- Email: [email protected]
D ratios
G Model
ARTICLE IN PRESS
FUSION-9272; No. of Pages 5
Fusion Engineering and Design xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Improving accuracy of Penning gauge spectroscopy for the determination of hydrogen isotope H/D ratios Gennady Sergienko ∗ , Hans Günter Esser, Marko Nonhoff, Alexander Huber, Michaele Freisinger, Christian Linsmeier Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung—Plasmaphysik, Partner of the Trilateral Euregio Cluster (TEC), 52425 Jülich, Germany
h i g h l i g h t s • Penning gauge spectroscopy of hydrogen and deuterium Balmer-alpha spectral lines was performed in the gas mixture pressures range of 10−6 –10−3 mbar.
• Subsequent measurements for identical gas pressures revealed that the H␣ line intensities are systematically higher the D␣ line intensities by a factor of about 1.28.
• H␣ line intensity for hydrogen gas having constant partial pressure is slightly increased with the total gas mixture pressure. • Proposed empirical functions, which take new observations into account, improve essentially the absolute accuracy of the mixture composition measurement.
a r t i c l e
i n f o
Article history: Received 2 October 2016 Received in revised form 12 March 2017 Accepted 16 March 2017 Available online xxx Keywords: Penning gauge spectroscopy Balmer-alpha line emission Hydrogen isotope gas mixture
a b s t r a c t Atomic spectral lines emitted from a Penning discharge are often used to quantify partial pressures and isotopes ratios in gases. To identify the potential of this method for thermal desorption studies, the hydrogen emission spectrum lines (H␣ and D␣ ) were examined by an Alcatel Penning gauge. The hydrogen/deuterium pressures were measured by both a capacitive vacuum gauge and the Penning gauge. Different gas mixtures were produced by varying of hydrogen/deuterium flows. The Balmer-alpha lines intensities were recorded with help of a high etendue spectrometer coupled to the Penning gauge using relay optics together with fiber bundle and equipped with Peltier cooled CCD camera. Subsequent measurements using hydrogen and deuterium gases revealed for identical pressures in the range of 10−6 –10−3 mbar that the H␣ line intensities are systematically higher the D␣ line intensities by a factor of 1.28 ± 0.01. This observation can be explained by the dissociative excitation of the molecular hydrogen by electron impact. In addition the H␣ line intensity for the H2 gas having constant partial pressure is slightly increased with total gas mixture pressure. Taking into account of both effects essentially improves the accuracy of the determination of partial pressures and isotope H/D ratios. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Deuterium-tritium gas mixture will be used as fuel in future fusion devises like ITER. Therefore, it is important to monitor hydrogen isotope ratios not only in fusion plasma and in the gas exhaust but also retained in the plasma facing components (PFC). Residual gas analysis by means of quadrupole mass spectrometer (QMS) is traditionally used to quantify the isotope species of the PFCs in
∗ Corresponding author. E-mail address: [email protected] (G. Sergienko).
the laboratory by means of thermal desorption spectroscopy (TDS). The drawback of this method is that the mass peaks of the isotopes cracking patterns and helium superimpose and complicate data analyses as well as accurate quantification. Atomic spectral lines emitted from a Penning discharge can be used for the measurement of partial pressures and isotopes ratios in gases. Penning gauge spectroscopy was primary developed for the research in the field of magnetic confinement fusion [1–7]. This diagnostic will be also used in ITER [8]. The main reason of using the Penning gauge spectroscopy in fusion study is better distinguishing of the helium signal from deuterium signal in comparison with conventional QMS. In addition, this method provides the measuring
http://dx.doi.org/10.1016/j.fusengdes.2017.03.092 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: G. Sergienko, et al., Improving accuracy of Penning gauge spectroscopy for the determination of hydrogen isotope H/D ratios, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.092
G Model FUSION-9272; No. of Pages 5
ARTICLE IN PRESS G. Sergienko et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
2
Fig. 1. Vacuum system used for Penning gauge calibration.
