CO2-LASER PRODUCED CHANNELS FOR
GUIDING LONG SPARKS IN AIR
D. Koopman, J. Greig, R. Pechacek, A. Ali, I. Vitkovitsky, R. Fernsler
To cite this version:
D. Koopman, J. Greig, R. Pechacek, A. Ali, I. Vitkovitsky, et al.. CO2-LASER PRODUCED
CHANNELS FOR GUIDING LONG SPARKS IN AIR. Journal de Physique Colloques, 1979,
40 (C7), pp.C7-419-C7-420. <10.1051/jphyscol:19797204>. <jpa-00219184>
HAL Id: jpa-00219184
https://hal.archives-ouvertes.fr/jpa-00219184
Submitted on 1 Jan 1979
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CoZZoque C7, suppZ6ment au n07, Tome 40, JuiZZet 2979, page C7- 419
JOURNAL DE PHYSIQUE
COZ-LASER PRODUCED CHANNELS FOR GUIDING LONG SPARKS IN AIR
D. ~ o o ~ r n a n ' J.
, Greig, R. Pechacek, A.Ali and I. Vitkovitsky, R. ~ernsler".
%Naval Research Laboratory, Washington, D. C. 20375, U.S. A., *'JACOR, Alexandria, Va., U.S. A.
I n s t i t u t e for Physical Science and TeehnoZogy, U n i v e r s i t y of Maryland, CoZZege Park, Mary Zand 20742
U. S. A.
The a b i l i t y of l a s e r r a d i a t i o n t o i n f l u e n c e
t o be formed by a complex s e r i e s of events, i n i -
t h e paths of e l e c t r i c a l discharges i n gases h a s
t i a t e d by l a s e r i o n i z a t i o n of a e r o s o l p a r t i c l e s .
been previously
Spherical b l a s t waves from t h e r e s u l t a n t plasma
E x c i t a t i o n and ion-
i z a t i o n were believed t o be a c t i v e when u l t r a violet3, v i s i b l e
4
, or
near-infrared
radiation
"beads" combine t o form a n e a r l y c y l i n d r i c a l
1
shock surrounding a t u r b u l e n t , low d e n s i t y , and
from Q-switched l a s e r s was focused t o d e f i n e a
p a r t i a l l y ionized core, which expands t o
p r e f e r r e d discharge path; long d u r a t i o n 10.6 m i -
diameter a t focus.
cron r a d i a t i o n i n an absorbing atmosphere was
photographs of t h i s process.
noted t o achieve guiding by r a r e f a c t i o n
5
.
In our
s t u d i e s 6 , pulsed 10.6 micron r a d i a t i o n from a TEA
-
4 cm
Figure 1 p r e s e n t s s c h l i e r e n
Holography confirms
t h a t t h e shock i s a compression f r o n t , and shows
t h e core i s a region of below-atmospheric d e n s i t y .
l a s e r has been used t o produce channels i n ambient
a i r which a r e capable of guiding d i s c h a r g e s nearl y orthogonal t o t h e i n i t i a l E-field,
over dis-
tances of 1-2 meters, a t average f i e l d s t r e n g t h s
t = 100 nsec
(a
a s low a s 1 KV/cm, and with average propagation
-
( e ) 1=19psec
-
v e l o c i t i e s a s h i g h a s 10' cm/sec.
The mechanism
by which t h e s e channels a r e produced, and t h e
p r o p e r t i e s of t h e channels which i n f l u e n c e elec-
-
( b 1 t = 400 nsec
-
t r i c a l d i s c h a r e e Drocesses. have been t h e s u b i e c t
-
~
- -
A
(f
t = 44psec
of continued i n v e s t i g a t i o n s .
A 1 K J , 10.6 micron p u l s e , w i t h a 100 nsec
i n i t i a l s p i k e containing about 30% of t h e energy,
followed by a 1.5 usec t a i l , i s focused by a 3
meter f.E.
l e n s from i t s o r i g i n a l 20 cm diameter
t o a f o c a l diameter of 2 cm.
About 80% of t h e
i n c i d e n t energy i s absorbed.
The channel appears
a s a tapered c l u s t e r of o p t i c a l breakdown "beads",
extending from
-
1.2 meters from t h e l e n s t o
meters beyond t h e f o c a l p o i n t .
-
(d)
0.1
Detailed time-
resolved d i a g n o s t i c s t u d i e s have shown t h e channel
F i g . 1.
t=Zpsec
-
1 cm
S c h l i e r e n photographs of c h a n n e l forma-
t i o n v s . t i m e a f t e r peak of l a s e r p u l s e .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797204
Optimum guiding of discharges occurs between 5
and
300 usec after the pulse.
Spectra initial1
show lines of 0 I1 and N 11, indicating T
tion to exist as free electrons.
few msec, continued mixing drops T below 500°K,
and electrons will recombine.
I
:
In the next
It is the free
30,00OoK. Later a continu~mspectrum, believed
electrons which allow electrical discharges to
due to electron-neutral bremsstrahlung from the
occur in the "return stroke" mode, explaining
core, decays with a time constant of
-
Microwave absorption measurements at
;\ =
indicate n
usec.
e
-
> 3xl0'~/cm~
10 ysec.
4 mm
in the core until t 3 5C
Combining the energy absorption, shock
dynamics, and core expansion, we compute an
average temperature of
-
After 300
usec, ambient air turbulently mixes into the core,
cooling it to
-
500°K by 1 msec.
Using the Saha
equilibrium relationship for the reaction e
02, we
An
example of guiding is shown in Figure 2, where a
near -90" turn is achieved by using intersecting
laser beams to define the desired spark path.
This ability to direct a discharge not only to,
1500°K and near-ambient
pressure within the expanded core.
guiding and the fast propagation velocity.
+ 02 +
but also away from, a designated isolated object
is essential for such applications as producing
the conductivity and magnetic field configuration
needed to propagate relativistic e-beams to pellets
in inertial fusion experiments.
find that these temperatures inhibit nega-
tive ion formation, allowing the residual ioniza-
1.
D. Koopman and K. Saum, J. Appl. Phys.
15
1149 (1973).
2.
G. Aleksandrov, et al., Sov. Phys. Tech. Phys.
22, 1233 (1977).
-
8,
3.
A. Akmanov, et al., JETP Lett.
258 (1968).
4.
W. Pendleton and A. Guenther, R. S. I. 36,
1546 (1965).
5.
LASER
BEAM
1
BEAM 2
K.
Saum and
2077 (1972).
6.
J. Greig, et al., Phys. Rev. Lett.
(1978).
Fig. 2.
A guided discharge.
Top: experimental
set-up; middle: laser-channels; bottom: electrical
spark.
D. Koopman, Phys. Fluids 15,
1,174