24pD40P
医療サイクロトロン用大電流負イオン源の開発
Development of a High Current Negative Ion Source for Medical Cyclotrons
衞藤晴彦1，尾内杜彰2，青木康1，三堀仁志1，荒川慶彦1，加藤隆典1，櫻庭順二1，
密本俊典1，矢島暁1，柴田崇統3，畑山明聖2，奥村義和4
H. Etoh1, M. Onai2, Y. Aoki1, H. Mitsubori1, Y. Arakawa1, T. Kato1, J. Sakuraba1,T. Mitsumoto1, S. Yajima1, T. Shibata3, A. Hatayama2, Y. Okumura4
1

住友重機械工業（株），2慶應義塾大学，3高エネ研，4原子力機構
1
Sumitomo Heavy Industries, Ltd., 2Keio Univ., 3KEK, 4JAEA

Negative hydrogen ion (H−) beam is widely used
for medical cyclotrons whose applications arecancer therapy and diagnostics. A high-current DC
H− source has been developed to improve these
cyclotrons’ beam intensity. The multi-cusp DC
arc-discharge type source has a beam extraction
system which consists of a plasma electrode, anextraction electrode and a grounded electrode. H−
ions are produced by the volume production process[1] and the surface production process with additionof caesium (Cs) [2]. In our previous studies, stable
10 mA of H− beam current has been obtained
without Cs addition [3], and it has reached 22 mA
in Cs-seeded operation [4].
In order to enhance H− current in both the Cs-free
and the Cs-seeded operations, some additional
studies have been done. First, the extracted H−
current dependences on the extraction voltage (Vex)and on the acceleration voltage (Vacc) is investigated.
Experimental results show that higher Vex and Vacc
have to be applied to extract higher H− currents (Fig.
1). These experimental results are compared withthe beam optics calculation by using BEAMORBT[5]. The optimum Vex which gives the highest H−
current at each Vacc, shows good agreements withthe simulation results. In the beam optics
calculation, the optimum Vex is determined so as tominimize the beam divergence. Extrapolating theVex dependence on H− current density, Vex of morethan 5.5 kV will be necessary to extract more than
30 mA of H−. The simulation also shows that thedistance between the extraction electrode and thegrounded electrode has to be shortened in order tominimize the beam divergence and the beam losses
at the extraction electrode.
Second, the relationship between the H−
production and the design/operating parameters(such as arc-discharge voltage and current,
magnetic filter field strength, and so on) is being
studied experimentally and numerically. A
systematic numerical study of electron energy
distribution function (EEDF) in the source plasma

has been conducted by KEIO-MARC simulation
code [6], and the resultant H− density through thevolume production process in the source has been
analyzed by the zero-dimensional (0D) rateequation for different arc-discharge conditions asshown in Fig. 2. The results of the analysis explain
the H− current dependences on the arc voltage andon the arc current obtained experimentally.

Fig. 1. H− current dependence on Vex and Vacc

Fig. 2. H− density vs. arc voltage[1]M. Bacal, A. Hatayama, and J. Peters, IEEE Trans. Plasma
Sci., 33, 1845-1871 (2005).
[2]S. R. Walther, K. N. Leung and W. B. Kunkel, J. Appl. Phys.
64, 3424 (1988).
[3]H. Etoh et al., Rev. Sci. Instrum. 85, 02B107 (2014).
[4]H. Etoh et al., Rev. Sci. Instrum., submitted.
[5]Y. Ohara, “Simulation code for beam trajectories in an ionsource”, JAERI-M 6757 (1976).
[6]T. Shibata, M. Kashiwagi, T. Inoue, A. Hatayama and M.
Hanada, J. Appl. Phys. 114, 143301 (2013).