of isotope composition in the hydrogen gas mixtures allowing the independent validation of the QMS data. The latter is very important for tritium content measurement in tritium experiments in JET and ITER as well. In the present paper, the investigation of a Penning gauge with spectroscopical detection of hydrogen and deuterium gases have been performed to qualify this method for TDS and particular for Laser Induced thermal Desorption (LID). 2. Experimental set-up The measurements presented here were performed on a vacuum system prepared for TDS which is shown on Fig. 1. Total volume of the stainless steel vacuum chamber with the quartz tube port was about of 25 l. The system was evacuated with the 345 l/s (nitrogen gas) turbomolecular pump which was connected to a dual-stage, rotary vane pump with a pumping speed of 1.7 l/s. The minimum vacuum pressure without system heating was about 10−7 mbar. For residual gas analysis, a differentially pumped QMS was used. The hydrogen and deuterium were injected into the vacuum chamber using the precision leak valves. Partial pressures of hydrogen and deuterium were varied by the change of hydrogen and/or deuterium gas flows. The pressure in the vacuum system has been monitored by means of different types of vacuum gauges: a hot cathode ionization gauge (Ionivac IE211), cold cathode ionization gauges (Penningvac PR36 and Alcatel CF 2P), a thermal conductivity gauge (Thermovac) and two capacitive vacuum gauges (Baratron and CMR 375). The capacitive vacuum gauges have different measurement ranges 10−4 –1 mbar and 10−5 –0.1 mbar and mainly used for setting of the desired hydrogen/deuterium composition and pressure of the gas mixture. The Alcatel model CF 2P Penning gauge in the combination with FA101 power supply was used to detect hydrogen and deuterium by spectroscopic means. The Penning gauge was mounted at the maximum distance from the gas injection port in the vicinity of the capacitive vacuum gauges to minimize the pressure differences. A quartz vacuum window ∅32 mm was installed at in front of the Alcatel Penning gauge to observe the plasma. The plasma light was collected by optical system shown on Fig. 2(a). Two achromatic lenses with focus lengths of 100 mm and 25 mm coupled the plasma light to ∅3.9 mm optical fiber bundle. The image of the plasma was built by lens 1 on the lens 2 and lens 2 imaged the lens 1 on the fiber bundle. Such optical arrangement illuminates more homogeneously the fiber bundle reducing the influence of plasma volume changes on the line intensity measurements. The exit of the fiber bundle had the rectangular shape with the size of 12 mm × 1 mm
and was used for the spectrometer entrance slit illumination with help of the optical system shown on Fig. 2(b), which adopted the fiber numerical aperture to the spectrometer numerical aperture minimizing the light loses. A Littrow-mounted plane grating spectrometer having large etendue (f/4.3) was used for the detection of the Balmer-alpha spectral lines. An achromatic lens with f = 750 mm collimated light from a 50 m entrance slit to a 1200 grooves/mm plane grating having the dimensions of 220 mm × 170 mm and a blaze angle of 17.27◦ . The spectrometer, operated in first diffractive order of the grating, had the dispersion of about 1 nm/mm and the spectral resolution of about 0.05 nm at the wavelength of 656 nm. Most presented measurements were recorded with the Peltier cooled CCD camera (camera Pluto from PixelVision Inc., 652 × 488 pixels sensor, pixel size of 12 m × 12 m, sensor temperature of −5 ◦ C) with the exposure time of 40 ms. In addition the spectra were averaged over 125 frames that is equivalent to 5 s time for a single measurement. This CCD camera was replaced later by low noise (11.6 electrons rms), high quantum efficiency (Q = 90%) Peltier cooled CCD camera with larger image sensor (camera PIXIS 400 B from Princeton Instruments, 1340×400 pixels sensor, pixel size of 20 m × 20 m, sensor temperature of −75 ◦ C). The exposure time of the new camera was selected in the range of 0.1–10 s depending of the spectral lines intensities.
3. Measurements and discussion The measurements of the Balmer-alpha spectral lines were performed with several hydrogen-deuterium gas mixtures in the pressures range of 10−6 –10−3 mbar. To produce desired gas mixture, the pressure in the vacuum chamber was adjusted by turning the precision leak valve of hydrogen gas then the pressure was increased by turning the precision leak valve of deuterium gas. The ratio of the pressures measured in first and second steps gives the hydrogen content in the produced gas mixture. The H␣ and D␣ spectral lines are closely spaced and partially overlapped due to Doppler broadening. The spectral shape of the Balmer-alpha line emitted under the collisions of the hydrogen molecules with the electrons is defined by two groups of atoms [9] (slow (“cold”) – with energy of about 0.2 eV and fast (“hot”) – with energy of about 6 eV) originated due to molecules dissociation. Therefore, the measured spectra were fitted using sum of 4 Gaussian functions to extract the H␣ and D␣ line intensities. The fitting function was used in the form:
Please cite this article in press as: G. Sergienko, et al., Improving accuracy of Penning gauge spectroscopy for the determination of hydrogen isotope H/D ratios, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.092
G Model
ARTICLE IN PRESS
FUSION-9272; No. of Pages 5
G. Sergienko et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
3
Fig. 2. Optical systems used for: (a) the Penning gauge plasma light collection and (b) the spectrometer slit illumination.
