Influence of Amorphous Carbon Deposition on the Probability for
Recombination of Neutral Oxygen Atoms on Aluminium Surfaces
A. Drenik, A. Vesel
Center of Excellence for Polymer Materials and Technologies
Tehnološki park 24, 1000 Ljubljana, Slovenia
[email protected], [email protected]
P. Panjan, M. Mozetič
“Jožef Stefan” Institute
Jamova 39, SI-1000 Ljubljana, Slovenia
[email protected], [email protected]
ABSTRACT
In fusion devices with carbon-based plasma facing components, the accumulation of
amorphous carbon deposits on various in-vessel surfaces is unavoidable. As it has been
identified as a major contribution to fuel retention, the carbon deposits should be regularly
removed. One of the suggested cleaning techniques is removal by neutral oxygen atoms.
Efficiency of this cleaning method will in a great way depend on the density of atoms in the
vicinity of carbon-covered surfaces. The atom density will, in turn, greatly depend on the
recombination coefficient of the surfaces. In this work, we study the impact that amorphous
carbon contamination has on the recombination coefficient of a solid surface. The influence of
surface coverage by an amorphous carbon deposit on the recombination coefficient of
aluminium for neutral oxygen atoms was investigated by measuring the recombination
coefficient of a pristine aluminium surface and a surface covered by an amorphous carbon
deposit. The recombination coefficient was determined by measuring the spatial distribution
of oxygen atoms in a side-arm lined with aluminium/amorphous carbon surface. Oxygen
densities were measured by means of Fiber Optic Catalytic Probes. An inductively coupled
radiofrequency discharge created in pure oxygen was used as a source of neutral oxygen
atoms at room temperature. The probability of recombination of a carbon-covered surface was
found to increase by a factor of four in regard to the pristine aluminium surface.
1
Introduction
In the effort to solve the problem of future energy supplies, ITER will no doubt play a
very important role. Among the many scientific and technological problems that are
occupying the research teams working on development of ITER is the question of the material
out of which plasma facing components will be constructed. Due to the low Z and excellent
607.1
607.2
thermal characteristics, carbon-based materials such as carbon fibre composites are very
attractive candidates for the plasma facing components[1, 2]. However, they are strongly
susceptible to chemical erosion by hydrogen atoms from the fusion plasma[3]. Hydrogen
atoms interact with carbon atoms from the plasma facing components, forming carbohydrate
complexes which are subsequently re-deposited on the inner walls of the reactor. Thus, thin
films of hydrogenated amorphous carbon deposits (a-C:H) are being formed. Depending on
the position in the reactor, the hydrogen content in the deposits can reach up to 40 %. In the
case of operation with D-T fuel mixtures, this leads to retention of tritium inside the reactor,
which is a very undesired effect[4]. In order to ensure the undisturbed operation of ITER, the
a-C:H deposits must be regularly removed.
A very promising method of removing such deposits is by oxidation[5-10]. While
standard forms of oxidation (baking in O2 atmosphere, oxygen/helium glow discharges) have
also proved successful in removing of a-C:H deposits, they are not applicable within the
limitations set by the ITER environment. The solution lies in oxidation by neutral oxygen
atoms. Neutral atoms on the other hand have been shown to achieve suitable rates of erosion
even within the temperature limits in ITER[11]. Namely, at the temperature of 575 K the
achieved erosion rate was 10 nm/s.
One of the key influences on the efficiency of the cleaning method will be the density of
oxygen atoms within the reactor[12]. In confined spaces, one of the main mechanisms of loss
of atoms is recombination on solid surfaces. Recombination is the event in which two neutral
oxygen atoms join to form an O2 molecule. Because recombination is an exothermic process
and due to laws of conservation of energy and momentum, a third body must be present to
absorb the excess energy. Since a three-body collision in the gas phase is an extremely
unlikely event at pressures below 100 Pa, recombination takes place almost exclusively on
solid surfaces.
