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The breakdown phase in a coaxial plasma gun
Volume 76A, number 5,6
PHYSICS LETTERS
14 April 1980
THE BREAKDOWN PHASE IN A COAXIAL PLASMA GUN A. DONGES, G. HERZIGER, H. KROMPHOLZ, F. RUHL and K. SCHONBACH Instirut fur Angewandte Physik, Technzsche Hochschule Darmstadt, Germany 55
Received 7 December 1979 Revised manuscript received 11 February 1980
55 55
The electrical breakdown in a coaxial plasma gun was investigated by means of optical and electrical measurements. The optimum start and operation conditions of the gun turned out to be strongly dependent on material and length of the cylindrical insulator.
In a coaxial plasma gun the capacitor bank was re-
L~
information on the breakdown phase could be g~ned using simple diagnostic techniques. Fig. I shows the electrical circuit. A coaxial cable with a wave impedanceZ lOclandalengthL=3mwaschargedup to U 20 kV. The voltage is switched by means of the pressurized spark gap S to the center electrode of the plasma gun which is grounded viaR1 = 1 M~Z. The length of the gun is 10 cm, the diameter of the center and outer electrode is 1.6 cm and 5 cm, respectively. The cylindrical insulator between the electrodes is 4.2 cm long. The device was filled with hydrogen at pressures ranging from 0.4 to 40 mbar. After switching S the voltage pulse produced by the cable discharge is reflected between plasma gun and power supply until the gas breaks down in the coaxial gun. During break-down the plasma resistance drops rapidly to values small compared to those of the series resistance R1 Z. Hence the cable discharge approaches matched conditions. That means the current flow with an amplitude of approximately U/2Z = 1 kA is restricted in time to about 2L/c 30 ns, c being the wave velocity in the cable. Thus the cable discharge represents the breakdown phase of a common plasma gun discharge. Photographs of the gas breakdown were made with an open shutter camera. The pictures show three distinct types of discharge (fig. 1) depending on the hy. drogen pressure. At low pressure, generally below 1 mbar, there is a discharge at the open end of the gun
R
2
I
____________
ii, R,
=
__________
II
_________
i... :.: .
Fig. 1. Electrical circuit of the plasma gun and the three types of discharge. Maximum energy density in the focus plasma is obtained with a type Il discharge.
(type I). With rising pressure type I is placed by a gliding discharge along the cylindrical insulator (type II). Finally at pressures between 5 and 10 mbar a discharge radial between the electrodes (type III) becomes visible. The three types of discharge sometimes are joined by filamentary discharges occurring randomly between the electrodes. Generally the discharge tends to shorten its path with increasing pressure. Hence by increasing or decreasing the insulator length the pressure range of the gliding discharge is shifted towards lower or higher pressure values, respectively, It was even possible to change the order of succession of type II and III by cutting the insulator to a length smaller than the radial distance between the electrodes. Not only the insulator length but also the insulator 391
Volume 76A, number 5,6
1000
PHYSICS LETTERS
——~—--r--—-—-————----—r—---—-—
HYDROGEN OON(
100 u.s
o’~~
PTFE
o
PYREX 0
~
—
type Ito type Ill. That means, the gliding discharge (type II) is always joined by a generally low intensity discharge at the end of the gun which is linking type I and type III. In case of weak gliding discharges its optical intensity becomes comparable to that of the type 11 discharge. The plasma resistance R~during flashover was determined by means of additional current measurements with a pick-up coil. R turned out to decline exponentially: p R~ exp (—t/r). with r 5 ns/p (mbar),
.
