JPRS ID: 9453 USSR REPORT PHYSICS AND MATHEMATICS

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• Sequence Number: 34
• Content Type: REPORTS

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JPRS L/9453
18 December 1980
USSR Re ort
p
PHYSICS AND MATHEMATICS
(F4UV 10/80)
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JPRS L/9453
18 Decembex 1980
a
USSR REPORT
- PHYSICS AND MATHEMATICS
(FOUO 10/80)
CONT~NTS
. CRYSTALS AND SEMICCNDUCTORS
Production and Properties of Thin Films . . . . . . . . . . . . . . . . . 1
I.ASERS AND MEiSERS
An Llectric-Diecharge Pul~ed C02 Reaearch Laser . . . . . . . . . . . . . 4
Limiting Characteristics of a Photochemical Xe0 Laser . . . . . . . . . . 9
Developmont of Optical Inhomogeneities in Flashtube Photolysis
Lasers . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . 21
- A Neodymium Pulse Laser That Produces a High-Frequency Series of
Nanosecond I'ulaes With Energy af tilOU J . . . . . . . . . . . . . . . . 29
_ Lasere Utilizing C02 Isotopes . . . . . . . . . . . . . . . . . . . . . . . 33
Study of a Large-Volume Flashlamp-Initiated H2-F2 Chemical Las~r. 41
Optical Resonatores and the Problem of Laeer Beam Divergenre 46
- MAGNETOHYDRpDYNAMICS
Magnetic Plasma Compre8aor With Exploaively-Driven Magnetic-Field
Compreesion Generator ~ ~ ~ ~ ~ ~ ~ . . , ~ ~ ~ , ~ ~ ~ . ~ ~ ~ ~ ~ ~ ~ 63
OPTICS AND SPEC~ROSCOPY
Interference Phenomena When Metals Are Heated by Laser in an Oxidative
Atmoepher~. � � � � � � � � � � ~ � � � � � � � � � � � � r a � . � � � 6$
Time for the Origin of Plaema With the Ef�ect of Laser Radiation o~
Vario~xe Wavelengtha on an Aluminum Obatacle in the Air, 78
I~ OPTOELECTRONICS
i Influence That Pumping Fluctuations I~ave on the Sensitivity of an
~ Infrared Receiver With Parametr.ic Frequency Conversion. . . . . . . . . 85
~ - a- [III - U~5R - 21H S&T FOUO]
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CRYSTALS AND SEMICONDUCTORS
PRODUCTION AND PROPFRTIES OF THIN FILMS
Kiev POLUCHENIYE I SVQYSTVA TONKIKH PLENOK in Ruasian 1979 aigned to presa
20 Dec 78
[Annotation and Table of Contente of collection of papers published by IPM
; (Inetitute of Problema in Material Science) of the UkSSR Academy of Sciences,
editore A. F. Andreyeva, V. Ya. Ayvazov, et al., 295 copiea, 179 pages]
, [Text] The present collection of papera ia the sixth volume of transactions
of. the all-union seminar on "Production and Properties of Thin Films" held at
the Inatitute of Problems in Material Science in 1978.
The collection presente tihe resulte of research on the methods for obtaining
and Ch~ propertiea of thin filme of oxides, nitr~.dea and certain other compounds
which are promieing in electronice and optica.
Thie volume is intended for the engineers, techniciane and acientiats of research
institutes and industrial enterprises and may also be used by graduate students
and undergraduates taking upper-level courses in micro-elecC�ronica and computer
engineering.
f
TABLE OF' CONTENTS ~
1. Capabili.Cies of the secondary i~n-ion emiasion method for analyais of,~concen-
tration profilea in th~n fi~.ms, M. A, vae~.r~ye~ and S. P. Chenakin ~
2. Proepsets for th~ use of REM oxide films in optics electronics, A. F. Andreyeva, ~
I. Ya. Gil'man and M. D. S~nolin
3~ On evaluation of the prospects for the use of diele~trics in microelectronics, -
D. I. Chernobrovkin
4. Predic~ing metalatea with specified properties, V. V. Vo~kova and D. I.
Chernob~avkin
5. SCr;zc~ural etudies o:E the initial stages of titanium film oxidation in vacuum, _
~ V. Z. Khitrova and S. A. Semiletov
- 1
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6. Low-temperature oxidation of titanium in rarefied molecular and atomic oxygen,
_ I. N, Frantsevich, V. L. Tikush, G. V. Rusakov, L. A. Gayevskaya, V. S.
