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A Kilovolt, Kiloampere Low Pressure Switch
NUCLEAR INSTRUMENTS AND METHODS 8
(1960) 236-238; NORTH-HOLLAND PUBLISHING CO.
A KILOVOLT, KILOAMPERE LOW PRESSURE SWITCHt GEORGE J. BRUCKER
United States Signal Corps Laboratories, Fort JVIonmouth, New Jersey and KENNETH C. ROGERS
Stevens Institute of Technology, Hoboken, New Jersey Received 9 April 1960
A low pressure switch is described that is capable of passing kiloamperes at voltages up to 30 kV. The effect of pressure and gap voltage on the switch closing time is discussed.
1. Introduction
The need for switching large currents at high voltages in thermonuclear research has lead to the development of the vacuum-gap switch1 •2 ). The Megatron accelerator work at Stevens Institute requires a similar device for switching the capacitor bank current to the field coil of the Megatron. It is the purpose of this paper to present the design of a practical switch, and the experimentally determined closing time for different gap voltages and pressures. Fig. 2. View of Stevens capacitor bank showing switches mounted on transmission lines and connected to pumping manifold.
2. Description of Switch
7"
•
~ .l F,;Th'ok Stainless Steel Plate INirr(Trigger)
:
--
•
2i ~
Epoxy Insulator ./""
Section A-A
Fig. 1. Details of switch.
t
Work performed under the auspices of the United States Army Signal Corps Laboratories, Fort Monmouth, New Jersey and the U.S. Atomic Energy Commission. tt Twenty-two part of sand, ten parts of Araldite 6050, three parts of Hardener 901 by weight.
The design of the Stevens capacitor bank3 ) has dictated the rectangular geometry of this switch. Fig. I is a drawing of the switch which consists of a rectangular box cast from a mixture of epoxy and sand tt with stainless steel electrodes and a lucite baffle plate. The switch is closed by a plasma burst from a pair of coaxial plasma 1) D. C. Hagerman and A. H. Williams, Rev. Sci. Instr. 30 (1959) 182. 2) W. R. Baker, Rev. Sci. Instr. 30 (1959) 700. 3) K. C. Rogers et al., Megatron Accelerator Progress Report, Signal Corps Contract DA36-039sc-78097 (June 3, 1958).
236
A KILOVOLT, KILOAMPERE LOW PRESSURE SWITCH
guns4 ). A high voltage pulse (2 kV to 10 kV) supplied by a hydrogen thyratron (5c22) and a 0.3 ,uF capacitor fires the guns. A view of the switches mounted on the transmission lines of the capacitor bank is shown in fig. 2. 3. Experimental Procedure The closing delay time Td and the "jitter" in T d were measured for ranges of voltage across the switch Vs, and for vacua from 0.02-3.3 microns Hg. The delay time T d, is defined to be the length of time from the inception of current flow to the plasma gun to the start of current through the switch. Both rates of current change (dlfdt) were picked up by means of Rogovsky coils, and the integrated signals were presented on a Tektronix 551 dual beam scope. The capacitor bank was prevented from ringing by a resistive load. All pressure readings were made by the same P. 1. G. located close to the switch. The data are plotted in figures 3, 4 and 5. Each point is the mean of not less than five separate measurements. The polarity of the applied voltage was varied so that the gun side of the switch was either positive or negative. 4. Discussion of Results The delay time versus voltage applied across the switch at a constant low pressure (0.02 microns) is shown in fig. 3. At this pressure it is apparent that the delay time, T d , varies inver-
1n
side poLive X Gun side negative 0
I
Pressure 0.02 mlcrons Hg
:::::; i:::::-- <--2--,
r----------- r-------- ~ r------
1
4 8 12 16 Voltage across sW'ltch in kilovolts
20
24
Fig. 3. Switch closing delay time versus voltage across switch for a constant pressure of 0.02 microns Hg and both positive and negative bank voltages.
