Influence of Amorphous Carbon Deposition on the Probability for Recombination of Neutral Oxygen Atoms on Aluminium Surfaces

Transcription

1 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 4, 1000 Ljubljana, Slovenia P. Panjan, M. Mozetič Jožef Stefan Institute Jamova 39, SI-1000 Ljubljana, Slovenia 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

2 607. thermal characteristics, carbon-based materials such as carbon fibre composites are very attractive candidates for the plasma facing components[1, ]. 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 O 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[1]. 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 O 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 Wdis + +, (1) where W dis is the dissociation energy (5.1 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 Eley- Rideal 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 1 st order reaction, meaning that its rate is proportional to the rate of atoms colliding with the surface from the gas phase: r = γ j, () 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

3 607.3 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. 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 7.1 MHz generator coupled to the reactor with a 1 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 % Figure 1: The experimental set-up. 1 oxygen bottle, reduction valve, 3 needle valve, 4 discharge chamber, 5 experimental chamber, 6 pressure gauge, 7 zeolyte trap, 8 high vacuum linear valve, 9 pump.

4 607.4 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. (). 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. γ 1 z L 3 γ + d 1 Figure : The side-arm of the reactor. 1 FOCP, 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, γ 1 recombination coefficient of the side-chamber wall, γ 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.

5 607.5 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 R. The recombination coefficient of the walls, at r = R, is γ 1 and the recombination coefficient of the endplate, at z = L, is γ. The equation that describes the density of atoms is the diffusion equation: n D n=, (3) t where n is the density of atoms. Since we are interested in the stationary case, it simplifies to the Laplace equation: ( ) n r, z = 0. (4) The boundary condition at the wall of the side-arm is: C nr ( R, z) = n( R, z), (5) R where the coefficient C is: n n + nm C = Rvγ1, (6) γ1 8D1 1 where v is the mean thermal velocity of oxygen atoms, γ 1 is the recombination coefficient of the wall of the side-arm, D 1 is the interdiffusion coefficient of O atoms in the gaseous mixture and n M is the density of oxygen molecules.

6 607.6 Analogously the boundary condition at the end-plate of the side-arm is: Q nz ( r, L) = n( r, L), (7) R and the coefficient is: n n + nm Q= Rvγ, (8) γ 8D1 1 where γ 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, n 0, and assuming the radial symmetry of the solution, we get: (, ) n r z Q L z L z r sinh αm + cosh αm J0 αm J1( αm) αn R R R = n 0, (9) m Q L L sinh αm + cosh αm R ( J0( αm) + J1 ( αm) ) αn R R where α m are coefficients determined by the boundary condition expressed in Eq. (6): ( ) C J ( ) α J α = α. (10) m 1 m 0 m 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: (, ) n r x = n 0 m Q d d r sinh αm + cosh αm J0 αm J1( αm) αn R R R Q x+ d x+ d sinh α + cosh α R J α + J α αn R R ( 0( ) 1 ( )) m m m m. (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 γ = The average value of the aluminium foil was found to be γ 1 = ± 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 7 mm in contrast to 43 mm in the

7 607.7 case of pristine aluminium. Fitting these results to the model, we obtained the average value of recombination coefficient of γ 1 = ± 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 (Center of Excellence Polymer Materials and Technologies).

8 References 1. J. Linke, "High heat flux performance of plasma facing materials and components under service conditions in future fusion reactors", Fusion Sci. Technol. 53, 008, T, pp U. Samm, "Plasma-wall interaction in magnetically confined fusion plasmas", Fusion Sci. Technol. 53, 008, T, pp A. Kirschner, D. Borodin, et al., "Modelling of tritium retention and target lifetime of the ITER divertor using the ERO code", J. Nucl. Mater. 363, 007, G. Counsell, P. Coad, et al., "Tritium retention in next step devices and the requirements for mitigation and removal techniques", Plasma Phys. Control. Fusion 48, 006, 1B, pp. B189-B J.A. Ferreira, F.L. Tabares, et al., "Removal of carbon deposits in narrow gaps by oxygen plasmas at low pressure", Journal of Vacuum Science & Technology A 5, 007, 4, pp J.S. Hu, J.G. Li, et al., "Oxygen removal with D--ICR cleanings after oxidation experiment in HT-7", Fusion Engineering and Design 8, 007,, pp C. Stancu, M. Teodorescu, et al., "Carbon layers cleaning from inside of narrow gaps by a RF glow discharge", Surface & Coatings Technology 05, 011, S435-S I. Tanarro, J.A. Ferreira, et al., "Removal of carbon films by oxidation in narrow gaps: Thermo-oxidation and plasma-assisted studies", J. Nucl. Mater , 009, A. Vesel, M. Mozetic, et al., "Etching of carbon-tungsten composite with oxygen plasma", Surface & Coatings Technology 04, 010, 9-10, pp R. Zaplotnik, A. Vesel, et al. "Interaction of Air Plasma with Graphite at Elevated Temperature", International Conference Nuclear Energy for New Europe A. Drenik, A. Vesel, et al., "Controlled carbon deposit removal by oxygen radicals", J. Nucl. Mater. 386, 009, A. Drenik, A. Tomeljak, et al., "Behaviour of neutral hydrogen atom density in the presence of a sample holder in a plasma reactor", Vacuum 84, 009, 1, pp A. Gelb and S.K. Kim, "Theory of Atomic Recombination on Surfaces", The Journal of Chemical Physics 55, 1971, 10, pp Y.C. Kim and M. Boudart, "Recombination of O,N, and H-Atoms on silica - kinetics and mechanism", Langmuir 7, 1991, 1, pp V. Guerra, "Analytical model of heterogeneous atomic recombination on silicalike surfaces", IEEE Trans. Plasma Sci. 35, 007, 5, pp I. Poberaj, M. Mozetic, et al., "Comparison of fiber optics and standard nickel catalytic probes for determination of neutral oxygen atoms concentration", J. Vac. Sci. Technol. A-Vac. Surf. Films 0, 00, 1, pp D. Babic, I. Poberaj, et al., "Fiber optic catalytic probe for weakly ionized oxygen plasma characterization", Review of Scientific Instruments 7, 001, 11, pp W.V. Smith, "The Surface Recombination of H Atoms and OH Radicals", The Journal of Chemical Physics 11, 1943, 3, pp