In brief, the recombination process starts when neutral oxygen atoms are adsorbed on
chemisorption sites on the surface. The surface is filled with both chemisorption and
physisorption sites. The bond which binds atoms onto the physisorption sites is not as strong
and allows them to either desorb into the gas phase or diffuse along the surface. The bond on
the chemisorption sites, on the other hand, is strong enough to keep the atoms localized until
they take part in the recombination reaction:
O+ O → O 2 + Wdis ,
(1)
where Wdis is the dissociation energy (5.12 eV), released at the reaction. The second atom in
the reaction can either come directly from the gas phase[13], which is known as the EleyRideal process, or from a neighbouring physisorption site, by surface diffusion, which is
known as the Langmuir-Hinshelwood process[14]. Except at very low neutral atom densities,
the surface chemisorption sites are fully occupied. Therefore, recombination becomes a 1st
order reaction, meaning that its rate is proportional to the rate of atoms colliding with the
surface from the gas phase:
r =γ j,
(2)
where r is the rate of recombination per surface area, j is the flux density of impinging neutral
oxygen atoms and γ is the recombination coefficient. The recombination coefficient is defined
as the probability that an atom, colliding with the wall, will find a partner and form a
molecule.
The probability depends on many microscopic parameters such as binding energy of the
chemisorption sites, mobility of atoms across the surface, desorption frequency, etc, as well as
Proceedings of the International Conference Nuclear Energy for New Europe, Bovec, Slovenia, Sept. 12-15, 2011
607.3
not-so microscopic ones, such as surface roughness[15]. In general, the recombination
coefficient is difficult to predict and empirical determination of its value can in some cases be
more accurate.
Moreover, the recombination coefficient can be highly susceptible to the contamination
of the surface with foreign species. In the case of the ITER environment, the inner walls of
the reactor will be eventually covered with amorphous carbon. It is reasonable to expect that
this will be the dominant influence on the probability of recombination on the walls.
Therefore, when predicting the propagation of neutral oxygen atoms throughout the reactor,
one should pay attention to the possible changes in the recombination coefficient of the
relevant solid surfaces.
In this paper, we present our experiments in which we experimentally determined the
influence of amorphous carbon contamination on the recombination coefficient of an
aluminium surface.
2
Experimental
The experimental set-up used in our experiments is presented in Fig. (1). The main part
of the experimental reactor is a cylindrical borosilicate glass tube with the inner diameter of
36 mm. The system was pumped with a two stage rotary pump with which we were able to
achieve the base pressure of around 5 Pa. A stream of partially dissociated oxygen at room
temperature was fed into the experimental chamber. The oxygen was dissociated in a weakly
ionized inductively coupled discharge, created by means of a 27.12 MHz generator coupled to
the reactor with a 12 turn coil. Oxygen of commercially available purity was leaked into the
discharge region at pressures between 40 Pa and 180 Pa. The degree of dissociation of oxygen
in the experimental chamber reached up to 11 %.
5
4
3
6
2
7
8
9
1
Figure 1: The experimental set-up. 1 – oxygen bottle, 2 – reduction valve, 3 – needle
valve, 4 – discharge chamber, 5 – experimental chamber, 6 – pressure gauge, 7 – zeolyte
trap, 8 – high vacuum linear valve, 9 – pump.
Proceedings of the International Conference Nuclear Energy for New Europe, Bovec, Slovenia, Sept. 12-15, 2011
607.4
The recombination coefficient of the sample surface was determined by measuring the
spatial distribution of the density of neutral oxygen atoms in the presence of the sample. The
measurement was performed in a side-chamber, perpendicular to the main part of the
experimental chamber. The inner wall of the side-chamber was lined with the observed
sample surface. The neutral oxygen atom density profile was measured by means of a
movable nickel-tipped fiber optic catalytic probe (FOCP)[16, 17]. The side-chamber set-up is
presented in Fig. (2). A Teflon disc is mounted approximately 10 mm below the probe tip.
