Available online at www.sciencedirect.com ScienceDirect Physics Procedia 71 (2015 ) 116 120 18th Conference on Plasma-Surface Interactions, PSI 2015, 5-6 February 2015, Moscow, Russian Federation and the 1st Conference on Plasma and Laser Research and Technologies, PLRT 2015, 18-20 February 2015 Concerning feasibility of water microleakage diagnostics by auto-oscillating discharge Ivan Sorokin*, Igor Vizgalov, Konstantin Gutorov, Fedor Podolyako National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe shosse 31, 115409, Moscow, Russia Abstract One of the perspective methods of water microleakages detection in thermonuclear devices is presented. Method is based on the secondary electron emission instability of Debye near-electrode layers in the contact of nonequilibrium plasma with electrode surface covered by thin dielectric film. Method is based on detection of beam-plasma discharge transition into auto-oscillation regime. It is observed when balance between oxidation and sputtering of a contact surface covered by thin dielectric film is shifted. The method has better sensitivity (< 10 16 mol/s) and response time (< 60 s) as compared with spectroscopic methods (> 10 16 mol/s and > 2000 s, respectively). In addition, it allows detecting negligible oxygen-containing admixture. In perspective, this method will allow to localize a water microleakage in vacuum chambers of thermonuclear devices and determine a water vapor flow by features of IV-curve. 2015 The The Authors. Published by Elsevier by Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) Physics Institute). Keywords: secondary emission auto-oscillating instability; N-shape IV-curve; water microleakage. * Corresponding author. Tel.: +7 499-324-70-24; fax: +7-499-324-21-11. E-mail address: iasorokin@mail.ru 1875-3892 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) doi:10.1016/j.phpro.2015.08.324
Ivan Sorokin et al. / Physics Procedia 71 ( 2015 ) 116 120 117 1. Introduction Early diagnostics and localization of microleakage in cooling system of thermonuclear devices is an important task to ensure safety and to increase devices lifetime. Presently there is no satisfactory method, which provides instant detection and further localization of leaks in cooling system. The spectroscopic methods based on hydroxyl spectral lines detection in plasma do not provide the required responsiveness, and lower limit value of the detected water vapour flow in this method exceeds the ITER requirements (10 16 molecules/s) by more than an order of magnitude. There are also methods using spectroscopic markers, such as small xenon admixture in water cooling systems. However, this method has a number of disadvantages associated with the markers high cost and the need for serious vacuum chamber treatment after each emergency activation event or diagnostics system test run because of the marker interaction with the wall [Voronov (2013)]. Oxidation processes due to the oxygen or oxygen-containing molecules presence in the residual gas and oxide dielectric film sputtering occur simultaneously on a non-equilibrium plasma-facing electrode surface during the discharge. Presence of such films on electrode surface leads to a significant increase in the effective secondary electron emission coefficient due to both increased secondary emission coefficient and electron tunnelling current through the film. In this case, IV-curve has N-shape with a negative differential resistance (NDR) region. Diagnostics of oxygen-containing compounds is based on transition detection of beam-plasma discharge into auto-oscillating regime (AOR) due to a shift in the balance between electrode surface oxidation and sputtering. Beam-plasma discharge transits into auto-oscillating regime with presence of a thin (10-50 nm) dielectric film on electron beam collector negatively biased to IV-curve NDR area [Gutorov (2010)]. In this case, the operating point is unstable, which leads to fluctuations in electrode power supply circuit. If balance is set in a transition region between sputtering and film growth (without oscillating regime), the system becomes very sensitive to the oxygencontaining admixture. In this instance, even small oxygen admixture can shift balance toward the film growth and initiate AOR. The voltage oscillations observed in experiments have amplitude greater than the electrode biasing. Current oscillations amplitude exceeds in the electrode circuit. This mode is characterized by high-energy input and increased plasma radiation. In addition, it increases the efficiency of optical diagnostics of hydroxyl spectral lines in plasma. 2. Experimental setup Research was carried out on the linear plasma device PR-2 (MEPhI) with axial magnetic field. Beam-plasma discharge is implemented. It is initiated by powerful electron gun with directly heated tantalum cathode. Working gas in the experiments was argon, residual gas pressure in the chamber is 10-3 Pa, working pressure is 2 10-1 Pa. Plasma parameters obtained at PR-2 are: ne is up to 10 13 cm -3, Te = 5-25 ev. In addition, plasma device is equipped with Langmuir probes, optical spectroscopy, static mass-spectrometer of plasma ions and quadrupole residual gas analyzer (cf. Fig. 1).