f () = ˛0 + 2
ln2
+
(1 − CHc ) exp Hh
+
(1 − CDc ) exp Dh
IH˛
CHc exp c
−
−
ln2 − H˛ ( ) 2 Hh
−
ln2 − D˛ ( ) 2 Dh
2
2
ln2 − H˛ ( ) 2 c
+ ID˛
,
2
CDc exp c
−
ln2 − D˛ ( ) 2 c
2
(1)
where IH˛ and ID˛ are the H␣ and D␣ total line intensities, H˛ = 656.279 nm and D˛ = 656.103 nm, c is Full Width at Half Maximum (FWHM) of the cold component, Hh and Dh are the FWHM of the hot component for H␣ and D␣ , CHc and CDc are cold component portion of H˛ and D˛ . The FWHM of the cold component for both H␣ line and D␣ line was unresolved by the spectrometer and was defined by the instrument function of the spectrometer. The example of the measured spectra and fittings for both CCD cameras are shown in Fig. 3. The IH˛ / (IH˛ + ID˛ ) ratios (the normalized H␣ line intensities) evaluated from this figure are 47.6 ± 3.4% and 45.7 ± 2.2% as measured respectively by PIXIS camera and by Pluto camera. One can see that these values are larger the hydrogen contents in the gas mixtures by about of 5%. These deviations have a systemic nature as seen in Fig. 4 were the normalized H␣ line intensity measured by the Pluto CCD camera are shown as a function of the hydrogen content in the gas mixture. The Balmer-alpha line intensities for this measurement as a function of total and partial pressures of H2 and D2 are shown in Fig. 5. For the injections of pure hydrogen gas or pure deuterium gas, the H␣ line intensity was higher D␣ line intensity at the same gas pressures resulted in the IH˛ /ID˛ ratio of 1.29 ± 0.02. This observation was confirmed by more precise measurements performed with help of PIXIS CCD camera shown on Fig. 6, which revealed the ratio of 1.28 ± 0.01. To extend the measurable pressure range to lower values, the Alcatel CF 2P Penning gauge current was measured and was proportional to the gas pressure measured by the 0.1 mbar baratron in the 10−5 –10−3 mbar pressure range. The current-to-pressure conversion factors of 1.27 ± 0.02 mbar/A and 1.39 ± 0.02 mbar/A were found respectively for hydrogen and for
Fig. 3. Spectral lines fitting; CH2 —hydrogen content in the H2 + D2 gas mixture, P—the gas mixture pressure.
deuterium. To fit the measurements shown on Fig. 5, the empirical functions were used:
IH˛ PH2 , ID˛ = A1 + A2 · PH2 + A3 + A4 · PH2 · I
D˛
ID˛ PD2 , IH˛ = B1 + B2 · PD2 + B3 + B4 · PD2 · I
H˛ ,
(2)
(3)
where PH2 and PD2 are hydrogen and deuterium partial pressures. The coefficients A1 and B1 is used to correct the backgrounds. The A2/B2 ratio is the IH˛ /ID˛ ratio, which is mentioned above. The coefficients A3 and A4 were introduced to take into account the observed increase of H␣ line intensity with the increase of the deuterium partial pressure for the fixed hydrogen partial pressure. The similar coefficients B3 and B4 were used for D␣ line intensity fitting. To calculate the hydrogen or deuterium partial pressure using
Please cite this article in press as: G. Sergienko, et al., Improving accuracy of Penning gauge spectroscopy for the determination of hydrogen isotope H/D ratios, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.092
G Model FUSION-9272; No. of Pages 5
ARTICLE IN PRESS G. Sergienko et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
4
Fig. 4. Normalized H␣ line intensity versus hydrogen content in the gas mixture.
the observed Balmer-alpha line intensities the following equations were derived from the Eqs. (2) and (3): PH2 (IH˛ , ID˛ ) =
IH˛ − A1 − A3 · ID˛ A2 + A4 · ID˛
(4)
PD2 (IH˛ , ID˛ ) =
ID˛ − B1 − B3 · IH˛ B2 + B4 · IH˛
(5)
The hydrogen and deuterium partial pressures calculated by Eqs. (4) and (5) are shown in Fig. 7 as a function of the hydrogen content in the gas mixture. One can see that the absolute accuracy of the mixture composition measurement was essentially improved and it is within the range of 1%–4%. The major impact on the accuracy improvement was taking measured H␣ /D␣ sensitivities ratio into account. The increased brightness of H␣ line in comparison with D␣ line is most probably due to dissociative excitation of molecular hydrogen by electron impact, which was previously observed in crossed molecular beam experiments [10]. According to the crossed molecular beam data, the H␣ /D␣ ratio observed in
Fig. 6. Balmer-alpha lines intensities measured by PIXIS camera versus the gas pressure.
