24pD45P
医療用負イオン源プラズマ中における高エネルギー電子が負イオン生成へ及ぼす影響の解析
Effect of High Energy Electrons on H- production
in a High Current DC Negative Ion Source for Cyclotron
尾内 杜彰1，衞藤 晴彦2，青木 康2，藤田 晋也1，柴田 崇統3，畑山
M. Onai, H. Etoh, Y. Aoki, S. Fujita, T. Shibata, A. Hatayama
1
慶應義塾大学2
住友重機械工業株式会社
3
高エネルギー加速器研究機構

明聖1

1

Faculty of Science and Technology, Keio University
2
Sumitomo Heavy Industries, Ltd.
3High Energy Accelerator Research Organization (KEK)In cyclotrons used for the medical application,negative hydrogens (H − ) are injected and accelerated toobtain higher extraction efficiently [1]. In such a medical
application of H − ions, it is highly required to enhance
the H − production in H − ion sources and to obtain thehigh H − beam current. Among various types of H − ionsources, in this study, we focus on the multi-cusp DC arcdischarge source [2]. A systematic study of the EEDF
(Electron Energy Distribution Function) in the arcdischarge plasma has been conducted. Especially, theeffect of the EEDF on the efficient H- production has beenstudied by the KEIO-MARC code (Kinetic modeling ofElectrons in the IOn source plasmas by the Multi-cuspARC-discharge) [3].
In Cs-free negative ion sources, H − ions are mainly
produced by the volume production. In the volumeproduction, the following two reactions take place
subsequently. The first reaction is H2 molecules’
vibrational excitation (EV reaction), which is caused by
the impact of the high-energy electron (several tens of eV)on the H2 molecule. In the second reaction, the H − ionis generated via the dissociative attachment (DA reaction)of the low energy electron (~1eV) to the vibrationallyexcited molecule H2 (𝑣) [4]. In our previous analyses
with the KEIO-MARC code, it is shown that Electron
Energy Distribution Function (EEDF) in JAEA 10A arcdischarge source has a high energy tail [3]. The highenergy tail, i.e., the non-equilibrium part of the EEDF,
possibly plays a key role for the efficient H − volume
production, because high energy electrons promote the EV
reactions, which lead to the increase in the H2(v) density
and resultant H − production. On the other hand, however,high-energy electrons also enhance H − destructions
through the electron detachment from the H − ion (EDreaction). Therefore, it is important to study the EEDF,
especially the high-energy part of the EEDF.
On the basis of the above discussion, the EEDF in theDC arc-discharge plasma has been analyzed by the KEIOMARC code. The KEIO-MARC code solves Boltzmannequation. In the KEIO-MARC code, collisions between
electron and hydrogen are taken into account by “nullcollision method” [5], while Coulomb collisions between
electrons by “binary collision method” [6]. By using theresults of the EEDF from the KEIO-MARC code, H −production rate/destruction rate is calculated by a systemof zero-dimensional (0D) rate equations, which include all
the H2 (𝑣) states. Effects of the arc-discharge
voltage/current on the EEDF and on the resultant H −production/destruction have been studied systematically.
The results of the analysis explain the experimental resultof H − extraction current reasonably. In the experiment,
the saturation of the extracted H − beam current isobserved at the high arc voltage region [7]. As shown inFig. 1, the saturation of H − is also observed in ouranalysis.
Furthermore, some improvements of the KEIOMARC code regarding neutral reactions and transport
have been conducted. These improvements of the code
will be also reported.

Fig. 1 H − density vs. Arc-Voltage (calculated)
[1]

[2]
[3]
[4]
[5]

[6]
[7]

M. E. Phelps, “Electronic Generators,” in PET: Molecular Imaging
and Its Biological Applications, (Springer, New York, 2004), pp.
227–229.
H. Etoh et al., Rev. Sci. Instrum. 85, 02B107 (2014).
T. Shibata et al., J. Appl. Phys. 114, 143301 (2013)
J. N. Bardsley et al., Phys. Rev. A 29, 106 (1984)
K. Nanbu et al., Proc. Soviet Union-Japan Symp. Comput. Fluid
Dynamics, (Computing Center of the USSR Academy ofSciences, 1989), Vol.2, pp.126-132
T. Takizuka and H. Abe,J. Comput, Phys. 25, 205 (1977).
H. Etoh, private discussion