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Application of vacuum arc in the study of unipolar arcs in tokamaks
Vacuum~volume 41/numbers 4-6/pages 1555 to 1557/1990
0042-207X/90S3.00 + .00 © 1990 Pergamon Press plc
Printed in Great Britain
A p p l i c a t i o n of v a c u u m arc in the study of unipolar arcs in t o k a m a k s M S Agarwal,
M K R a d h a k r i s h n a n and A K S i n g h , Department of Electrical Engineering, liT,
Bombay 400 076, India
The regime of plasma parameters in a high-current vacuum arc is such that it may be used to conduct some studies related to unipolar arcs, which cause contamination of the plasma in tokamaks. Such an arc provides a very convenient and low-cost facility for this purpose as it has a simple linear and variable geometry. Easy diagnostic access to the plasma allows one to investigate various plasma parameters,
1. Introduction For commercial exploitation of fusion plasma, proper confinement is essential. The toroidal geometry is considered to be most effective for this purpose. However, the plasma confinement to the extent necessary to produce fusion energy output higher than the electrical energy input has not yet been achieved, even in this geometry, since the plasma energy is lost due to several known and unknown processes. One of the processes leading to plasma dilution and energy loss by radiation is impurity production by unipolar arcing at the walls and limiters. It is, therefore, still appropriate to study the behaviour of unipolar arcs in a simple linear geometry, such as that provided by a vacuum arc. In the present work, magnetic-field dependence of cathode-spot motion, metal migration, arc energy transport and arcing voltage have been studied in a vacuum arc on several high-purity metals, namely AI, Bi, Cd, Cu, Mo, Ni, Sn and stainless steel, covering a wide range of thermophysical properties.
2. Experimental apparatus and procedure The experiments were conducted in a stainless-steel vacuum chamber, with the electrodes having plane faces and arranged in a circular butt-contact configuration. The gap length was varied by means of bellows at both the ends. The electrodes were made of high-purity (5 N) metals and the chamber was evacuated by LN2-trapped diffusion and rotary pumps. The arcs were initiated by a third-electrode trigger arrangement near the cathode and the current pulse supplied from a capacitor bank or a d c generator. An insulated feedthrough with four conductors was arranged to bring out the signals from two thermocouples attached to the electrodes to measure their temperatures for arc-energy transport studies. A copper rod of 4.8 mm diameter covered with a PTFE sleeve of 8 mm outer diameter was passed through the central holes of the electrodes for carrying the current for producing a transverse magnetic field. A four-channel Nicolet 4094B digital storage oscilloscope was used to record various electrical parameters. A high-speed
camera, both in framing and streak modes, was used for recording the arc-spot motion.
3. Experimental results and discussion 3.1. Cathode spot motion. Once the arc was initiated at the edge of the cathode, a cathode spot following the retrograde motion, under the influence of the magnetic field, travelled radially towards the centre of the disc. The velocity of the cathode spot calculated from the streak photographs was found to depend upon the magnetic field. Table 1 gives the cathode spot velocities on copper, cadmium and stainless-steel electrodes for different magnetic fields. The magnetic field is low at the edge (35 mm from the axis) and high near the surface of the PTFE sleeve (4 mm from the axis); the values given in Table 1 are at a point midway (19.5 mm from the axis) on the radial track of the cathode spot. The retrograde motion and track of a cathode spot in vacuum arc are similar to those observed on tokamak walls ~. On wall surfaces which are parallel to the toroidal field B, they run along the /~ x J direction, i.e. the characteristic retrograde motion of cathode spots in vacuum arcs, where J i s the current carried by the arc. Therefore, useful information and insight into these characteristics can easily be obtained using electrodes in the vacuum arc of those metals/materials which are likely to be used in tokamaks.