Dvernyakov and Ye. S. Lugovakaya
7. Poroaity of c~pper filme d~posited under conditlons of cuncurrer~t ion bom~
bardment, V. Peta and A. Flerning
8. On the influence of a conatant electric field on the growth ot insular
metallic filme, V. Peta, M. Friedrich and K. G. Dang ~
- 9. Production and properties of chromium si~icide resistive films, H. Helms,
V. Bretschneider, G. Beddia and P. Bogdanova
10. On the question of low impurity levels in oxide filma, V. A. Ogorodnik
~ 11. Some propertiea ~f films of praseodymium oxide allo~ed with aluminum, V. A.
Ogorodnik
12. Use of accelerated ion source for obtaining intermeCallic thin films, V. G.
Grigor'yan, G. I. Kazanets, V. I. Minakov and V. A. Obukhov ~
13. Calculation of cathod~ dark apace length for discharg~ devices with roci- _
type electrodes, 0. Ya. Gavrilyuk and Yu. G. Kononenko
14. Dependence of depoaition rate on the geomeCric dimensi~ns of a rod-tvpe
diacharge device, 0. Ya. Gavrilyuk and Yu. G. Kononenko
15. Production of :.esistive thin films by cathode sputtering in a rod-type die-
charge device, V. G. Kobka, G. I. Koatrygina, A. A. Smakovenko and 0. F.
Taranenko
16. Production and study of dielectric films based on REM metallate compounds
_ for mic~ocircuit elemente, D, I. Chernobrcvkin, M. N. Piganov and I. A. Korzh
17. On the eCructure and properties of filma obtained by catalytic oxidation
~f titanium, N. P. Pekeheva _
18. Mechanical atreasea in A1XGa1_,rAs filma gro.an on subatra~ea of GAL1s with
- various dopanta, N. D. Vasilenko, A. M. D'yar.henko and B. P. Nfaser.ko
19. SCudy of gallium microinclusions in epitaxial GaAs L�~.lms, G. V. B~.renshtein,
N. D. Vasilenko, A. M. D'yachenko and 0. K. Gorodnichenko
_ 20. Some properties of thin films of indium antimonide, 0. G. I~yubutsin, I. I.
K1eCcherikov and F. Plevako
21. Temperature dependence of the brightness of GdF2-Eu3+ electroluminescent
filma, N. A. Vlasenko, Z. L. DenisovA and V. S. Khomchenko
= 22. Electroluminescent prop~~rties uf ZnS:MnF2 filma, T. P. Vistievu ~
2 ~
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23. On the effectiveness and etching characteriatice of semi-i.nsulating semi-
conductor cryetals in a low-prasaure r-f plasma discharge, N. V. Gavrilenko,
' A. F. Onipko ai.d E. B. Tal' yanakiy
2.4. Study of tha parametere of impurity centere in CdSexTel_X fflma produced in
their owa vapors in r-f plaema, N. V. Gavrilenko, A. F. Gnipko, I, I. Sava
and M. G. ICollenik
25. Phase composition and photoelectric properties of SbXlnl_XSe filme produced
in their own vapore in r-f plasmA, N. V. Gavrilenko, A. F. Onipko, T. I. Sava
and I. V. 2hornovyi
~ 26. Thermaresieti.ve fsrrite films obtained by e.lectron beam vaporization, Yu. N.
' Okunevekiy, S. A. Pilipko and L. N. Tul'chinakiy
27. Study of ref].ection apectra of alumin~im condeneatea in the UV part of the
apectrum, V. M. Nedoetup and N. N, Smirnova
28. Determination of anisotropy conefiand and magneCization level of thin �ilms,
A. I. Beresnyakov, N. S. Troitskiy and V. N. Gurin
29. Production of film materials by exposure to concentrated solar radiation
flux, I. N. Frantaevich, V. S. Dvernyakov, I. E. Kasich-Pilipenko, V. L.
Tikush, G. V. Rueakov, L. A. Gayavakaya, L. R. Sharinyan and R. S. Biryukova
30. klas.ma electron gun baeed on the WP-2K univereal vacuum facility, R. V.
- Dashtoyan, V. I. Mel'nik and R. A. Kadzhoyan
; 31. Study of the factore detbrmin~.ng the chemical etability of filma, N. P.
