J. Plasma Fusion Res. SERIES, Vol. 8 (2009)

Interaction of Helium Ion Beam with Argon Gas andPlasma
Atsushi OKAMOTO, Takehiro ISONO, Takashi KOBUCHI, Sumio KITAJIMA andMamiko SASAO
Department of Quantum Science and Energy Engineering, Tohoku University, Sendai 980-8579 Japan(Received: 1 September 2008 / Accepted: 20 March 2009)

A helium ion beam is extracted and is injected into an argon gas and a plasma in a linear devicein terms of experimental study of interaction between energetic ions in divertor plasmas and gases forradiator. The beam with the energy of 0-10 keV passing through the argon gas is attenuated by chargeexchange and elastic collisions with increasing the ﬁlling gas pressure. The electron temperature (∼ 10eV) and density (∼ 107 cm−3 ) of a bulk plasma degrade under the ion beam in some conditions. Abeam-produced plasma is also observed. Although mechanism of the degradation is unresolved, theargon gas in the divertor is expected to moderate the momentum ﬂux of energetic ion.
Keywords: Ion Beam, Beam Injection, Beam-Plasma Interaction, Divertor Plasma
in DT-ALPHA since 2008. Those are described inSec. 2. Experiments on plasma production and ion
beam extraction are explained in Sec. 3, where resultsof the ion beam injection into an argon gas and anargon plasma are also shown, followed by discussion
in Sec. 4.

1. Introduction
Interactions between energetic particles and plasmas are an interesting topic in the research ﬁeld of
the magnetically conﬁned plasma as well as in thatof the basic plasma physics. Energetic particle ﬂux
into the divertor is considered to change the balanceof atomic processes in the detached divertor plasma[1].
In order to reduce heat ﬂux onto the divertor plate,radiative cooling of the divertor plasma is performed
prior to its volumetric recombination. Therefore theradiator, puﬀed noble gases, also interacts with theenergetic particle; eﬃciency of the radiative coolingis potentially aﬀected. It is important to understandbehavior of the radiator against the energetic particle
as well as to understand mechanism of plasma detachment and the recombination processes. While the energetic particle originates from a pedestal plasma andits energy reaches several keV in ITER[2], puﬀed argon is a candidate of the radiator. Thus, interaction
between several-keV ion and partially ionized argon
plasma is a signiﬁcant process in the divertor region.
In order to investigate such research ﬁelds, a simple experimental apparatus with an ion beam is useful
to complement results of real divertor in magnetic conﬁnement devices. We have launched an experimental
study on the beam-plasma interaction using a linear
plasma device with an ion source. In this paper, development of ion beam injection system applicable tothe linear plasma device is described. Then interaction of a helium-ion beam with argon gas and thatwith plasma are presented. Signiﬁcant beam attenuation due to atomic interactions is indicated, whilea plasma modiﬁed in some conditions is shown. The
linear plasma device, named DT-ALPHA, has been
constructed and has been used for experiment since2007 JPY[3], while an ion source has been installed

2. Experimental Setup
2.1 DT-ALPHA device
The experiments were performed in a linear
plasma device equipped with an ion source. The former was originally designed as a “Diagnostic Tool Assisted by Linear Plasma device for Helium Atom beam(DT-ALPHA)”[3]. A schematic of the experimental
setup is shown in Fig. 1. The vacuum chamber consists of an ion source, a diﬀerential pumping chamber equipped with a Faraday cup, a quartz pipe (0.4m in length) coupling an antenna to a plasma, anda main chamber (1.1 m in length) made of stainlesssteel. An aperture with an inner diameter of 10 mm
made of stainless steel is equipped at the entrance ofthe quartz pipe, z = 0.49 m, where z is the distancefrom the ion source. The aperture is connected tothe ground through a 1-kΩ resistor. An end-plate assembly, which consists of the center plate and four
azimuthally segmented plates, is installed at the endof the main chamber, z = 2.02 m. All the end-plates
are made of stainless steel and are connected to theground through a 1-kΩ resistor.
A magnetic ﬁeld, which is a mirror conﬁguration
with the mirror ratio RM = 1.4 and the mirror throats
located at z = 0.60, 1.65 m, is applied along the axial direction. Magnetic ﬁeld intensity is B = 0.11
T at the center (z = 1.17 m). A so-called Nagoya
type III antenna[4] (l = 0.15 m) is used in the presentexperiment, which is connected to a 13.56 MHz radio frequency (RF) oscillator through a matching cir-author’s e-mail: atsushi.okamoto@qse.tohoku.ac.jp