The disc effectively prevents further diffusion of neutral oxygen atoms further along the sidechamber, which makes subsequent calculations considerably easier as the well-defined disc
makes for an easily describable boundary condition.
z
γ1
L
3
γ2
d
+
2
1
Figure 2: The side-arm of the reactor. 1 – FOCP, 2 – holder, 3 – Teflon endplate. z –
distance from the beginning of the side-arm to the probe tip, d – distance from the probe
tip to the Teflon endplate, L – effective length of the side-arm, γ1 – recombination
coefficient of the side-chamber wall, γ2 recombination coefficient of the Teflon endplate.
Beside the movable FOCP in the side-chamber another nickel-tipped FOCP was
mounted in the main part of the experimental chamber to monitor the neutral oxygen atom
density during the course of the experiment.
The first set of measurements was performed with a cylinder of pristine aluminium foil,
inserted in the side-chamber. The second set was performed after the aluminium foil was
deposited with a 500 nm thick layer of amorphous carbon. The deposition was done in a
thermionic arc sputtering system, by sputtering a graphite target in an argon atmosphere.
3
Results and Discussion
The measured density profiles in an aluminium-lined side-chamber are presented in Fig.
(3). Depending on the source gas pressure, the densities could be measured up to the distance
from the opening of the side-chamber of about 40 mm. After that point, the density falls
below the threshold of detection of the FOCP.
Proceedings of the International Conference Nuclear Energy for New Europe, Bovec, Slovenia, Sept. 12-15, 2011
607.5
Figure 3: Neutral atom densities vs. position, in the side-chamber lined with an
aluminium surface, recorded at various source gas pressures.
In order to evaluate the recombination coefficient, we used Smith’s diffusion
model[18]. The main assumption of the model is that the net mass flow through the sidechamber is zero and the only way of propagation of neutral atoms through the side-chamber is
diffusion. Since our side-chamber is placed perpendicularly to the gas flow, the assumption is
justified. Let us thus consider the side-chamber of the length L and a circular cross-section
with the diameter of 2R. The recombination coefficient of the walls, at r = R, is γ1 and the
recombination coefficient of the endplate, at z = L, is γ2. The equation that describes the
density of atoms is the diffusion equation:
D∇ 2 n = −
∂n
,
∂t
(3)
where n is the density of atoms. Since we are interested in the stationary case, it simplifies to
the Laplace equation:
∇2n ( r, z ) = 0 .
(4)
The boundary condition at the wall of the side-arm is:
nr ( R, z ) = −
C
n ( R, z ) ,
R
(5)
where the coefficient C is:
n
n + nM
C = Rv γ 1
,
⎛ γ1 ⎞
8D12 ⎜1 − ⎟
2⎠
⎝
2−
(6)
where v is the mean thermal velocity of oxygen atoms, γ1 is the recombination coefficient of
the wall of the side-arm, D12 is the interdiffusion coefficient of O atoms in the gaseous
mixture and nM is the density of oxygen molecules.
Proceedings of the International Conference Nuclear Energy for New Europe, Bovec, Slovenia, Sept. 12-15, 2011
607.6
Analogously the boundary condition at the end-plate of the side-arm is:
nz ( r , L ) = −
Q
n ( r, L ) ,
R
(7)
and the coefficient is:
n
n + nM
Q = Rv γ 2
,
⎛ γ2 ⎞
8D12 ⎜1 − ⎟
2⎠
⎝
2−
(8)
where γ2 is the recombination coefficient of the end-plate.
Taking into account that the density at the opening of the side-arm is constant and equal
to the density of atoms in the main part of the experimental chamber, n0, and assuming the
radial symmetry of the solution, we get:
⎛Q
L−z⎞
L − z ⎞⎞ ⎛
r⎞
⎛
⎛
⎜ sinh ⎜ α m
⎟ + cosh ⎜ α m
⎟ ⎟ J 0 ⎜ α m ⎟ J1 (α m )
α
R ⎠
R ⎠⎠ ⎝
R⎠
⎝
⎝
,
n ( r , z ) = n0 ∑ ⎝ n
⎛Q
L⎞
L ⎞⎞ 2 2
m
⎛
⎛
2
⎜ sinh ⎜ α m ⎟ + cosh ⎜ α m ⎟ ⎟ R ( J 0 (α m ) + J1 (α m ) )
R⎠
R ⎠⎠
⎝
⎝
⎝ αn
(9)
where αm are coefficients determined by the boundary condition expressed in Eq. (6):
α m J1 (α m ) = C J 0 (α m ) .