the Penning gauge discharge should corresponds to the electron kinetical energy of about 40 eV. The total ionization cross section of the molecular hydrogen at this electron energy reaches its maximum. The Penning gauge pumps hydrogen due to hydrogen ions implantation into the cathode, therefore, the hydrogen density in the gauge will increase due to the limited conductance of the vacuum connection to the gauge. The pumping speeds of 0.204 l/s and 0.186 l/s respectively for hydrogen and for deuterium were estimated using the current-to-pressure conversion factors mentioned above. The effective conductance of about 0.0088 l/s (for H2 ) and 0.0062 l/s (for D2 ) required to produce observed IH˛ /ID˛ ratio was calculated using these pumping speeds. The vacuum conductance of about 11 l/s (for H2 ) and 7.9 l/s (for D2 ) were estimated using the geometrical dimensions of the vacuum connection. Thus, the conductance is much higher and cannot be responsible for observed IH˛ /ID˛ sensitivities ratio. The hydrogen content in the Penning gauge can deviate from the injected gas mixture due to a formation
Fig. 5. Balmer-alpha lines intensities versus the gas mixture pressure.
Please cite this article in press as: G. Sergienko, et al., Improving accuracy of Penning gauge spectroscopy for the determination of hydrogen isotope H/D ratios, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.092
G Model FUSION-9272; No. of Pages 5
ARTICLE IN PRESS G. Sergienko et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
5
pressures range of 10−6 –10−3 mbar. The empirical functions were proposed to fit measured Balmer-alpha intensities as function of partial pressures. For the pure hydrogen or deuterium gas injections, the IH˛ /ID˛ line intensity ratio at constant absolute pressure was 1.28 ± 0.01. Using the proposed fitting functions, the absolute accuracy of the mixture composition measurement was essentially improved down to values of 1%–4%. The accuracy can be additionally improved by a factor of 3 by using more sensitive PIXIS camera. References
Fig. 7. Calculated hydrogen content versus hydrogen content in the gas mixture.
of HD molecules, because the HD formation rate could be much higher than the gauge pumping. The empirical functions used for Balmer-alpha intensities fitting are partially taking this effect into account. 4. Summary The measurements of the H␣ and D␣ line emission were performed in the hydrogen-deuterium gas mixtures in the mixture
[1] N. Ogiwara, et al., Direct pressure measurement near a plasma of JT-60 by using a Penning discharge sustained by the toroidal magnetic field, Shinku, J. Vacuum Soc. Jpn. 31 (5) (1988) 406–409. [2] N. Ogiwara, M. Maeno, Hydrogen pressure measurement using the intensity of the light emitted from a Penning discharge, J. Vacuum Sci. Technol. A 7 (1989) 2804–2807. [3] K.H. Finken, et al., Measurement of helium gas in a deuterium environment, Rev. Sci. Instrum. 63 (1) (1992) 1–7. [4] T. Denner, et al., Helium partial pressure measurement in a deuterium environment, Rev. Sci. Instrum. 67 (1996) 3515–3520. [5] C.C. Klepper, et al., Application of a species-selective Penning gauge to the measurement of neon and hydrogen-isotope partial pressures in the plasma boundary, Rev. Sci. Instrum. 68 (1) (1997) 400–403. [6] D.L. Hillis, et al., Tritium concentration measurements in the Joint European Torus divertor by optical spectroscopy of a Penning discharge, Rev. Sci. Instrum. 70 (1) (1999) 359–362. [7] N.H. Brooks, et al., Partial pressure measurements with an active spectrometer, Rev. Sci. Instrum. 70 (1) (1999) 423–426. [8] T.R. Younkin, et al., Description of the prototype diagnostic residual gas analyzer for ITER, Rev. Sci. Instrum. 85 (2014) 11E816. [9] G.N. Polyakova, et al., Kinetic-energy distribution of excited atoms produced when H2 and D2 molecules are dissociated by electron impact, Sov. Phys. J. Exp. Theor. Phys. 46 (6) (1977) 1117–1122. [10] C. Karolis, E. Harting, Electron impact dissociation cross sections in hydrogen and deuterium, leading to Balmer alpha and beta emission, J. Phys. B 11 (2) (1978) 357–369.
