- Email: [email protected]
low pressure discharge lamps in a manufacturing test cell
surface ELSEVIER
.~ience
Applied Surface Science 111 (1997) 311-317
Studies of oxide-cathode/low pressure discharge lamps in a manufacturing test cell W. Yu, G. Gregory, P. Ingram, R. Devonshire * High Temperature Science Laboratories, Chemisto' Department, UniversiO' of Sheffield, Sheffield $3 7HF, UK
Received 24 June 1996;revised 1 August 1996; accepted 23 August 1996
Abstract A UHV manufacturing test cell (MTC) has been constructed for in-situ studies of the preparation, processing, activation and operation of Ba-Sr-Ca triple oxide and other cathodes in gas discharge lamps and vacuum tubes. A wide range of variables critical to cathode/device performance (e.g. cathode processing, gas pressure and composition, component configuration and electrical parameters) can be established accurately and altered systematically under highly controlled conditions. A wide range of diagnostic techniques, including gas analysis, plasma emission spectroscopy, noise analysis of radiant emission and plasma power, spectrally resolved imaging and laser based diagnostics of both the plasma and the cathodes surface, are incorporated into, or have access to, the MTC. The MTC's capabilities are illustrated by several results from a study of low pressure deuterium (D 2) gas discharge lamps, namely: (i) gas analysis during cathode breakdown and activation; (ii) emission spectroscopy of the negative glow and positive column and (iii) instabilities in the UV output. The MTC is giving insight into critical aspects of gas discharge device design, processing and operation. PACS: 52.70.-m; 52.80; 73.50.Td; 81.40.-z.Rs Keywords: Deuterium; Cathode; Ultraviolet; Discharge; Plasma; Diagnostics; Emission; Spectroscopy
1. Introduction Triple-oxide (BaO; SrO; CaO) and related cathodes are in widespread use in vacuum tubes and low pressure discharge lamps. There exists a considerable body of published work on the preparation and properties of such cathodes but uncertainties remain and optimisation of their role in particular applications generally requires investigation. To undertake a comprehensive and integrated study of the preparation,
* Corresponding author. Tel.: +44-114-2824468; fax: +44114-2701563; e-mail: [email protected].
processing and operation of triple-oxide and related cathodes in practical devices we have constructed a UHV manufacturing test cell (MTC) in which it is possible to create and operate low pressure discharge or vacuum tube devices under highly controlled conditions. A wide range of techniques for diagnosis of the electrical and optical properties of a device and its associated cathode are incorporated into, or have access to, the MTC [I]. In the present paper, results from a study of low pressure deuterium (D 2) gas discharges are reported. This discharge is exploited in deuterium lamps used widely as a broadband source of UV and VUV radiation in analytical instrumentation.
016%4332/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII SO 169-4332(96)007 10-6
312
W. Yu et aL / Applied Surface Science 111 (1997) 311-317
2. Experimental
configuration, etc., is controlled precisely by a linear motion drive in the remaining horizontal port and a linear/rotation drive mounted axially in the top flange of the chamber. The minimum detectable gas partial pressure of the RGA (VG SX200 QMS) is l 0 -14 mbar and 16 different gas species can be monitored simultaneously. The gas handling system makes possible backfilling of the analysis chamber to any required pressure from l 0 3 to 10 - 6 mbar and any composition based on 5 separate, attached, gas cylinders. A baratron capacitance manometer (BCM) measures gas pressure in the range from 0.1 mbar to 100 mbar with an accuracy of 0.005 mbar. After system bakeout, a base pressure of less than 2 × 10 - 9 mbar in the analysis chamber is achieved and maintained, using a 600 l / s diffusion pump above which an LN 2 cold trap is mounted horizontally. A 240 l / s turbo molecular pump services both the RGA and the gas handling system• Sample
2.1. Manufacturing test cell (MTC) The current configuration of the MTC is represented in Fig. I. The MTC comprises four major parts: the analysis chamber, a vacuum system, a gas handling system and an RGA (residual gas analyser). The analysis chamber (Fig. l a, b) is a cylindrical stainless steel container ( ~ 150 × 200 mm) with eight equally spaced ports in a single horizontal plane normal to the cylinder's axis (CVT). Five of the ports have silica or glass windows to provide access to the discharge device for CCD imaging, laser based plasma and surface diagnostic techniques and measurements of UV-VIS emission intensity and spectra, noise on the radiant output and surface temperatures. Two ports are used for connection to an RGA and for manometry. The orientation of a discharge device with respect to a given port, device
i _ / . ~ - opticalfibre monitor (c) CCO IICCDII I X-Y-Z Bara!ron ~ positioner .~/Nresidual ~mm PnaoCimta~~a nga~yserla~--mp~ ~ f~T~X-Y-~YI r"---J',~ ii be nr ]I----'~ 'pehosphorescent ~ L'~' optical (a)
optical ~ f ~LF~/~ "~ optogalvanomet.
