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Electrode phenomena in a medium voltage spark
Sp&rochlmicaActa, Vol.28B, pp.13 to 15. PergamonPress197% Printedin NorthernIreland
Electrode phenomena in a medium voltage spark A. STRASREIM and F. BLUM National Physical Research Laboratory, Council for Scientific and Industrisl Research, Pretoria, South Africa (Received 17 April 1972) Ab8tr&-The surface of alumini urn samples containing approximately 5% copper has been studied by means of high speed photography while it was being struck by single medium voltage spark discharges under atmospheric pressure. The formation of cathode spate was found to be dependent on the condition of the sample electrode surface and changed during the duration of one single discharge. Cathode spots have been observed under the impact of oscillating and damped spark discharges. INTRODUCTION
of electrode erosion under the impact of single and multiple spark discharges has been the subject of many investigations, and different theories have been established concerning the removal of material from the sample electrode [l-4]. Observations of luminous spots on the sample surface before and during the process of sampling have been reported mainly in low pressure gas discharges, different theories were proposed to explain these phenomena [5-lo]. Due to the variation with time of these phenomena observations must be made on an instantaneous basis. Some phenomena occurring on the sample electrode under the impact of single medium voltage spark discharges under atmospheric pressure will be presented in this paper and the results compared with those reported in other types of gas discharges. THE STUDY
EXPERIMENTAL
CONDITIONS
Series of up to 59 single pictures were taken, using a Barr and Stroud high-speed framing camera, of the surface of the aluminium sample electrode while it was being struck by a medium voltage spark as described previously [4].The spark parameters were the following: e.m.f. = 1000 V, C = 2pF, L = 100 PH and R = O-5!A(residual). The oscillating discharge with 4 current half-cycles had a duration of 140 pet. Increasing the resistance R to 10 &2produces a damped spark discharge having a duration of 100 ,usec. A graphite counter electrode was used and the sample electrode [l]
[2] [3] [4] [B] [6] [7]
K. YOSHINO and T. TA~AHASHI,i%i. tight Tokyo 14, 114 (1965). P. H~~LLER,Spectrochim. Acta 2SB, 1 (1967). S. W. BREWER, JR and J. P. WALTERS, Anal. Chem. 41,1980(1969). A. STRASHEIMand F. BLUM, Spectrochim. Acta 26B, 685 (1971). G. EC~ER, 2. Phys. 136,556 (1953). 0. FARISH and D. J. TEDFORD, Br. J. A&. Phys. 17,965 (1966). R. BA~HAROV,E. N. GAVRILOVS~AYA,0. A. ~~KIN and E. S. TREKEOV, Soviet Phya. Tech.
Phye. 10, 1428 (1966). [S] L. A. SENA, Soviet Phys. Tech. Phya. 15,1613 (1971). [Q] N. M. ZY~OVA, V. V. BANTSEL’, V. I. RAKHOVSKII,I. F. SELIYERSTVOVAand A. P. UsnMETS, Soviet Phys. Tech. Phya. 15, 1844 (1971). [lo] B. E. DJAKOV and R. HOLMES, J. Phys. (D) 4, 504 (1971). 13
A. STRASEEI~~and F. BLUM
14
was always negative at the first breakthrough. The analytical gap width was 6 mm. The 35 mm Eastman Kodak 4-X fYm type 5244 (500 ASA) was developed in Vromicrol”* line grain developer. The standard solution was diluted 1: 3 and the film was developed for 14 min at 20°C. The photographic densities were kept constant by changing the aperture of the optical system of the camera for better comparison of the results. EXPERIMENTAL
RESULTS
The formation of cathode spots during the first half-cycle of a single spark discharge photographed with a framing interval of 4 psec is shown in Fig. 1. Two different types of spot formation are observed depending on the structure of the sample surface. A pattern of single spots which compare roughly with the veins of a leaf were observed on highly polished surfaces (polished with “Alumina”, particle size 5 pm) under the impact of the first current half-cycle as seen in series A of Fig. 1. The cathode spot during the first half-cycle (24 ,usec duration) is completely formed after 16 psec. The area covered is fairly large, but the surface of the sample is only slightly etched. If the sample surface shows small grooves and ridges due to grinding with fine sandpaper, the first cathode spot travels linearly along these ridges with a velocity of approximately 1.2 x lo5 cm/see as demonstrated in series B of Fig 1. This spot was also completely formed after 16 psec. A second cathode spot is formed during the third current half-cycle 80 psec after spark initiation when the sample electrode is negative again. The formation of this second spot on the polished surface (series A) and on the ground surface (series B) is illustrated in Fig. 2 together with the erosion marks left under the impact of the first current half-cycle. These spots are concentrated in a smaller area than the first and cause deeper erosion of the sample surface. The erosion of a polished sample surface after the impact of one single spark discharge is seen in Fig. 3. The larger and less bright area has been etched under the impact of the first current half-cycle while the smaller bright area (indicated by an arrow) is the result of the deeper erosion during the course of the third half-cycle. To study the cathode spot formation in greater detail, photographs of the events occurring on or very close to the sample surface were taken with a framing interval of 1 psec as seen in Fig. 4. The following is observed from these pictures: (1) The breakthrough is completely formed after 1 psec as seen in frame 1.