39©2009 by The Japan Society of Plasma
Science and Nuclear Fusion Research

A. Okamoto et al., Interaction of Helium Ion Beam with Argon Gas and Plasma(a)BHe gasLangmuir probe-3

Faraday cupAr gasVlens

electron density [cm ]Vacc

End plates
Main chamber
AAntenna
to TMP

ApertureDifferential
pumping chamber
0to TMP
Quartz pipe

0.34 0.49

1.17z [m]Fig. 1 Schematic of the experimental setup.

cuit. An argon plasma is produced under the condition of the RF power up to 500 W and with working
pressure of 0.04-0.2 Pa. Blue plasmas are partially
produced near the antenna, which imply strong emission of ArII[5] and is regarded as a marker of helicondischarge[6, 7, 8, 9].
Plasma parameters are evaluated using the current (I)- voltage (V) characteristics of a single Langmuir probe installed at z = 1.17 m. The probe madeof a molybdenum wire (l � 1.2mm, d � 0.6mm) isbiased in a range of -120 V to 70 V.

2.2

101110109108107-10(b)2.02

electron temperature [eV]

Ion source

101.8x10 Pa-2
4.8x10 Pa
100200300

400500100200300
RF power [W]

400

500

201000Fig. 2 RF power dependence of electron density and temperature of an argon plasma measured in two different pressures.

3. Experimental Result
3.1 Plasma Parameter without Beams
RF power dependence of the electron density andtemperature are shown in Fig. 2. The open square
represents the result obtained in a high working-gas
pressure (1.8 × 10−1 Pa) experiment. The electron
density reaches 3 × 1011 cm−3 in the RF power PRF >300 W region. The electron temperature is about 15eV for PRF < 50 W and is gradually decreases withthe density increasing.
In the low pressure (4.8 × 10−2 Pa) operation,
the density increases up to 3 × 109 cm−3 with the RFpower, while the electron temperature is about 10-20eV. These densities are two or three orders lower thanthat obtained in the high pressure operation. However, in order to sustain stable arc discharge in theion source, the lower gas pressure condition was selected in the present experiment as a target plasma of
the beam injection.

Ion Source

A multicusp ion source is installed in the end of
the quartz pipe through a diﬀerential pumping chamber. The ion source is made of copper and has 80 mm
diameter and 90 mm long inside volume[10, 11]. Permanent magnets attached at the wall form line cusp
magnetic ﬁeld for plasma conﬁnement. A 0.2 standard cubic centimeter per minute (sccm) of helium
gas is injected into the ion source chamber. Then thepressure of the chamber is estimated to be 0.1 Pa. A
hot tungsten ﬁlament serves as the cathode of an arc
discharge. A constant-voltage (Varc = 80 V) powersupply is used for the arc discharge; up to Parc = 130
W (Iarc = 1.6 A) of arc power are injected into theion source.
Helium ion beams extracted from an 6 mm diameter extraction hole are injected into target plasmas.
The extraction system consists of three electrodes, theplasma electrode, the lens electrode, and the groundelectrode. The ion source chamber is connected to apotential of the acceleration power supply, Vacc , which
varies over 0-10 kV. The voltage of the lens electrode,which is negative −Vlens with respect to the groundelectrode, is adjusted to the optimum beam current.
The ion beam current is measured by both the movable Faraday cup located at z = 0.34 m and the center
end-plate.

