J. Plasma Fusion Res. SERIES, Vol.7 (2006) 114-117

Discharge in Hydrogen-Noble Gas Mixture as the Method
to Increase Efficiency of Conditioningof the In-Vessel Components of Large-Scale Fusion Devices
VOITSENYA Vladimir, NAIDENKOVA Diana, MASUZAKI Suguru1 ,KUBOTA Yusuke1 , SAGARA Akio1 and YAMAZAKI Kozo1∗
Institute of Plasma Physics, NSC KIPT, 61108 Kharkov, Ukraine
1 National Institute for Fusion Science, Toki 509-5292, Japan（Received: 5 October 2004 / Accepted: 5 January 2006）

AbstractFusion devices are needed very long wall conditioning time for preparation to experiments with hot plasma.
The conditioning procedure is based on chemical reactions when hydrogen atoms and ions of low temperatureplasma interact with contaminants coating the walls. With that, the eﬃciency of removal of carbon atoms
is quite small, 〜 2 − 3 %. The possible way to enhance the eﬃciency of C film cleaning is to use theplasma produced by discharge in mixture of hydrogen with one of heavy rare gases, as was suggested anddemonstrated in [1]. This approach is acceptable in fusion reactor projects, where the addition of a rare gas(heavier than helium) is considered as the method to control the periphery plasma parameters.
In the present paper we analyze the published data on the most important reactions in a low-temperatureplasma produced in mixture of hydrogen with gases: Ne, Ar, Xe, Kr. The peculiarity of such plasma is
the appearance of a group of hydride ions, NeH+ , ArH+ , XeH+ , KrH+ , which probably are important factor
increasing the rate of C film removal compare to pure hydrogen plasma. The comparison of data on crosssections of diﬀerent reactions demonstrates clearly that among all rare gases the highest concentrations ofhydride ions would be in the Ar-H2 mixture plasma.
Keywords:
conditioning, discharge, hydrogen-noble gas mixture, noble-gas-hydride ion, carbon-carbon bond,chemical erosion
bon molecules production is quite small, 〜 2 − 3 % [3]that is why the regular provided conditioning procedures take the time orders of magnitude exceeding thetime of experiments with hot plasma.
One possible way to increase the eﬃciency of cleaning surfaces from carbon-containing films is to use forconditioning a discharge in a mixture of hydrogen (deuterium) with one of heavy noble gases as was shown for
the first time by authors of [1]. This approach is acceptable for a fusion reactor, where the use of a noble gas
addition is suggested to control the parameters of theperiphery plasma [4]. The eﬀect in [1] was diﬀerent for
diﬀerent noble gases added to hydrogen, Ne, Kr, or Xe.
The partial pressures of impurity-containing moleculesin plasma, as well as the rate of conditioning, depended
on the composition of mixture (i.e., on ratio of densities of noble gas atoms to hydrogen molecules). Table 1
shows the ratios of partial pressure of indicated masses

1．Introduction
In fusion devices, to exclude an influx of metallic
impurities into the confined plasma, the protective tiles
from carbon-based materials are widely used (e.g., [2]).
As a result, carbon is the main component in the erosion product. It is transported inside the vacuum vessel and deposits on surfaces remote from locations of
the strongest plasma impact. Due to co-deposition ofhydrogen isotopes and other impurity atoms that arein a background atmosphere or are the part of composition of in-vessel components, the deposit can havea complicated composition with carbon predominance.
The methods of surface cleaning from a carbon film
are based on high chemical activity of hydrogen (deuterium) atoms and ions when volatile molecules containing carbon (mainly CH4 ) and oxygen (H2 O) are created, which can be evacuated out by standard pumping equipment. However, the eﬃciency of hydrocar-Corresponding author’s e-mail: voitseny@ipp.kharkov.ua∗ Present address: Department of Energy Engineering and Science, Graduate Schoolof Engineering, Nagoya University, Nagoya 464-8603, Japan114

C 2006 by The Japan Society of Plasma

Science and Nuclear Fusion ResearchDischarge in Hydrogen-Noble Gas Mixture as the Method to Increase Eﬃciency of Conditioning of the In-Vessel Components of Large-Scale Fusion Devices

Table 2 Rate coefﬁcients, k, for reactions in the low temperature plasma of a discharge in the argon - hydrogen mixture taken from references in [5].