Seven individual spots are seen in frame 2-2 ,usec after breakthrough-near to the place of the first impact spot of which five are rather weak. (3) Frame 3 reveals that 1 psec later the fist impact spot and four weak spots have disappeared, leaving only 3 spots which are seen to grow in the following frames. After 10 psec the individual spots cannot be seen separately due the brightness of the spark channel. (2)
Sequential formation of cathode spots during the course of the first half-cycle of one single discharge is seen in Fig. 6, together with the erosion marks left by the preceding spark which act as reference points on the sample surface for these spots. The following is observed from these pictures : * Manufactured by Maybaker, Port Elizabeth, South Africa.
I Fig.
1.
Formation
Fig.
2. Formation
I
5mm
I
I
I
I
of cathode spots during the an oscillating single spark of cathode spots during the an oscillating single spark
course of the first half-cycle discharge. course of the third half-cycle discharge.
of of
14
9
o ~
L~
e
~
C C
EE t 0
~D
I
2
3
4
5
6
5 mm
Fig. 5. Sequential spot formation on a polished sample surface.
5mm
1 Fig. 6. “After-glow”
I
I
I
I
I
of cathode spots of a damped single spark discharge.
Electrodephenomena in a medium voltage spark
15
(1) The first impact spot only is seen above the old erosion marks in picture 1. (2) At least six individual spots can be distinguished 2 psec later in picture 2. (3) New spots are created in the following pictures while the older ones are expanding and becoming more diffuse.
The formation of cathode spots under the impact of a damped spark discharge is initially similar to that of an oscillatory discharge as described in Figs. 4 and 5. However, an “after-glow” of the cathode spots occurs just before current cut-off as shown in Fig. 6. The framing interval in this case is 4 psec. Five rather bright spots are distinguishable in frame 1. The spot number decreases by one spot per frame in the following frames. This observation could not be made in the case of oscillating spark discharges. DISCUSSIONOF TEE RESULTS The following conclusions can be drawn from a large number of high-speed photographs taken : (1) Luminous spots are observed on the sample surface during the course of each half-cycle when the sample is cathodic. (2) The surface of the sample considerably influences the formation of the cathode spots in the course of the first half-cycle. The cathode spot is fairly uniform on a highly polished surface, but if the ground surface is roughened the ridges preferentially act as spark roots along which the cathode spots spread linearly. (3) Though the sample surface after the impact of the frrst current half-cycle appeared only slightly etched under the microscope, sufficient material was removed to give high spectral line intensity as was found by studying time-resolved spectral radiation [41. (4) The cathode spots can be formed sequentially at the beginning of the discharge on highly polished surfaces. These spots remain in fixed localities t~oughout their life-times, which may vary considerably. Life-times between ~1 and 4 ,usec for individual spots have been observed. This agrees well with the findings in a low pressure pulsed discharge [7], while other authors claim the spots to be mobile [9]. The spots are small and intense when formed but become larger and diffuse as jets of material are released. Some spots fail to reach the diffuse stage and die before releasing material. This dying out of spots has been observed in vacuum arcs as well [lo]. (5) Current densities in a gas discharge calculated on the basis of the size and structure of the eroded areas only [7] do not give very reliable values. The timedependent cathode spots should therefore be taken as an additional variable to calculate time-de~ndent cu~ent-densities.