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.
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Fig. 1. Schematic of the manufacturing test cell. (a) Top view; (b) front view; (c) optical diagnostic equipments and (d) configuration of commercial deuterium discharge lamp.
W. Yu et al. / Applied Surface Science 111 (1997) 311-317
changing in the MTC can be accomplished in less than 2 h and the analysis chamber can be pumped down from atmosphere to UHV within 30 min. 2.2. MTC optical diagnostics
A versatile optical system (Fig. lc) mounted on an X - Y - Z positioner makes possible: (i) continuous visual monitoring, or VCR recording, of images of the whole or selected regions of the discharge in selected wavelength ranges and (ii) spatially resolved measurements (0.4 mm resolution) of both the spectral distribution and noise characteristics of the radiant emission. In this system, radiation from the discharge is divided between two paths. 90% of the total is coupled into an optical fibre (0.4 mm core diameter) and delivered to a spectrophotometer (modified UNICAM UV-2) for spectral analysis in the wavelength range from 180 to 920 rim. Linking of the spectrometer's output signal to an FFT analyser (Tektronix 2642A) provides noise analysis in the range from 0 to 200 kHz. 10% of the emission is reflected and focused onto a phosphorescent screen; the UV-excited green image is relayed to a CCD camera and VCR, The camera image is also used for targeting discharge regions for spatially resolved spectral analysis and for accurately measuring distances between device components. 2.3. Experimental deuterium discharge device
A schematic of a commercial deuterium lamp which provides a UV continuum from 180 to 370 nm is given in Fig. ld. A multi-pin glass baseplate supports a structure comprising three major elements: a Mo anode plate, a coiled-coil W filament coated with an activated triple-oxide mixture and a Mo centre plate with a cup shaped depression with a central hole of ~ 1 mm diameter through which the positive column of the discharge is channelled. The centre plate aperture acts as the source point of radiation for equipment external to the lamp. The cathode is offset usually to allow the 'cup' to be viewed without obstruction. The structure is sealed into a glass/silica envelope filled with pure D 2 at --- 10 mbar.
313
In the present study a linear alignment of these three major elements was achieved. The cathode (6 turns; 7 mm length; 3 mm outer diameter; 0.5 mm turn diameter; coating of triple carbonate in a binder solution; 10 V rating) was mounted on the horizontal linear motion system; the anode (0.1 mm Mo plate with • 3 mm hole in the centre to make discharge visible from behind anode) and centre plate (0.1 mm Mo plate with Q 2.5 mm depression in the centre reducing to a Q3 1 mm hole at a depth of -- 2 ram) at a fixed separation of 4 mm were mounted on the vertical linear drive and positioned so that the cathode can be moved along the discharge axis defined by the anode and centre plate apertures. 2.4. Automation of MTC based experiments
A 486 PC running Lab View (National Instruments) and other software and fitted with a multifunction I / O board (NI AT-MIO-64E-3) is used to control, monitor and collect and analyse data from the MTC. 2.5. Experiments on deuterium lamp structures in the MTC
Following assembly in the MTC of the lamp structures described in Section 2.3 the system was pumped and baked to reach stable UHV conditions of 3 × 10 - 9 mbar. Cathode 'breakdown' was achieved using a stepped heating cycle typical of industrial protocols. The heater voltage was increased from 1 to 7 V in 1 V steps every 5 min and maintained at 7 V for ~-30 min. The maximum cathode temperature at each step was measured using an infrared pyrometer (Minolta-Land Cyclops 52, emissivity setting 0.52) which was pre-calibrated during manufacture using a standard calibration source. The readings were corrected by a multiplying factor of 0.94 to take the surface reflection of the quartz window into account. The partial pressures of H 2, C, H20, CO(N2), O~ and CO 2 gases in the analysis chamber were monitored throughout the process. Following the breakdown procedure the chamber was pumped to 5 × 10 -9 mbar and then backfilled with deuterium gas (99.7%) to 10 mbar. The cathode was activated by striking a discharge and then the chamber was flushed and refilled and
314
W. Yu et al. / Applied Surface Science I I I (1997) 311-317
the 'lamp' operated at a constant discharge current of 300 mA in cold cathode mode. Measurements of the plasma emission in the range 200 to 900 nm were made using the optical system described in Section 2.2. In spatially resolved (0.2 mm resolution X - Y grid) total emission measurements the light from the fibre optic was taken directly to the photomultiplier (PM) in the UV-VIS spectrophotometer. The measurement of the noise on this total emission was achieved in two parts: noise frequency components above 10 Hz were analysed in real-time mode by connecting the PM output to the FFT analyser; lower frequency components were identified by simultaneously recording the PM's output and an image of the discharge. In spatially resolved measurements (0.2 mm resolution X - Y grid) of emission spectra (2 nm resolution), the light from the fibre optic was taken to the entrance slit of the monochromator in the spectrophotometer.
