Nuclear Instruments and Methods in Physics Research B 240 (2005) 44–47

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Proof examination of alternating phase focusing

Kazuo Yamamoto *, Toshiyuki Hattori, Noriyosu Hayashizaki, Toshiki Hata,Hirotsugu Kashiwagi, Yasuyuki Takahashi

Tokyo Institute of Technology (TITech.), Tokyo 152-8550, Japan

Available online 22 August 2005

Abstract

We designed and constructed a new compact IH-LINAC with Alternating Phase Focusing (APF). The LINAC wasdesigned to accelerate O+ ions up to 77 keV/u from 10 keV/u as a proof of the APF (0.5 m in length, 0.6 m in diameterand 63 MHz in operation frequency). We constructed a test bench for accelerating protons. It included that a PIG ionsource, a focusing lens in front of the cavity and a bending magnet to analyze the accelerated beams. We succeeded inthe world�s first acceleration with the IH-LINAC using APF. This paper describes the design of the linac and the resultof the proof examination.� 2005 Elsevier B.V. All rights reserved.

PACS: 29.17.+w; 29.27.�a

Keywords: Heavy ion accelerator; IH linac; Alternating phase focusing structure

1. Introduction

Recently, cancer therapy is one of the majorapplications of hadron accelerators. In the typicalconventional design, a chain of linacs occupies alarge area in cancer-therapy facilities. For in-stance, an injector of HIMAC (Heavy Ion MedicalAccelerator in Chiba), consists of RFQ linac andAlvarez linac, accelerates C4+ ions up to 6 MeV/u;

0168-583X/$ - see front matter � 2005 Elsevier B.V. All rights reservdoi:10.1016/j.nimb.2005.06.086

* Corresponding author. Address: 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan. Tel./fax: +81 3 5734 3055.

E-mail address: 02d19092@nr.titech.ac.jp (K. Yamamoto).

the length is over 30 m [1]. Particularly in such amedical accelerator complex, not only machineperformance, but also cost performance is veryimportant. Therefore a compact and reliable linachas been strongly required. For this purpose, wedesigned an Interdigital-H mode linac with alter-nating-phase focusing as a high-efficiency injector.The IH structure has an advantage of a high-shuntimpedance in the low-energy region. The APF hasnot yet successfully accelerated beams, although ithas been proposed for the design of short low-betastructures due to eliminating the external trans-verse focusing by drift-tube quadrupoles [2–5].

ed.

K. Yamamoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 44–47 45

As the first proof examination in the world, a pro-ton beam was successfully accelerated up to77.2 keV from 10.8 keV. This paper describes thedesign of the IH-LINAC with APF and the resultof the acceleration test.

Fig. 2. Measurement result of the electric field distribution ofthe linac.

Table 1Design parameters of the linac

Particle =1/16

Input energy 10.8 keV/uOutput energy 77.2 keV/uFrequency 63.3 MHzNumber of cell 16Cavity length 530 mmDiameter 630 mmTransverse acceptance 88p mm radLongitudinal acceptance 30 degreeGap voltage 74–145 kV

2. Design and manufacture

At first, the basic parameters were determinedfor O+ acceleration, which were mainly an injec-tion energy of 10 keV/u, an acceleration energyof 77 keV/u and an operation frequency of63.3 MHz. For stable operation and efficient accel-eration, the electric-field strength in the gaps andthe effective acceleration voltage were determinedto be twice the Kirpatric limit and 4 MV/m,respectively. Based on these conditions, the linacwas designed to be 0.53 m in length. The numberof cells is 16, which was determined by the linaclength divided by the average cell length. Eachgap voltage and length was designed as follows:(1) The 1st gap length was determined to be halfof the 1st cell length. (2) The nth gap voltage dis-tribution was determined to maintain the totalacceleration voltage. (3) All of the gap lengthwas determined to be twice the Kilpatric limit ofthe electric-field strength. Based on the above de-sign condition, we optimized the phase pattern inorder to work APF effectively using the thin-lensapproximation. The result is shown in Fig. 1.