Please cite this article in press as: G. Sergienko, et al., Improving accuracy of Penning gauge spectroscopy for the determination of hydrogen isotope H/D ratios, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.092
ARTICLE IN PRESS
FUSION-9272; No. of Pages 5
Fusion Engineering and Design xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Improving accuracy of Penning gauge spectroscopy for the determination of hydrogen isotope H/D ratios Gennady Sergienko ∗ , Hans Günter Esser, Marko Nonhoff, Alexander Huber, Michaele Freisinger, Christian Linsmeier Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung—Plasmaphysik, Partner of the Trilateral Euregio Cluster (TEC), 52425 Jülich, Germany
h i g h l i g h t s • Penning gauge spectroscopy of hydrogen and deuterium Balmer-alpha spectral lines was performed in the gas mixture pressures range of 10−6 –10−3 mbar.
• Subsequent measurements for identical gas pressures revealed that the H␣ line intensities are systematically higher the D␣ line intensities by a factor of about 1.28.
• H␣ line intensity for hydrogen gas having constant partial pressure is slightly increased with the total gas mixture pressure. • Proposed empirical functions, which take new observations into account, improve essentially the absolute accuracy of the mixture composition measurement.
a r t i c l e
i n f o
Article history: Received 2 October 2016 Received in revised form 12 March 2017 Accepted 16 March 2017 Available online xxx Keywords: Penning gauge spectroscopy Balmer-alpha line emission Hydrogen isotope gas mixture
a b s t r a c t Atomic spectral lines emitted from a Penning discharge are often used to quantify partial pressures and isotopes ratios in gases. To identify the potential of this method for thermal desorption studies, the hydrogen emission spectrum lines (H␣ and D␣ ) were examined by an Alcatel Penning gauge. The hydrogen/deuterium pressures were measured by both a capacitive vacuum gauge and the Penning gauge. Different gas mixtures were produced by varying of hydrogen/deuterium flows. The Balmer-alpha lines intensities were recorded with help of a high etendue spectrometer coupled to the Penning gauge using relay optics together with fiber bundle and equipped with Peltier cooled CCD camera. Subsequent measurements using hydrogen and deuterium gases revealed for identical pressures in the range of 10−6 –10−3 mbar that the H␣ line intensities are systematically higher the D␣ line intensities by a factor of 1.28 ± 0.01. This observation can be explained by the dissociative excitation of the molecular hydrogen by electron impact. In addition the H␣ line intensity for the H2 gas having constant partial pressure is slightly increased with total gas mixture pressure. Taking into account of both effects essentially improves the accuracy of the determination of partial pressures and isotope H/D ratios. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Deuterium-tritium gas mixture will be used as fuel in future fusion devises like ITER. Therefore, it is important to monitor hydrogen isotope ratios not only in fusion plasma and in the gas exhaust but also retained in the plasma facing components (PFC). Residual gas analysis by means of quadrupole mass spectrometer (QMS) is traditionally used to quantify the isotope species of the PFCs in
∗ Corresponding author. E-mail address: [email protected] (G. Sergienko).
the laboratory by means of thermal desorption spectroscopy (TDS). The drawback of this method is that the mass peaks of the isotopes cracking patterns and helium superimpose and complicate data analyses as well as accurate quantification. Atomic spectral lines emitted from a Penning discharge can be used for the measurement of partial pressures and isotopes ratios in gases. Penning gauge spectroscopy was primary developed for the research in the field of magnetic confinement fusion [1–7]. This diagnostic will be also used in ITER [8]. The main reason of using the Penning gauge spectroscopy in fusion study is better distinguishing of the helium signal from deuterium signal in comparison with conventional QMS. In addition, this method provides the measuring
http://dx.doi.org/10.1016/j.fusengdes.2017.03.092 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: G. Sergienko, et al., Improving accuracy of Penning gauge spectroscopy for the determination of hydrogen isotope H/D ratios, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.092
G Model FUSION-9272; No. of Pages 5
ARTICLE IN PRESS G. Sergienko et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
2
Fig. 1. Vacuum system used for Penning gauge calibration.
