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Submitted on 1 Jan 1979
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STUDIES OF THE BASIC PROCESSES RESPONSIBLE FOR LASER-TRIGGERED
BREAKDOWN IN GASES
P. Williams, R. Crumley, M. Gundersen
To cite this version:
P. Williams, R. Crumley, M. Gundersen. STUDIES OF THE BASIC PROCESSES RESPONSIBLE
FOR LASER-TRIGGERED BREAKDOWN IN GASES. Journal de Physique Colloques, 1979, 40
(C7), pp.C7-305-C7-306. �10.1051/jphyscol:19797150�. �jpa-00219125�
JOUMAL
D E PHYSIQUE ColZoque C7, suppl6ment
aun07, Tome
40,JuiZZet
1979,page
~ 7 , 305SNDIES OF THE BASIC PROCESSES RESPONSBLE FOR LASW-TRIGGERED'BREAKWWN IN GAS%
P.F. Williams,
R.J.
Crumley and M.A. Gundenen.Department of EZectricaZ Engheering, Texas Tech. University Lubbock, T e r n , U.S.A.
A number of technologically important appli- cations require high voltage, high current switches which operate with low delay and jitter in the closure time. Laser-triggered spark gaps offer significant advantages for use in these applica- tions in that the switching of 50 kV with a delay in the range of 3.0 ns. and jitter in the 0.1 ns.
range has been demonstrated.' In this paper some experimental studies intended to elucidate the basic physical processes important in such devices are described.
In a typical laser-triggered discharge exper- iment a spark gap is placed in a vacuum-tight en- closure which may be filled with gas to any de- sired pressure up to several atmospheres. A static voltage is applied to the gap, and the gap is in- duced to break down by the focussed output beam of a pulsed laser. Only modest laser energy is re- quired (-1 mJ.) and breakdown may be readily in- duced even for voltages well below the static breakdown voltage for the gap (V20.6 VSB). De- pending on the gap voltage, fill gas pressure and chemical composition, gap geometry and dimensions, and laser energy and focussing, the delay between the triggering laser and the closure of the gap may range from 1 ns. to several ,Us.
In order to investigate the basic physical processes occurring in laser-induced breakdown, the experimental set-up shown in Fig. 1 was used. A spark gap consisting of two aluminum electrodes machined with a Rowgowski profile was enclosed in a stainless steel vacuum/pressure cell. Triggering was accomplished with an N laser which delivered
2
10 ns., 5 mJ. pulses of 3361 radiation. The beam was focussed through a window and then a small hole in the upper gap electrode onto the lower electrode where it produced a small plasma "fireball" when tightly focussed. For all experiments we report here the gap separation was 1 cm.
A charged coaxial cable system was used to apply voltage to the gap. With careful matching of the 50 fi load resistor a clean current pulse with no reflection-induced after-pulses was obtained.
Gap current was determined by measuring the voltage across the load resistor. Optical access to the
O ~ U ~ ~ D . ~ O W
L7%+,-1
Fig. 1 Schematic drawing of experimental setup.
discharge for spectral analysis of the emission was provided by a second window transverse to the gap axis. An 0.5 m. spectrograph coupled with a computer-controlled optical multichannel analyzer was used to obtain spectra of the discharge.
Gating of the SIT detector of the analyzer provided temporal resolution down to 50 ns.
Although preconditioning of the arc channel by the triggering laser probably plays a role in the breakdown process, the small plaqma fireball pro- duced by the focussed laser striking the lower electrode is primarily responsible for the gap closure. We have conducted a number of experiments designed to characterize the plasma in the fire- ball. In one set of experiments the vacuum cell was evacuated (P<l/CO and a low voltage applied to the gap. Under these conditions the triggering laser did not cause the gap to breakdown, but a current polse due to the plasma of the fireball was observed. Charge multiplication from ioniza- tion and other, secondary, processes was unimpor- tant so that the integrated current of the pulse was indicative of the free charge in the fireball.