~ Pekeheva
� ~2. Production af dielectric filma on gal7.ium araen~.de by the electro~hemical
oxidation method, V. V. Novichkov and N. P. Novichkova
~ 33. ProperCiss oP NIDS etructu~r~a w~.th anodic Gat`~e oxide cflating, M. K. 3amokhvalov,
- V. Novichkov, A. ~I. Sverd7.ovr~ p:.d N. P. Novichkova
34. Growth of anodic films on GaAs in acid, alkali~ and neutral electrolytes,
V. V. Ovchinnikov, L~ V. Kozl~itov, D. LT. Kamalova and V. V. Ventsov
35. Electron diffract~on study of intermeta~:lic phas~e in Pt and Ni films deposited
on ai~.icon, B. G. Doniahev, A. Ye. Likh~tman, Ye. I. Kotenko, Yu, N. Makogon
and S. I. Sidorenko
_ 36. Study of high-~reai.atance thin-film resistors based on new cermets, N. D.
- Mi.chsanin, D. I. Chernobrovkin and Ye. V. Kootryukov
37. X-ray diffraction phase analysis o~ films obtainad by vacu~+.am vaporization
- of somr~ multicompunent copper-baee alloys, V. I. Popov, V. G. Tinyayev and
, A. A. Popova
_ COPYRIGHT: ~nstitut probiem materi~lovedeniya AN USSR (IP'M), 1978
[6-9576]
C806 1f362 3
'Rf1R AFx'T~'TAT TTC~' /1ATr V
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LA3ERS AND MA~ERS
,
i
UDC 621.378.3
AN ELECTRIC-DISCHARGE PULSED C02 RESEARCH LASER I
Moscow KVANTOVAYA ELEKTRONIKA in Russia.. Vol 7, No 7(97), Jul 80 pp 1456-1460
- manuscript receivea 10 Dec 79
[Article by Z. I. Ashurly, Yu. M. Vas'kovskiy, I. A. Gordeyeva, L. V. Malyshev,
R. Ye. Rovinskiy and A. A. Kholodilov]
[Text] Thia paper describea a pulsed high-pressure ~lectric-
diecharge C02 laser that provides a wide range of beam output
parameters. By sectionalizing the electrode system and power
supply, proviaions are made for ad~usting the length of the active
- part and for producing two or more pulses that follow one another
with a predetermined delay. Stable ad~ustment of tr.e duration and
shape of the emission pulse is achieved by proper selection of the
mixture compoaition. An investigation is made of the way that the
weak-signal gain of.the active medium depends on time and on the
total presaure of the mixture for different proportions of the
gae componente. Treatment of the results is based on a simplified
model of ki.neric processes in the discharge.
In research aeaociated with the practical application of infrared lasc.rs for pr.o-
duction purposea, high-preasure electric-discharge CO2 leaera with tr~nsverse dis-
charge have given a good account of themaelves. Capabilities for con:r~].ling the
parameters of output radiation o� electric-diacharge lasers are appreciably en-
hanced by aectionalizing the ~lectrode eystem of the laser and the part of the
power aupply circult that ahapes the electric pulses. This article describes a
laboratory electric-diacharge laser with a sectionalized electrode system and
demonetraCes its research capabilities.
The electric-diacharge section of the laboratory laser facility wit11 length of
about 45 cm consiets of an anode in the form of a Duralumin plate witli workin~; sur-
face measuring 42 x 15 cm, a cathode of the same material measuring 40 x 12 cm, and
an auxiliary electrode for which parallel transverse grooves 2.6 mm wide and ap-
proximately ae deep are milled on the working surface of the cathode. The insu-
lated wire o� the auxiliary electrode is stretched in these grooves so that half-
its diameter extends above the surface of the cathode.
A diagram of the power supply section is shown in Fig. 1. The electric pulse is
shaped by comtnutating devices K1 and K2 that supply approximately twice the voltage
Up of storage capacitor C1 to the diecharge gap of the laser ~ectio:i. Initially,
4
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~
R~ Rt
A ~1
U�SD~wB ~t K~ K2 ~ 3 IP,, F~$. 1. Electric supply
� ~t K circuit of the laser
kV
aection -
+
Rr
thie cauaes d~velopment of an auxiliary corona discharge between the wire of -
ionizing electrode 3 and cathode plate K, followed by $ high-pressure glow dis-
- charge in the main discharge gap A-K.
