Experimental verification of the method for detection of water

ISSN 1063780X, Plasma Physics Reports, 2012, Vol. 38, No. 3, pp. 197–201. © Pleiades Publishing, Ltd., 2012. Original Russian Text © A.B. Antipenkov, O.N. Afonin, V.N. Ochkin, S.Yu. Savinov, S.N. Tskhai, 2012, published in Fizika Plazmy, 2012, Vol. 38, No. 3, pp. 221–225.

TOKAMAKS

Experimental Verification of the Method for Detection of Water Microleakages in Plasma Vacuum Chambers by Using the Hydroxyl Spectrum A. B. Antipenkova, O. N. Afoninb, V. N. Ochkinb, S. Yu. Savinovb, and S. N. Tskhaib a

b

ITER Organization, Route de VinonsurVerdon, St. PaullezDurance, 13115 France Lebedev Physical Institute, Russian Academy of Sciences, Leninskii pr. 53, Moscow, 119991 Russia Received July 5, 2011

Abstract—Experimental determination of the sensitivity of the method for detection of water microleakages in the cooling systems of the plasma vacuum chambers of complex electrophysical devices (such as tokamaks, fuel elements of nuclear reactors, and plasmachemical reactors) is considered. It was shown that the spectro scopic method for detection of water microleakages by using the hydroxyl radiation spectrum makes it possi ble to detect leakages at a level of 10–5 Pa m3 s–1. The spatial resolution of the method allows one to localize defects with an accuracy of several centimeters. DOI: 10.1134/S1063780X12020018

1. INTRODUCTION Micronsize leakages in the walls of vacuum and hermetic chambers can be detected using conven tional diagnostics [1]. However, these diagnostics fail to provide rapid detection of microleakages, especially in largevolume chambers of complex devices. As a rule, microleakages practically do not influence the performance of such devices, because the rate of gas leakage through them is usually comparable with the flux of adsorbed molecules from the chamber wall, so such leakages may not manifest themselves in quiet conditions. However, the problem of detection of microleakages is rather challenging for such devices as tokamaks, fuel elements of nuclear reactors, plasma chemical reactors, and spacecraft. The chamber walls of these devices undergo enormous stresses; as a result, the existing microleakages can widen and new ones may appear, thereby leading to emergency conditions. The problem of rapid detection of microleakages in plasma vacuum chambers was noticed in the ITER Final Design Report [2] as one of the important tasks to be resolved. In [2], microleakages through which water vapor can leak into the chamber from the cool ing system were discussed. The idea of spectroscopic method for detection of water leakages is as follows. When a plasma (or an electron) flow interacts with the chamber wall, water vapor is decomposed and radia tion of its components (H2O, H, O, OH, or a marker) is excited, the characteristic spectral region of which can be detected against the minimal noise back ground. If the plasma parameters are appropriate for efficient excitation, then a halo radiating within the wavelength range Δλ appears around the leakage. In this case, the problem is reduced to the choice of the

detected component and optimization of its excitation conditions and spectrum measurements. It is more convenient to do this during the conditioning of the inner surface of the chamber wall with an auxiliary dis charge, although it is also possible to continuously monitor the wall in the standard operating mode. Esti mates and preliminary experiments have shown that local leakages at a level of 10–2 Pa m3 s–1 (or 2 × 1018 s–1) can be detected in ITER (when the first wall is cleaned using an auxiliary glow discharge) [3–5]. In [5], the possibility of achieving the required sensitivity and spatial resolution by measuring the radiation spectrum in the 0–0 band of the OH* radical was considered. The present work is devoted to experimental determi nation of the sensitivity of the method based on the detection of hydroxyl radiation in a glow discharge with a hollow cathode on the Tech’ setup, which mod els the conditions near the ITER first wall during its cleaning with a glow discharge. 2. DETERMINATION OF THE SENSITIVITY OF THE METHOD FOR DETECTION OF WATER MOLECULES BY USING HYDROXYL SPECTRAL LINES Figure 1 shows the scheme of the vacuum system of the Tech’ setup, in the chamber of which the discharge unit was placed. In these experiments, we used a dis charge with a hollow cathode (see Fig. 2). The total volume of the chamber was 3 × 104 cm3, and the dimensions of the cathode cavity were 10 × 5 × 1 cm. The discharge unit modeled the gap between the mod ules of the ITER first wall in order to study mecha

