Plasma and Fusion Research: Regular Articles

Volume 11, 2406063 (2016)

Study of H− Production in DC H− Source of Medical Cyclotron∗)Haruhiko ETOH, Moriaki ONAI1) , Yasushi AOKI, Hitoshi MITSUBORI, Yoshihiko ARAKAWA,Junji SAKURABA, Takanori KATO, Toshinori MITSUMOTO, Satoru YAJIMA,Takanori SHIBATA2) , Akiyoshi HATAYAMA1) and Yoshikazu OKUMURA3, a)
Sumitomo Heavy Industries, Ltd., Tokyo 141-6025, Japan
Graduate School of Science and Technology, Keio University, Yokohama 223-8522, Japan2)
High Energy Accelerator Research Organization (KEK), Tsukaba 305-0801, Japan3)Fusion Research and Development Directorate, Japan Atomic Energy Agency, Aomori 039-3212, Japan1)(Received 30 November 2015 / Accepted 20 February 2016)
A filament driven multi-cusp negative hydrogen ion source has been developed for proton cyclotrons. In order to increase the H− beam current, the dependences of H− beam current on various design/operating parameters
has been studied experimentally and numerically. In this paper, the eﬀects of arc-discharge voltage and current on H− production via the volume production process are investigated by three-dimensional kinetic modelingof electrons in the source plasma and zero-dimensional rate equations. This numerical analysis reproduces theexperimental results of H− beam current dependence on the arc-discharge condition, and also gives reasonable
explanations for their characteristics.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: negative hydrogen ion source, medical cyclotron, EEDF, KEIO-MARC code
DOI: 10.1585/pfr.11.2406063

1. Introduction

Negative hydrogen ion (H− ) beam is widely used for
medical proton cyclotrons, whose applications are cancer therapy and diagnostics, such as boron neutron capture therapy (BNCT) and the radioisotope production formolecular imaging technology. These applications require
high intensity beam extracted from cyclotron to shorten theirradiation time for BNCT and increase the production rateof medical radioisotope.
A DC arc-discharge multi-cusp H− source has beendeveloped for improving the cyclotron’s beam intensity.
Since the addition of Caesium (Cs) into this type of sourceis known to increase the production rate of H− on the surface of the plasma electrode [1], Cs-free and Cs-seeded
operations have been tested on our source. 10 mA of H−
beam current has been obtained stably at an arc-discharge
power of about 3 kW in the Cs-free operation [2], and H−
current has reached 22 mA at a lower arc-discharge powerof 2.6 kW with less gas flow rate and fewer co-extracted
electrons in the Cs-seeded operation [3].
In order to enhance H− beam current, the relationship between the H− production and the design/operating
parameters of our source (such as arc-discharge power,
magnetic field configuration, and so on) has been studied. The experimental results show the saturation of H−
beam current at higher arc power in the Cs-free operation
author’s e-mail: haruhiko.etoh@shi-g.com∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).
a) Present aﬃliation: National Institute for Quantum and Radiological
Science and Technology[2, 3]. Therefore, it is necessary to investigate the eﬀectsof the arc voltage (Varc ) and the arc current (Iarc ) on the H−
production to improve H− beam current. In the Cs-freeoperation, H− ions are produced by the volume production process that consists of the following two-stepreactions [4]:(a)(b)

H2 (v) + e → H2 (v ) + e,−H2 (v ) + e → H + H .

(EV)(DA)The first reaction (a) is the vibrational excitation (EV)by the impact of the high energy electron (several tens ofeV) on the H2 molecule. The second reaction (b) is theH− production via the dissociative attachment (DA) of thelow energy electron (∼1 eV) to the vibrationally excitedmolecule H2 (v). The cross section of DA reaction becomeslarge at higher vibrationally excitation level (v > 6) [5].
In the present paper, the H− production via these reactions at diﬀerent arc-discharge conditions are investigatedby the numerical analysis of electron energy distributionfunction (EEDF) in 3D3 V (three-dimensional real and velocity) space and zero-dimensional (0D) rate equations forH2 (v) and H− densities. The high energy tail of the EEDF
plays a key role for the eﬃcient H− volume production,
because high energy electrons promote the EV reaction.
Therefore, it is important to study the EEDF, especiallythe high-energy part of the EEDF. The EEDF analysis isconducted by KEIO-MARC code (Kinetic modeling ofElectrons in the IOn source plasmas by the Multi-cuspARC-discharge) [6–8]. There are several previous studies on modeling the volume production process that have2406063-1

c 2016 The Japan Society of Plasma
Science and Nuclear Fusion Research

Plasma and Fusion Research: Regular Articles

Volume 11, 2406063 (2016)provided the fundamental comprehensions of H− production in the source [9–19]. These achievements and related
works are summarized in Ref. 20. However, these studies
have been conducted by 0D numerical models without thespatial distribution of electrons. The transport of electrons
has been modeled by the simple global confinement time.
In contrast, KEIO-MARC code can directly solve the electron trajectories in the three-dimensional (3D) real space
to reproduce a more realistic physics in the source.