Several euch electric-diacharge sectione set up in aeries inslde a common bulb with
the mirrors of an aptical cavity on the end facea make up the laboratory laser. A -
trigger and synchronization unit enaurea simultaneous operation of the controlled
commutators K1 and all sections of the electric-discharge laser. � _
The secLionalized laser design enablee the use of a variety of schemes for trigger-
ing the aections. For example the length of the active part of the laser can b~
regulated by energizing only some of the aections. Individual ~ections or indi-
vidual groupe of aections can be triggered with controllable delay relative to one
another from 0.5 us to any required value.
The controll.able change of the duration and shape of the emission pulse is con-
ducive to expaneion o� the research ~capabilities of the laboratory laser. (Pulse
duxation i~ takan as the time during whi~h 90% of its energy is released). One way
to change pulse shape and duration is to superimpose two sequential pulses produced
by two groupe of la~er sections; however, reproducibility of the reaults is poor in
' thie cgee since the spread in triggering delay hae an effect. Reaearch has ahown _
that more reliable results are realized by varying the proportion of components of
~ the gae mixture and altering electric discharge conditions.
TABLE 1 -
/ tloMep ~QTMQW~NN� KOMItONlMTO/
\1~ CMlCM B~ A7R n E N. MIfC
Cp~ N~ H~
~ 2 0 8 80 0,66 0,76
2 ~ l g 80 0,36 l,3
3 4 l lb 200 0,25 t,8
4 ( 2 3 390 0,2 2,1
~ g ~ 4 5 430 . 0,15 3,6
360 0,13 4,5
- 7 1 ib 10 160 0,1 7,8
1--Mixture number 4--Ratio of peak energy to total pulse energy
2--Froportion of components S--Pulse duration, us
3--~, ,joules
In studying this technique for regulating emission ~ulse shape and duration, the
puloe shape was recorded by a germanium photoreceptor with photon charge enhance-
metit, and Che total Qnergy in a pulse was measured by a TPI-2-5 graphite calorim-
eter. 'J'he time-reaolved values of the discharge current and the voltage between
eloc~radea were r~imultaneously measured by a Rogowski loap and voltage divider.
i 5
~rnR (1Fl~Tf'TAT. TTSF. (IAIf,Y -
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us
0 ~ 2 J 4 5 Q t,M~rc
' as Fig. 2. Oacillograms of emission
~ 2 3 f? 6 7 5 pu].ses in reduced form
- r
I,amM rd. re1. units
'Ihe gae mixture ca~cpositiona that were studied and resultant emission pulse dura-
tione are aummarized in Table 1. Fig. 2 shows oscillograms of the correspondin~
pulgee in raduced f.orm. These give a graphic idea of the way that the emission
pulae is deformed wi.th a change in gas mixture composition. When nitrogen content
is am~.ll or totally absent, a considerable part of the radianC energy is released
in the initial peak, and the pulse has almost no "tai1." The .ratio of peak ener~y
Co Cata1 pulse energy reachea nearly 70Y. As the percentage of nitrogen in the
. ttiixture ia increase3 there is an increase in the proportion of the "tail" part of
the piilse, which is accompanied by elongation. With the maximum relative nitrogen
content (mixture 7) the initial spike is weak, containing no more than 10~ of the
tot�al energy, and the pulse has the greatest duration. In a11 cases the total
preseure of the gas mixture was 630 mm Hg.
The emission energy, and the effic:iency with which the electrical energy investec~
in th~ diecharge i~ converted to radiant energy at first incr~ase with increasing
relaCive contenC of nitrogen in the mixture, reaching a maximum in mixture 5, and
then fall. off. The energy input to the discharge is practically independent of the
mixture compoeition except for the most extreme cases of nitrogen-rich or nitrogen-
, poor mixturea. In thess caees the energy input falls off noticeably.
Since the duretion and shape of the emis3ion pulse ahowed good reproducibility with
ev~ry predetermined mixture compoaition, these characteristics of the electric-
discharge laser could be confi,dently ad~uated by selecting the necessary proportion
of ges compoaents. In the optimum case (from the standpoint of maximizing output
_ energy and efflciency) with a ratio of CO ;N :He*~1:2:3 the laboratory laser pro-
- duced a etable energy output of more than 30 J/Z at a total efficiency of conver-
aion of the energy stored in the capacitore to emiseion of about 15~. Emission
puls~ duration in thie cas~e wae ~2 {1s. The total range of variation of pulse dura-
tion in these experiments was from M0.1 to ~8 �s. Pulse duration can also be
varied by changing the initial voltage acrosa the diacharge gap; however, this
_ method of ad~uetme:tt ie not very e�ficient since for a given preasure of the gas
mixture and stable axietence of the glow discharge the technique can be realized
only aver a very narrow voltage r~nge, and involveg a conaiderable change in the
energy pattern of the einisaion.
