22P089-P
Charge Fraction Measurements for Au Ion Beams Generated
by tandem accelerator
タンデム加速器で生成された金イオンチャージフラクションの測定
Akira Taniike(1), Tomoaki Shinobu(1), Syo Mochizuki(1), Takeshi Ido(2),Masaki Nishiura(2), Akihiro Shimizu(2), Yuichi Furuyama(1), Akira Kitamura(1)谷池 晃(1)，忍 友彰(1)，望月 翔(1)，井戸 毅(2)，西浦正樹(2)，清水昭博(2)，古山雄一(1)，北村 晃(1)(1)

Maritime Sciences, Kobe University
5-1-1, Fukae Minami-machi, Higashinada-ku, Kobe 658-0022, Japan神戸大学大学院・海事科学研究科 〒658-0022 神戸市東灘区深江南町5-1-1(2)
National Institute for Fusion Science322-6 Oroshi-cho, Toki 509-5292, Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
The Heavy ion beam probe method is one of the methods of measuring the plasma potential, and themethod can be useful to study a mechanism of plasma confinement. In this study the fundamental
properties of the generation process of the charged heavy ion beams were examined. Charge fractions ofAu ions were measured with a tandem accelerator at Kobe University. After Au- ion whose energy wasbelow 1 MeV was collided with atoms in a gas cell at high voltage terminal, the positive Au ions weregenerated. These ionization and recombination cross sections in collision of Au ions/neutral and target gas
were estimated from the experimental results.

1. Introduction
The Heavy Ion Beam Probe (HIBP) method is amethod of the plasma diagnostics, which measuresplasma potential distribution. In HIBP method forLarge Helical Device, a high energy Au+ ion beam
is injected into the LHD plasma, and the Au++ ionsare detected. It is expected that the method isapplied to the higher density plasma in future [1].
An enhanced Au+ beam current is needed for thepurpose. The LHD-HIBP system has a tandem
accelerator with a gas cell which is a gas
confinement system for charge stripping of ions.
Our tandem accelerator which has a gas cell canbe used in preliminary experiments forenhancement of the Au+ ion beam current.
Measurements of the ratio of the ion beam currents
for some charge state ions, i.e. charge fraction, canbe conducted.
The measurement system of the ion beams in thepresent work, taking account of the atomic /molecular collision process in the gas cell, isavailable to obtain the physical value such as theionization cross sections. Determination of the gas
thickness is one of the most important items toobtain the cross sections. The results will bepresented in a poster of 22P090-P.
2. Experimental Apparatus
In the present work tandem accelerator system at
Kobe University has been used. The accelerator isof the pelletron type and the maximum terminal
voltage is 1.7 MV. There are five beam lines atintervals of 15 degrees for the experiments such as
the accelerator analyses or the ion beam irradiation.
The ion beam is bended by a switching magnet (SWMag.) to each beam line. Because mass of the Au
atom is too heavy, the ions with energies of theorder of MeV cannot be bent into the existing beamline. The maximum energy should be about 300
keV, and the incident energy of Au- into the gas cellis 150 keV [2]. Therefore, an ion detection systemwith a small deflection angle was installed at 0degree (straight beam line) [3]. The system isshown in Fig. 1. The system allows detection of thehigh energy Au ions.

Fig. 1. The detection system for high energy Auions
When the charge fraction is measured, thefollowing attention is necessary. Because thebending angle of ions is small, non focused single
charged ions, for example, are overlapping to the
doubly charged ions. Consequently, the current of
Au++ ion beam should be corrected for theNormalized Beam Current [nA/nA]

3. Results and Discussion
Au- ions were generated by a Cs sputter typenegative ion source and were injected into thetandem accelerator. N2 gas was introduced as target(stripper) gas. The energy of Au- ion at the gas cellwas 500 keV, the terminal voltage was 479 kV. An
attenuation curve of Au- beam current as a functionof the target gas pressure was obtained and isshown in Fig. 2. In the figure the horizontal axis
indicates the pressure, PHE, measured at thepost-acceleration beam-duct, and is not calibrated togas thickness. The Au- beam current is normalized
by the current measured at LE FC, which is the
Faraday cup placed between the ion source and thetandem column.
0.020.2
Au -0.015
Au +AuAu+
Au++
Au+++

0.1
Au +++
0.05

Au ++0004628Gas Pressure, PHE [10-7 Torr]

0.2
Au +
Au ++

0.15

Au+
Au++
Au+++

0.1
Au +++
0.05

002468SW Mag. Current [A]1012Fig. 3. Momentun spectra for Au+, Au++, and Au+++
ion beams.
The gas pressure dependences of Au+, Au++, andAu+++ ion beams are shown in Fig. 2, together withthat of Au-. The current was normalized by thecurrent of LE FC and the charge state. Some crosssections can be deduced from these data and that ofneutral particles. Since an experiment for theneutral particles was not conducted, someassumption is needed for obtaining cross sections.
In our poster, an estimation of the cross sections
will be presented in detail.

0.15

0.01

0.005

It is shown in these results that the distribution of
the Au++ ions was overlapped by that of the Au+ions. Therefore, when the gas pressure dependenceof Au++ ion beam is measured, a correction in abeam current is needed. The same is true in the casewith the Au+++ ion beam.
Normarized Bean Current [nA/nA]

spreading Au+ beam current.
There is a restriction on selection of stripping gas
species. When the terminal voltage is applied, atube for transporting the stripper gas to the terminal
sometimes suffers from high-voltage breakdown
and SF6 gas to insulate the accelerator columns is
introduced into a stripper gas cell. In the case of Ar
gas this phenomenon is avoided by raising thepressure of a gas cylinder to 1 MPa. But in the caseof Ne, the phenomenon is caused even if the same
treatment was made. In present work, N2 gas wasused as the stripper gas, because the gas has
resistance to high voltage operation.

10Fig. 2. Charge fractions for Au-, Au+, and Au++
beams as a function of gas pressure.
A current through SW Mag. was set so that theAu+ ions could be detected. Then, the parameters ofEinzel lens, Q-doublet lens, and so on was adjusted,
so that the Au+ beam current became maximum.
The SW Mag. current was swept, and the energy(momentum) spectrum of the Au+ beam wasmeasured. The result is shown in Fig. 3, together
with the results for Au++ and Au+++ using the sameprocedure.

4. Summary
The Au ion beams were generated bycharge-exchange collisions with N2 gas introduced
into the gas cell. The charge fractions of Au ionswere measured in our tandem pelletron accelerator.
The gas pressure dependence of some charge stateions was measured correctly by removing a
influence of the overlapped other ions. In our poster,an estimation of the cross sections and theexperiments for other gas and energy will bepresented in detail.

References[1] T. Ido et al., J. Plasma Fusion Res. Vol.86, No.9，pp.507-516(2010)[2] A. Taniike et al., Plasma Fusion Res. 5,S2087(2010)[3] A. Taniike et al., Rev. of the Faculty of Mar.
Sci., Kobe Univ., No. 8, pp.47-58(2011).