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On the expansion of gaseous plasmas
Volume bA, number 1
PHYSICS LETTERS
5 February 1979
ON THE EXPANSION OF GASEOUS PLASMAS P. RICE-EVANS and I.J. FRANCO Department of Physics, Bedford College, University of London, London NW1, UK Received 8 December 1978
The expansion of plasma in neon plus helium under a modulated high voltage pulse regime has been studied. Photomultiplier studies of the striated appearance show that the bright regions are developed in sequence.
It is surprising that several centuries after the discovery of electrical discharges in gases many features are still not understood. Considerable controversy surrounds the basic mechanisms of the discharge: e.g. whether avalanches develop into streamers, and if so, when and how [1,2]. Even the origins of the familiar pattern of striations seen in the positive glow of a dc discharge remain unexplained: no convincing theoret. ical model has emerged [1]. In this letter we report observations that shed light on both these questions. In 1972 we [3] announced the discovery of dark rings superimposed on Lichtenberg Figures emanating
from cosmic ray tracks in a streamer chamber [4]. Last year we reported on the extension of these studies to a point.plane geometry [5] ; we had been able to obtain a series of dark spaces in columnar discharges. We used single high-voltage pulses with a high.fre. quency modulation superimposed. By arresting the discharges we were able to trace the growth of the bright—dark pattern from the point into the column, and to associate each bright patch with a peak in the oscillation. An important feature of these results is that the striated pattern developed both when the point was positive and when it was negative. As a
~:1 3rj4
8j 4
a
b
Fig. I. Photographs of complete discharges for (a) negative polarity at the wire electrode, and (b) positive polarity at the wire electrode. The numbers identify regions viewed with the photomultiplier.
20
Volume bOA, number I
PHYSICS LETTERS
result conventionally held ideas of discharge mechanisms are called into question. In the present experiment a point-plane gap of 12.5 cm has been employed. The plane was covered with a 1.2 cm thick sheet of perspex. The radius of the gold plated tungsten wire forming the point was lOjim. The gas was the usual spark chamber gas, i.e. 70% neon, 30% helium, at a pressure of 39 cm Hg. Single voltage pulses, with a maximum amplitude of 18 kV, rise time 40 ns, and overall duration 7 j.ts, were applied to the point. The nature of the superimposed oscillation is shown in fig. 2; it is seen to have a period of about 170 ns. The pulse shapes were virtually the same for both polarities. In spite of the oscillation the sign of the voltage does not vary; the oppositely phased potential at the earth-electrode is much smaller and does not cause the field to reverse. Fig. 1 shows open-shutter photographs of the dis charges arising from the application of the voltage pulses. Each photograph shows fifteen discharges a fact necessitated by the limited film speed (400 asa). The time development of the striated pattern has been studied with a 56 AVP photomultiplier. Spaces of 1 mm were focussed on to a 1 mm slit before the photocathode. Fig. 2 shows the photomultiplier output when different regions in the discharge were studied. The individual traces (1—8) correspond to light emitted from the regions identified in fig. 1. All the traces were triggered by the initial part of the high voltage pulse. The traces (2—8) shows the remarkable sequential development of the discharge from the point towards the plane, for both negative (a) and positive (b) point polarities. Field emission at the wire tip appears to initiate the discharges. Region (2), the centre of the first bright space appears first. No significant light is emitted from the first dark space (3); but region (4) emits light about 160 ns after region (2). Similarly, little light emerges from the second dark space (5), but light from region (6) appears about 340 ns after region (2). Traces (7) and (8) record the emission from the corresponding fourth and fifth bright regions. At the actual tip of the wire (1), light appears for each modulation peak in the voltage pulse. This is in contrast with the bright columnar spaces (2,4,6,7,8) where the major light emission occurs only at the moment the discharge front traverses the region. Near the wire, we can therefore conclude that an ionisation —
5 February 1979
V _____
____________
______________________
2 ~
3 4 -_______
5 6
8 A
B
Fig. 2. At the top (V) are shown high voltage pulses at the wire electrode for (a) negative polarity and (b) positive polarity, with 200 ns/div and 5000 V/div. Osdillograms (1—8) show the photomultiplier output corresponding to regions indicated in fig. I, with 100 ns/div.