Studies of Che gain of the medium provide additional information on the influence
that the proportion of gas mixture components has on the nature of proceasea in the
active medium of a spctionalized laboratory electric-diacharge laser. To n~easure
the gain, plane-parallel NaCl platea were installed at the Brewster angle in place
af Ch~ mirrore of the optical cavity ~f the laser. The bulb was illuminated along
the optical axi.e by a thin probing beam, the source of which was a C02 electric-
diech~rge laser operating in the pulee mode on a'single tranaverse mode. The pre-
ionization eystem for the probing laaer was the same as for the main bulb. The
- laser operaCed ak a relatively low pressure of Che working mixture (of the order of
200 mm hg or lees), which wae conducive to a fairly narrow luminescence line. The
6
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laser had a ahorC reaonator and emitte~l close to the threshold. The energy in the
emiaeion p~a1s~ wae 0.7 J at a duration of ~2 us on a level of 35X maximum.
The opCical cavity of the probing laser was made up of a di�f.raction grating of
100 lines/mm and a flat germanium mirror. Within the cavity iteelf were two NaCl -
- lenses forming a teleacopic eyatea~ with approximately 2x magnification, and an
~ ad~umtable irie. Individual vibrational-rotational transitions, in particular the
traaeition P(20), were isolated by rotating the diffraction grating. The probing
~ lae~er was triggered with ad,juetable delay relative to the triggering of the dis-
charge in the main bulb. Gain was determined from the ratio of amplitudes of the
probing signal and the aignal amplified after passage through the cell with an `
accuracy af about 151'. Abaence of eaturation of the amplified signal was verified
by increasing the amplitude of the probing pulse by a considerable factor as com-
pared with the level on which the gain measurements were made. The amplitude of
the entsrinz radiation pulse was regulated by using calibrated film absorbers of
infrared radiation.
~M~ Fig. 3 shows the resultant curves for gain
; 2~. as a function of the delay time of the
probing pulse relative to the beginning of ~
~ ' 4 diacharge in t~e bulb at different gas
' ~ ~~n~., mixture compositinns. All curves were
~ q5 ~ plotted at the same value of E/p (E is
0' 2 4 Q t'~us ' field strength, p is gas pressure). For
the different investigated mixture compo-
Fig. 3. Gain as a function of the sitions the poaition of the maximum of the
_ delay time of the probing pulse curves and their initial displacement de-
relative to Che beginning of dis- pend in a complicated way on the composi-
charge in the bulb for gas mixtures tion of the active medium.
; C02: N2: He ~ 1:1: 3(1) ; 2:1: 5(2) ; -
1:1:2 (3); 1:1:1 (4); 1:2:3 (5) K,M' 4~ ~
and 1: 2:1 (6) , ~
~
- Fig. 4 ehows the gain in different 20 ' "
~ media as a function of total gas ' ?