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Fig. 1. Scheme of the vacuum system of the Tech’ setup: (1) vacuum chamber, (2) TDS022 pumpingout system, (3) magneticdischarge pump, (4) shutoff valve, and (5) needle inlet valve.

needle valve. The gas was supplied through a 800μm diameter channel, and, after 64 s, the plasma radiation spectrum was recorded (see Fig. 3). The spectrum was recorded using an AvaSpec256 spectrometer with 1200grooves/mm diffraction grating, which provided 0.8nm spectral resolution in the first diffraction order. A 256element CCD linear array was used as a detector. The spectrum recording time was 500 ms. To determine the water leakage rate, we used the (0–0) band of the A2Σ⎯X2Π electronic transition of the hydroxyl molecule (306–310 nm) and measured the time during which the pressure in the chamber changed by 0.01 mbar. For example, if this time is t = 1200 ± 100 s and the above change in the pressure cor responds to a change in the gas density of ~2.7 × 1020 m–3, then the leakage rate is

nisms of water vapor excitation inside the gap and determine the sensitivity of the method for detection of microleakages in their most probable locations. To estimate the sensitivity of the method, a gas mixture is required in which hydroxyl (OH) radiation is either absent or minimal. To this end, the preoutgassed vac uum chamber of the Tech’ setup was evacuated using the TDS022 oilfree pumpingout system to a pres sure of 3.5 × 10–5 mbar, after which it was filled with the buffer gas (a 9 : 1 He–Ar mixture) at a pressure of 7 × 10–1 mbar. The pressure was measured with a Pfe iffer TRP 280 gauge with an accuracy of 0.01 mbar. The gas flow rate was controlled using an NRP1.6

11 −1 −3 (1) Q = n = 2.7 × 10 ≈ 2.25 × 10 s cm . t 1200 Thus, over the time interval τ = 64 s, the gas density in the chamber increases by ~1.44 × 1014 cm–3. For the conditions typical of the experiments car ried out on the Tech’ setup, the water content in air was about 1%; hence, and at the time at which the spectrum was measured, the number density of water molecules in the chamber was about 1.44 × 1011 cm–3. The region from which radiation was collected was determined as follows (see Fig. 4). Mercuryvapor lamp 1, used as a light source, was placed in case 5 and covered with a shutter with 1mmdiameter aperture 2.

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Fig. 2. Discharge unit with a hollow cathode. The cone indicates the aperture of the recording system. PLASMA PHYSICS REPORTS

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3. SPATIAL LOCALIZATION OF MICROLEAKAGES In order to solve the problem of spatial localization of a microleakage, we performed experiments with different leakage rates. For this purpose, an inlet valve

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Extrapolating our experimental conditions to ITER, such a number density of water molecules cor responds to the average density of the water molecules expanding isotropically from a microleakage into the 10cmdiameter nearwall hemisphere (according to preliminary estimates, such a radiation zone is pro duced in a glow discharge by a leakage in the first wall) at a leakage rate of 3.5 × 1015 s–1 (or about 3 × 10 ⎯5 Pa m3 s–1). Thus, for a standard fiberoptical AVASPEC spectrometer with a CCD matrix, the sen sitivity of the method for detection of water microleakages from hydroxyl radiation at wavelengths of 306–310 nm in a glow discharge is about 300 better than that adopted in the project for this type of dis charge (10–2 Pa m3 s–1); i.e., the method is applicable for ITER. When using a photomultiplier to record the hydroxyl band in the investigated spectral range, the sensitivity of the spectroscopic method can be addi

tionally enhanced by two to three orders of magnitude, which meets the acceptable level of the water leak rate for ITER nominal operation (10 ⎯7 Pa m3 s–1).

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At a distance of 15 cm from the source, optical fiber 3 was placed. By moving the end of the optical fiber along line 4, we measured the profile of the radiation intensity (see Fig. 5), from which the angular aperture of the recording system was found to be α = 8°. The aperture of the recording system is shown as a cone in Fig. 2. The hollowcathode discharge filled the entire gap; therefore, the region from which radiation was collected was limited, on the one hand, by the aperture of the recording system and, on the other hand, by the boundaries of the hollow cathode itself. The radiation was recorded from a ~13cm3volume region, which contained about 2 × 1012 water molecules.