of isotope composition in the hydrogen gas mixtures allowing the independent validation of the QMS data. The latter is very important for tritium content measurement in tritium experiments in JET and ITER as well. In the present paper, the investigation of a Penning gauge with spectroscopical detection of hydrogen and deuterium gases have been performed to qualify this method for TDS and particular for Laser Induced thermal Desorption (LID). 2. Experimental set-up The measurements presented here were performed on a vacuum system prepared for TDS which is shown on Fig. 1. Total volume of the stainless steel vacuum chamber with the quartz tube port was about of 25 l. The system was evacuated with the 345 l/s (nitrogen gas) turbomolecular pump which was connected to a dual-stage, rotary vane pump with a pumping speed of 1.7 l/s. The minimum vacuum pressure without system heating was about 10−7 mbar. For residual gas analysis, a differentially pumped QMS was used. The hydrogen and deuterium were injected into the vacuum chamber using the precision leak valves. Partial pressures of hydrogen and deuterium were varied by the change of hydrogen and/or deuterium gas flows. The pressure in the vacuum system has been monitored by means of different types of vacuum gauges: a hot cathode ionization gauge (Ionivac IE211), cold cathode ionization gauges (Penningvac PR36 and Alcatel CF 2P), a thermal conductivity gauge (Thermovac) and two capacitive vacuum gauges (Baratron and CMR 375). The capacitive vacuum gauges have different measurement ranges 10−4 –1 mbar and 10−5 –0.1 mbar and mainly used for setting of the desired hydrogen/deuterium composition and pressure of the gas mixture. The Alcatel model CF 2P Penning gauge in the combination with FA101 power supply was used to detect hydrogen and deuterium by spectroscopic means. The Penning gauge was mounted at the maximum distance from the gas injection port in the vicinity of the capacitive vacuum gauges to minimize the pressure differences. A quartz vacuum window ∅32 mm was installed at in front of the Alcatel Penning gauge to observe the plasma. The plasma light was collected by optical system shown on Fig. 2(a). Two achromatic lenses with focus lengths of 100 mm and 25 mm coupled the plasma light to ∅3.9 mm optical fiber bundle. The image of the plasma was built by lens 1 on the lens 2 and lens 2 imaged the lens 1 on the fiber bundle. Such optical arrangement illuminates more homogeneously the fiber bundle reducing the influence of plasma volume changes on the line intensity measurements. The exit of the fiber bundle had the rectangular shape with the size of 12 mm × 1 mm
and was used for the spectrometer entrance slit illumination with help of the optical system shown on Fig. 2(b), which adopted the fiber numerical aperture to the spectrometer numerical aperture minimizing the light loses. A Littrow-mounted plane grating spectrometer having large etendue (f/4.3) was used for the detection of the Balmer-alpha spectral lines. An achromatic lens with f = 750 mm collimated light from a 50 m entrance slit to a 1200 grooves/mm plane grating having the dimensions of 220 mm × 170 mm and a blaze angle of 17.27◦ . The spectrometer, operated in first diffractive order of the grating, had the dispersion of about 1 nm/mm and the spectral resolution of about 0.05 nm at the wavelength of 656 nm. Most presented measurements were recorded with the Peltier cooled CCD camera (camera Pluto from PixelVision Inc., 652 × 488 pixels sensor, pixel size of 12 m × 12 m, sensor temperature of −5 ◦ C) with the exposure time of 40 ms. In addition the spectra were averaged over 125 frames that is equivalent to 5 s time for a single measurement. This CCD camera was replaced later by low noise (11.6 electrons rms), high quantum efficiency (Q = 90%) Peltier cooled CCD camera with larger image sensor (camera PIXIS 400 B from Princeton Instruments, 1340×400 pixels sensor, pixel size of 20 m × 20 m, sensor temperature of −75 ◦ C). The exposure time of the new camera was selected in the range of 0.1–10 s depending of the spectral lines intensities.
3. Measurements and discussion The measurements of the Balmer-alpha spectral lines were performed with several hydrogen-deuterium gas mixtures in the pressures range of 10−6 –10−3 mbar. To produce desired gas mixture, the pressure in the vacuum chamber was adjusted by turning the precision leak valve of hydrogen gas then the pressure was increased by turning the precision leak valve of deuterium gas. The ratio of the pressures measured in first and second steps gives the hydrogen content in the produced gas mixture. The H␣ and D␣ spectral lines are closely spaced and partially overlapped due to Doppler broadening. The spectral shape of the Balmer-alpha line emitted under the collisions of the hydrogen molecules with the electrons is defined by two groups of atoms [9] (slow (“cold”) – with energy of about 0.2 eV and fast (“hot”) – with energy of about 6 eV) originated due to molecules dissociation. Therefore, the measured spectra were fitted using sum of 4 Gaussian functions to extract the H␣ and D␣ line intensities. The fitting function was used in the form:
Please cite this article in press as: G. Sergienko, et al., Improving accuracy of Penning gauge spectroscopy for the determination of hydrogen isotope H/D ratios, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.092
G Model
ARTICLE IN PRESS
FUSION-9272; No. of Pages 5
G. Sergienko et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
3
Fig. 2. Optical systems used for: (a) the Penning gauge plasma light collection and (b) the spectrometer slit illumination.