peak occurs with each voltage peak, and hence a peaked flow of current must be expected in the gap. The observations show that the discharge fronts advance at an average velocity of about 6.106 cm s1, for both field directions. The relevant fields are hard to assess. Using the relation E,, = V/r, one can say that the maximum nominal field at a distance of 6 cm from the wire tip is 3 kY/cm. But the effects of space charge are uncertain and the oscillatory nature of the field will reduce the average field. 21
Volume 70A, number 1
PHYSICS LETTERS
The present velocities are much lower than those found at higher fields in streamer chambers; for example [6,5] in fields of 12 kV cm~in neon, isolated streamers advance in both directions with comparable velocities in the region of 108 cm The findings raise the question of the discharge mechanism. The Townsend avalanche theory may be invoked to explain the anode-directed front, but certainly not the cathode-directed front. Photons must be involved in the ionisation. Furthermore, the regular uniformity of the luminous boundaries implies that the ionisation process must concern large numbers of photons with comparatively short mean free paths. An implication of the dark spaces is that the discharge continues its advance even in very low fields~ The conventional view that streamers require 108 electrons to produce the necessary space-charge fields is thus brought into question. The results suggest that in mixtures of noble gases even in low fields a suck and spit model is appropriate for an expanding plasma. The ionization outside the avalanche is probably that proposed by Lozanskii [7,5], i.e. it is due to the formation of molecular ions in the collision of excited and neutral atoms: ~
*
The advance of the peaks in fig. 2 shows how the plasma extends into the gap, with the frontier advancing through a combination of photon effects and electron multiplication mechanisms, in a manner independent of the distant electrode. The low level of light emission once the frontier has passed indicates how rapidly the space charge fields develop as the plasma expands, sufficiently at least to damp further ionisation and excitation within. We believe the technique employed in this work has great potential and we are planning a new series of systematic measurements.
We are pleased to thank Professor C. Grey Morgan and Dr. J.K. Nelson for their interest and Professor E.R. Dobbs for his continuing support. References [11 E. Nasser, Fundamentals of gaseous ionisation and plasma electronics (Wiley, New York, 1971).
[21 J.M. Meek and J.D. Craggs, eds., Electrical breakdown of gases (Wiley, Chichester, 1978). Phys. Lett. 38A (1972) [3] P. Rice-Evans and l.A. Hassairi, 196.
Rice-Evans, Spark, streamer, proportional and drift chambers (Richeheu Press, London, 1974). [5] P. Rice-Evans and I.J. Franco, Phys. Lett. 63A (1977) [4] P.
+
The patterns of dark spaces observed in the photographs of fig. 1 (and especially in fig. 2 of ref. [5]) show some similarity with dc striations. However stationary dc striations exhibit a continuous oscillatory light emission due to the meeting of oppositely moving positive and negative striations [8] and nothing like the singular peaks of fig. 2. The phenomena are therefore distinct. ,
22
5 February 1979
[6]
Rudenko and V.1. Smetanin, Soy. Phys. JETP 34 (1972) 76. [71E.D. Lozanskii, Soy. Phys. Tech. Phys. 13 (1969) 1269; Soy. Phys. Dokl. 13 (1969) 1134. [8] T. Donahue and G.H. Dieke, Phys. Rev. 81(1951) 248.
PHYSICS LETTERS
5 February 1979
ON THE EXPANSION OF GASEOUS PLASMAS P. RICE-EVANS and I.J. FRANCO Department of Physics, Bedford College, University of London, London NW1, UK Received 8 December 1978
The expansion of plasma in neon plus helium under a modulated high voltage pulse regime has been studied. Photomultiplier studies of the striated appearance show that the bright regions are developed in sequence.
It is surprising that several centuries after the discovery of electrical discharges in gases many features are still not understood. Considerable controversy surrounds the basic mechanisms of the discharge: e.g. whether avalanches develop into streamers, and if so, when and how [1,2]. Even the origins of the familiar pattern of striations seen in the positive glow of a dc discharge remain unexplained: no convincing theoret. ical model has emerged [1]. In this letter we report observations that shed light on both these questions. In 1972 we [3] announced the discovery of dark rings superimposed on Lichtenberg Figures emanating
from cosmic ray tracks in a streamer chamber [4]. Last year we reported on the extension of these studies to a point.plane geometry [5] ; we had been able to obtain a series of dark spaces in columnar discharges. We used single high-voltage pulses with a high.fre. quency modulation superimposed. By arresting the discharges we were able to trace the growth of the bright—dark pattern from the point into the column, and to associate each bright patch with a peak in the oscillation. An important feature of these results is that the striated pattern developed both when the point was positive and when it was negative. As a
~:1 3rj4
8j 4
a
b
Fig. I. Photographs of complete discharges for (a) negative polarity at the wire electrode, and (b) positive polarity at the wire electrode. The numbers identify regions viewed with the photomultiplier.