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Fig. 4. Scheme of measurements of the angular aperture (all dimensions are in millimeters): (1) mercuryvapor lamp, (2) aperture, (3) optical fiber of the recording sys tem, (4) observation plane, and (5) protective case.

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0.7 mbar by puffing the gas through the needle valve. This process was recorded using a video camera. The duration of gas puffing was about 1 s, and the leakage rate was about 1019 molecules/s (or about 0.1 Pa m3 s–1). Figure 6 shows three consecutive frames taken at dif ferent instants after the appearance of a microleakage.

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was designed that allowed us to introduce water vapor directly into the hollowcathode discharge. In this case, a UV was observed, which was recorded by a CCD matrix at a frame rate of 30 s–1. The hydroxyl radiation spectrum was also recorded using the AVASPEC spectrometer at a frame rate of 5 s–1, the exposure time being 100 ms. Two series of experiments were carried out. In the first series, the microleakage was localized visually, while in the second, the leakage was detected by using the hydroxyl radiation spectrum recorded during water puffing into the chamber. In the former case, the camber was filled with a buffer gas to a pressure of about 0.09 mbar (the mini mum pressure at which the discharge could be initi ated). The discharge radiation intensity was very low. Then, the pressure of the buffer gas was increased to

A similar experiment was performed with water vapor. In this case, the pressure of the buffer gas was about 0.45 mbar. The choice of this pressure was dic tated by the volume occupied by the hollowcathode discharge, which filled the entire cathode cavity. The inlet valve was connected to a water vessel. The plasma radiation spectrum began to be recorded just after the valve was opened. Over 10 s during which the inlet valve was open, the gas pressure in the chamber increased to 0.7 mbar, the leakage rate being one order of magnitude lower than in the previous case (about 1018 molecules/s, i.e., 10–2 Pa m3 s–1). A typical time dependence of the intensity of the 306 to 310nm hydroxyl radiation band in this series of experiments is shown in Fig. 7. One can see how the spectrum varies after the appearance of water vapor. The rapid increase in the signal intensity allows one to reliably determine the localization and rate of water leakage into the vac uum chamber. 4. CONCLUSIONS We have shown that the spectroscopic method for detection of water microleakages in vacuum chambers by using the hydroxyl radiation spectrum allows one to detect leakages in the range of 10–2–10–5 Pa m3 s–1. The spatial resolution of the method makes it possible to localize the defects with an accuracy of several cen timeters. Our experiments have demonstrated that this method can be used to detect and localize microcracks in the cooling systems of the vacuum chambers of complex electrophysical devices with strict require ments to impurity concentrations.

Fig. 6. Photographs of the discharge glow taken (a) 0, (b) 3.3, and (c) 6.6 ms after the beginning of gas puffing. The arrow shows the position of the inlet valve needle. PLASMA PHYSICS REPORTS

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Fig. 7. Time dependence of the intensity of the 306 to 310nm hydroxyl band during water vapor puffing. The upper left and lower right insets show the spectra recorded before and after water vapor puffing, respectively.

ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research (project no. 100800886a), the Council of the Russian Federation Presidential Grants (grant no. MK2352.2012.2), and the Ministry of Science and Education of the Russian Federation (state contract no. 032221).

1. Nondestructive Testing, Vol. 2: Hermeticity Testing, Ed. by V. V. Klyuev (Mashinostroenie, Moscow, 2003) [in Russian].

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3. A. B. Antipenkov, O. N. Afonin, I. V. Vizgalov, et al., Vopr. At. Nauki Tekh., No. 3, 46 (2006). 4. A. B. Antipenkov, O. N. Afonin, I. V. Vizgalov, et al., Vopr. At. Nauki Tekh., No. 4, 44 (2007). 5. A. B. Antipenkov and O. N. Afonin, Bull. Lebedev Phys. Inst. 37, 291 (2010).

REFERENCES

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2. ITER Final Design Report No. G 31 DDD 14 0107 19 W 0.1, Section 3.1: Vacuum Pumping and Fuelling Systems (IAEA, Vienna, 2001).

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Translated by E.V. Voronova