237
Ln
side p10sitive X
Gun side negative 0 Pressure 3.3 microns Hg I
I I
I i
2 1
4
12 16 8 Voltage across switch in kilovolts
20
24
Fig. 4. Switch closing delay time versus voltage across switch for a constant pressure of 3.3 microns Hg and both positive and negative bank voltages.
sely with the applied voltage V s. If electrons are responsible for closing the switch then reversing the polarity of V s so that the gun side of the switch is positive should show a longer time delay which is independent of voltage. This is not the case at low pressure. The evidence in support of the theory that electrons are responsible for switch closure becomes stronger for high pressure operation. Fig. 4 displays this independence of delay time on switch voltage when the gun side of the switch is positive; in this case T d is comparable to plasma transit time. Reversal of polarity at this pressure (3.3 microns) brings about an increase of Td with voltage. J. A. Mather5 ), who observed this behavior in' a vacuum-gap switch at Los Alamos, has suggested that the increasing delay time is due to the dependence of the electron-atomic ionization cross section a(v) on the applied voltage. The picture does not seem to be this simple as is indicated by fig. 5. The shape of these curves is not explicable on the basis of the ionization cross section voltage dependence, although the precision of this experiment is not sufficient to say with 4) K. C. Rogers et al., Megatron Accelerator Progress Report, Signal Corps Contract DA36-039sc-78097 (April I, 1959). 5) J. A. Mather and A. H. Williams, Rev. Sci. Instr. (to be published).
238
GEORGE
J.
BRUCKER AND KENNETH C. ROGERS
certainty that the curve of fig. 5 is the true behavior of the switch. The mechanism responsible for switch closure should be similar to that occurring in the breakdown of an over-voltaged low pressure gap. The theory of current growth between parallel plates has been discussed in detail by Davidson6 ,7,8) and in the monograph of Llewellyn-Jones9 ). The treatment is quite complete and probably can be applied to our vacuum gap switches over the entire range of pressures. A detailed comparison with this theory is difficult because of the (unknown) effect of electrode contamination on y, the second Townsend coefficient. By a more elementary treatment that should apply under low pressure conditions p «0.1 microns, the time T required for the switch to "close" is given by T
+
= t+
Mt+ In (~) . I+M M-l
M = y(e
where d: electrode spacing, IX: first Towsend coefficient, y: second Townsend coefficient, t+: anode to cathode ion transit time. If the electrodes are of steel and Fe+ ions are responsible for (secondary) electron liberation at the cathode then for a three inch electrode gap, t+ =
vs =
2.3
X
Vs!
10-6 sec,
gap voltage in kilovolts.
v<>lge across
lCh La
0.01
0.02
'1 ~ ~ "--
---
0.04 0.05
01
0.2 0.3
Pressure in switch in microns Hg
2
5. Conclusions The data in fig. 5 points out the desirability of operating the switch at a pressure higher than 0.1 microns. An operating pressure of 0.3 microns would give a switch with a low value of delay time, and one which only depends to a small degree on voltage and pressure. The fluctuation (jitter) in Td at this pressure is 0.1 #sec. This type of switch was test fired over a 1000 times at 30 kV passing a current of 30 000 amperes. The inductance and resistance of the switch is approximately 20 millimicrohenries and 0.03 ohms respectively. No special voltage conditioning is required. More development work is necessary as the switch is far from optimum, for example decrease of gap spacing will decrease the closing delay time. At this time the switch satisfies the requirements of the Megatron experiment but further development work is being conducted. Acknowledgements The development of this switch was initiated by several ideas and proposals of Professors David Finkelstein and Winston H. Bostick. Their assistance is gratefully appreciated. The authors also wish to express their thanks to I. Mansfield and L. Ferrari for their help in the construction and study of the switch.
8
20kVx
,/
-~
Clearly the ion transit time determines the time scale. At 20 kV and 0.02 microns pressure t+ = 0.5 microsecond. This requires M = 1 + 2 e-8 if the above expression for T is used. It is our belief that at all pressures switch closure is triggered by fast electrons from the "plasma gun". Metallic plasma ejected by the gun probably plays a minor role. At low pressures an avalanche is initiated by positive ions liberated from the anode. At high pressures both the anode and the gas in the gap contribute positiveions. Ion bombardment ofthe cathode releases secondary electrons and breakdown proceeds.
3.3
Fig. 5. Switch closing delay time versus pressure for two constant bank voltages with gun side of switch negative.
6) P. M. Davidson, Brit. J. App. Phys. 4 (1953) 170. 7) P. M. Davidson, Phys. Rev. 99 (1955) 1072. 8) P. M. Davidson, Phys. Rev. 103 (1956) 1897. 9) F. Llewellyn-Jones, Ionization and Breakdown in Gases (Methuen, London, 1957).