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3. Results and discussion
Fig. 2. Residual gas analysis of cathode breakdown and activation processes. (a) Gas evolution in MTC during the stepped heating cycles; (b) C O / C O 2 ratios calculated from RGA measurements.
3.1. RGA studies o f cathode breakdown process
Major features of the reaction mechanism that result in gas evolution during the preparation of triple-oxide cathodes were investigated in a milestone paper by Cayless and Watts [2]. They highlighted the importance of the ratio of CO to CO 2 in the evolved gas as an indicator of the nature and progress of the thermal reactions taking place. Fig. 2a shows the changes in partial pressure (i.e. dynamic RGA partial pressure, as pumping is continuous) of the three major gas species, CO 2, CO(N 2) and H20, evolved during the stepped heating cycle. Both CO and N 2 signals appear at a mass of 28 in the RGA spectrum. N 2 gas from dissociation of binder materials may be generated during the very early stages of the process ( T ~ 124°C), but the major contribution ( > 95%) at this mass above 600°C is from CO due to the cathode breakdown [2]. The partial pressures of the H z and O z gases are two orders lower than those of CO 2, CO and H20, except in the first few seconds of the process. H~O, evidently moisture adsorbed by the carbonate paste, appears with the CO 2 and CO at 5 min, then reduces steadily to a low background level over a period of
15 min. The dominant evolved gases a r e C O 2 and CO; their rates of evolution both peaking after ~ 15 min of process time. Assuming the pumping speeds for CO and CO 2 are identical [3], then the C O / C O 2 molar ratio, rcco/co~_ I, at any given process time is: Pco - a28 Pcoz °'cot r~co/co,~ =
Pco,
"
°'co
(1)
where Pco and Pco2 are the instrumental 'partial pressures' of CO and CO2; c~28 is the CO fraction arising from CO 2 cracking (0.08 for our RGA [4]); ¢co and ~rco~ are the sensitivity factors for CO and CO 2 (1.05 and 1.4, respectively, for our RGA [4]). Fig. 2b shows the value of r(co/co:) derived from the partial pressures in Fig. 2a. CO 2 evolution is dominant in the earlier part of the process cycle before waning to reveal a background process leading to CO evolution which persists to higher temperatures. Different trends in the rates of decay of gas evolution once a new higher voltage is established reflect the several competing physical and chemical processes taking place [2]. These trends are being related to results of thermogravimetric analysis
315
IV. Yu et al. / Applied Surface Science 111 f1997) 311-317
(TGA) obtained separately and will contribute to the development of a model of the process.
=- 24 h after activation viewed normally to the discharge axis. The cathode to centre plate separation was m 5 ram. The strongest of the three peaks in Fig. 3a is the negative glow (NG) in the region of the cathode; the other two peaks are the two regions of the positive column (PC) separated by the centre plate. The NG and PC are separated by the Faraday
3.2. Emission spectroscopy of deuterium lamp structu res
Fig. 3a shows the spatial distribution of the total discharge emission in a deuterium lamp structure
4.0E+6-
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~
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200 300 400 500 600 700 800 900
200 300 400 500 600 700 800 g00
WAVELENGTH (nm)
WAVELENGTH (nm)
Fig. 3. Optical measurements of D 2 discharge. (a) 2D mapping of emission intensity distribution: (b) emission spectrum of positive columl (PC) and (c) emissior~ spectrum of negative glow (NG).