Table 1 Rise of partial pressure for indicated mass numbers of impurity-containing molecules [1].

MixtureRatios of partial pressure values ofsome impurities with and without
noble gas addition
15* 16* 17* 18*28*

H2 + Ne

1.9

1.4

1.9

1.3

1.6

H2 + Xe

2.8

2.6

1.5

1.8

2.6

H2 + Kr

2.6

4.8

7.7

3.1

3.1

No10.62; 0.68; 1.6; 1.6; 1.7;

Ar+ + H2 →ArH+ + H

0.98; 0.54; 1.1; 1.0; 1.3;
0.82; 0.95; 1.7; 2.0; 1.2

2

H+2 + Ar →ArH+ + H

2.3; 1.24; 2.0; 2.2; 1.8;
2.2; 2.0

3ArH+ + H2 →H+3 + Ar

0.5; 0.32; 0.8; 0.34;
0.35; 0.5

* Mass of impurity-containing molecules.

during discharges with rare gas addition to values without rare gas, i.e., when discharge was provided in purehydrogen (the data are taken from Fig. 7 of [1]). It isseen that the highest conditioning eﬃciency was with
Kr added.
However, the physical principle of increasing theconditioning eﬃciency with noble gas addition was notclearly explained in [1]. Besides, the experiments with
Ar-H2 mixture were not provided. Therefore, from [1]
impossible to make a conclusion on the optimal choiceof the rare gas-H2 mixture. Meanwhile, the analysis
of published experimental and theoretical data on processes in plasma of the mixture of hydrogen with a raregas, shows that the most promising for conditioning efficiency increase should be addition of argon to hydrogen (or to deuterium). Below a detailed analysis is presented.

k at〜room temperature,
10−9 cm3 molecule−1 s−1Reaction

4Ar+ + H2 →H+2 + Ar

0.27; 0.1
0.15

5H+2 + Ar →Ar+ + H2〜0.27; ≤ 0.1; 0.27

0.25

6H+2 + H2 →H+3 + H

2.05; 2.4; 2.02.2

7

H+3 + Ar →ArH+ + H20.37; 0.06; 0.50.2

8e + ArH+ →Ar + H〜100

Table 3 Rate coefﬁcients, k, for most important reactionsin the low temperature plasma of a discharge in arare gas-hydrogen mixture taken from references
listed in [5].

2．Reactions in the plasma of H 2-noble gas
mixtureThe rate coeﬃcient of reactions in cold plasma with
low ionization degree produced by glow or ECR (Electron Cyclotron Resonance) discharge in the mixture ofnoble gas with hydrogen are shown in Table 2 by theexample of Ar-H2 mixture. The data on the most important reactions for other rare gases are compared inTable 3. These data were taken from diﬀerent publishedpapers which are referenced in [5].
As seen, the distinctive feature of such plasma is thatit contains a group of noble gas hydride ions, XH+ ,which are a quite long-lived plasma component, but disintegrate quickly after the ion becomes neutral, as show
data of Table 4 [6]. A priori, we may suppose that justexistence of noble gas ions, X+ , and XH+ ions, does determine, to a considerable degree, the increase of conditioning eﬃciency in comparison with the case when
during conditioning the discharge in a monocomponentgas (H2 or D2 ) is in use. If so, to increase the eﬃciencyof conditioning (i.e., the rate of cleaning the walls fromcarbon films) one has to reach the maximal concentra-

k at 〜room temperature,
10−9 cm3 molecule−1 s−1Reaction
HeNeArKrXe

H+2 + X →
+

0.12
0.16
0.1

1.1
2.3

2.3
1.2
2.0
2.2

2.9
2.3

2.4
2.1

X + + H2 →XH+ + H〜0a〜0ab

0.7
1.6
1.6
1.00.15
0.6
0.17
0.2

< 0.02a1.1
0.9

0.17
0.28

XH + H

1.8

XH+ + H2 →H+3 + X〜01.4〜00.90.3

a : < 0.0035, b :< 0.008

tion in the discharge just of these ions.
From Table 3 one can see that the probability for
XH+ ions to be produced is lowest for He-H2 plasma.

115

Discharge in Hydrogen-Noble Gas Mixture as the Method to Increase Eﬃciency of Conditioning of the In-Vessel Components of Large-Scale Fusion Devices[8]. For further analysis of reactions that occur in theAr-H2 mixture we will use some “average” values ofrate coeﬃcients, which are shown in Table 2 as the boldfigures.