Table 1. Magnetic-field dependence of cathode spot motion Cathode metal
Arc current (A)
Flux density ( x I0-3 T)
Averagevelocity (ms -1)
Copper
63 63 63 95 9.8 9.8 72 72 72
3.76 4.67 6.07 5.02 6.24 6.97 4.4 3.43 6.07
32.6 37.6 54.4 46.4 1.5 1.7 57.0 77.5 88.4
Bismuth Stainless steel
1555
M S Agarwal et al: Unipolar arcs in tokamaks
Table 2. Average cathode erosion rates of the metals investigated Erosion rate Og/C) Cathode metal
Arc current (A)
Average duration of arcing (ms)
Zn Sn A1 Cu Ni Mo W Cd Stainless steel
11.7 16.7 30.0 28.3 19.6 41.7 35.8 30.5 140.0
531 581 560 581 543 552 537 24.6 47.3
Table 3. Values of Vco. and
Electrode metal
Current (A)
Cadmium
19.3 18.7 20.0 16.8 19.2 101 89 93
13.8 14.0 14.0 13.8 6.1 6.2 2.3 8.6
Copper
Erosion (mg)
Present work
Other investigations (refs 2-4)
15 20 18 15 15 20 20 20 20
14.0 79.6 15.8 15.4 10.8 4.6 4.8 10.I 7.2
150 410 5762 68 10 12 672 54
215-320 650-1120 120 32-240 I00 47 62 400-655 65
V¢o,/V=. ¢ for the cathode
Arcing time (s)
Bismuth
Number of arcings
V¢o.
Vcon/Var¢ (for cathode)
V~r~ (V)
(for cathode) (V)
Present work
10.0 10.3 10.8 9.6 9.8 20.0 20.5 20.4
2.5 2.6 2.7 2.4 2.4 6.7 6.8 6.8
0.25 0.25 0.25 0.25 0.25 0.33 0.33 0.33
Other investigations 0.25 (Daalder 5) -0.31 (Daalder 5) 0.33 (Reece6)
3.2. Cathode erosion. The cathode erosion rate for various metal cathodes was determined by weighing the cathode prior to, and after, the arcing. The cathode was arced by currents ranging from 8 to 140 A, for durations ranging from 8 to 550 ms. The cathode was then removed and reweighed. The erosion rate was determined by comparing the cathode material loss with the total charge passed through the arc. The erosion measurements are given in Table 2. Since this is a study of cathode erosion at low currents the data obtained here may be compared with the low-current erosion studies reported in the literature. The range of lowcurrent erosion values reported in the literature is also shown in Table 2; the comparison of which with the results obtained in this investigation shows that there is a reasonable agreement between the present experimental values and those reported earlier. The cathode erosion data presented here would be helpful in selecting suitable metals for wall materials in tokamaks.
negligibly small. Thus the arc energy distributes itself between the anodeand cathode, and manifests itself in the form of a temperature rise of electrodes. This temperature rise of the electrodes has consequences on the erosion and mass loss from their surfaces. Also, the power transfer to the cathode is relevant to the study of the cathode spot mechanism. With this idea, a brief study of the phenomenon of energy dissipation at the electrodes in the case of vacuum arc was made for cadmium, bismuth and copper. Using thermocouples, the temperature rise at the electrodes subsequent to arcing was measured. The outputs of the thermocouples were recorded on the digital storage oscilloscope or observed on microvoltmeters. The calculated condition-loss values are given in Table 3. This temperature rise study showed that the ratio of the energy dissipated at the cathode to that at the anode was about 1 : 2 in the case of copper, and about 1 : 3 for cadmium and bismuth.
3.3. Arc energy transport. The electrical energy input into the vacuum arc, which is the product of arcing voltage, arc current and arc-burning time, is dissipated only through conduction by the electrodes. Because of high-vacuum conditions prevailing inside the chamber, convection is totally absent, and, because of low arc temperature, the radiation loss is also
3.4. Arcing voltage. Arc voltages obtained for cathode materials investigated are listed in Table 4. Also listed in the same table are the voltages reported in the literature. In the case of copper and stainless-steel electrodes, arc voltages were found to be dependent upon magnetic field7.