(1960) 236-238; NORTH-HOLLAND PUBLISHING CO.
A KILOVOLT, KILOAMPERE LOW PRESSURE SWITCHt GEORGE J. BRUCKER
United States Signal Corps Laboratories, Fort JVIonmouth, New Jersey and KENNETH C. ROGERS
Stevens Institute of Technology, Hoboken, New Jersey Received 9 April 1960
A low pressure switch is described that is capable of passing kiloamperes at voltages up to 30 kV. The effect of pressure and gap voltage on the switch closing time is discussed.
1. Introduction
The need for switching large currents at high voltages in thermonuclear research has lead to the development of the vacuum-gap switch1 •2 ). The Megatron accelerator work at Stevens Institute requires a similar device for switching the capacitor bank current to the field coil of the Megatron. It is the purpose of this paper to present the design of a practical switch, and the experimentally determined closing time for different gap voltages and pressures. Fig. 2. View of Stevens capacitor bank showing switches mounted on transmission lines and connected to pumping manifold.
2. Description of Switch
7"
•
~ .l F,;Th'ok Stainless Steel Plate INirr(Trigger)
:
--
•
2i ~
Epoxy Insulator ./""
Section A-A
Fig. 1. Details of switch.
t
Work performed under the auspices of the United States Army Signal Corps Laboratories, Fort Monmouth, New Jersey and the U.S. Atomic Energy Commission. tt Twenty-two part of sand, ten parts of Araldite 6050, three parts of Hardener 901 by weight.
The design of the Stevens capacitor bank3 ) has dictated the rectangular geometry of this switch. Fig. I is a drawing of the switch which consists of a rectangular box cast from a mixture of epoxy and sand tt with stainless steel electrodes and a lucite baffle plate. The switch is closed by a plasma burst from a pair of coaxial plasma 1) D. C. Hagerman and A. H. Williams, Rev. Sci. Instr. 30 (1959) 182. 2) W. R. Baker, Rev. Sci. Instr. 30 (1959) 700. 3) K. C. Rogers et al., Megatron Accelerator Progress Report, Signal Corps Contract DA36-039sc-78097 (June 3, 1958).
236
A KILOVOLT, KILOAMPERE LOW PRESSURE SWITCH
guns4 ). A high voltage pulse (2 kV to 10 kV) supplied by a hydrogen thyratron (5c22) and a 0.3 ,uF capacitor fires the guns. A view of the switches mounted on the transmission lines of the capacitor bank is shown in fig. 2. 3. Experimental Procedure The closing delay time Td and the "jitter" in T d were measured for ranges of voltage across the switch Vs, and for vacua from 0.02-3.3 microns Hg. The delay time T d, is defined to be the length of time from the inception of current flow to the plasma gun to the start of current through the switch. Both rates of current change (dlfdt) were picked up by means of Rogovsky coils, and the integrated signals were presented on a Tektronix 551 dual beam scope. The capacitor bank was prevented from ringing by a resistive load. All pressure readings were made by the same P. 1. G. located close to the switch. The data are plotted in figures 3, 4 and 5. Each point is the mean of not less than five separate measurements. The polarity of the applied voltage was varied so that the gun side of the switch was either positive or negative. 4. Discussion of Results The delay time versus voltage applied across the switch at a constant low pressure (0.02 microns) is shown in fig. 3. At this pressure it is apparent that the delay time, T d , varies inver-
1n
side poLive X Gun side negative 0
I
Pressure 0.02 mlcrons Hg
:::::; i:::::-- <--2--,
r----------- r-------- ~ r------
1
4 8 12 16 Voltage across sW'ltch in kilovolts
20
24
Fig. 3. Switch closing delay time versus voltage across switch for a constant pressure of 0.02 microns Hg and both positive and negative bank voltages.