316
W. Yu et al. /Applied Surface Science I 11 (1997) 311-317
dark space and this enables distinct emission spectra to be obtained from the two luminous regions. Fig. 3b, c show normalised emission spectra from the PC and the NG, respectively (N.B. the apparent intensity loss at wavelength below 250 nm is caused by the detection system). The broad UV continuum from the PC exploited in commercial deuterium lamps is prominent in both spectra as is the strong D,(32D 22p) line at 657 nm. The higher intensity of the latter in the NG may be attributed to a higher degree of D 2 dissociation a n d / o r an increase in the excitation rate of D atoms by the more energetic electrons in this region. The D,~ line emerges from molecular spectral features spanning the entire visible range. Features at 337 and 350 nm in the NG arise possibly from N 2 derived from the cathode binder, coinciding as they do with N 2 2nd positive system lines [5]. However, CO 2 and CO + [5] released from the cathode are also
strong candidates, having features at 337, 350 [CO 2] and 289 nm [CO~]. A strong visible band system peaking at -= 575 nm in the NG coincides with strong features of the N 2 1st positive system [5]. Resonance lines of the alkaline earth metals and their positive ions are not observed. These measurements of the PC and NG emissions will be repeated using a high resolution spectrometer to make definite spectral assignments and to reveal more detail.
3.3. Emission smbili~ of deuterium lamp structures The stability of the emission from the deuterium lamp structures, a performance criterion for commercial lamps, was studied by monitoring the total PC emission intensity for periods of time at different 'ages' of lamp operation. During the same periods the PC emission spectrum and CCD images of the discharge were both recorded.
(b)
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10
20
30
40
TIME (minutes)
50
60
200
300
400
500
600
700
800
900
W A V E L E N G T H (nm)
Fig. 4. Noise analysis of radiant output of D 2 lamp of different lamp ages. (a) 60 min periods of total light output from PC and (b) emission spectra from PC.
w. Yu et al. / Applied Surface Science 111 (1997) 311-317
Three types of low frequency ( < 1 Hz) noise were observed at different ages of the lamp and can be seen in Fig. 4a. The first, an oscillatory noise of magnitude 15-30%, appeared during the first 24 h of lamp operation. It could be suppressed by increasing the cathode to centre plate separation, disappearing when this distance was ---- 10 mm. The behaviour of the feature at 337 nm (Fig. 4b) during lamp operation supports the idea that it arises from CO 2. A study of the effects of different gases on cathode emission [6] showed CO and CO 2 to have different roles. This may account for the oscillatory behaviour as a competition may be set up between the reactions of CO with BaO and of CO 2 with Ba. The second and third noise characteristics, drift and step-changes, of magnitude 1% and 2 - 5 % , respectively, were revealed at lamp ages > 72 h (Fig. 4a). The step-changes were captured by the CCD (inset in Fig. 4a) and revealed to be associated with sudden movements of the plasma attachment area on the cathode's surface.
4. Conclusions A versatile experimental system has been established for in-situ investigations of cathode science
317
and cathode applications in discharge devices and electron tubes.
Acknowledgements The authors wish to thank the EPSRC D & IP Programme for financial support and Cathodeon and Unicam for their interest in and material support for this work.
References [1] W. Yu, G. Gregory, P. Ingram and R. Devonshire, Proc. 7th Int. Syrup. on Sci. and Techn. of Light Sources, Kyoto (1995) p. 76. [2] M.A. Cayless and B.N, Watts, Br. J. Appl. Phys. 7 (1956) 351. [3] N.S. Harris, Modern Vacuum Practice (McGraw-Hill, Maidenhead, 1989). [4] Information supplied by Vacuum Generators, East Sussex, (1995). [5] R.W.B. Pearse and A.G. Gaydon, The Identification of Molecular Spectra (Chapman and Hall, London, 1965). [6] S. Itoh, M. Yokoyamaand K. Morimoto, J. Vac. Sci. Technol. 5 (1987) 3430.