1556
M S Agarwal et al: Unipolar arcs in tokamaks
Table 4. Arc voltage for some cathode materials investigated Arc voltage as obtained in the present work (V)
Arc voltage (V)
Cathode metal
Without magnetic field
With magnetic field
ref 2
ref 5, 6
ref 7
Cu Cd Bi Stainless steel
20.0-20.4 10.0-10.8 9.6- 9.8 30.0
54.0 10.4 -54.0
20.0 I1.4 ---
21.5 10.0 -33.0
19.5 9.0 9.0 --
4. Conclusions In the present work the metal migration, arc-energy transport and magnetic-field dependence of cathode spot motion in a vacuum arc have been studied on several high-purity metals, namely AI, Bi, Cd, Cu, Mo, Ni, Sn and stainless-steel, cover
ing a wide range of thermophysical properties. It is found that the retrograde cathode spot motion and tracks are similar to those observed on tokamak chamber walls due to unipolar arcs, and the cathode spot velocity increases both with the magnetic field and the discharge current. The velocities measured are between 1 and 90 m/s. The erosion rates measured lie in the range 10-670 p g / C for the metals studied. It is also observed that for all the metals studied here, 2 5 - 3 3 % of arc energy is dissipated at the cathode and the balance at the anode. References D Y Fang, A W Nurenberg and V H Bauder, J Nucl Materials, 111-112, 517 (1982). 2W G J Rondeel, J Phys D: Appl Phys, 6, 1705 (1973). 3 C W Kimblin, J Appl Phys, 44, 3074 (1973). 4 S K Sethuraman, P A Chatterton and M R Barrault, J Nucl Materials, 111-112, 510 (1982). 5 j E Daalder, J Phys D: Appl Phys, 10, 2225 (1977). 6 M P Reece, Proc IEE, I10, 793 (1963). 7 M S Agarwal and R Holmes, J Phys D: Appl Phys, 17, 757 (1984).
1557
0042-207X/90S3.00 + .00 © 1990 Pergamon Press plc
Printed in Great Britain
A p p l i c a t i o n of v a c u u m arc in the study of unipolar arcs in t o k a m a k s M S Agarwal,
M K R a d h a k r i s h n a n and A K S i n g h , Department of Electrical Engineering, liT,
Bombay 400 076, India
The regime of plasma parameters in a high-current vacuum arc is such that it may be used to conduct some studies related to unipolar arcs, which cause contamination of the plasma in tokamaks. Such an arc provides a very convenient and low-cost facility for this purpose as it has a simple linear and variable geometry. Easy diagnostic access to the plasma allows one to investigate various plasma parameters,
1. Introduction For commercial exploitation of fusion plasma, proper confinement is essential. The toroidal geometry is considered to be most effective for this purpose. However, the plasma confinement to the extent necessary to produce fusion energy output higher than the electrical energy input has not yet been achieved, even in this geometry, since the plasma energy is lost due to several known and unknown processes. One of the processes leading to plasma dilution and energy loss by radiation is impurity production by unipolar arcing at the walls and limiters. It is, therefore, still appropriate to study the behaviour of unipolar arcs in a simple linear geometry, such as that provided by a vacuum arc. In the present work, magnetic-field dependence of cathode-spot motion, metal migration, arc energy transport and arcing voltage have been studied in a vacuum arc on several high-purity metals, namely AI, Bi, Cd, Cu, Mo, Ni, Sn and stainless steel, covering a wide range of thermophysical properties.