237
Ln
side p10sitive X
Gun side negative 0 Pressure 3.3 microns Hg I
I I
I i
2 1
4
12 16 8 Voltage across switch in kilovolts
20
24
Fig. 4. Switch closing delay time versus voltage across switch for a constant pressure of 3.3 microns Hg and both positive and negative bank voltages.
sely with the applied voltage V s. If electrons are responsible for closing the switch then reversing the polarity of V s so that the gun side of the switch is positive should show a longer time delay which is independent of voltage. This is not the case at low pressure. The evidence in support of the theory that electrons are responsible for switch closure becomes stronger for high pressure operation. Fig. 4 displays this independence of delay time on switch voltage when the gun side of the switch is positive; in this case T d is comparable to plasma transit time. Reversal of polarity at this pressure (3.3 microns) brings about an increase of Td with voltage. J. A. Mather5 ), who observed this behavior in' a vacuum-gap switch at Los Alamos, has suggested that the increasing delay time is due to the dependence of the electron-atomic ionization cross section a(v) on the applied voltage. The picture does not seem to be this simple as is indicated by fig. 5. The shape of these curves is not explicable on the basis of the ionization cross section voltage dependence, although the precision of this experiment is not sufficient to say with 4) K. C. Rogers et al., Megatron Accelerator Progress Report, Signal Corps Contract DA36-039sc-78097 (April I, 1959). 5) J. A. Mather and A. H. Williams, Rev. Sci. Instr. (to be published).
238
GEORGE
J.
BRUCKER AND KENNETH C. ROGERS
certainty that the curve of fig. 5 is the true behavior of the switch. The mechanism responsible for switch closure should be similar to that occurring in the breakdown of an over-voltaged low pressure gap. The theory of current growth between parallel plates has been discussed in detail by Davidson6 ,7,8) and in the monograph of Llewellyn-Jones9 ). The treatment is quite complete and probably can be applied to our vacuum gap switches over the entire range of pressures. A detailed comparison with this theory is difficult because of the (unknown) effect of electrode contamination on y, the second Townsend coefficient. By a more elementary treatment that should apply under low pressure conditions p «0.1 microns, the time T required for the switch to "close" is given by T
+
= t+
Mt+ In (~) . I+M M-l
M = y(e
where d: electrode spacing, IX: first Towsend coefficient, y: second Townsend coefficient, t+: anode to cathode ion transit time. If the electrodes are of steel and Fe+ ions are responsible for (secondary) electron liberation at the cathode then for a three inch electrode gap, t+ =
vs =
2.3
X
Vs!
10-6 sec,
gap voltage in kilovolts.
v<>lge across
lCh La
0.01
0.02
'1 ~ ~ "--
---
0.04 0.05
01
0.2 0.3
Pressure in switch in microns Hg
2
5. Conclusions The data in fig. 5 points out the desirability of operating the switch at a pressure higher than 0.1 microns. An operating pressure of 0.3 microns would give a switch with a low value of delay time, and one which only depends to a small degree on voltage and pressure. The fluctuation (jitter) in Td at this pressure is 0.1 #sec. This type of switch was test fired over a 1000 times at 30 kV passing a current of 30 000 amperes. The inductance and resistance of the switch is approximately 20 millimicrohenries and 0.03 ohms respectively. No special voltage conditioning is required. More development work is necessary as the switch is far from optimum, for example decrease of gap spacing will decrease the closing delay time. At this time the switch satisfies the requirements of the Megatron experiment but further development work is being conducted. Acknowledgements The development of this switch was initiated by several ideas and proposals of Professors David Finkelstein and Winston H. Bostick. Their assistance is gratefully appreciated. The authors also wish to express their thanks to I. Mansfield and L. Ferrari for their help in the construction and study of the switch.
8
20kVx
,/
-~
Clearly the ion transit time determines the time scale. At 20 kV and 0.02 microns pressure t+ = 0.5 microsecond. This requires M = 1 + 2 e-8 if the above expression for T is used. It is our belief that at all pressures switch closure is triggered by fast electrons from the "plasma gun". Metallic plasma ejected by the gun probably plays a minor role. At low pressures an avalanche is initiated by positive ions liberated from the anode. At high pressures both the anode and the gas in the gap contribute positiveions. Ion bombardment ofthe cathode releases secondary electrons and breakdown proceeds.
3.3
Fig. 5. Switch closing delay time versus pressure for two constant bank voltages with gun side of switch negative.
6) P. M. Davidson, Brit. J. App. Phys. 4 (1953) 170. 7) P. M. Davidson, Phys. Rev. 99 (1955) 1072. 8) P. M. Davidson, Phys. Rev. 103 (1956) 1897. 9) F. Llewellyn-Jones, Ionization and Breakdown in Gases (Methuen, London, 1957).