�5
preasure in the pressure range where 6
, ~he glow diecharge ia atable. The
mAximum gain is displaced toward
higher presaures as the helium con- ~ ' ' ' '
tent i~ the mixture increasea. Data g4 0,5 0,6 0,7 O,B p,amM
atm
showing how the gain dEp~nds on the
quantity E/p for �aur mixture compo- Fig. 4. Gain a~ a function of the total
s~tione are eummarized in Ta.bl.e 2. gas preasure for different gas mixture
TABLE 2 compoaitions. Notation ia the same as on
Fig. 3
~~cr~~ nMec� (2)~+/A I -1 KEY TO TABLE 2: -
- ~ H~(CM � MN I K~. N -
- CO~ I N~ He pr. cr.l
1--Mixture composition
1 I l 22t0,S 2,4-3,0 2--E/P, V/ (~~m~mm Hg)
I 2 I 2St1,0 2,2-3,3
~ 1 2 3 ' l8t(1;5 2,0-2,5
2 I b lai~,4 2,1-2,T
i
i
7
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The vAluee f4r the gain that are given in the table are for the range of initial
valCa~es acrosa the electrodes of the bulb from 94 to 100 kV. An imgediment to a
mnre deteiled investigation of the way that gain depende on E/p is the fact that
Che region of stable exiatence of the glow diacharge is limited Co a very narrow
change in f ield strengCh E or preasure p. The region of stable exiatence of the
diecharge narrows with an increa8e of nitrogen content in the mixture, and to an `
eveii greater extent with an increase in helium content. -
The observed peculiaritiea in the way that gain depends on time, gas mixture compo-
eirion, total pressure p, and the parameter E/p are determined by the nature of
populaCion and deactivation of the laser levels of C02 molecules as they interact
with the electrona and molecules of the active medium. It can be shown that the
following simplified model givea a eatisfactory qualitative descriptiun of kinetic
- processes. The levela of sytmaetric, asy~netric and deformatlonal vibrational modes
- of the C02 molecules are populated, and the N2 molecules are vibrationally excited
~ under the effect of time-variable discharge current in the volumetrically homo-
geneous active medium. The distribution of vibrational energy by levels is not
conaidered. Working levels 10~0 and 02~0 are unified into a single "lower" level.
- The energy of excitation is invested directly in the upper laser level 00~1 of C02i
(v =1) of N2, and the lower laser level. This assumption is justified by the rapid
procese of exchange between 00~1 of C02 and (v = 1) of N2, the strong interaction of
the symmetric and deformation modes, and also rapid intermodal exchange and the low
level of vibrational temperatures.
~,~i rnK The results of calculation of the gain for
,pp 360 two mixtures of gases done by numerical
/ methods basec~ on the simplif.ied kinetic
o p
' � � � ~ '~40 havioraof theWCalculated5values ofmgain,
,~0 Q~~;-.-~----~'g~4 � the presence of .3 max:mum and its position
oi~i~ o d20 as well as the absolute value of the gain
;0 agree satiafactorily with experiment,
/i which favors the chosen model, at least
300
p ? 4 6 t,M~rc for sectiona of increase and attainment of
us maximum gain. However, the experimental ~
i~ig. S. Theoretical dependences of plot falls off much more rapidly than zhe
~c~in (:L, 3) and gae temperature Tr theoretical curves. This probably due to
(2,4) ~s a function of the elapsed the fact that the actiial rise in vibra-
time after onset of the probing tional temperature is more rapid than the
pu].ea for mixtures of C02:N :He = calculation implies.
1:2:3 (1,2) and 1:2:1 (3,4);
o---expcrimental values of gain for The authora thank A. A. Bakeyev for ~lseful _
the first, and o---for the second discussion of the materials of the paper,
mixture and also V. G. Kulikov, G. A. Bogatova and
N. N. Vorob'yeva for assiatance with the
experiments.
COPYRIGHT: Izdatel'stvo "Sovetskoye radio", "Kvantovaya elektronika", 1980
[3-6610J
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UDC 621.378.33
LIMITING CHARACTERISTICS OF A PHOTOCHEMICAL Xe0 LASER
~ Moacow KVANTOVAYA ELEKTRONIKA in Ruasian Vol 7, No 7(97), Jul 80 pp 1482-1491
manuscript received 30 Dec 79
i [Article by V. S. Zuyev, L. D. Mikheyev and I. V. Pogorel'skiy, Physics Institute _
~ imeni P. N. Lebedev, U5SR Academy of Sciences, Moscow]
[Text] An experimental and theoretical study i^ done on the
~ nature of internal losses in the active medium of a photochemical
Xe0 laser with pumping by an open high-current electric~discharge.
It is shown Chat the specif ic efficiency of the Xe0 laser can be
~ increased by reducing internal losses. Pumping is analyzed and
the anticipated lasing energy is determined for an Xe0 laser with
a flat resonator. The optimum compoaition of the mixture is found
as well as the tranemission of mirrora that ahould yield a lasing
energy of ~40 J in a pulse with duration of ~10 us, which corre-
' eponds to a total.laser efficiency of ~0.1;6. Possibilities are
~ mentioned for further improving the eff iciency of tre given laser
~ to 0.2�d when isotopically pure xenon is used.
! h~ ~1,~1'I},fNM~ 1. Introduct~ion
: nm .