f () = ˛0 + 2
ln2
+
(1 − CHc ) exp Hh
+
(1 − CDc ) exp Dh
IH˛
CHc exp c
−
−
ln2 − H˛ ( ) 2 Hh
−
ln2 − D˛ ( ) 2 Dh
2
2
ln2 − H˛ ( ) 2 c
+ ID˛
,
2
CDc exp c
−
ln2 − D˛ ( ) 2 c
2
(1)
where IH˛ and ID˛ are the H␣ and D␣ total line intensities, H˛ = 656.279 nm and D˛ = 656.103 nm, c is Full Width at Half Maximum (FWHM) of the cold component, Hh and Dh are the FWHM of the hot component for H␣ and D␣ , CHc and CDc are cold component portion of H˛ and D˛ . The FWHM of the cold component for both H␣ line and D␣ line was unresolved by the spectrometer and was defined by the instrument function of the spectrometer. The example of the measured spectra and fittings for both CCD cameras are shown in Fig. 3. The IH˛ / (IH˛ + ID˛ ) ratios (the normalized H␣ line intensities) evaluated from this figure are 47.6 ± 3.4% and 45.7 ± 2.2% as measured respectively by PIXIS camera and by Pluto camera. One can see that these values are larger the hydrogen contents in the gas mixtures by about of 5%. These deviations have a systemic nature as seen in Fig. 4 were the normalized H␣ line intensity measured by the Pluto CCD camera are shown as a function of the hydrogen content in the gas mixture. The Balmer-alpha line intensities for this measurement as a function of total and partial pressures of H2 and D2 are shown in Fig. 5. For the injections of pure hydrogen gas or pure deuterium gas, the H␣ line intensity was higher D␣ line intensity at the same gas pressures resulted in the IH˛ /ID˛ ratio of 1.29 ± 0.02. This observation was confirmed by more precise measurements performed with help of PIXIS CCD camera shown on Fig. 6, which revealed the ratio of 1.28 ± 0.01. To extend the measurable pressure range to lower values, the Alcatel CF 2P Penning gauge current was measured and was proportional to the gas pressure measured by the 0.1 mbar baratron in the 10−5 –10−3 mbar pressure range. The current-to-pressure conversion factors of 1.27 ± 0.02 mbar/A and 1.39 ± 0.02 mbar/A were found respectively for hydrogen and for
Fig. 3. Spectral lines fitting; CH2 —hydrogen content in the H2 + D2 gas mixture, P—the gas mixture pressure.
deuterium. To fit the measurements shown on Fig. 5, the empirical functions were used:
IH˛ PH2 , ID˛ = A1 + A2 · PH2 + A3 + A4 · PH2 · I
D˛
ID˛ PD2 , IH˛ = B1 + B2 · PD2 + B3 + B4 · PD2 · I
H˛ ,
(2)
(3)
where PH2 and PD2 are hydrogen and deuterium partial pressures. The coefficients A1 and B1 is used to correct the backgrounds. The A2/B2 ratio is the IH˛ /ID˛ ratio, which is mentioned above. The coefficients A3 and A4 were introduced to take into account the observed increase of H␣ line intensity with the increase of the deuterium partial pressure for the fixed hydrogen partial pressure. The similar coefficients B3 and B4 were used for D␣ line intensity fitting. To calculate the hydrogen or deuterium partial pressure using
Please cite this article in press as: G. Sergienko, et al., Improving accuracy of Penning gauge spectroscopy for the determination of hydrogen isotope H/D ratios, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.092
G Model FUSION-9272; No. of Pages 5
ARTICLE IN PRESS G. Sergienko et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
4
Fig. 4. Normalized H␣ line intensity versus hydrogen content in the gas mixture.
the observed Balmer-alpha line intensities the following equations were derived from the Eqs. (2) and (3): PH2 (IH˛ , ID˛ ) =
IH˛ − A1 − A3 · ID˛ A2 + A4 · ID˛
(4)
PD2 (IH˛ , ID˛ ) =
ID˛ − B1 − B3 · IH˛ B2 + B4 · IH˛
(5)
The hydrogen and deuterium partial pressures calculated by Eqs. (4) and (5) are shown in Fig. 7 as a function of the hydrogen content in the gas mixture. One can see that the absolute accuracy of the mixture composition measurement was essentially improved and it is within the range of 1%–4%. The major impact on the accuracy improvement was taking measured H␣ /D␣ sensitivities ratio into account. The increased brightness of H␣ line in comparison with D␣ line is most probably due to dissociative excitation of molecular hydrogen by electron impact, which was previously observed in crossed molecular beam experiments [10]. According to the crossed molecular beam data, the H␣ /D␣ ratio observed in
Fig. 6. Balmer-alpha lines intensities measured by PIXIS camera versus the gas pressure.
