INVESTIGATION

OF

LASER

RADIATION

DENSE

PLASMA

V. V.

S. P.

Burakov, Ivanov,

THE

TRANSMISSION

THROUGH

OF H I G H - P O W E R

AN O P T I C A L L Y

P . A. N a u m e n k o v , a n d G. A. K o I o s o v s k i i

UDC 621.378:533.9

High-power l a s e r beams are widely used to solve problems in p l a s m a diagnostics: to investigate Thomson scattering, to obtain a hologram image of a plasma, and to determine the absorbing p r o p e r t i e s of a plasma. In alI these c a s e s the high density of the radiation pulse can lead to perturbation p r o c e s s e s in the plasma being investigated. These p r o c e s s e s were f i r s t observed and explained in [1, 2] for a xenon p l a s m a at a t e m p e r a t u r e T e ~ 1 eV. In the present investigation we made an experimental study of the t r a n s m i s s i o n of the radiation of a ruby l a s e r (density of 106-5 9108 W/cm 2 and duration 3 . 1 0 -8 sec) through a q u a s i - s t a t i o n a r y p l a s m a at a t e m p e r a t u r e of 3-4 eV and with a comparatively high absorption coefficient (4-7 cm-1); the mechanism of the observed illumination of the plasma is discussed. The investigations were made on an a r r a n g e m e n t which included a ruby l a s e r with a passive shutter (galliumphthalocyanininchlorobenzene), a p l a s m a obtained in an 1~V-39 s o u r c e [3], and a two-channel r e cording system. The rad[ation of the plasma was made monochromatic using a diffraction spectral apparatus or interference filters with a passband of 10-20 A in the region of 694.3 nm. The radiation r e c e i v e r s were FI~U-36 photomultipliers; the signals were r e c o r d e d on an S I - l l oscilloscope using a delay line. The t r a n s m i s s i o n of the p l a s m a was found by c o m p a r i n g the radiation which passed through the p l a s ma and through a c o m p a r i s o n channel. The ratio of the fluxes [n both the channels was m e a s u r e d f r o m the maximum of their o s c i l l o g r a m s . The p l a s m a radiation was cut off using a permanent stack of neutral filt e r s placed in front of the photomuittplier in the m e a s u r e m e n t channel. To further reduce the l a s e r beam and to calibrate the measuring s y s t e m we used a set of calibrated neutral filters or a cuvette with a solution of "brilliant g r e e n " dye. Separate changeable stacks of these filters were placed in front o r behind the plasma. This enabled us to i n c r e a s e the a c c u r a c y of the m e a s u r e m e n t s , since the photomultipliers r e corded approximately the same l a s e r flux, although its value [n the plasma varied by m o r e than two o r d e r s . Figure 1 shows the t r a n s m i s s i o n of the p l a s m a to l a s e r radiation as a function of the l a s e r flux L Maximum illumination of the p l a s m a is observed for a flux of approximately 2 9107 W/cm 2. T h e r e [s no disturbance of the p l a s m a by the probing l a s e r flux for flux densities of the o r d e r of 10 ~ W/cm 2. The linear section (shown dotted in Fig. 1) was found by extrapolation of the value of the t r a n s m i s s i o n m e a s u r e d with a ruby l a s e r under f r e e - r u n n i n g conditions (the point on the extreme left of the graph) to the n e a r e s t value of the t r a n s m i s s i o n measured with a single-pulse l a s e r . F o r low flux densities of the illuminating l a s e r beam under Q-switched conditions it was difficult to m e a s u r e the t r a n s m i s s i o n for fixed p a r a m e t e r s of the r e c o r d i n g apparatus because of the high optical density of the plasma. The qualitative behavior of the t r a n s m i s s i o n c o r r e s p o n d s to the illumination region of a xenon p l a s ma observed in [2]. However, the specific features of the p l a s m a which we used (high t e m p e r a t u r e s of about 4 eV, particle densities of about 2.1019 c m -3, and high degree of ionization) give certain unique features to the mechanism of p l a s m a illumination. The composition of the p l a s m a is determined by the material of the walls of the capillary made of textolite which contains components of H, C, and O (the d i a m e t e r Translated f r o m Zhurnal Prikladnoi Spektroskopii, Vol. 16, No. 2, pp. 239-242, F e b r u a r y , 1972. Original article submitted January 21, 1971. 9 1974 Consultants Bui'eau, a division of Plenum Publishing Corporation, 227 [['est 17th Street, New York, :~'. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. ,-i copy of this article is available from the publisher for $15.00.

