22P090-P
Measurement of the ionization cross section for a negative Au ion beam withthe tandem accelerator
タンデム加速器を用いた金負イオン電離断面積の測定
Tomoaki Shinobu(1), Akira Taniike(1), Syo Mochizuki(1), Takeshi Ido(2),Masaki Nishiura(2), Akihiro Shimizu(2), Yuichi Furuyama(1), Akira Kitamura(1)忍 友彰(1)，谷池 晃(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
In this study, we investigated the fundamental process of heavy ion beam generation by our tandem
accelerator. The gas thickness in the gas cell was calibrated by the measurement with using BARATRON,
the conductance calculations, and the reference experiments with known cross sections. The ionization
cross sections of the negative Au ion was obtained with the beam attenuation method, and the energy wasbelow 1000 keV.

1. Introduction
A heavy ion beam probe (HIBP) method is adiagnostics method to measure the plasma potential
directly. On HIBP system for large helical devices(LHD) which is one of the plasma confinement
devices, a tandem accelerator of 3 MV is used. A
beam of 1.5 MeV Au+ ions has been used for
diagnostics in plasma confined magnetic field of
1.5 T (6 MeV is for 3 T) [1]. Recently HIBP has
measured the potential of the plasma whose
electron density is about 0.1×1019 cm-3. In order toapply it to a high-density plasma, it is necessary toincrease the beam current. When an Au- ion collides
with an atom in the gas cell of the tandem
accelerator, the phenomena such as charge
exchange, ionization, electron capture, andscattering occur. Therefore, in order to investigate
the fundamental process of the heavy ion beam
generation in this study, we obtained the total
ionization cross section of Au- ion using the ion
beam current detection system at the 0 degree beamline of our tandem accelerator.

Fig. 1. Schematic view of the tandem accelerator
system at Kobe University.

Figure1 shows Schematic view of the tandem
accelerator system at Kobe University.
2. Calibration of the gas thickness in a gas cell
In order to calibrate the gas thickness in the gascell, first we investigated the pressure of central
part of the gas cell. Fig. 2 is Schematic view of thegas cell part.

Fig. 2. Schematic view of the gas cell.

In order to measure the pressure of the central partof the gas cell directly, the pressure transducer(BARATRON) was installed in the gas cell upper
part of the high-pressure terminal. WhenBARATRON is installed, it is not easy to measure
the pressure in the tandem operation. Then, therelationship between of BARATRON and theionization gas gauge controller (HE IGC) installedin outside of accelerating tube was investigated forsome kind of gas. Because HE IGC is placed at
ground potential, it can measure the pressure in theoperation, and the pressure of the gas cell central
part is determined by the value. To take into
account the pressure distribution in the gas cell,
conductance calculations conducted. From theseresults, the gas thickness in the gas cellcorresponding to the gas species was calibrated.
Details of the experiments is done in a poster
presentation.
3. Measurement of the ionization cross sections
for a negative Au ion beam
In the rate equation taking into account of Au-,Au0+, Au1+, Au2+, Au3+ and Au4+, σ0,-1F0 becomes
negligible in the case of the small gas thickness,and the fraction of Au- can express in the following
approximation.

F  eThe positive charged Au ions are increased withdecreasing of Au- ions. A value deviates fromexponential curve because the positive Au ions are
simultaneously measured as gas thickness becomeslarge. In each energy, There data is fit byequation(2) in the range of 0 - 0.1 × 10-16 cm-2, andthe total ionization cross sections of Au- in each
impact energy were determined. The results areshown in Fig. 3. Comparing the total ionization
cross sections with that indicated in reference [3],our data in this experiment is somewhat small, but
it is in agreement with less than 13%.

Data of reference [3] ( 1, 0  1,1  1, 2  1, 3  1, 4 ) x.
(1)
The fraction of Au- decreases exponentially withincrease in the gas thickness. The data of Aucurrents for some energy were fit by the following
exponential functions.
I = ILE exp(-σx).
(2)
Here, I is the current measured in experimental
chamber, I0 is the current measured by LE FC, σ isthe total ionization cross section, x [cm-2] = nl is gas
thickness.[2]4. Result
In these experiments, Ar was used as stripping gas.
Au- beam was transported to the experimental
chamber and it was focused on the FC in a
detection chamber. Then, the pressure in the gascell was changed and Au- ion beam current wasmeasured. Moreover, experiments were conducted
for various impact energies of Au- ions. The results
were shown in Fig. 2.

Our data

Fig. 3. Dependences of total cross sections and thecoefficient on the impact energy.

5. Conclusion
Investigating the relationship between pressure
measured by BARATRON and HE IGC, theabsolute pressure inside the gas cell in operation
was obtained, and the gas thickness in the gas cell
was calibrated by the pressure of HE IGC.
Moreover, using the gas thickness, the Au- ion totalionization cross sections of various impact energies
were obtained from the Au- beam attenuation
measurements. Although there are almost equal tothe value of reference [2], our data is somewhat
smaller. In order to determine the various crosssections in collision, in addition to the resultspresented in this study, measurements for theneutral particles and the positive ions are needed.
[4]
References

Fig. 2. Measurement for some impact energies.

[1] T. Ido et al., J. Plasma Fusion Res. Vol.86, No.9 ，pp.507-516(2010)[2] M. Nishiura et al., NIFS Research Report, NIFS
Series 884(2008).
[3] M M Sant’Anna et al., Plasma Phys Control.
Fusion, 51, 045007(2009)[4] A. Taniike et al.,Plasma Fusion Res. 5,S2087(2010)