118 Ivan Sorokin et al. / Physics Procedia 71 ( 2015 ) 116 120 Fig. 1. Scheme of the PR-2: 1 - magnetic field coils, 2 - electron gun, 3 - collector, 4 - gas inlet system, 5 - static mass-spectrometer, 6 - quadrupole mass-spectrometer, 7 - thermochemical calibrated water vapour source. Calibrated thermochemical water vapour source developed for an early microleakage detection system in thermonuclear devices [Voronov (2013) and Kurnaev (2013)] was used as a water vapour source. Source is based on a thermal decomposition of calcium hydroxide. It has fine-tuning of a water vapour flow in the range from 10 12 molecules/s to 10 22 molecules/s by a heater temperature. Low limit of the range is caused by pressure of a saturated vapour of a calcium hydroxide (10-5 Pa). Test experiments of water leakage detection system in the main ITER operating modes should be conducted at a flow rate up to 10 17 molecules/s [Voronov (2013)]. In this work, the water microleakage corresponding to ITER requirements was simulated. The source was mounted by a linear feedthrough into PR-2 chamber. The water vapour flow was let in directly into the vacuum vessel. An equivalent electrical scheme for a discharge transition into auto-oscillation regime is shown in Fig. 2. Fig. 2. Electrical scheme for discharge detection in auto-oscillating regime: 1 - vacuum chamber, 2 - electron beam collector, 3 - plasma column, 4 - Rogowski coil. Electron beam collector was biased to NDR CVC by source of EMF - with internal resistance - R. C is output capacitance of a power supply, inductance (L) is intended to change voltage oscillations parameters generated on an electrode. High voltage divider (R1 and R2) and Rogowski coil are provided for measuring voltage and current in the collector circuit, respectively. In addition, an optical spectrometer AvaSpec-2048x14 was used for obtaining plasma radiation spectrum.
Ivan Sorokin et al. / Physics Procedia 71 ( 2015 ) 116 120 119 3. Results During the experiments, operating point in NDR CVC area of an aluminium collector has been selected in lowpower discharge regime. Discharge power was 200 W, working pressure in the chamber was 8.4 10-3 Pa. In 48 seconds after switching on the thermochemical source beam-plasma discharge transits into the autooscillation regime. In addition, oscillations of collector potential and current in the measuring circuit were observed. Electrode potential oscillations during the beam-plasma discharge transition are shown in Fig. 3. After switching off the water vapour source oscillations existed for 30-100 s, and then disappear after collector surface purification by sputtering in argon plasma. Water molecules flow, leading to the oscillations was about 10 16 molecules/s. Pressure variation in the chamber during experiments was less than 10-4 Pa. Fig. 3. Voltage signal of electron beam collector at the time of beam-plasma discharge transition into auto-oscillation regime. In addition, optical spectroscopy was carried out. During the discharge transition into the auto-oscillation regime, appearance of water lines and significant increase in total intensity of plasma emission (cf. Fig. 4) were observed. Presence of Ar II indicates a dramatic increase in plasma energy input. Fig. 4. Plasma spectral lines: a beam-plasma discharge, b auto-oscillation regime. In addition, oscillation modes for tungsten, tantalum and aluminium electrodes with own oxide, and for stainless steel electrode with a surface of silicon oxide film were experimentally obtained.