2. Experimental apparatus and procedure The experiments were conducted in a stainless-steel vacuum chamber, with the electrodes having plane faces and arranged in a circular butt-contact configuration. The gap length was varied by means of bellows at both the ends. The electrodes were made of high-purity (5 N) metals and the chamber was evacuated by LN2-trapped diffusion and rotary pumps. The arcs were initiated by a third-electrode trigger arrangement near the cathode and the current pulse supplied from a capacitor bank or a d c generator. An insulated feedthrough with four conductors was arranged to bring out the signals from two thermocouples attached to the electrodes to measure their temperatures for arc-energy transport studies. A copper rod of 4.8 mm diameter covered with a PTFE sleeve of 8 mm outer diameter was passed through the central holes of the electrodes for carrying the current for producing a transverse magnetic field. A four-channel Nicolet 4094B digital storage oscilloscope was used to record various electrical parameters. A high-speed
camera, both in framing and streak modes, was used for recording the arc-spot motion.
3. Experimental results and discussion 3.1. Cathode spot motion. Once the arc was initiated at the edge of the cathode, a cathode spot following the retrograde motion, under the influence of the magnetic field, travelled radially towards the centre of the disc. The velocity of the cathode spot calculated from the streak photographs was found to depend upon the magnetic field. Table 1 gives the cathode spot velocities on copper, cadmium and stainless-steel electrodes for different magnetic fields. The magnetic field is low at the edge (35 mm from the axis) and high near the surface of the PTFE sleeve (4 mm from the axis); the values given in Table 1 are at a point midway (19.5 mm from the axis) on the radial track of the cathode spot. The retrograde motion and track of a cathode spot in vacuum arc are similar to those observed on tokamak walls ~. On wall surfaces which are parallel to the toroidal field B, they run along the /~ x J direction, i.e. the characteristic retrograde motion of cathode spots in vacuum arcs, where J i s the current carried by the arc. Therefore, useful information and insight into these characteristics can easily be obtained using electrodes in the vacuum arc of those metals/materials which are likely to be used in tokamaks.
Table 1. Magnetic-field dependence of cathode spot motion Cathode metal
Arc current (A)
Flux density ( x I0-3 T)
Averagevelocity (ms -1)
Copper
63 63 63 95 9.8 9.8 72 72 72
3.76 4.67 6.07 5.02 6.24 6.97 4.4 3.43 6.07
32.6 37.6 54.4 46.4 1.5 1.7 57.0 77.5 88.4
Bismuth Stainless steel
1555
M S Agarwal et al: Unipolar arcs in tokamaks
Table 2. Average cathode erosion rates of the metals investigated Erosion rate Og/C) Cathode metal
Arc current (A)
Average duration of arcing (ms)
Zn Sn A1 Cu Ni Mo W Cd Stainless steel
11.7 16.7 30.0 28.3 19.6 41.7 35.8 30.5 140.0
531 581 560 581 543 552 537 24.6 47.3
Table 3. Values of Vco. and
Electrode metal
Current (A)
Cadmium
19.3 18.7 20.0 16.8 19.2 101 89 93
13.8 14.0 14.0 13.8 6.1 6.2 2.3 8.6
Copper
Erosion (mg)
Present work
Other investigations (refs 2-4)
15 20 18 15 15 20 20 20 20
14.0 79.6 15.8 15.4 10.8 4.6 4.8 10.I 7.2
150 410 5762 68 10 12 672 54
215-320 650-1120 120 32-240 I00 47 62 400-655 65
V¢o,/V=. ¢ for the cathode
Arcing time (s)
Bismuth
Number of arcings
V¢o.
Vcon/Var¢ (for cathode)
V~r~ (V)
(for cathode) (V)
Present work
10.0 10.3 10.8 9.6 9.8 20.0 20.5 20.4
2.5 2.6 2.7 2.4 2.4 6.7 6.8 6.8
0.25 0.25 0.25 0.25 0.25 0.33 0.33 0.33
Other investigations 0.25 (Daalder 5) -0.31 (Daalder 5) 0.33 (Reece6)
3.2. Cathode erosion. The cathode erosion rate for various metal cathodes was determined by weighing the cathode prior to, and after, the arcing. The cathode was arced by currents ranging from 8 to 140 A, for durations ranging from 8 to 550 ms. The cathode was then removed and reweighed. The erosion rate was determined by comparing the cathode material loss with the total charge passed through the arc. The erosion measurements are given in Table 2. Since this is a study of cathode erosion at low currents the data obtained here may be compared with the low-current erosion studies reported in the literature. The range of lowcurrent erosion values reported in the literature is also shown in Table 2; the comparison of which with the results obtained in this investigation shows that there is a reasonable agreement between the present experimental values and those reported earlier. The cathode erosion data presented here would be helpful in selecting suitable metals for wall materials in tokamaks.