2. EEDF Analysis by KEIO-MARC
The geometrical model of our source is shown inFig. 1. The source consists of a cylindrical plasma chamber
made of copper, multi-cusp permanent magnets, and threeelectrodes system. These electrodes are a plasma electrode, an extraction electrode, and a grounded electrode.
Dipole magnets called as filter magnets are located at theextraction region of the chamber to create the magnetic filter field [21] whose direction is perpendicular to the central axis of the chamber. The magnetic filter divides thechamber volume into high energy electron region (driverregion) for the EV reaction, and low energy electron region (extraction region) for the DA reaction. The Z-axis isdetermined as the central axis of the chamber and its value
Z is defined as the distance from the inner surface of theplasma electrode as shown in Fig. 1. The internal diameter and length of the chamber are 100 mm and 160 mm, respectively. Hydrogen plasma is generated by arc-discharge
between two tungsten filaments and the chamber wall. The
3D calculation of magnetic cusp and filter fields is conducted by ANSYS software package [22]. Figure 2 shows
the calculated magnetic field on the Z-axis. The peak valueof the magnetic filter field is about 110 G at the plasma
electrode’s surface (Z = 0 mm). The other peak at the bottom side of the chamber (120 mm < Z < 160 mm) is dueto the magnetic field generated by cusp magnets locatedunder the filaments.
In KEIO-MARC code, the equations- of motion foreach electron is followed by Eq. (1),medve= −e(ve × B) + (collision force),dt(1)where me , ve , and B are the electron mass, velocity, themagnetic field, respectively. Equation (1) is solved by
the leap-frog method [23], and Coulomb collision process between electrons is calculated by the binary collision
method [24]. More than five hundred reactions between
the electrons and the hydrogen species are modeled by thenull collision method [25]. In Eq. (1), the electric field is
not included because quasi-neutrality is assumed. Sheathpotentials at the wall and the surface of the filaments aretaken into account by simple models [8]. The initial energyof electron is set to be the same as the Varc . The number ofelectrons emitted from the filaments is determined by theIarc .
The number of test particles is set to be 1 × 104 . The

Fig. 1 The cross sectional view of the source.

Fig. 2 The magnetic field on the Z-axis.
Table 1 Densities and temperatures of hydrogen species.

hydrogen gas pressure is 1.6 Pa, which is estimated fromthe actual value of gas flow rate measured experimentally.
Other parameters of neutral particles and ions, such as
their gas temperatures and densities, are given from Ref. 4.
These parameters are summarized in Table 1. The internal space of the chamber is divided into small cubic cells(2 mm × 2 mm × 2 mm). The wall boundary is specified by
the cell located at the chamber wall. The EEDF and the resultant electron density are calculated for each spatial cellwith the number of test electrons and the cell-volume.
Figure 3 shows an EEDF dependence on the arc voltage Varc at the driver region (Z = 80 mm). The arc current
Iarc is constant as 20 A. High energy tail of the EEDF isshifted toward the higher energy side as Varc rises. Figure 4shows an EEDF calculation results at Iarc = 20 A in the extraction region (Z = 16 mm). The high energy electrons
are reduced by the magnetic filter field in the extraction region. Figures 5 and 6 show an EEDF dependence on Iarc at2406063-2

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Fig. 3 EEDF dependence on arc voltage at the driver region (Iarcis 20 A).

Fig. 5 EEDF dependence on arc current at the driver region (Varcis 60 V).

Fig. 4 EEDF dependence on arc voltage at the extraction region(Iarc is 20 A).

Fig. 6 EEDF dependence on arc current at the extraction region(Varc is 60 V).

Varc of 60 V in the driver region and the extraction region,respectively. These results show that Iarc does not change
the high energy tail of the EEDF, but has a role to increase
the number of electrons.