Next, we constructed a half-scale cold model inorder to find the length of an end-ridge tuner toobtain the required electric-field distribution. The

Fig. 1. Phase pattern in the gap of the linac.

measurement was carried out by a perturbationmethod; a small aluminium ball was put onto theacceleration axis, and the electric field at this

Fig. 3. Photograph of the IH linac with APF.

46 K. Yamamoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 44–47

position was obtained by measuring the variationin the frequency. The electric-field distributionwas obtained by changing the ball position. Theoptimum electric-field distribution was obtainedunder the cut-area length of 150 mm. The resultis shown in Fig. 2 and the final parameters aresummarized in Table 1. Based on these parame-ters, the beam dynamics was re-calculated.

The cavity was processed within an accuracy of±0.1 mm. After plating Cu on the cavity, the drifttubes and the length of each gap were alignedunder an accuracy of ±1% (Fig. 3). The vacuumwas checked after assembling the side plates. Thefrequency and Q-values were 63.32 MHz andaround 15370, respectively.

Fig. 4. Photograph of the experimental set-up.

Fig. 5. Layout of the ex

3. Acceleration test and the results

The effective shunt impedance of the linac is ex-pected to be 405 MX/m. An acceleration test of aproton beam was carried out in the test benchshown in Fig. 4. A Penning Ionization Gauge(PIG) ion source with an extraction voltage of10.8 kV was utilized to supply protons. For low-energy beam transport, a solenoid lens was pre-pared. The acceleration energy after the IH linacwas analyzed by a bending magnet after focusingby an electric-quadrupole lens. Slits were preparedafter the magnet to improve the resolution (Fig. 5).

Fig. 6 shows the energy spectrum of acceleratedions. It is clearly shown from the figure that theprotons were accelerated up to the design energy.During this measurement, the focusing-lensstrength was adjusted so that the accelerated-ionscurrent would be maximum at the Faraday cup(FCN3) after the bending magnet. Since it washard to decrease noises, the slit in front of theFCN3 was fully opened. The signal widths in theenergy spectrum do not mean the energy spread.Fig. 7 shows the accelerated energy and the ioncurrent as a function of the RF power. The gapvoltage, given by the measured Q-value, corre-sponds to an RF power of 45 W. The accelerationefficiency of the IH linac was around 6%. It shouldbe noted that the PIG ion source supplied beamsunder DC operation while the phase acceptanceof the linac was 30�. By pulsing the beam with abuncher so as to match the beam-pulse length withthe acceptance, the efficiency is expected to eventu-

perimental setup.

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100 120 140

Energy [keV]

Cur

rent

[arb

.]

H3+

Accelerated proton

H2+

H+

Fig. 6. Energy spectrum of the accelerated beam. The acceler-ated proton is shown at an energy of 77 keV.

Fig. 7. Current and energy of the accelerated proton beam as afunction of the RF-power.

K. Yamamoto et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 44–47 47

ally be around 70%. Also, by improving the trans-verse matching, the acceleration efficiency is esti-mated to be improved up to 8% even under DCoperation of the PIG ion source.

4. Conclusion

A proof test of IH linac with APF was success-fully completed, which is the first acceleration of abeam by using the APF principle in the world.The results of the proof examination with low-current proton beams are in good agreement witha numerical prediction. In order to evaluate theperformance of the IH linac with APF, however,high-current and high-energy acceleration testsare strongly required. We are planning to acceler-ate C4+ ions up to a few MeV/u with a few mAusing a new linac and a new ECR ion source.

References

[1] S. Yamada et al., Proc. APAC, Tsukuba, 1998, p. 885.[2] T. Hattori et al., Nucl. Instr. and Meth. B 161 (2000) 1174.[3] N. Hayashizaki et al., Rev. Sci. Instrum. 71 (2000) 990.[4] V.V. Kushin et al., Proc. PAC, Washington DC, 1993, p.

1798.[5] D.A. Swenson, Part. Accel. 7 (1976) 61.