negligibly small. Thus the arc energy distributes itself between the anodeand cathode, and manifests itself in the form of a temperature rise of electrodes. This temperature rise of the electrodes has consequences on the erosion and mass loss from their surfaces. Also, the power transfer to the cathode is relevant to the study of the cathode spot mechanism. With this idea, a brief study of the phenomenon of energy dissipation at the electrodes in the case of vacuum arc was made for cadmium, bismuth and copper. Using thermocouples, the temperature rise at the electrodes subsequent to arcing was measured. The outputs of the thermocouples were recorded on the digital storage oscilloscope or observed on microvoltmeters. The calculated condition-loss values are given in Table 3. This temperature rise study showed that the ratio of the energy dissipated at the cathode to that at the anode was about 1 : 2 in the case of copper, and about 1 : 3 for cadmium and bismuth.
3.3. Arc energy transport. The electrical energy input into the vacuum arc, which is the product of arcing voltage, arc current and arc-burning time, is dissipated only through conduction by the electrodes. Because of high-vacuum conditions prevailing inside the chamber, convection is totally absent, and, because of low arc temperature, the radiation loss is also
3.4. Arcing voltage. Arc voltages obtained for cathode materials investigated are listed in Table 4. Also listed in the same table are the voltages reported in the literature. In the case of copper and stainless-steel electrodes, arc voltages were found to be dependent upon magnetic field7.
1556
M S Agarwal et al: Unipolar arcs in tokamaks
Table 4. Arc voltage for some cathode materials investigated Arc voltage as obtained in the present work (V)
Arc voltage (V)
Cathode metal
Without magnetic field
With magnetic field
ref 2
ref 5, 6
ref 7
Cu Cd Bi Stainless steel
20.0-20.4 10.0-10.8 9.6- 9.8 30.0
54.0 10.4 -54.0
20.0 I1.4 ---
21.5 10.0 -33.0
19.5 9.0 9.0 --
4. Conclusions In the present work the metal migration, arc-energy transport and magnetic-field dependence of cathode spot motion in a vacuum arc have been studied on several high-purity metals, namely AI, Bi, Cd, Cu, Mo, Ni, Sn and stainless-steel, cover
ing a wide range of thermophysical properties. It is found that the retrograde cathode spot motion and tracks are similar to those observed on tokamak chamber walls due to unipolar arcs, and the cathode spot velocity increases both with the magnetic field and the discharge current. The velocities measured are between 1 and 90 m/s. The erosion rates measured lie in the range 10-670 p g / C for the metals studied. It is also observed that for all the metals studied here, 2 5 - 3 3 % of arc energy is dissipated at the cathode and the balance at the anode. References D Y Fang, A W Nurenberg and V H Bauder, J Nucl Materials, 111-112, 517 (1982). 2W G J Rondeel, J Phys D: Appl Phys, 6, 1705 (1973). 3 C W Kimblin, J Appl Phys, 44, 3074 (1973). 4 S K Sethuraman, P A Chatterton and M R Barrault, J Nucl Materials, 111-112, 510 (1982). 5 j E Daalder, J Phys D: Appl Phys, 10, 2225 (1977). 6 M P Reece, Proc IEE, I10, 793 (1963). 7 M S Agarwal and R Holmes, J Phys D: Appl Phys, 17, 757 (1984).
1557