176

I .lO3 I5 /0

t

zo*

!

zO ~

)

/08 Z, w l c m 2

Fig. 1

3.2

qO T./O-e

Fig. 2

Fig. 1. T r a n s m i s s i o n of the l a s e r radiation through the p l a s m a as a function of the flux of the l a s e r illuminating radiation [. Fig. 2. Absorption coefficient of different components of the p l a s m a as a function of the t e m p e r a t u r e : components HI (2); C[ (3); OI (4); CII (5), overall absorption (1). of the capillary is 2.9 mm and the length is 10 mm). It is not only atoms but also ions of certain c o m p o nents of the m a t e r i a l of the walls of the c a p i l l a r y which contribute to the overall absorption coefficient of the plasma. Using the quantum-defect method [4, 5] we estimated the absorption coefficients of the p l a s m a for t e m p e r a t u r e of 25,000-42,000~ and over a wide range of variation of the total concentration of material in the discharge channel for the equilibrium electron density. Satisfactory a g r e e m e n t between the theoretical and experimentally m e a s u r e d values of the absorption coefficients of the p l a s m a (with the methods u s e d ) h a s been repeatedly obtained p r e v i o u s l y [6, 7]. Figure 2 shows the absorption coefficient as a function of the p l a s m a t e m p e r a t u r e for fixed c o n c e n trations of the atoms of the textolite material in the d i s c h a r g e (N = 1.4 9 1019 c m - 3 ) , c o r r e s p o n d i n g to the experimentally realized value. Because of the brief time for which the single-pulse l a s e r radiation acts, the concentration of textolite material in the d i s c h a r g e channel obviously r e m a i n s unchanged. Under these conditions the main absorption o c c u r s due to the components HI (2), CI (3), CH (5), and OI (4). The contribution dueto the components CIII and OII is e x t r e m e l y small. The overall value of k~ (cm -1) is shown by curve 1. The absorption of the l a s e r radiation in the p l a s m a should lead to an i n c r e a s e in the t e m p e r a t u r e (energy) of the electronS. When the l a s e r beam acts for a time which enables t h e r m o d y n a m i c equilibrium to be established, the i n c r e a s e in t e m p e r a t u r e should be accompanied by an i n c r e a s e in the absorption of the p l a s m a due to the component CII, despite some reduction in k k due to CI and OI. The illumination of the p l a s m a observed experimentally obviously indicates that the time during which the l a s e r radiation acts (the scale of the leading edge of the pulse is approximately 10 -8 sec) is small c o m p a r e d with the relaxation time of the individual p r o c e s s e s . The c h a r a c t e r i s t i c equalization time of the t e m p e r a t u r e s of the electrons, the e l e c t r o n s and ions, and of the establishment of ionization equilibrium and the Boltzmann distribution for the s e p a r a t e c o m p o nents can be estimated using the relations and data given in [8]. The time taken for equilibrium to be established in the electron gas is given by Tee = T3/2/3.8 N e in ~, where In ~. is the Coulomb logarithm, T is the t e m p e r a t u r e , and N e is the electron density. Under our exp e r i m e n t a l conditions (T ~ 40,000~ and Ne ~- 2.1019 cm -3) Tee ~ 10-13-10 -1~ sec. The c h a r a c t e r i s t i c time taken for the t e m p e r a t u r e s of the e l e c t r o n s and ions to reach the same level can be found from the expression 3.15.10SAT3 ~ Tei

--

_

_

N iz 2 In

where A is the atomic weight, N i is the density, z is the charge of the ions, and T is in eV.