Table 4 Measured and calculated energy of dissociationof rare gas hydride ions and molecules [6].

HydridesDissociation energies, ∆dis , eV
Experimental
Calculated

NeH+
NeH

2.15

ArH+

≥ 2.65; ≥ 2.14

0.17

4．Conditioning by ECR discharge plasma
Taking into consideration the data of Table 2, the balance equation for the hydride ions (ArH+ ) in low temperature hydrogen plasma is:

0.26

ArH

3.03

KrH+
KrH

3.42 ≤ ∆dis ≤ 4.09

3.46
0.12

∂n(ArH+ )/∂t =k1 · n(Ar+ )n(H2 ) + k2 · n(H+2 ) · n(Ar)

XeH+XeH

1.2 ≤ ∆dis ≤ 3.19

2.66
0.15

+ k7 · n(H+3 ) · n(Ar) − k3 · n(ArH+ )· n(H2 ) − k8 ne · n(ArH+ )− n(ArH+ )/τ(ArH+ ).

Much higher reaction rates are in the Ne-H2 and Xe-H2mixtures. For these gases, practically only one reaction (2) leads to a positive result, i.e., to production ofXH+ ions. For Kr-H2 mixture both reactions give thedesired products, but asymmetry is still very high between both brunches, 〜one order in magnitude. And
only for Ar-H2 mixture the ArH+ ions are formed byboth reactions, (1) and (2), with rate coeﬃcients which
are not too much diﬀerent.

(a)In this equation there is no term corresponding to ionization of metastable argon atoms; the term τ(ArH+ )designates the life time of ArH+ ions.
For the mixture with identical initially both components, Ar50 % + H2 50 %, which relates to the experimental conditions described in [1] with n(H2 ) ≈n(Ar) ≈ 1/2 [4 · 1012 cm−3 ], the approximate equality n(H2 )  n(Ar) has to fulfill during all the discharge time as the relative concentration of every ion
component satisfies inequalities: n(Ar+ )/n(H2 )  1,n(H+ )/n(H2 )  1, n(ArH+ )/n(H2 )  1. Besides, at thestationary state of discharge the left part of this equation is zero, thus, after substitution of all numbers theequation (a) becomes:

3．Reactions in argon-hydrogen mixture
plasmaIn the Ar-H2 mixture the hydride ions can be produced by several reactions known for plasma of a raregas - H2 mixture, if the temperature of ions and neutrals (Ar atoms and H2 molecules) is below 1 eV (whatis frequently realized in discharges used for wall conditioning). The ArH+ ions are formed as a result ofexoteric reactions of Ar atoms with H+2 ions in all available vibrational states, by reaction of Ar+ ions in ground(2 P3/2 ) and excited (2 P1/2 ) states with H2 , and in reaction with H+3 ions, which are frequently the most abundant ion component in low temperature H2 plasma. Under the term ”low temperature plasma” we imply everywhere below in the text the plasma with near room temperature of ion and atom components, with the electron
temperature near two orders in magnitude higher.
It is seen from Table 2 that on the average the observed rates for the reaction Ar+ + H2 → H+2 + Ar are
lower than for reaction H+2 + Ar → ArH+ + H. Roughly
the value of k(Ar+ + H2 )/k(H+2 + Ar) ratio is close to 1/2
what is in a quite good agreement with results of calculations in [7] where this ratio was found to be 〜 0.7
for reactions in H2 -Ar and D2 -Ar mixtures. However,
the energy dependence of theoretical and experimental
cross sections of reactions (2) and (4) in Table 2 are very
diﬀerent, i.e., the experimental cross section decreases
with energy much faster than theory predicts (〜 E −1/2 )10{1.2 + 2 + 0.2 − 0.5 − 1.5} = 10−2 /τ(ArH+ ),and from this relation we see that for the given rate coeﬃcients the degree of ionization near 1 % is close toa critical value, i.e., for better realization of the conditions that would provide the accumulation of ArH+ ionsin the discharge, the degree of ionization of the Ar-H 2
mixture has to be below 1 %.
Two indispensable conditions are necessary to provide the production and accumulation of ArH+ ions inaccordance with reactions (1)-(7) in Table 2 : (i) thelong enough life time of hydride ions, what means thatthey have rather high dissociation energy, and (ii) thelife time of ArH molecules is short, i.e., their dissociation energy is small. The data presented in Table
4 demonstrate that these conditions are really satisfy:∆Edis (ArH)  ∆Edis (ArH+ ).
Under conditions of the above cited example, due toacceleration by the sheath potential the ions will gain
energy 〜15 eV. When hitting the surface, the heavy
Ar+ and ArH+ ions will give a much larger portion of
their energy to absorbed impurity molecules than thelight H+2 ions, what follows from the kinematic ratio:
116Discharge in Hydrogen-Noble Gas Mixture as the Method to Increase Eﬃciency of Conditioning of the In-Vessel Components of Large-Scale Fusion Devices

