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Submitted on 1 Jan 1979
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LASER/FIBER OPTIC BREAKDOWN OF A PULSE CHARGED 90% Ar-10% N2 GAS SWITCH
L. Hatfield, H. Harjes, M. Kristiansen, A. Guenther, K. Schönback
To cite this version:
L. Hatfield, H. Harjes, M. Kristiansen, A. Guenther, K. Schönback. LASER/FIBER OPTIC BREAK- DOWN OF A PULSE CHARGED 90% Ar-10% N2 GAS SWITCH. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-483-C7-484. �10.1051/jphyscol:19797234�. �jpa-00219217�
JOURNAL DE PHYSIQUE CoZZoque C7, s u p p l 6 m e n t au n 0 7 , Tome 4 0 , J u i Z Z e t 1979, page C7- 483
LASERfFIBER OPTIC BREAKDOWN OF A P U S € CHARGED 90% AT-1096 N2 GAS SWITCH
L.L. Hatfield, H.C. ~ a r j e s '
,
M. ~ristiansen', A.H. ~uenther'and K.H. chi in back'".*Department P h y s i c s ,
*,Department E l e c . Eng. T e x a s T e c h n i c a Z U n i v e r s i t y , Lubbock, T e x a s U.S.A. 79409.
T e c h n i s c h e HochschuZe i?urmstadt, F. 8. G.
Abstract nously, o r t o i n i t i a t e several parallel arc chan- nels simultaneously, appears t o be t r i v i a l exten- Precise l a s e r i n i t i a t i o n of the breakdown in a
sions of the technique.
pulse charged gas switch i s described. A novel feature i s the transport of the 40 ns triggering l a s e r pulse through a 1.5 m long 1 mm diameter op- t i c a l f i b e r . Optimization of gas breakdo-wn charac- t e r i s t i c s was necessary a s a r e s u l t of the limited power transported through the f i b e r . Low, nanose- cond delay and j i t t e r were achieved a t reduced f i e l d s approaching 189 V/cm-Torr i n the overvolted gas gap through t h e use of 90% argon/l0% nitrogen gas mixtures. The influence of l a s e r power, gas mixture, focal point location, pressure, time of l a s e r i n s e r t i o n e t c . on switch performance i s presented.
Laser triggered switching of high voltage spark gaps was f i r s t demonstrated more than a de- cade ago by Guenther and Griffin and has since been the subject of many investigations l y 2 . The prin- cipal advantages of t h i s type of triggering a r e e l e c t r i c a l i s o l a t i o n of t h e t r i g g e r from high volt- ages, controlable switching delay, and subnano- second j i t t e r . On the other hand, optical access to the gap must be available, usually through occa- sionally complex system design, and care taken t o avoid misalignment o r environmental degradation of optical elements.
To reduce greatly the above complexity, a novel l a s e r triggered spark gap has been construct- ed featuring the use of an optical f i b e r t o trans- port the l a s e r l i g h t along a circuitous route t o the gap. Problems of optical alignment and the in- fluence of t h e environment a r e a l l but eliminated.
In-addition, the use of multiple f i b e r s illuminated by t h e same l a s e r to t r i g g e r several gaps synchro-
* Supported in p a r t by a contract from the Department of Energy, administered by Lawrence Livermore Laboratories.
The spark gap employed i s a modified e l e c t r i - c a l l y triggered gap normally used to switch a water d i e l e c t r i c Blumlein l i n e producing a 40 ns output pulse. The Blumlein i s pulse charged by a Marx generator with a 250 ns erection time. The maximum voltage and energy a r e 250 k V and about
1 kJ. The l a s e r beam i s introduced coaxially through a 6 mm aperture in the grounded cathode and focused 1 mm inside the surface of the charged machinable tungsten anode. A schematic of the optical system i s shown in Fig. 1 . The lenses a r e U-V grade quartz and the 1.5 m long f i b e r i s a 1 mm dia. s i n g l e quartz element.
Measurements with an integrating energy meter indicate a 25% throughput efficiency in bringing the l a s e r l i g h t i n t o the gap. The maximum allow- able l a s e r power i s determined by the permissable power density which does not cause damage t o the entrance surface of the quartz f i b e r . This ap-
9 2
pears t o be > 5 x 10 W/cm
.