120 Ivan Sorokin et al. / Physics Procedia 71 ( 2015 ) 116 120 4. Discussion Experiments show that the described method based on the oscillations detection can be successfully applied for a quick indication of water microleakage in thermonuclear devices cooling system or oxygen-containing admixture into the vacuum chamber of physical facilities in accordance with the ITER requirements (<10 16 molecules/s). The discharge transition is initiated by potential oscillations excitation on electrode and discharge power increase. This allows using spectroscopic diagnostics more efficiently due to an increased total radiation intensity of plasma column. In future, this method may be also used to determine the flux of oxygen-containing impurities according to dielectric film thickness on electrode surface. It was noted that when film thickness is less than 5 nm no oscillation mode occurs, with thickness in the range of 10-100 nm oscillations occur and with film thickness more than 150 nm a breakdown occurs [Gutorov (2010)]. More precise information about thickness can be obtained from the CVC characteristic parts [Gutorov (2015)]. However, this method requires additional equipment. Working gas, capable of sputtering or oxidizing the surface is an essential requirement for the method. In this case, it is possible to choose an operating point in the transition region between the removal and the film growth. 5. Conclusion Microleakage detection method in water cooling system of thermonuclear devices, based on unstable charge exchange between non-equilibrium plasma and contact surface of electrode with thin dielectric film, was developed. Method has good sensitivity and response time as compared to previously proposed methods. Method is suitable for detecting oxygen-containing compounds occurrence in the vacuum chambers of physical facilities. Dielectric films with suitable parameters may exist with beryllium and tungsten, which are the first ITER wall materials. Oscillation regimes were experimentally observed with tungsten, tantalum and aluminium electrodes with own oxide films, and stainless steel electrode with surface of silicon oxide film. Using this approach, in perspective, one can determine not only the presence of the cooling system leakage, but also the flow by the dielectric film thickness estimating, which may be obtained by the electrode CVC, and, coupled with the auxiliary plasma source use will allow precise localization of a specific leak. Acknowledgements This work was conducted in accordance to contract with Lebedev Physical Institute of the Russian Academy of Sciences (LPI RAS) within grant 14-12-00784 of the Russian Scientific Fund. References Kurnaev, V., Afonin, O., Antipenkov, A., Koborova, N., Mukhammedzyanov, T., Ochkin, V., Pearce, R., Pleshkov, E., Podolyako, F., Sorokin, I., Urusov, V., Vizgalov, I., Voronov, G., Vukolov, K., Worh, L., L-2M team, 2013. Spectroscopic localization of water leaks in ITER. Fusion Engineering and Design 88, No. 6-8, 1414-1417. Gutorov, K.M., Vizgalov, I.V., Markina, E.A., Kurnaev, V.A., 2010. Influence of thin dielectric films on electronic emission and stability of plasma-surface contact. Bulletin of Russian Academy of Sciences 74 No. 2, 188-191. Gutorov, K.M., Vizgalov, I.V., Sorokin, I.A., Podolyako, F.S., 2014. Current-voltage characteristic of the contact of a plasma with an electrode with a thin dielectric film on the surface. JETP Letters 100, No. 11, 708-711. Voronov, G.S., Berezhetskii, M.S., Bondar, Y.F., Vafin, I.Yu., Vasil kov, D.G., Voronova, E.V., Grebenshchikov, S.E., Grishina, I.A., Larionova, N.F., Letunov, A.A., Logvinenko, V.P., Meshcheryakov, A.I., Pleshkov, E.I., Khol nov, Yu.V., Fedyanin, O.I., Tsygankov, V.A., Shchepetov, S.V., Kurnaev, V.A., Vizgalov, I.V., Urusov, V.A., Sorokin, I.A., Podolyako, F.S., Antipenkov, A., Pearce, R., Worth, L., 2013. Testing of the method for water microleakage detection from OH hydroxyl spectral lines at the L-2M stellarator. Plasma Physics Reports 39 No. 4, 277-288.