The oscillograms in Figs. 2a and 2b show the current pulses observed under these conditions with +23 and -23 volts respectively applied to the gap.
The diagrams to the side of each oscillogram clar- ify the polarity used in each. For both polarities the integrated current is essentially the same,
-2 X C O U ~ .
,
supporting the contention that charge multiplication was not important in these experiments. This amount of charge is significant and strong space-charge fields, such as appearArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797150
y ; - ";;
-I
5 E'
0 f
250 ns/Div time
Fig. 2 Current pulses observed in an evacuated gap with
+
23 volts. Voltage polarity is indicated at the left.n e c e s s a r y t o e x p l a i n t h e a b i l i t y of t h e l a s e r t o cause gap breakdown f o r v o l t a g e s s i g n i f i c a n t l y below t h e s t a t i c breakdown v o l t a g e , can be produced i n t h e plasma s h e a t h s . S i m i l a r experiments have been c a r r i e d o u t i n hydrogen a t p r e s s u r e s up t o a few Torr. I n t h e s e experiments f o r v o l t a g e p o l a r - i i y such t h a t i l s g a t i v e cha~yges t r a v e r s e t h e gap, c l e a r evidence f o r i o n i z a t i o n - i n d u c e d c u r r e n t i s seen even f o r gap v o l t a g e s a s low a s 10 v o l t s .
We a r e a l s o c a r r y i n g o u t a program t o d e t e r - mine t h e e l e c t r o n d e n s i t y d u r i n g t h e a r c phase of l a s e r - t r i g g e r e d d i s c h a r g e s i n hydrogen u s i n g S t a r k broadening measurements. With t h i s t e c h n i q u e we have o b t a i n e d temporally-resolved e l e c t r o n d e n s i t y information d u r i n g t h e a r c , and f o r times e x t e n d i n g t o approximately 1 ,Us. i n t o t h e a f t e r - g l o w .
E l e c t r o n d e n s i t y d a t a o b t a i n e d i n t h i s f a s h i o n a r e shown i n Fig. 3 , a l o n g w i t h t h e gap c u r r e n t p u l s e , f o r 300 T o r r of hydrogen and gap v o l t a g e approximately 80% of t h e s t a t i c breakdown v a l u e . A s might be expected, t h e s e r e s u l t s a r e s i m i l a r t o t h o s e observed by o t h e r workers f o r o v e r - v o l t e d gaps i n hydrogen.2 To o b t a i n :he s p e c t r a from which t h e s e r e s u l t s were d e r i v e d , t h e image of t h e a r c was c e n t e r e d on t h e e n t r a n c e s l i t of t h e s p e c t r o g r a p h , and t h e s e d e n s i t i e s r e p r e s e n t , t h e r e - f o r e , a weighted average over t h e diameter and l e n g t h of t h e a r c channel. Abel i n v e r s i o n t e c h - n i q u e s have been used t o unfold t h e r a d i a l v a r i a - t i o n of t h e e l e c t r o n d e n s i t i e s , and p r e l i m i n a r y r e s u l t s i n d i c a t e t h a t t h e v a r i a t i o n s a c r o s s t h e diameter of t h e luminous column a r e not l a r g e , being of t h e o r d e r of 25%. V a r i a t i o n s a l o n g t h e
Current and Electron Density
Fig. 3 Electron density (points) and gap current (solid line). 300 torr Hz.
l e n g t h of t h e column may be determined d i r e c t l y , and i n our experiments t h e e l e c t r o n d e n s i t y d i s - played e s s e n t i a l l y t h e same time behavior a t a l l p o i n t s monitored along t h i s dimension.
1. A.H. Guenther and J . R . B e t t i s , Proc. IEEE
59,
689 (1971); and r e f e r e n c e s contained t h e r e i n .
2. J . Meyer, B r i t . J . Appl. Phys.
18,
801 (1967).Work supported by AFOSR, and Research Corporation.