.~ience
Applied Surface Science 111 (1997) 311-317
Studies of oxide-cathode/low pressure discharge lamps in a manufacturing test cell W. Yu, G. Gregory, P. Ingram, R. Devonshire * High Temperature Science Laboratories, Chemisto' Department, UniversiO' of Sheffield, Sheffield $3 7HF, UK
Received 24 June 1996;revised 1 August 1996; accepted 23 August 1996
Abstract A UHV manufacturing test cell (MTC) has been constructed for in-situ studies of the preparation, processing, activation and operation of Ba-Sr-Ca triple oxide and other cathodes in gas discharge lamps and vacuum tubes. A wide range of variables critical to cathode/device performance (e.g. cathode processing, gas pressure and composition, component configuration and electrical parameters) can be established accurately and altered systematically under highly controlled conditions. A wide range of diagnostic techniques, including gas analysis, plasma emission spectroscopy, noise analysis of radiant emission and plasma power, spectrally resolved imaging and laser based diagnostics of both the plasma and the cathodes surface, are incorporated into, or have access to, the MTC. The MTC's capabilities are illustrated by several results from a study of low pressure deuterium (D 2) gas discharge lamps, namely: (i) gas analysis during cathode breakdown and activation; (ii) emission spectroscopy of the negative glow and positive column and (iii) instabilities in the UV output. The MTC is giving insight into critical aspects of gas discharge device design, processing and operation. PACS: 52.70.-m; 52.80; 73.50.Td; 81.40.-z.Rs Keywords: Deuterium; Cathode; Ultraviolet; Discharge; Plasma; Diagnostics; Emission; Spectroscopy
1. Introduction Triple-oxide (BaO; SrO; CaO) and related cathodes are in widespread use in vacuum tubes and low pressure discharge lamps. There exists a considerable body of published work on the preparation and properties of such cathodes but uncertainties remain and optimisation of their role in particular applications generally requires investigation. To undertake a comprehensive and integrated study of the preparation,
* Corresponding author. Tel.: +44-114-2824468; fax: +44114-2701563; e-mail: [email protected].
processing and operation of triple-oxide and related cathodes in practical devices we have constructed a UHV manufacturing test cell (MTC) in which it is possible to create and operate low pressure discharge or vacuum tube devices under highly controlled conditions. A wide range of techniques for diagnosis of the electrical and optical properties of a device and its associated cathode are incorporated into, or have access to, the MTC [I]. In the present paper, results from a study of low pressure deuterium (D 2) gas discharges are reported. This discharge is exploited in deuterium lamps used widely as a broadband source of UV and VUV radiation in analytical instrumentation.
016%4332/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII SO 169-4332(96)007 10-6
312
W. Yu et aL / Applied Surface Science 111 (1997) 311-317
2. Experimental
configuration, etc., is controlled precisely by a linear motion drive in the remaining horizontal port and a linear/rotation drive mounted axially in the top flange of the chamber. The minimum detectable gas partial pressure of the RGA (VG SX200 QMS) is l 0 -14 mbar and 16 different gas species can be monitored simultaneously. The gas handling system makes possible backfilling of the analysis chamber to any required pressure from l 0 3 to 10 - 6 mbar and any composition based on 5 separate, attached, gas cylinders. A baratron capacitance manometer (BCM) measures gas pressure in the range from 0.1 mbar to 100 mbar with an accuracy of 0.005 mbar. After system bakeout, a base pressure of less than 2 × 10 - 9 mbar in the analysis chamber is achieved and maintained, using a 600 l / s diffusion pump above which an LN 2 cold trap is mounted horizontally. A 240 l / s turbo molecular pump services both the RGA and the gas handling system• Sample
2.1. Manufacturing test cell (MTC) The current configuration of the MTC is represented in Fig. I. The MTC comprises four major parts: the analysis chamber, a vacuum system, a gas handling system and an RGA (residual gas analyser). The analysis chamber (Fig. l a, b) is a cylindrical stainless steel container ( ~ 150 × 200 mm) with eight equally spaced ports in a single horizontal plane normal to the cylinder's axis (CVT). Five of the ports have silica or glass windows to provide access to the discharge device for CCD imaging, laser based plasma and surface diagnostic techniques and measurements of UV-VIS emission intensity and spectra, noise on the radiant output and surface temperatures. Two ports are used for connection to an RGA and for manometry. The orientation of a discharge device with respect to a given port, device
i _ / . ~ - opticalfibre monitor (c) CCO IICCDII I X-Y-Z Bara!ron ~ positioner .~/Nresidual ~mm PnaoCimta~~a nga~yserla~--mp~ ~ f~T~X-Y-~YI r"---J',~ ii be nr ]I----'~ 'pehosphorescent ~ L'~' optical (a)
optical ~ f ~LF~/~ "~ optogalvanomet.