(10)
It should be noted that in our experimental configuration, the length of the side-chamber
changes when the FOCP is moved. Thus, taking into account that the length of the sidechamber is L = x + d , Eq. (9) becomes:
⎛Q
d⎞
d ⎞⎞ ⎛
r⎞
⎛
⎛
⎜ sinh ⎜ α m ⎟ + cosh ⎜ α m ⎟ ⎟ J 0 ⎜ α m ⎟ J1 (α m )
R⎠
R ⎠⎠ ⎝
R⎠
⎝
⎝
⎝ αn
.
n ( r , x ) = n0 ∑
x+d ⎞
x + d ⎞⎞ 2 2
⎛
⎛
m ⎛ Q
2
⎜ sinh ⎜ α m
⎟ + cosh ⎜ α m
⎟ ⎟ R ( J 0 (α m ) + J1 (α m ) )
R ⎠
R ⎠⎠
⎝
⎝
⎝ αn
(11)
When fitting the model to the experimentally obtained results, the diffusion coefficients
were calculated from the measurements of the degree of dissociation in the main part of the
experimental chamber, and the source gas pressure. The value of the recombination for Teflon
we used was γ2 = 5 · 10−4. The average value of the aluminium foil was found to be
γ1 = 4.2 · 10−4 ± 0.8 · 10−4.
The density profiles measured in the second part of the experiment, when the
aluminium foil, inserted in the side-chamber, was covered by a 500 nm thick film of
amorphous carbon, are presented in Fig. (4). From the first glance one can see that the depth
range in which the profiles could be measured is much shorter. The neutral oxygen atom
density drops below the level of detection at the depth of 27 mm – in contrast to 43 mm in the
Proceedings of the International Conference Nuclear Energy for New Europe, Bovec, Slovenia, Sept. 12-15, 2011
607.7
case of pristine aluminium. Fitting these results to the model, we obtained the average value
of recombination coefficient of γ1 = 1.6 · 10−3 ± 0.3 · 10−3.
Figure 4: Neutral atom densities vs. position, in the side-chamber lined with a surface,
contaminated with amorphous carbon, recorded at various source gas pressures.
4
Conclusion
Contamination of surfaces with amorphous carbon deposits will present a significant
problem in fusion devices with carbon-based plasma facing components. Among other
effects, the contamination could impact the recombination coefficient for neutral oxygen
atoms of the inner walls of the reactor and therefore change the efficiency of fuel removal
methods that are based on oxidation by neutral oxygen atoms.
We have studied the impact of coverage by amorphous carbon by measuring the
recombination coefficient of an aluminium surface, and an aluminium surface covered by an
amorphous carbon deposit. The recombination coefficient was observed by measuring the
density profile of neutral oxygen atoms in a closed side-chamber of a plasma reactor, where
the wall was lined with the sample material. The value of the recombination coefficient was
determined by using Smith’s diffusion model.
We noticed that while the recombination coefficient of the contaminated surface was
still relatively low, it was nonetheless greater than that of the pristine surface by a factor of 4.
Even in our experimental system with relatively small dimensions, this had a drastic impact
on the decay length of atom densities in the side-chamber. This illustrates the fact that even at
relatively low values of the recombination coefficient, the recombination can have a profound
effect on neutral atom densities in confined spaces.
5
Acknowledgement
The author acknowledges the financial support from the Ministry of Higher Education,
Science and Technology of the Republic of Slovenia through the contract No. 3211-10000057 (Center of Excellence Polymer Materials and Technologies).
Proceedings of the International Conference Nuclear Energy for New Europe, Bovec, Slovenia, Sept. 12-15, 2011
607.8
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Proceedings of the International Conference Nuclear Energy for New Europe, Bovec, Slovenia, Sept. 12-15, 2011