S1-5
新たな負イオン研究の応用
Emerging Application Fields of Negative Ion Research
和田 元Motoi Wada
同志社大学理工学研究科
Graduate School of Science and Engineering, Doshisha University
1. Introduction
Intense beams of negative hydrogen (H-) ionsare required for two research fields: magneticallyconfined fusion, and high energy proton
storage/acceleration in ring accelerators. Sources
to produce H- ions often confine plasma of power
density exceeding 10 W/cm3, while they reduce thetemperature of plasma near the beam extractor
below 2 eV. Fundamental processes of H- ion
formation in H- ion sources have been summarized[1], and the development status of the ion source isroutinely reported [2]. As the direct application of
intense high energy negative ion beam, Si wafer“smart-cut” process has been demonstrated at Japan
Atomic Energy Research Institute [3]. However,species other than H- are necessary for industrial
applications of negative ions. Ishikawa [4] hadsummarized the possible application areas ofnegative ion beams that can be extracted from his
surface production type negative ion source.
Negative ion research includes wide variety of
study area in physics and chemistry. Atomic andmolecular physics is critically important inunderstanding formation and destruction processes
in the ion source plasma.
Plasma surface
interaction at the source wall also plays a decisiverole in determining overall production efficiency ofnegative ions.
Work function, in particular,determines the negative ionization efficiency forparticles reflected, desorbed and sputtered. These
pieces of information on fundamental processes canbe also applied to other field of science andengineering.
Here is reported the candidate
application areas where science and technology
related to negative ion formation and transport canbe utilized.
2. Merits of negative ions
There are two major advantages of using
negative ions instead of positive ions. An extra
electron in the negative ion affinity level can beeasily released as the affinity level if much smaller
than ionization potentials of typical atoms andmolecules; the electron is loosely bound. The

small binding energy makes the cross section forelectron stripping in gas molecules for H- large atenergy exceeding 100 keV, which makes the Hbased high energy neutral beam injection system forfusion plasma heating to be efficient enough. Also,
negative ions can be neutralized by photon
irradiation in the wavelength region of intenselasers.
The other virtue of negative ion is its negativeelectric charge. When a solid surface is irradiatedby an ion beam, it produces secondary electrons.
Electrons going out of the surface charges aninsulator surface up to positive electrical potential,
which will be further enlarged as the positive ionsstrike the surface. In the case of negative ions
irradiate an insulating surface, a single electron
emission by an incident negative ion is equivalent
to an injection of a neutral particle produced
through electron detachment from the incident
negative ion. As the result, the surface electricalpotential of an insulator under negative ion
irradiation is much smaller than that under positiveion irradiation [5].
3. Application to mass analysis
Negative ions have been used as probe beamfor SIMS (Secondary Ion Mass Spectrometry)
application. Beams of oxygen negative ions areproduced by cold cathode duoPlasmatron ion
source, and surface ionized sputtered atoms aredetected by mass analyzers. On the other hand,
the incident beam can produce negative ions at thesurface when Cs ions are injected onto the surface
as the primary beam. As the surface negative
ionization phenomena have been studied for Hproduction, optimization of the measurement
system based on negative SIMS can be made.
Mass analysis investigations on bio-materials
requires incident beam of large cluster ions like C60+
to scrape off large molecules from sample surface.
Positive and negative ion modes are utilized formass analysis to identify molecular structure [7].
Mapping of lipid species has been successfully
accomplished by negative ion mode of MALDI-ion

mobility imaging system [8].
Fragmentation
analyses of large molecules are also done bydetecting negative ions [9]. In these applications,
understanding of the formation processes ofnegative ions is important in identifying themolecular structure of the sample.
Realization of a negative ion beam of C60 is achallenging research topic for a negative ion
physicist. The electron affinity of C60 should be ashigh as 2.68 eV [10], and efficient production ofnegative ion of C60 is expected. As the parent
molecule is fragile, low temperature plasma suitablefor electron volume process may produce negative
C60 ions with an enough reaction rate.
4. Application in material science
Plasma based material synthesis has achieved
a significant progress. Atomic and molecular
process in a plasma coupled to target surface
science determine the material synthesis capabilityof the production system.
The behavior ofnegative oxygen ions in a plasma is foundimportant in preparation of ZnO film [11].
Introduction of negative ion species into a plasma
will yield another tuning knob to control reactionsin the plasma.
Knowledge obtained in thenegative ion research can be applicable to develop abetter material production system.
Plasma surface treatment has become popular,and functional surfaces are realized by plasma
exposures.
For example, adhesion between
polyimide and metal is improved by irradiating thesurface of polyimide film with an oxygen plasma[12]. The negative ions in plasma determines thesheath potential, and the surface erosion due tosputtering and chemical etching. Methods toproperly control spatial distributions of plasma
parameters are readily applicable to realize desireda flux of reactive ion species.
5. Space charge neutralization
Low energy large current ion beams easily
diverge due to a space charge effect. Ions withelectrical charge of the opposite sign of the beamions will neutralize the space charge, and mitigate
the ion beam to diverge. Space charge arisingfrom concentration of positive ions can be mitigatedby electrons, but high mobility of electrons causes
oscillations to make the plasma unstable.
Negative ions may be more suitable to reduce spacecharge due to a positive ion beam.
The problem of “charge-up” is also serious inelectric space propulsion.
An electron gun
neutralizer is combined with an ion-engine for aspace satellite to avoid charge up of the space craft,

but part of the extracted ions can inside of theneutralizer to damage by ion bombardment. The
positive ion negative ion alternate extraction systemremoves additional component to electricallyneutralize the space craft. Characteristics of an
ion-ion plasma have been extensively studied in Hion source for fusion experiments [14]. Efficientbeam extraction from ion-ion plasma of future ion
engine can be studied from H- ion source physics.
[1] M. Bacal and M. Wada, Applied Physics
Reviews, 2, 021305, (2015).
[2] M. Kashiwagi, et al., Fusion Engineering andDesign, 96-97, 107(2015).
[3] 奥村義和, 原子力eye, (日本原子力研究所)，
Vol. 46 61(2000).
[4] J. Ishikawa, Rev. Sci. Instrum.79, 02C506,(2008).
[5] J. Ishikawa, Surf. Coat. Technol. 203,2351(2009), and references therein.
[6] http://www.eag.com/[7] P.M. Wehrli et al., Surf. Interf. Analysis, 46,173(2014).
[8] S.N. Jackson et al., Analytical methods, 6,5001(2014).
[9] L. Abranko, J. Mass Spetrom. 46, 478(2011).
[10] D.L. Hang et al., J. Chem. Phys., 140, 224315,(2005).
[11] A. Bikowski et al., J. Appl. Phys., 114, 223716,(2013).
[12] Y, Nakamura et al., Thin Solid Films, 290-291,
367 (1997).
[13] A. Aanesiand, A. Meige, P Chabert, Journal ofPhysics: Conference Series, 162, 012009,(2009).
[14] Tsumori et al., Rev. Sci. Instrum., 83, 02B116,(2012).