the Penning gauge discharge should corresponds to the electron kinetical energy of about 40 eV. The total ionization cross section of the molecular hydrogen at this electron energy reaches its maximum. The Penning gauge pumps hydrogen due to hydrogen ions implantation into the cathode, therefore, the hydrogen density in the gauge will increase due to the limited conductance of the vacuum connection to the gauge. The pumping speeds of 0.204 l/s and 0.186 l/s respectively for hydrogen and for deuterium were estimated using the current-to-pressure conversion factors mentioned above. The effective conductance of about 0.0088 l/s (for H2 ) and 0.0062 l/s (for D2 ) required to produce observed IH˛ /ID˛ ratio was calculated using these pumping speeds. The vacuum conductance of about 11 l/s (for H2 ) and 7.9 l/s (for D2 ) were estimated using the geometrical dimensions of the vacuum connection. Thus, the conductance is much higher and cannot be responsible for observed IH˛ /ID˛ sensitivities ratio. The hydrogen content in the Penning gauge can deviate from the injected gas mixture due to a formation
Fig. 5. Balmer-alpha lines intensities versus the gas mixture pressure.
Please cite this article in press as: G. Sergienko, et al., Improving accuracy of Penning gauge spectroscopy for the determination of hydrogen isotope H/D ratios, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.092
G Model FUSION-9272; No. of Pages 5
ARTICLE IN PRESS G. Sergienko et al. / Fusion Engineering and Design xxx (2017) xxx–xxx
5
pressures range of 10−6 –10−3 mbar. The empirical functions were proposed to fit measured Balmer-alpha intensities as function of partial pressures. For the pure hydrogen or deuterium gas injections, the IH˛ /ID˛ line intensity ratio at constant absolute pressure was 1.28 ± 0.01. Using the proposed fitting functions, the absolute accuracy of the mixture composition measurement was essentially improved down to values of 1%–4%. The accuracy can be additionally improved by a factor of 3 by using more sensitive PIXIS camera. References
Fig. 7. Calculated hydrogen content versus hydrogen content in the gas mixture.
of HD molecules, because the HD formation rate could be much higher than the gauge pumping. The empirical functions used for Balmer-alpha intensities fitting are partially taking this effect into account. 4. Summary The measurements of the H␣ and D␣ line emission were performed in the hydrogen-deuterium gas mixtures in the mixture
[1] N. Ogiwara, et al., Direct pressure measurement near a plasma of JT-60 by using a Penning discharge sustained by the toroidal magnetic field, Shinku, J. Vacuum Soc. Jpn. 31 (5) (1988) 406–409. [2] N. Ogiwara, M. Maeno, Hydrogen pressure measurement using the intensity of the light emitted from a Penning discharge, J. Vacuum Sci. Technol. A 7 (1989) 2804–2807. [3] K.H. Finken, et al., Measurement of helium gas in a deuterium environment, Rev. Sci. Instrum. 63 (1) (1992) 1–7. [4] T. Denner, et al., Helium partial pressure measurement in a deuterium environment, Rev. Sci. Instrum. 67 (1996) 3515–3520. [5] C.C. Klepper, et al., Application of a species-selective Penning gauge to the measurement of neon and hydrogen-isotope partial pressures in the plasma boundary, Rev. Sci. Instrum. 68 (1) (1997) 400–403. [6] D.L. Hillis, et al., Tritium concentration measurements in the Joint European Torus divertor by optical spectroscopy of a Penning discharge, Rev. Sci. Instrum. 70 (1) (1999) 359–362. [7] N.H. Brooks, et al., Partial pressure measurements with an active spectrometer, Rev. Sci. Instrum. 70 (1) (1999) 423–426. [8] T.R. Younkin, et al., Description of the prototype diagnostic residual gas analyzer for ITER, Rev. Sci. Instrum. 85 (2014) 11E816. [9] G.N. Polyakova, et al., Kinetic-energy distribution of excited atoms produced when H2 and D2 molecules are dissociated by electron impact, Sov. Phys. J. Exp. Theor. Phys. 46 (6) (1977) 1117–1122. [10] C. Karolis, E. Harting, Electron impact dissociation cross sections in hydrogen and deuterium, leading to Balmer alpha and beta emission, J. Phys. B 11 (2) (1978) 357–369.
Please cite this article in press as: G. Sergienko, et al., Improving accuracy of Penning gauge spectroscopy for the determination of hydrogen isotope H/D ratios, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.092