23P173-B
Particle Reflections at the Extraction Electrode Surface of an Ion Source
イオン源引出電極面での粒子反射
Motoi Wada1, Toshiro Kasuya2, Takahiro Kenmotsu1 and Shoichi Tokushige2
和田 元1, 加藤周一2，粕谷俊郎2，剣持貴弘1，徳重翔一1
1

Faculty of Life and Medical Sciences, Doshisha University
2
Graduate School of Engineering, Doshisha University
Kyotanabe, Kyoto 610-0321, Japan
同志社大学1生命医科学部・2工学研究科，〒610-0321 京田辺市多々羅都谷1-3
Effects upon ion beam characteristics due to particle reflections at the plasma electrode surface are being
investigated both experimentally and theoretically. Based on the results recently reported by Bacal et al.
[1], negative hydrogen ion (H-) yields at the plasma electrode surface have been reexamined. The
negative ionization efficiency at the surface depends upon the H- velocity component perpendicular to thesurface. The strength of the filter magnetic field had turn out to play a decisive role in H- extraction, as it
determines the incident angle for incoming hydrogen ions onto the plasma grid surface.

1. Introduction
Highly positive plasma potential was observed in
an RF driven ion source for negative hydrogen ionThe measured plasma(H-) production [1].
potential as high as 60 V accelerates hydrogenplasma ions toward the plasma electrode surface
biased at 15 V. Thus, 45 eV H+, H2+ and H3+ ionsare back scattered at the surface as 45 eV, 22.5 eV
and 15 eV H atoms, as the mean free paths for theseions can be comparable to the ion source dimension[2]. Bacal had assumed the negative ionization
fraction based on the normal incidence of these ionsat full energy [3].
However, the velocity
distribution function of surface produced H atomsshould determine the negative ionization of H
atoms leaving the surface, and a study to estimate
the velocity distribution function has been started

ions should be largely bent by strong magnetic fieldof the filter magnets [5]. Sheath structure of theextraction electrode also affects the H- current, andthe extraction model must include all these aspects.

2. ACAT calculation
The energy distribution functions of hydrogenatoms reflected from Cs-H-Mo surface, together
with those of hydrogen atoms desorbed from thesurface have been calculated with a Monte-CarloSimulation code ACAT [4]. The result is shownin Fig. 1, which indicates broader energy
distributions for higher energy hydrogen ions.
The change in energy distribution functions due tocoverage of hydrogen and Cs did not largely affectthe particle reflection coefficient, but the Cs overlayer on hydrogen saturated Mo had enhanced thehigh energy component in the distributions.

Fig.1. Energy resolved H atom yield for reflection(broader distribution) and fro ion induced desorption(sharp peak) due to impact of hydrogen ions at shownenergies.

3. Effect due to Filter FieldMoreprecise
information
on velocity
distributions on incident hydrogen ions arenecessary to accurately compute the negative ionyield due to surface production, as the incoming

References[1] Martha Bacal, “Effect of fast positive ions incident
on cesiated plasma grid of negative ion source”:
Book of Abstracts for 14th International Conference
on Ion Sources, (2011) 160.
[2] M. Bacal, R. McAdams and B. Lepetit, AIP
Conference Proceedings Volume 1390, (2011) 13.
[3] M. Seidl, H. L. Cui, J.D. Isenberg, H.J. Kwon, B.S.
Lee, S.T. Melnychuk, J. Appl. Phys. 79 (6), (1996)
2896.
[4] Y. Yamamura, Y. Mizuno, Plasma Physics,
IIPJ-AM-40, Inst, Nagoya Univ., 1985.
[5] W. Kraus, U. Fantz , D. Wunderlich and the NNBI
team, AIP Conference Proceedings Volume 1390,(2011) 303.