(b)
.
\.
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, ~,X~[-
~~
_ ~
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~
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..".................................~UVglass i envelope i cup ] J ~perture
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~
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O
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Fig. 1. Schematic of the manufacturing test cell. (a) Top view; (b) front view; (c) optical diagnostic equipments and (d) configuration of commercial deuterium discharge lamp.
W. Yu et al. / Applied Surface Science 111 (1997) 311-317
changing in the MTC can be accomplished in less than 2 h and the analysis chamber can be pumped down from atmosphere to UHV within 30 min. 2.2. MTC optical diagnostics
A versatile optical system (Fig. lc) mounted on an X - Y - Z positioner makes possible: (i) continuous visual monitoring, or VCR recording, of images of the whole or selected regions of the discharge in selected wavelength ranges and (ii) spatially resolved measurements (0.4 mm resolution) of both the spectral distribution and noise characteristics of the radiant emission. In this system, radiation from the discharge is divided between two paths. 90% of the total is coupled into an optical fibre (0.4 mm core diameter) and delivered to a spectrophotometer (modified UNICAM UV-2) for spectral analysis in the wavelength range from 180 to 920 rim. Linking of the spectrometer's output signal to an FFT analyser (Tektronix 2642A) provides noise analysis in the range from 0 to 200 kHz. 10% of the emission is reflected and focused onto a phosphorescent screen; the UV-excited green image is relayed to a CCD camera and VCR, The camera image is also used for targeting discharge regions for spatially resolved spectral analysis and for accurately measuring distances between device components. 2.3. Experimental deuterium discharge device
A schematic of a commercial deuterium lamp which provides a UV continuum from 180 to 370 nm is given in Fig. ld. A multi-pin glass baseplate supports a structure comprising three major elements: a Mo anode plate, a coiled-coil W filament coated with an activated triple-oxide mixture and a Mo centre plate with a cup shaped depression with a central hole of ~ 1 mm diameter through which the positive column of the discharge is channelled. The centre plate aperture acts as the source point of radiation for equipment external to the lamp. The cathode is offset usually to allow the 'cup' to be viewed without obstruction. The structure is sealed into a glass/silica envelope filled with pure D 2 at --- 10 mbar.
313
In the present study a linear alignment of these three major elements was achieved. The cathode (6 turns; 7 mm length; 3 mm outer diameter; 0.5 mm turn diameter; coating of triple carbonate in a binder solution; 10 V rating) was mounted on the horizontal linear motion system; the anode (0.1 mm Mo plate with • 3 mm hole in the centre to make discharge visible from behind anode) and centre plate (0.1 mm Mo plate with Q 2.5 mm depression in the centre reducing to a Q3 1 mm hole at a depth of -- 2 ram) at a fixed separation of 4 mm were mounted on the vertical linear drive and positioned so that the cathode can be moved along the discharge axis defined by the anode and centre plate apertures. 2.4. Automation of MTC based experiments
A 486 PC running Lab View (National Instruments) and other software and fitted with a multifunction I / O board (NI AT-MIO-64E-3) is used to control, monitor and collect and analyse data from the MTC. 2.5. Experiments on deuterium lamp structures in the MTC
Following assembly in the MTC of the lamp structures described in Section 2.3 the system was pumped and baked to reach stable UHV conditions of 3 × 10 - 9 mbar. Cathode 'breakdown' was achieved using a stepped heating cycle typical of industrial protocols. The heater voltage was increased from 1 to 7 V in 1 V steps every 5 min and maintained at 7 V for ~-30 min. The maximum cathode temperature at each step was measured using an infrared pyrometer (Minolta-Land Cyclops 52, emissivity setting 0.52) which was pre-calibrated during manufacture using a standard calibration source. The readings were corrected by a multiplying factor of 0.94 to take the surface reflection of the quartz window into account. The partial pressures of H 2, C, H20, CO(N2), O~ and CO 2 gases in the analysis chamber were monitored throughout the process. Following the breakdown procedure the chamber was pumped to 5 × 10 -9 mbar and then backfilled with deuterium gas (99.7%) to 10 mbar. The cathode was activated by striking a discharge and then the chamber was flushed and refilled and
314
W. Yu et al. / Applied Surface Science I I I (1997) 311-317
the 'lamp' operated at a constant discharge current of 300 mA in cold cathode mode. Measurements of the plasma emission in the range 200 to 900 nm were made using the optical system described in Section 2.2. In spatially resolved (0.2 mm resolution X - Y grid) total emission measurements the light from the fibre optic was taken directly to the photomultiplier (PM) in the UV-VIS spectrophotometer. The measurement of the noise on this total emission was achieved in two parts: noise frequency components above 10 Hz were analysed in real-time mode by connecting the PM output to the FFT analyser; lower frequency components were identified by simultaneously recording the PM's output and an image of the discharge. In spatially resolved measurements (0.2 mm resolution X - Y grid) of emission spectra (2 nm resolution), the light from the fibre optic was taken to the entrance slit of the monochromator in the spectrophotometer.
