of the pre-extraction
control.

Fast Beam Shutter
The fast beam shutter was developed to reduce the deadtime produced by switching from pre-extraction toirradiation. The open/close-mechanism of this beam
shutter is constructed with two parallel plates on a 90
degree rotary table, as shown in Fig. 4. The shutter plates
and rotary table are made in integral structure by cutting
an aluminum block. The magnetic fluid seal is used as avacuum seal for a shaft which drives the rotary table. The
driving mechanism of the shaft is a pneumatic rotaryactuator. The space between the parallel plates on therotary table is the beam aperture when the shutter is open.
An ionization chamber is mounted on one of plates inorder to measure the beam intensity during the shutter

Figure 5: Photograph of the fast beam shutter installedin the HIMAC extraction line.

EXPERIMENTAL RESULTS
The experimental test was carried out with a 140
MeV/n carbon ion beam at HIMAC to confirm theperformance of the developed beam shutter. The
experimental parameters are given in Table 1.

ISBN 978-3-95450-119-9
2Feedbacks and Beam Stability

Proceedings of IBIC2012, Tsukuba, Japan

Energy of 12C6+140 MeV/n

Betatron tune (Qx/Qy)

3.68/3.13

Stored particle number

6 × 109

Extraction beam-intensity

1 × 108 pps

The beam extraction gates for pre-extraction andirradiation were opened alternately in the experimental
test, as shown in Fig. 6. The intervals between the preextraction and irradiation gates were the times for driving
the beam shutter and sequential processing by the preextraction control unit. The actual driving times were 104
ms and 101 ms for the opening and closing of the shutter,respectively.
The beam spill was measured with the two ionizationchambers which are mounted on the beam shutter plate
and placed in the irradiation port. Both values measured
by these ionization chambers are sent to the RF-knockout
controller. In Fig. 6, the beam intensity was kept constantat 1 × 108 particles per second (pps) during pre-extractionin addition to irradiation. It means that the two
measurement values were successfully used for thefeedback control of the beam intensity in the RFknockout controller. In this experimental test, it wasconfirmed that the developed shutter prevented the beam
delivering to the irradiation port during pre-extraction.

overshoot was prevented by the pre-extraction technique,as shown in Fig. 7. The pre-extraction time was 200 ms.
We could remove the particles causing the spill overshootbeforehand by pre-extraction. As a result, we could obtainthe stable beam spill with a fast ramping.

Figure 7: Comparison between the beam spills withand without pre-extraction at the start of extraction.

CONCLUSION
In slow beam extraction for scanned carbon-ion therapy,
an overshoot of the beam spill is undesirable because it
brings dose hot spot inside the target volume. The preextraction system was introduced into the HIMAC
synchrotron to remove beforehand the particles inducing
the spill overshoot. In addition, a fast rotating beam
shutter was developed for the reduction of the dead time
due to the pre-extraction technique, and it was installed inthe HIMAC extraction line. The switching time, from preextraction to irradiation, of 100 ms was achieved by thedeveloped shutter, and the reliable beam-intensity control
was realized by the pre-extraction technique. Thistechnique is also effective in the multiple-energy
synchrotron operation [5], because it can prevent the spillovershoot induced by the emittance growth due to thestepwise beam deceleration.

ACKNOWLEDGMENT

Figure 6: Experimental test of the fast beam shutter
equipping the ionization chamber.
The effect of the pre-extraction technique was also
verified by the beam experiment at HIMAC. Although thehorizontal beam scraper in the synchrotron had beenutilized to limit the beam size in the usual condition, it
was not used in this experiment to confirm the preextraction effect clearly. The spill overshoot wasobserved without pre-extraction, as shown in Fig. 1. The
horizontal beam emittance was surely over the separatrix
area at the start of extraction due to the non-use of thescraper. Nevertheless, the generation of the spill

The authors would like to express their thanks to themembers of the Accelerator Engineering Corporation(AEC) for their skillful operation of the HIMAC
accelerator complex. We are grateful to Mr. N. Shinozaki,Dr. T. Miyoshi, Mr. A. Takubo and Mr. C. Kobayashi at
AEC for their support concerning the development of thefast beam shutter. This work was carried out as a part ofthe Research Project with Heavy Ions at NIRS-HIMAC.

REFERENCES[1] C. Schoemers et al., in: Proceedings of IPAC2011, San
Sebastian, Spain, 2011, 2851.
[2] T. Furukawa et al., Med. Phys. 37, 5672 (2010).
[3] M. Tomizawa et al., Nucl. Instr. and Meth. A 326, 399(1993).
[4] S. Sato et al., Nucl. Instr. and Meth. A 574, 226 (2007).
[5] Y. Iwata et al., Nucl. Instr. and Meth. A 624, 33 (2010).

ISBN 978-3-95450-119-9
Feedbacks and Beam Stability

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Table 1: Experimental Parameters

MOPA37