3.2Ion Beam Extraction

Typical radial distribution of the beam current
density measured by the Faraday cup is shown inFig. 3(a). The condition of the beam extraction
are arc current Iarc = 1.1 A, acceleration voltage
Vacc = 4.5 kV, and lens voltage Vlens = 0.25 kV. Total beam current and the half width at 1/e times themaximum in the beam proﬁle, which are obtained bydeconvolution of the instrumental function, are 0.52
mA and 0.68 cm, respectively.

40

A. Okamoto et al., Interaction of Helium Ion Beam with Argon Gas and Plasma

(a)

0.4
0.3
0.2
0.10(b)10.82IFC [mA/cm ]Iendplate [μA/cm2]

2IFC [mA/cm ]100.601r [cm]26430 -410Iarc
1.0A
1.3A
1.6A

2-310-2

10Pressure [Pa]-1

10Fig. 4 End-plate current-density vs. ﬁlling gas pressure.
Ion beams are extracted with various acceleration voltage. Solid line represents Iendplate =I0 exp(−αp) with α = 58.5 Pa−1 , I0 = 17.6 μA/cm2for Vacc = 4.0 kV, dash-dotted line with α =46.4 Pa−1 , I0 = 6.58 μA/cm2 for Vacc = 2.0 kV,
respectively.

0.246Vacc [kV]810of the end-plate is proportional to the beam current
density. Figure 4 shows ﬁlling gas pressure dependence of the end-plate current. The current density
gradually increases with the ﬁlling gas pressure andsaturates around 0.01 Pa. Because the space chargeof the ion beam is canceled by weakly ionized ambient gas, the current density measured with the gas islarger than that without gas.
On the other hand, the current density decreases
in the higher pressure region. Curves indicating an
exponential function, Iendplate = I0 exp(−αp), agreewith the data within the parameter α = 50 ± 10 Pa−1
in the both beam energy. The result shows that theion beams are attenuated by interactions with weakenergy-dependence. Then the eﬀective cross-section,σ = αp/nl = 1 × 10−15 cm2 is obtained, where nl
represents the line integrated density of the argon
gas. Since both the cross-section of charge-exchange
reaction[12] and that of elastic scattering[13] are orderof 10−15 cm2 , the beam attenuation observed in Fig. 4
is considered to be caused by those atomic processes.
Under the working gas pressure for plasma production, p = 4.8 × 10−2 Pa, the ion beam suﬃciently
interact with argon gas and is attenuated.
Eﬀect of ion beam injection on target plasma parameter is investigated. Up to 8 keV of helium ion
beams are injected into various densities of argon plasmas. Figure 5(a) shows the electron density, which
increases with the RF power. Ion beam injection decreases electron density to some extent, while the electron temperature shown in Fig. 5(b) also decreases bythe beam injection. In these experiments, a matching condition was unchanged against the beam injection. However the changes of the plasma param-Fig. 3 (a) Radial distribution of beam current density
measured under the conditions of Varc = 80 V,
Iarc = 1.1 A, Vacc = 4.5 kV, and Vlens = 0.25 kV.
(b) Acceleration voltage dependence of beam current density under the condition of Varc = 80 V.
The lenz voltage is adjusted to maximize the beamcurrent density for each acceleration voltage.

Acceleration voltage dependence of the beam current density is shown in Fig. 3(b). Optimized acceleration voltages for each arc current exist, in which thecurrent density has local maxima. While one maximum locates at Vacc = 4.5 kV for Iarc = 1.0 A, twolocal maxima exist for Iarc ≥ 1.3 A. Ones observed
around Vacc = 6 − 8 kV for Iarc ≥ 1.3 A are a series of that observed at Vacc = 4.5 kV for Iarc = 1.0
A. The optimized acceleration voltage for these increases with the arc current. Other local maxima appear around Vacc = 2.0 kV, where the extraction voltage (Vext ≡ Vacc + Vlens ) is a little smaller than that
optimized for Vacc = 4 − 8 kV, suggesting that a beam
optics suitable for the low energy beam has a shallow
meniscus at the plasma electrode. Maximum current
density of 0.64mA/cm2 is extracted when Vacc = 7.0kV and Iarc = 1.3 A.