Electrode phenomena in a medium voltage spark A. STRASREIM and F. BLUM National Physical Research Laboratory, Council for Scientific and Industrisl Research, Pretoria, South Africa (Received 17 April 1972) Ab8tr&-The surface of alumini urn samples containing approximately 5% copper has been studied by means of high speed photography while it was being struck by single medium voltage spark discharges under atmospheric pressure. The formation of cathode spate was found to be dependent on the condition of the sample electrode surface and changed during the duration of one single discharge. Cathode spots have been observed under the impact of oscillating and damped spark discharges. INTRODUCTION
of electrode erosion under the impact of single and multiple spark discharges has been the subject of many investigations, and different theories have been established concerning the removal of material from the sample electrode [l-4]. Observations of luminous spots on the sample surface before and during the process of sampling have been reported mainly in low pressure gas discharges, different theories were proposed to explain these phenomena [5-lo]. Due to the variation with time of these phenomena observations must be made on an instantaneous basis. Some phenomena occurring on the sample electrode under the impact of single medium voltage spark discharges under atmospheric pressure will be presented in this paper and the results compared with those reported in other types of gas discharges. THE STUDY
EXPERIMENTAL
CONDITIONS
Series of up to 59 single pictures were taken, using a Barr and Stroud high-speed framing camera, of the surface of the aluminium sample electrode while it was being struck by a medium voltage spark as described previously [4].The spark parameters were the following: e.m.f. = 1000 V, C = 2pF, L = 100 PH and R = O-5!A(residual). The oscillating discharge with 4 current half-cycles had a duration of 140 pet. Increasing the resistance R to 10 &2produces a damped spark discharge having a duration of 100 ,usec. A graphite counter electrode was used and the sample electrode [l]
[2] [3] [4] [B] [6] [7]
K. YOSHINO and T. TA~AHASHI,i%i. tight Tokyo 14, 114 (1965). P. H~~LLER,Spectrochim. Acta 2SB, 1 (1967). S. W. BREWER, JR and J. P. WALTERS, Anal. Chem. 41,1980(1969). A. STRASHEIMand F. BLUM, Spectrochim. Acta 26B, 685 (1971). G. EC~ER, 2. Phys. 136,556 (1953). 0. FARISH and D. J. TEDFORD, Br. J. A&. Phys. 17,965 (1966). R. BA~HAROV,E. N. GAVRILOVS~AYA,0. A. ~~KIN and E. S. TREKEOV, Soviet Phya. Tech.
Phye. 10, 1428 (1966). [S] L. A. SENA, Soviet Phys. Tech. Phya. 15,1613 (1971). [Q] N. M. ZY~OVA, V. V. BANTSEL’, V. I. RAKHOVSKII,I. F. SELIYERSTVOVAand A. P. UsnMETS, Soviet Phys. Tech. Phya. 15, 1844 (1971). [lo] B. E. DJAKOV and R. HOLMES, J. Phys. (D) 4, 504 (1971). 13
A. STRASEEI~~and F. BLUM
14
was always negative at the first breakthrough. The analytical gap width was 6 mm. The 35 mm Eastman Kodak 4-X fYm type 5244 (500 ASA) was developed in Vromicrol”* line grain developer. The standard solution was diluted 1: 3 and the film was developed for 14 min at 20°C. The photographic densities were kept constant by changing the aperture of the optical system of the camera for better comparison of the results. EXPERIMENTAL
RESULTS
The formation of cathode spots during the first half-cycle of a single spark discharge photographed with a framing interval of 4 psec is shown in Fig. 1. Two different types of spot formation are observed depending on the structure of the sample surface. A pattern of single spots which compare roughly with the veins of a leaf were observed on highly polished surfaces (polished with “Alumina”, particle size 5 pm) under the impact of the first current half-cycle as seen in series A of Fig. 1. The cathode spot during the first half-cycle (24 ,usec duration) is completely formed after 16 psec. The area covered is fairly large, but the surface of the sample is only slightly etched. If the sample surface shows small grooves and ridges due to grinding with fine sandpaper, the first cathode spot travels linearly along these ridges with a velocity of approximately 1.2 x lo5 cm/see as demonstrated in series B of Fig 1. This spot was also completely formed after 16 psec. A second cathode spot is formed during the third current half-cycle 80 psec after spark initiation when the sample electrode is negative again. The formation of this second spot on the polished surface (series A) and on the ground surface (series B) is illustrated in Fig. 2 together with the erosion marks left under the impact of the first current half-cycle. These spots are concentrated in a smaller area than the first and cause deeper erosion of the sample surface. The erosion of a polished sample surface after the impact of one single spark discharge is seen in Fig. 3. The larger and less bright area has been etched under the impact of the first current half-cycle while the smaller bright area (indicated by an arrow) is the result of the deeper erosion during the course of the third half-cycle. To study the cathode spot formation in greater detail, photographs of the events occurring on or very close to the sample surface were taken with a framing interval of 1 psec as seen in Fig. 4. The following is observed from these pictures: (1) The breakthrough is completely formed after 1 psec as seen in frame 1.