20
Volume bOA, number I
PHYSICS LETTERS
result conventionally held ideas of discharge mechanisms are called into question. In the present experiment a point-plane gap of 12.5 cm has been employed. The plane was covered with a 1.2 cm thick sheet of perspex. The radius of the gold plated tungsten wire forming the point was lOjim. The gas was the usual spark chamber gas, i.e. 70% neon, 30% helium, at a pressure of 39 cm Hg. Single voltage pulses, with a maximum amplitude of 18 kV, rise time 40 ns, and overall duration 7 j.ts, were applied to the point. The nature of the superimposed oscillation is shown in fig. 2; it is seen to have a period of about 170 ns. The pulse shapes were virtually the same for both polarities. In spite of the oscillation the sign of the voltage does not vary; the oppositely phased potential at the earth-electrode is much smaller and does not cause the field to reverse. Fig. 1 shows open-shutter photographs of the dis charges arising from the application of the voltage pulses. Each photograph shows fifteen discharges a fact necessitated by the limited film speed (400 asa). The time development of the striated pattern has been studied with a 56 AVP photomultiplier. Spaces of 1 mm were focussed on to a 1 mm slit before the photocathode. Fig. 2 shows the photomultiplier output when different regions in the discharge were studied. The individual traces (1—8) correspond to light emitted from the regions identified in fig. 1. All the traces were triggered by the initial part of the high voltage pulse. The traces (2—8) shows the remarkable sequential development of the discharge from the point towards the plane, for both negative (a) and positive (b) point polarities. Field emission at the wire tip appears to initiate the discharges. Region (2), the centre of the first bright space appears first. No significant light is emitted from the first dark space (3); but region (4) emits light about 160 ns after region (2). Similarly, little light emerges from the second dark space (5), but light from region (6) appears about 340 ns after region (2). Traces (7) and (8) record the emission from the corresponding fourth and fifth bright regions. At the actual tip of the wire (1), light appears for each modulation peak in the voltage pulse. This is in contrast with the bright columnar spaces (2,4,6,7,8) where the major light emission occurs only at the moment the discharge front traverses the region. Near the wire, we can therefore conclude that an ionisation —
5 February 1979
V _____
____________
______________________
2 ~
3 4 -_______
5 6
8 A
B
Fig. 2. At the top (V) are shown high voltage pulses at the wire electrode for (a) negative polarity and (b) positive polarity, with 200 ns/div and 5000 V/div. Osdillograms (1—8) show the photomultiplier output corresponding to regions indicated in fig. I, with 100 ns/div.
peak occurs with each voltage peak, and hence a peaked flow of current must be expected in the gap. The observations show that the discharge fronts advance at an average velocity of about 6.106 cm s1, for both field directions. The relevant fields are hard to assess. Using the relation E,, = V/r, one can say that the maximum nominal field at a distance of 6 cm from the wire tip is 3 kY/cm. But the effects of space charge are uncertain and the oscillatory nature of the field will reduce the average field. 21
Volume 70A, number 1
PHYSICS LETTERS
The present velocities are much lower than those found at higher fields in streamer chambers; for example [6,5] in fields of 12 kV cm~in neon, isolated streamers advance in both directions with comparable velocities in the region of 108 cm The findings raise the question of the discharge mechanism. The Townsend avalanche theory may be invoked to explain the anode-directed front, but certainly not the cathode-directed front. Photons must be involved in the ionisation. Furthermore, the regular uniformity of the luminous boundaries implies that the ionisation process must concern large numbers of photons with comparatively short mean free paths. An implication of the dark spaces is that the discharge continues its advance even in very low fields~ The conventional view that streamers require 108 electrons to produce the necessary space-charge fields is thus brought into question. The results suggest that in mixtures of noble gases even in low fields a suck and spit model is appropriate for an expanding plasma. The ionization outside the avalanche is probably that proposed by Lozanskii [7,5], i.e. it is due to the formation of molecular ions in the collision of excited and neutral atoms: ~
*
The advance of the peaks in fig. 2 shows how the plasma extends into the gap, with the frontier advancing through a combination of photon effects and electron multiplication mechanisms, in a manner independent of the distant electrode. The low level of light emission once the frontier has passed indicates how rapidly the space charge fields develop as the plasma expands, sufficiently at least to damp further ionisation and excitation within. We believe the technique employed in this work has great potential and we are planning a new series of systematic measurements.
We are pleased to thank Professor C. Grey Morgan and Dr. J.K. Nelson for their interest and Professor E.R. Dobbs for his continuing support. References [11 E. Nasser, Fundamentals of gaseous ionisation and plasma electronics (Wiley, New York, 1971).
[21 J.M. Meek and J.D. Craggs, eds., Electrical breakdown of gases (Wiley, Chichester, 1978). Phys. Lett. 38A (1972) [3] P. Rice-Evans and l.A. Hassairi, 196.
Rice-Evans, Spark, streamer, proportional and drift chambers (Richeheu Press, London, 1974). [5] P. Rice-Evans and I.J. Franco, Phys. Lett. 63A (1977) [4] P.
+
The patterns of dark spaces observed in the photographs of fig. 1 (and especially in fig. 2 of ref. [5]) show some similarity with dc striations. However stationary dc striations exhibit a continuous oscillatory light emission due to the meeting of oppositely moving positive and negative striations [8] and nothing like the singular peaks of fig. 2. The phenomena are therefore distinct. ,
22
5 February 1979
[6]
Rudenko and V.1. Smetanin, Soy. Phys. JETP 34 (1972) 76. [71E.D. Lozanskii, Soy. Phys. Tech. Phys. 13 (1969) 1269; Soy. Phys. Dokl. 13 (1969) 1134. [8] T. Donahue and G.H. Dieke, Phys. Rev. 81(1951) 248.