10
14 April 1980
PYREX AL
2 03 I
0.4
1
10
40
PRESSURE p Imbor] Fig. 2. Breakdown delay time versus hydrogen pressure and pressure range of the gliding discharge for teflon, pyrex and A1203 insulators. The pyrex~line indicates the pressure range of pyrex with an additional electron emitter. material is affecting the pressure range and moreover the optical intensity of the gliding discharge. Pyrex, Al203, PTFE and PVC were used as insulating material. Corresponding pressure ranges are shown in fig. 2. No gliding discharge was observed with PVC in the pressure range from 0.4 mbar to 40 mbar. With a PTFE insulator a gliding discharge was recorded between 2,7 and 4.7 mbar, the optical intensity decreasing towards the upper pressure limit. Gliding discharges with pyrex insulator showed comparable optical intensity in the pressure range from 3.9 to 4.1 mbar. Intensities larger by one order of magnitude and the largest operating range with respect to pressure (4 to 9.3 mbar) were achieved with A1203 as insulator material, The pressure range for a gliding discharge can be extended by using simple electron emitters. With a ring shaped edge at the bottom of the gun close to the Pyrex insulator the lower limit of the operating range could be expanded from 3.9 to 1 .3 mbar. Furthermore an increase in optical intensity was observed compared to the set up without the edge. The voltage across the plasma resistance in the gun was measured by means of a coaxial capacitive divider, The delay rD between the switching of the spark gap S and the electrical breakdown in the gun turned out to be independent of the different types of discharge. There is only a pressure dependence as shown in fig. 2: 1’3 (mbar1’3). = 220 ns/p Obviously there is a continuous transition from 392
valid in the pressure range from 1.5 to 6.5 mbar. The long delay times at low pressure and the relatively slow fall of the resistance cannot be explained by well-known breakdown and spark models [1 —31. In these models loss processes such as recombination and diffusion are either neglected in the current carrier .
balance ~r diffusion is not considered as a loss process. At low pressure, on the other hand, the diffusion process which leads to a depletion of electrons in strong space-charge field regions becomes important. The electron generation rate characterized by the ionization coefficient in return is reduced. Hence at low pressure the overall growth rate of the electron density is noticeably lowered causing a slow fall of the plasma resistance. An intensive gliding discharge along the insulator without filamentary discharges is necessary to get a single homogeneous plasma layer propagating along the coaxial gun. In plasma focus devices, on the other hand, the homogeneity of the accelerated plasma layer is an essential condition for a focus plasma with high energy density [4]. In case of prerunning filaments the magnetic field built up in front of the layer is compressed on the focus axis by the collapsing hot plasma, thus preventing maximum energy density in the focus plasma itself. Hence the stochastic behaviour of the plasma focus is induced by the random properties of the prerunning filaments which in return are determined by the behaviour of the discharge during breakdown.
a
References
[Il E. Nasser, Fundamentals of gaseous ionization and plasma electronics 1971) Cli. 9, 10. ]2] M. Toepler,(Wiley, Ann. Phys. 21(1906)193. [31 R. Rompe und W. Weizel, Z. Phys. 122 (1944) 636. [4] G. Herziger et al., to be published. as as as as as as as as as
PHYSICS LETTERS
14 April 1980
THE BREAKDOWN PHASE IN A COAXIAL PLASMA GUN A. DONGES, G. HERZIGER, H. KROMPHOLZ, F. RUHL and K. SCHONBACH Instirut fur Angewandte Physik, Technzsche Hochschule Darmstadt, Germany 55
Received 7 December 1979 Revised manuscript received 11 February 1980
55 55
The electrical breakdown in a coaxial plasma gun was investigated by means of optical and electrical measurements. The optimum start and operation conditions of the gun turned out to be strongly dependent on material and length of the cylindrical insulator.
In a coaxial plasma gun the capacitor bank was re-
L~
information on the breakdown phase could be g~ned using simple diagnostic techniques. Fig. I shows the electrical circuit. A coaxial cable with a wave impedanceZ lOclandalengthL=3mwaschargedup to U 20 kV. The voltage is switched by means of the pressurized spark gap S to the center electrode of the plasma gun which is grounded viaR1 = 1 M~Z. The length of the gun is 10 cm, the diameter of the center and outer electrode is 1.6 cm and 5 cm, respectively. The cylindrical insulator between the electrodes is 4.2 cm long. The device was filled with hydrogen at pressures ranging from 0.4 to 40 mbar. After switching S the voltage pulse produced by the cable discharge is reflected between plasma gun and power supply until the gas breaks down in the coaxial gun. During break-down the plasma resistance drops rapidly to values small compared to those of the series resistance R1 Z. Hence the cable discharge approaches matched conditions. That means the current flow with an amplitude of approximately U/2Z = 1 kA is restricted in time to about 2L/c 30 ns, c being the wave velocity in the cable. Thus the cable discharge represents the breakdown phase of a common plasma gun discharge. Photographs of the gas breakdown were made with an open shutter camera. The pictures show three distinct types of discharge (fig. 1) depending on the hy. drogen pressure. At low pressure, generally below 1 mbar, there is a discharge at the open end of the gun
R
2
I
____________
ii, R,
=
__________
II
_________
i... :.: .
Fig. 1. Electrical circuit of the plasma gun and the three types of discharge. Maximum energy density in the focus plasma is obtained with a type Il discharge.