3. H2 (v) Density
The H2 (v) population density is calculated by rateequations for each vibrationally state (0 ≤ v ≤ 14)dN(v)= N(e)N(v )R(eV; v → v)
dt+ N(e)N(v )R(EV; v → v)
− N(e)N(v)R(eV; v → v )− N(e)N(v)R(EV; v → v )− N(e)N(v)R(diss; v)− N(e)N(v)R(DA; v)− N(v)N(H+ )R(IC; v) − γN(v)+ S,τ(2)where N(e) is the electron density, N(v) and N(v ) are densities of hydrogen molecules at diﬀerent vibrationally excitation levels v and v respectively. In addition, N(H+ )is the H+ density, γ and τ are the reflection rate of vibrationally excited hydrogen molecules at the wall boundary
and their confinement time, respectively. Finally, S is theneutral source determined by the gas flow rate. The electron density N(e) is calculated from the EEDF at the driver
region. In Eq. (2), N(H+ ) is adjusted to be equal to N(e),so as to keep the charge neutrality at each spatial cell. Thereactions taken into account in the analysis are listed inTable 2. The reaction rate coeﬃcients R in Eq. (2) are calculated from the cross section data from the references inTable 2 and the EEDF at the driver region. The confinement time τ is calculated as L/vH2 , where L is the radiusof the plasma chamber and vH2 is the average velocity of
H2 molecules. γ is the parameter given parametrically [9],and is set to be 0.8 in the analysis.
Figure 7 shows the dependence of H2 (v) density onVarc at each excitation level. The arc current Iarc is 20 Ain this analysis. As shown in Fig. 7, The H2 (v) density increases with Varc increasing. This is caused by the increasein the high energy tail of the EEDF with Varc as shown inFig. 3. Figure 8 shows same data from Fig. 7 replotted witha linear scale, the H2 (v) density saturates at higher Varc dueto the H2 (v) destruction caused by high energy electrons.
Figure 9 shows the dependence of H2 (v) density on Iarc at
Varc = 60 V. As seen from Fig. 9, the population density ofH2 (v) does not depend so much on Iarc . These results show

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Table 2 Reactions taken account in Eqs. (2) and (3).

Fig. 9 H2 (v) density at each vibrationally excitation level for different arc currents (Varc is 60 V).

Fig. 7 H2 (v) density at each vibrationally excitation level for different arc voltages (Iarc is 20 A).

Fig. 10 Normalized H− density as a function of arc voltage calculated by Eq. (3).

Fig. 8 H2 (v) density at each vibrationally excitation level (5 ≤v ≤ 14) from Fig. 7 replotted with a linear scale.

that the high energy tail of the EEDF mainly enhance thepopulation density of H2 (v) with the relatively high vibrationally excited number (v > 4).

4. H− Density

The H− density is calculated by the rate equation at
the steady state [29]N(H− ) =N(e)N(v)R(DA; v),
N(H+ )R(MN)+N(H)R(AD)+N(e)S (ED)+ 1τ
(3)where N(H− ) and N(H+ ) are the H− density and the H+density, respectively. In Eq. (3), N(e) is given by the EEDF
at the extraction region, while N(v) is given by the H2 (v)density discussed in the previous section.
Figure 10 shows the H− density as a function of Varc
for diﬀerent Iarc = 20 A, 30 A, and 40 A. Figure 11 showsthe H− beam current measured at the Faraday Cup as afunction of Varc . In both figures, data points are normalized
by the values of the maximum H− density and beam current (Varc = 80 V, Iarc = 40 A). The simulation result of theH− density reproduces the H− beam current dependences
on Varc and Iarc obtained experimentally. In the simulation
result, the H− density is increased by Varc and saturates athigher Varc . These characteristics are possibly explainedby the H2 (v) density dependences on Varc . The H− density
is also increased with Iarc , which is caused mainly by theenhancement of the DA reaction, because Iarc increases thelow energy electrons at the extraction region as shown in
Fig. 6, and does not change the H2 (v) density as shown in
Fig. 9.

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Fig. 11 Normalized H− beam current as a function of arc voltage
obtained experimentally.

5. Conclusion

The numerical study of H− production via the volume
production process gives the reasonable explanation of theH− beam current dependencies on Varc and Iarc obtained
experimentally. Arc voltage has a role to enhance high energy tail of the EEDF, which promotes H2 (v) production
through the EV reaction. Because of this enhancement ofH2 (v), H− density increases with Varc . The saturation of H−
density is due to the H2 (v) destruction caused by high energy electrons. On the other hand, Iarc does not change thehigh energy tail of the EEDF but increase the number ofelectrons. The increase in low energy electrons at the extraction region enhances the H− production thorough theDA reaction.
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