Seven individual spots are seen in frame 2-2 ,usec after breakthrough-near to the place of the first impact spot of which five are rather weak. (3) Frame 3 reveals that 1 psec later the fist impact spot and four weak spots have disappeared, leaving only 3 spots which are seen to grow in the following frames. After 10 psec the individual spots cannot be seen separately due the brightness of the spark channel. (2)
Sequential formation of cathode spots during the course of the first half-cycle of one single discharge is seen in Fig. 6, together with the erosion marks left by the preceding spark which act as reference points on the sample surface for these spots. The following is observed from these pictures : * Manufactured by Maybaker, Port Elizabeth, South Africa.
I Fig.
1.
Formation
Fig.
2. Formation
I
5mm
I
I
I
I
of cathode spots during the an oscillating single spark of cathode spots during the an oscillating single spark
course of the first half-cycle discharge. course of the third half-cycle discharge.
of of
14
9
o ~
L~
e
~
C C
EE t 0
~D
I
2
3
4
5
6
5 mm
Fig. 5. Sequential spot formation on a polished sample surface.
5mm
1 Fig. 6. “After-glow”
I
I
I
I
I
of cathode spots of a damped single spark discharge.
Electrodephenomena in a medium voltage spark
15
(1) The first impact spot only is seen above the old erosion marks in picture 1. (2) At least six individual spots can be distinguished 2 psec later in picture 2. (3) New spots are created in the following pictures while the older ones are expanding and becoming more diffuse.
The formation of cathode spots under the impact of a damped spark discharge is initially similar to that of an oscillatory discharge as described in Figs. 4 and 5. However, an “after-glow” of the cathode spots occurs just before current cut-off as shown in Fig. 6. The framing interval in this case is 4 psec. Five rather bright spots are distinguishable in frame 1. The spot number decreases by one spot per frame in the following frames. This observation could not be made in the case of oscillating spark discharges. DISCUSSIONOF TEE RESULTS The following conclusions can be drawn from a large number of high-speed photographs taken : (1) Luminous spots are observed on the sample surface during the course of each half-cycle when the sample is cathodic. (2) The surface of the sample considerably influences the formation of the cathode spots in the course of the first half-cycle. The cathode spot is fairly uniform on a highly polished surface, but if the ground surface is roughened the ridges preferentially act as spark roots along which the cathode spots spread linearly. (3) Though the sample surface after the impact of the frrst current half-cycle appeared only slightly etched under the microscope, sufficient material was removed to give high spectral line intensity as was found by studying time-resolved spectral radiation [41. (4) The cathode spots can be formed sequentially at the beginning of the discharge on highly polished surfaces. These spots remain in fixed localities t~oughout their life-times, which may vary considerably. Life-times between ~1 and 4 ,usec for individual spots have been observed. This agrees well with the findings in a low pressure pulsed discharge [7], while other authors claim the spots to be mobile [9]. The spots are small and intense when formed but become larger and diffuse as jets of material are released. Some spots fail to reach the diffuse stage and die before releasing material. This dying out of spots has been observed in vacuum arcs as well [lo]. (5) Current densities in a gas discharge calculated on the basis of the size and structure of the eroded areas only [7] do not give very reliable values. The timedependent cathode spots should therefore be taken as an additional variable to calculate time-de~ndent cu~ent-densities.