(type I). With rising pressure type I is placed by a gliding discharge along the cylindrical insulator (type II). Finally at pressures between 5 and 10 mbar a discharge radial between the electrodes (type III) becomes visible. The three types of discharge sometimes are joined by filamentary discharges occurring randomly between the electrodes. Generally the discharge tends to shorten its path with increasing pressure. Hence by increasing or decreasing the insulator length the pressure range of the gliding discharge is shifted towards lower or higher pressure values, respectively, It was even possible to change the order of succession of type II and III by cutting the insulator to a length smaller than the radial distance between the electrodes. Not only the insulator length but also the insulator 391
Volume 76A, number 5,6
1000
PHYSICS LETTERS
——~—--r--—-—-————----—r—---—-—
HYDROGEN OON(
100 u.s
o’~~
PTFE
o
PYREX 0
~
—
type Ito type Ill. That means, the gliding discharge (type II) is always joined by a generally low intensity discharge at the end of the gun which is linking type I and type III. In case of weak gliding discharges its optical intensity becomes comparable to that of the type 11 discharge. The plasma resistance R~during flashover was determined by means of additional current measurements with a pick-up coil. R turned out to decline exponentially: p R~ exp (—t/r). with r 5 ns/p (mbar),
.
10
14 April 1980
PYREX AL
2 03 I
0.4
1
10
40
PRESSURE p Imbor] Fig. 2. Breakdown delay time versus hydrogen pressure and pressure range of the gliding discharge for teflon, pyrex and A1203 insulators. The pyrex~line indicates the pressure range of pyrex with an additional electron emitter. material is affecting the pressure range and moreover the optical intensity of the gliding discharge. Pyrex, Al203, PTFE and PVC were used as insulating material. Corresponding pressure ranges are shown in fig. 2. No gliding discharge was observed with PVC in the pressure range from 0.4 mbar to 40 mbar. With a PTFE insulator a gliding discharge was recorded between 2,7 and 4.7 mbar, the optical intensity decreasing towards the upper pressure limit. Gliding discharges with pyrex insulator showed comparable optical intensity in the pressure range from 3.9 to 4.1 mbar. Intensities larger by one order of magnitude and the largest operating range with respect to pressure (4 to 9.3 mbar) were achieved with A1203 as insulator material, The pressure range for a gliding discharge can be extended by using simple electron emitters. With a ring shaped edge at the bottom of the gun close to the Pyrex insulator the lower limit of the operating range could be expanded from 3.9 to 1 .3 mbar. Furthermore an increase in optical intensity was observed compared to the set up without the edge. The voltage across the plasma resistance in the gun was measured by means of a coaxial capacitive divider, The delay rD between the switching of the spark gap S and the electrical breakdown in the gun turned out to be independent of the different types of discharge. There is only a pressure dependence as shown in fig. 2: 1’3 (mbar1’3). = 220 ns/p Obviously there is a continuous transition from 392
valid in the pressure range from 1.5 to 6.5 mbar. The long delay times at low pressure and the relatively slow fall of the resistance cannot be explained by well-known breakdown and spark models [1 —31. In these models loss processes such as recombination and diffusion are either neglected in the current carrier .
balance ~r diffusion is not considered as a loss process. At low pressure, on the other hand, the diffusion process which leads to a depletion of electrons in strong space-charge field regions becomes important. The electron generation rate characterized by the ionization coefficient in return is reduced. Hence at low pressure the overall growth rate of the electron density is noticeably lowered causing a slow fall of the plasma resistance. An intensive gliding discharge along the insulator without filamentary discharges is necessary to get a single homogeneous plasma layer propagating along the coaxial gun. In plasma focus devices, on the other hand, the homogeneity of the accelerated plasma layer is an essential condition for a focus plasma with high energy density [4]. In case of prerunning filaments the magnetic field built up in front of the layer is compressed on the focus axis by the collapsing hot plasma, thus preventing maximum energy density in the focus plasma itself. Hence the stochastic behaviour of the plasma focus is induced by the random properties of the prerunning filaments which in return are determined by the behaviour of the discharge during breakdown.
a
References
[Il E. Nasser, Fundamentals of gaseous ionization and plasma electronics 1971) Cli. 9, 10. ]2] M. Toepler,(Wiley, Ann. Phys. 21(1906)193. [31 R. Rompe und W. Weizel, Z. Phys. 122 (1944) 636. [4] G. Herziger et al., to be published. as as as as as as as as as