The conditions f o r switching gas f i l l e d spark gaps with short delay and subnanosecond j i t t e r a r e reviewed i n references7 and 2. The gas used should have a 1 arge secondary electron coeff i -
c i e n t , the switch should be triggered a t a large f r a c t i o n of the s e l f-breakdown voltage, the 1 a s e r pulse should be longer than the switching delay, and the l a s e r power should usually be greater than
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6 MW f o r DC charged gaps.Since our study indicates an efficiency of
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25% i n the overall optical system from l a s e r out- put t o entrance i n t o the spark gap the require- ment of 6 MW of l a s e r power i n the gap implies a rather modest imput l a s e r power of 24 MW. The i n t e n s i t y a t the input surface of the optical
9 2
f i b e r i s then about 3 x 10 w/cm which i s below the estimated threshhold f o r surface damage. No surface damage has been observed a t the f i b e r a t
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797234
t h i s i n t e n s i t y , although higher i n t e n s i t i e s have produced cracks and internal damage. The e f f i c i
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ency of the optical system i s limited by i n s u f f i - c i e n t c o l l e c t i o n , r e f l e c t i o n a t the surfaces o f lenses, attenuation In t h e f i b e r due t o absorption and s c a t t e r i n g , and t h e spreading introduced as the bean passes through the f i b e r . The beam emerges from the 1 mm quartz f i b e r with a 60 degree f u l l
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angle divergence a t t h e e-' points. To render t h i s beam p a r a l l e l would require an f / l lens but a s a practical compromise an f / 3 l e n s i s used in t h i s system, thus lowering the efficiency. Another com- promise i s necessary when focusing the beam onto the anode through a hole i n t h e cathode. As t h e entrance a ~ e r t u r e i n the cathode i s made l a r g e r t o accomodate a f a s t e r optical system t h e e l e c t r i c f i e l d i n the gap becomes increasingly ?on-uniform.
This system has a 6 mm aperture in t h e 10 cm dia- meter cathode and a f / 3 lens is used i n t h i s case, a s well. In an e f f o r t t o improve t h e over-all throughput efficiency, t h e use of a f i b e r which in- troduces ? e s s divergence i s being explored together with the use of a n t i - r e f l e c t i o n coated o p t i c s . Re- s u l t s using a 0.4 mm dia. f i b e r which produces an output f u l l angle of < 29 degrees will be presented.
The use of the optical f i b e r t o transport the l a s e r l i g h t makes simultaneous triggering of many spark gaps more a t t r a c t i v e because i t eliminates most of the optical alignment problems associated with transport of the l a s e r beam and greatly re- duces t h e cost. Environmental problems with the o p t i c s and personnel s a f e t y problems a r e a l s o a l l e v i a t e d . However, i f many f i b e r s are t o be illuminated w i t h t h e same l a s e r t h e power renrrir- ed could become q u i t e large. Therefore, the optimization of gas breakdown conditions t o min- imize the required l a s e r power i s of great import.
Typical switch conditions employed during t h i s study were as follows: 1 cm gap spacing; gas pres- sure
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686 Torr; gas mixture, 90% Ar-10% !I2; gap nulse charaed t o voltages < 120 kV; l a s e r pulse length of 30-40 ns; and a l a s e r power i n the gap of 0.5-6 Mk4. Gas breakdown i s produced in t h e overvoltaaed gap while i t i s being charged by the Marx bank by f i r i n g the l a s e r a t an appropriate time a f t e r i n i t i a t i n g erection of the R r x . With a l a s e r p w e r of only 2 Mbl i n t h e gap we have ob- served nanosecond j i t t e r and switching delay a s short a s 10 ns. (The reason f o r t h e good perfor- mance a t t h i s low l a s e r power i s t h a t the gap i spulse charged and overvolted compared t o the DC breakdown ~ o l t a g e . ~ ) Results a s a function of l a s e r power will be presented. The e f f e c t s of changing the gas pressure and the gas mixture t o 50% Ar
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50% N p will be discussed. The s e n s i t i v i t y of the triggering of the gap t o the position of the focal point of the l a s e r will a l s o be reported.
Time integrated photographs of t h e gas break- down show both s i n g l e channel and multichannel modes depending on the condition of operation.
Streak camera photographs of the breakdown will a l s o be shown.
1 . A. H. Guenther and J . R. B e t t i s , J . Phys. D.
11, 1577 (1978).
2. Electrical Breakdown of Gases, J . M. Meeks and J . D. Craggs, Eds., John Idiley & Sons (1978) Ch. 7 and 9.
3 - L. P. Bradley and T. J. Davies, IEEE JQE r)E-7, 464 (1971) and L. P. Bradley, J. Appl.
Phys
9,
886 (1972).DnrlTlvF I FNS NEOATlVE
Fig. 1. Schematic Diagram of LaserIFiber Optic Triggered High Voltage Switch Assembly.