E = E0 · {4M1 M2 /(M1 + M2 )2 }, with M1 and M2 as
masses of ion and impurity molecules and E 0 - the ion
energy, E - the energy which the particle on the surface will get after collision with ion. If the surface iscoated by a carbon film, the C atom will get 0.7E 0 energy due to collision with ArH+ ion and only 〜 0.2E0
due to collision with H+2 ion. The energy E0 ≈ 15 eV,that ions gain passing through the sheath potential whenT e = 5 eV, is not enough for sputtering of absorbedmolecules, but is suﬃcient to break their bonds. Thus,
we can suppose that the eﬀect of increasing eﬃciency
of conditioning in [1] is based on the following mechanism. The heavy ArH+ ions after acceleration by thesheath potential hit the adsorbed impurity molecules
and break some intermolecular bonds. At this process
they become neutral and quickly dissociate to Ar and H
atoms (see Table 4). The free Ar atoms go into plasma,
but the free H atoms occupy the released bonds andform new molecules, part of which are volatile and canbe pumped out. The Ar+ ions do also break bonds ofcontaminants and thus facilitate formation of volatile
molecules by those H atoms which are produced in theplasma by dissociation of H+2 ions and H2 molecules
and hit surface as single atoms. Without heavy Ar+ andArH+ ions, in pure hydrogen plasma with identical ion
flux density to the contaminated surface, similar process
would have significantly lower eﬃciency.

film became about one order in magnitude higher thandue to physical sputtering by Ar+ alone. The increase
of C film cleaning eﬃciency occurs because the Ar ionsbreak the C-C-bonds which are passivated by hydrogenatoms.
6．Conclusion
As follows from above shown data, in the low temperature plasma of an Ar-H2 mixture, the ArH+ ionscan be formed due to several reactions. The processof ArH+ ion production in conditions that can be realized during conditioning procedure in fusion devices ismore eﬀective in the Ar-H2 mixture compare to mixtureof H2 with other rare gases. The heavy ions, present inplasma, can gain energies corresponding to the sheathpotential, and highly increase the probability of sputtering and desorption of impurities from the surfaces
subjected to plasma containing hydrogen ions. Furthermore, the H atoms liberated due to disintegration ofArH molecule on the surface, would facilitate the generation of volatile molecules of light impurities which
can be pumped out.
We suppose that such a process has resulted in
an increase of desorption rate of impurity-containing
molecules in the HL-1 tokamak [1] when the rare gas- H2 mixture was used as a working gas.
The use of noble gases for wall conditioning is compatible with requirements in fusion devices, e.g., fullexclusion of oxygen.

5．Conditioning by glow discharge plasma
In glow discharge the plasma characteristics are notmuch diﬀerent as compare to the ECR plasma. The only
important diﬀerence is a quite high sheath potential ( 〜150 − 300 volts) exceeding strongly that for the ECR
plasma. Thus, all ions hitting the wall will gain thehigh energy, and the processes of sputtering and desorption can occur. To avoid an uncontrolled sputtering ofthe walls by Ar+ and ArH+ ions in such conditions, therelative Ar concentration has to be maintained at lower
level than was supposed above. For example, in [9] wasshown that sputtering of stainless steel walls of the vacuum chamber by ions of glow discharge plasma (anodepotential 300 V at pressure of Ar+H2 mixture 10−2 Torr)
was at the low level right up to the Ar+ /H+2 flux ratio〜 2. With increasing the argon to H2 ratio above 〜 2
the sputtering rate was observed to rise significantly.
Authors of [10] showed that addition of Ar+ flux
with energy 200 eV consisting only of 0.25 % to fluxof H atoms resulted in significant rise of the erosion
rate of the carbon film. The absolute erosion rate of C

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