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3. Results and discussion
Fig. 2. Residual gas analysis of cathode breakdown and activation processes. (a) Gas evolution in MTC during the stepped heating cycles; (b) C O / C O 2 ratios calculated from RGA measurements.
3.1. RGA studies o f cathode breakdown process
Major features of the reaction mechanism that result in gas evolution during the preparation of triple-oxide cathodes were investigated in a milestone paper by Cayless and Watts [2]. They highlighted the importance of the ratio of CO to CO 2 in the evolved gas as an indicator of the nature and progress of the thermal reactions taking place. Fig. 2a shows the changes in partial pressure (i.e. dynamic RGA partial pressure, as pumping is continuous) of the three major gas species, CO 2, CO(N 2) and H20, evolved during the stepped heating cycle. Both CO and N 2 signals appear at a mass of 28 in the RGA spectrum. N 2 gas from dissociation of binder materials may be generated during the very early stages of the process ( T ~ 124°C), but the major contribution ( > 95%) at this mass above 600°C is from CO due to the cathode breakdown [2]. The partial pressures of the H z and O z gases are two orders lower than those of CO 2, CO and H20, except in the first few seconds of the process. H~O, evidently moisture adsorbed by the carbonate paste, appears with the CO 2 and CO at 5 min, then reduces steadily to a low background level over a period of
15 min. The dominant evolved gases a r e C O 2 and CO; their rates of evolution both peaking after ~ 15 min of process time. Assuming the pumping speeds for CO and CO 2 are identical [3], then the C O / C O 2 molar ratio, rcco/co~_ I, at any given process time is: Pco - a28 Pcoz °'cot r~co/co,~ =
Pco,
"
°'co
(1)
where Pco and Pco2 are the instrumental 'partial pressures' of CO and CO2; c~28 is the CO fraction arising from CO 2 cracking (0.08 for our RGA [4]); ¢co and ~rco~ are the sensitivity factors for CO and CO 2 (1.05 and 1.4, respectively, for our RGA [4]). Fig. 2b shows the value of r(co/co:) derived from the partial pressures in Fig. 2a. CO 2 evolution is dominant in the earlier part of the process cycle before waning to reveal a background process leading to CO evolution which persists to higher temperatures. Different trends in the rates of decay of gas evolution once a new higher voltage is established reflect the several competing physical and chemical processes taking place [2]. These trends are being related to results of thermogravimetric analysis
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IV. Yu et al. / Applied Surface Science 111 f1997) 311-317
(TGA) obtained separately and will contribute to the development of a model of the process.
=- 24 h after activation viewed normally to the discharge axis. The cathode to centre plate separation was m 5 ram. The strongest of the three peaks in Fig. 3a is the negative glow (NG) in the region of the cathode; the other two peaks are the two regions of the positive column (PC) separated by the centre plate. The NG and PC are separated by the Faraday
3.2. Emission spectroscopy of deuterium lamp structu res
Fig. 3a shows the spatial distribution of the total discharge emission in a deuterium lamp structure
4.0E+6-
(a)
3.5E+63.0E+6C
~
2.5E+6 2.0E+6 1.5E+6
Z
Ud I"" Z
1.0E+6
10 12
5.0E+5
¢
0.0E+C
~(mml .w
ii
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(b) PC
19
21
23
25
(c) NG
r" 13
Z III
z t/a I.z
I"Z
200 300 400 500 600 700 800 900
200 300 400 500 600 700 800 g00
WAVELENGTH (nm)
WAVELENGTH (nm)
Fig. 3. Optical measurements of D 2 discharge. (a) 2D mapping of emission intensity distribution: (b) emission spectrum of positive columl (PC) and (c) emissior~ spectrum of negative glow (NG).