3.3

4.5keV
4 keV
2 keV

20.4008Eﬀect of Beam Injection

Ion beam injection into an argon gas was performed in order to investigate beam attenuation inthe argon gas and to obtain experimental window for
beam injection into an argon plasma. The center endplate was used to monitor the beam current passing
through the chamber of DT-ALPHA. Current density

41

A. Okamoto et al., Interaction of Helium Ion Beam with Argon Gas and Plasma

(a)

(a)

(b)
-2

10-2-310-3electron density [cm ]10electron temperature [eV](b)109100200300

400-310w/o beam
Eb=8keV
4keV
2keV

80

electron current [mA]

10500-410-500probe voltage [V]-410-500probe voltage [V]15Fig. 6 (a) Electron current characteristic curve with 8keV-He+ beam injection. Filled circle represents
that obtained with an Ar plasma produced by
PRF = 25 W. Open square represents without
plasma. Solid line shows a slope corresponding to
the bulk temperature. (b) the same as (a) but without an ion beam.

10500

100

200300RF power [W]400500The charge-exchange process neutralizes the energetic ions resulting in reduction of momentum ﬂux
along the magnetic ﬁeld in the divertor region. Thus
the argon gas puﬀed into the divertor is expected tomoderate the momentum ﬂux as well as to reduce electron energy by radiation.
Mechanism of the electron temperature and density degradation under ion beam injection is unresolved. Precise and wide-parameter measurements including beam detection in a plasma and some optical methods will be requreid for detailed discussion.
Fluctuations might be a key in terms of the waveparticle interactions, which excite instabilities aﬀecting the plasma.
In summary, a several-keV of helium ion beam isinjected into an argon gas and a plasma. The beampassing through the argon gas is attenuated by chargeexchange and elastic collisions. The bulk plasma degrade under ion beam in some condition. However
why the bulk plasma is aﬀected by the beam and thebeam-produced plasma remains our future work. The
argon gas in the divertor is expected to moderate themomentum ﬂux of energetic ion.

Fig. 5 RF power dependence of electron density and temperature of an argon plasma with various energy ofion beam injection.

eter are within the order of error and no signiﬁcantchange is observed for a middle density (PRF � 200W) plasma. Further investigation is required for quantitative discussion.
In some beam injections, a tail component is observed in the probe characteristic curve. Filled circle in Fig. 6(a) corresponds to a result with beam
injection, where the bulk temperature is Te = 7 eV
and a tail component has a slope of 30 eV. The tailcomponent also emerges only with the ion beam injection as indicated by a probe characteristic curve (opensquare). On the other hand, the same plasma production but no beam injection, clear tail component is
not observed as shown in Fig. 6(b). The presence ofthe tail component implies that low density plasma isproduced by the ion beam but is not fully thermalized.

4. Discussion

Acknowledgements

Between the charge-exchange and the elastic collisions, which interaction dominates the beam attenuation is an interest problem. However it is also a diﬃcult problem, because accurate cross-sections of thosecollisions are insuﬃcient especially in the energy range
1-10 keV. The cross-section of the elastic collision has,in general, a monotonic dependence σel ∼ E −2 , whilethat of the charge-exchange σCX (E) has a peak at
E = 20 − 30 keV[12]; competition between the decreasing function σel (E) and the increasing function
σCX (E) is expected in several keV.

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This work was partly supported by the Ministry ofEducation, Culture, Sports, Science and Technologyof Japan (MEXT) Grant-in-Aid for ”Priority area ofAdvanced Burning Plasma Diagnostics” (16082201).
Authors acknowledges for the support for young scientists by the school of engineering in Tohoku University.

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