177

Taking N i ~ 1019 c m -3, and A = 12 (carbon atoms), we have Tei ~ 10 -9 see. A calculation for the hydrogen nucleus for the same t e m p e r a t u r e gives ~-ei ~ 10-1~ see. The latter quantity will also mainly assign the scale of the time taken for the t e m p e r a t u r e to equalize between the electron and ion gases of the plasma. Hence, when the p l a s m a is being heated by l a s e r radiation the t e m p e r a t u r e of the ions and electrons change p r a c t i c a l l y in synchronism. However, in this case it may not be possible to establish an equiiibrium value of the atom and ion densities, which, according to [9] can lead to illumination of the plasma. It is not possible to use the relations obtained in [9] to explain the results of our investigations because the p l a s m a contains s e v e r a l components and a small portion of unionized atoms. Hence, the relaxation times of the mutual p r o c e s s e s were only considered qualitatively. The rate of ionization of atoms (ions) f r o m the unexcited state, ts c o m p a r a t i v e l y small, despite the c o m p a r a t i v e l y high initial t e m p e r a t u r e of the p l a s m a (T O = 3.45 eV). The time scale for ionization equilibrium to be established for the unexcited a t o m s i s of the o r d e r of 20-50 nsee, i . e . , c o m p a r a b l e with the duration of the l a s e r radiation. The s t e p - b y - s t e p ionization p r o c e s s through excited states takes less time. The time taken for equilibrium to be established between neutral atoms and ions is of the o r d e r of 10 - l ~ sec, while between single and double ions of carbon it is of the o r d e r of 10 -8 see. This means that because of the i n c r e a s e in the p l a s m a t e m p e r a t u r e due to the l a s e r radiation, the components HI, CI, and OI, on the whole, cause a drop in the absorption coefficient (Fig. 2) even when the electron density is close to the equilibrium value. F o r tons of CII we might also expect a reduction in the absorption coefficient due to depletion of the upper excited levels of the s y s t e m and the c o m p a r a t i v e l y slow rate at which they are populated by ions f r o m a lower state. This mechanism explains the increase in the t r a n s m i s s i o n shown in Fig. 1. tion gives rise to a c o m p a r a t i v e l y small increase in t e m p e r a t u r e .

Here the l a s e r r a d i a -

An estimate of the i n c r e a s e in t h e p l a s m a t e m p e r a t u r e due to the action of the radiation was made a s suming a II-shaped l a s e r pulse. It can be seen [10] that the main mechanism of energy loss in the discharge is emission. A change in the e m i s s i v e power of the p l a s m a then c o r r e s p o n d s to the value of the absorbed laser radiation. F o r a l a s e r flux [ ~ 2.107 W/cm 2 for a p l a s m a t e m p e r a t u r e T = 39,000~ the values of AT is 0.2-0.3 eV. Maximum illumination of the p l a s m a o c c u r s in this case. F o r higher l a s e r fluxes (I ~ 10a-109 W/em 2) it is possible to increase the t e m p e r a t u r e by several e l e c t r o n - v o l t s . This gives rise to a considerable i n c r e a s e in the rate of population of the upper levels of the CII s y s t e m and to an inc r e a s e in the absorbing power of the plasma, c o r r e s p o n d i n g to the descending branch of the curve in Fig. 1. It should be noted that due to the high optical density of the plasma, attenuation of the illuminating radiation along the beam is possible. F o r linear absorption the intensity of the b e a m along the length of the p l a s m a in the capillary is reduced by approximately two o r d e r s of magnitude. As a result of this, nonlinear phenomena will manifest~themselves strongly in a confined region of the p l a s m a and wili become weaker as the radiation p a s s e s through the plasma. Hence, for a multicomponent strongly ionized p l a s m a deviations f r o m the equilibrium value of the electron density can occur due to individual components which are difficult to ionize. In our ease CII plays this role. The nonlinearity of the absorption p r o c e s s e s observed indicates the need for careful p r o c e d u r e when using high-power l a s e r b e a m s for p l a s m a diagnostics. At the same time, this nonlinearity could provide another method for extending p l a s m a diagnostics.

LITERATURE 1. 2. 3: 4. 5. 6. 7. 9 8. 9. I0. 178

CITED

N.A. Generalov, G. I. Kozlov, and Yu. P. Raizer, Pisma Zh. l~ksp. Teor. Fiz., 8, 138 (1969). N.A. Generalov, G. I. Kozlov, and Yu. P. Raizer, Zh. I~ksp. Teor. Fiz., 56, 769-(1969). N.N. Ogurtsova, I. V. Podmoshenskii, and M. [. Demidov, OMP, No. I, I-~960). A. Burgess and M, J. Seaton, Rev. Mod. Phys., 30, 992 (1958). L.M. Biberman and G. E. Norman, Usp. Fiz. Nau-k, 91, 193 (1967). V.S. Burakov, P. A. Naumenkov, and V. P. Ivanov, Z-h. Prikl. Spektrosk., 8, 738 (1968). V.S. Burakov, V. P. Ivanov, and P. A. Naumenkov, Zh. PriM. Spektrosk., 9, 1134 (1969). Ya. B. Zel'dovich and Yu. P. Raizer, Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena [in Russian], Nauka, Moscow (1966). N.A. Generalov, G. I. Koslov, and Yu. P. Raizer, Prik[. Mat. Teor. Fiz., No. 3, 27 (1970). N.N. Ogurtsova, I. V. Podmoshenskti, and V. M. Melemina, TVT, 6, 400 (1968).