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W. Yu et al. /Applied Surface Science I 11 (1997) 311-317
dark space and this enables distinct emission spectra to be obtained from the two luminous regions. Fig. 3b, c show normalised emission spectra from the PC and the NG, respectively (N.B. the apparent intensity loss at wavelength below 250 nm is caused by the detection system). The broad UV continuum from the PC exploited in commercial deuterium lamps is prominent in both spectra as is the strong D,(32D 22p) line at 657 nm. The higher intensity of the latter in the NG may be attributed to a higher degree of D 2 dissociation a n d / o r an increase in the excitation rate of D atoms by the more energetic electrons in this region. The D,~ line emerges from molecular spectral features spanning the entire visible range. Features at 337 and 350 nm in the NG arise possibly from N 2 derived from the cathode binder, coinciding as they do with N 2 2nd positive system lines [5]. However, CO 2 and CO + [5] released from the cathode are also
strong candidates, having features at 337, 350 [CO 2] and 289 nm [CO~]. A strong visible band system peaking at -= 575 nm in the NG coincides with strong features of the N 2 1st positive system [5]. Resonance lines of the alkaline earth metals and their positive ions are not observed. These measurements of the PC and NG emissions will be repeated using a high resolution spectrometer to make definite spectral assignments and to reveal more detail.
3.3. Emission smbili~ of deuterium lamp structures The stability of the emission from the deuterium lamp structures, a performance criterion for commercial lamps, was studied by monitoring the total PC emission intensity for periods of time at different 'ages' of lamp operation. During the same periods the PC emission spectrum and CCD images of the discharge were both recorded.
(b)
7_= e-
72 hrs
i
>.. I-
o')
Z ILl I-Z
,:5.
I
0
10
20
30
40
TIME (minutes)
50
60
200
300
400
500
600
700
800
900
W A V E L E N G T H (nm)
Fig. 4. Noise analysis of radiant output of D 2 lamp of different lamp ages. (a) 60 min periods of total light output from PC and (b) emission spectra from PC.
w. Yu et al. / Applied Surface Science 111 (1997) 311-317
Three types of low frequency ( < 1 Hz) noise were observed at different ages of the lamp and can be seen in Fig. 4a. The first, an oscillatory noise of magnitude 15-30%, appeared during the first 24 h of lamp operation. It could be suppressed by increasing the cathode to centre plate separation, disappearing when this distance was ---- 10 mm. The behaviour of the feature at 337 nm (Fig. 4b) during lamp operation supports the idea that it arises from CO 2. A study of the effects of different gases on cathode emission [6] showed CO and CO 2 to have different roles. This may account for the oscillatory behaviour as a competition may be set up between the reactions of CO with BaO and of CO 2 with Ba. The second and third noise characteristics, drift and step-changes, of magnitude 1% and 2 - 5 % , respectively, were revealed at lamp ages > 72 h (Fig. 4a). The step-changes were captured by the CCD (inset in Fig. 4a) and revealed to be associated with sudden movements of the plasma attachment area on the cathode's surface.
4. Conclusions A versatile experimental system has been established for in-situ investigations of cathode science
317
and cathode applications in discharge devices and electron tubes.
Acknowledgements The authors wish to thank the EPSRC D & IP Programme for financial support and Cathodeon and Unicam for their interest in and material support for this work.
References [1] W. Yu, G. Gregory, P. Ingram and R. Devonshire, Proc. 7th Int. Syrup. on Sci. and Techn. of Light Sources, Kyoto (1995) p. 76. [2] M.A. Cayless and B.N, Watts, Br. J. Appl. Phys. 7 (1956) 351. [3] N.S. Harris, Modern Vacuum Practice (McGraw-Hill, Maidenhead, 1989). [4] Information supplied by Vacuum Generators, East Sussex, (1995). [5] R.W.B. Pearse and A.G. Gaydon, The Identification of Molecular Spectra (Chapman and Hall, London, 1965). [6] S. Itoh, M. Yokoyamaand K. Morimoto, J. Vac. Sci. Technol. 5 (1987) 3430.