_ Nt0 ~ Ref. 1 gives the results of a study of the
specif ic characteristi.cs of a cooled photo-
~ chemical Xe0 laser ~vith the kinetic scheme
shown in Fig. 1. The laser was pumped by
~'z~Xl 0(~sol ,re the vacuum ultraviolet radiation of a
high-current pulsed electric discharge fed
by a capacitor bank with stored energy of
57 kJ. Laser operation was studied in an
xe0(2'~',! arrangement in which the volume of active
, medium taking part ~n lasing was restricted
_ ~r~ bJ (540~) by the small diameter of the laser chamber
(5 cm). The maximum lasing energy in a
Xe0(fr~'') pulse with half-amplitude duration of 8 us
equal to 2.2 J was attained by using a
' planar-spherical cavi*_y and a working mix-
~1+ ture of N O:Xe:He = 1:75:170 at a total c~n-
Fig. le Kinetic scheme of photo- centration of 3 amagat units and tempera-
chemical Xe0 laser [Ref. 1] ture of 160 K.
~ .
9
~ , FnR (1FFTr,TAT. TT ;F. (11~TI.Y
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'1'he.expe:imentally measured weak-signal gain reached Ky~ = 3.5�10-3 cm 1. In the
abAence nf a lasing field, the concentraCions of molecules af Xe0 (21E+) and oxygen
aComs (1Sp) corresponding ko such a value of Ky~ are equal to 2�101`' and 1.5�10~5 =
_ cm"3 xespectively. The time for quenching of oxygen atoms (1Sp) by nitrous oxide
N20 ~:nd phatolyeie products is known, and under rhe conditions of the experiment is
~r~~~ 200--4;1Q ns. Thus at tlie instant when maximum lasing power is reached, the rate
- of I`ormaCion of atoms of 0(1Sp) averaged over the cross section oi the active
medi~im (kh~ pumping rate) i.s ~=[0 (1Sp) ]/TT �(6 � 2)'1021 cm"~�s-1. Comparing this
_ quAntity with the speclfic peak lasing pawer WyA = 300 kW/Z = 0.75�1021 photons per _
cm3�s, we find that the efficiency of converting the pumping per unit volume of
actJ.ve medium to stimulated emiasion is n= 15 � 5%.
Tf we consider that r.he coeffic:Lent oF internal losses in the active medium under
tt~~~ c:onditions of the experiment is yn = 10-3 cm 1, and the distributlon of losses
!.n kransmission of the mirrors is y~ a 3.5�10-`' cm`1 [Ref. 1], it can be readily
yhown that the experimental value of n is due chiefly to internal losses. Thus a
further reduction~ ~.n ~yn ehauld improve the efficiency of the Xe0 laser.
I:n this paper we exan?ine the nature of internal losses in the active medium of an
XeU laser and the feasibility of reducing them. An examinarion is also made uf the `
- pos:~ibility of a further. considerable increase in the lasing energy of the Xe0
laser by increasing the volume o� the active medium while using the same pwnping
source a8 was used in previous experiments. It is shown on the basis of calcula-
ti.ona Chat an energ;~ of 40 J can be attained in the laser pulse, whicYi corresponds
to a total laser eff iciency o� ~0.1%. Mention is also made ~f the possibility for
- further i.ncreseing the efficiency of the given laser to 0.2% by using isotopically
pure xenon in the working mixture. ~
2. Internal Lossea in the Active Medium _
~'tatir~ ~r~~ccI~ient of ihe index of refraetiort. It was ~oted above that the efficiency
witli whict~ pumpin~ in a unit vulume of active medium of a photochemical Xe0 laser
- i.e converted to lasing is limited by lossea within the resonato�r. The greatest
lev~1 af loese~ ie aseociated mainly with the optical inhomogenei~y of the working
medLum that nriaes as it ie cooled in the laser chamber. This inhomogeneity shows
up In particular when the chamber is expoaed to the probing beam of a helium-nec~n _
l~ser. Deflection of such a beam was uaed to measure the grddient of the index of
refrsc~lan (grad n) in the medium.
An investigation was made of the change in grad n heightwise of the chamber in gas _
mi.xtures of various compositions. Fig. 2 shows graphs of the change in gr.ad n in
Xe and Ar with displacement from the center of the ch~mber up~aar~ (Z ~ 0) and down-
warci (Z < 0). Grad n is directed downward ar all points, which corresponds to an _
i.ncrease in the density of the medium in this direction.
Gr;id n averaged over the cross section of the laser chamber was on a level of
" 10"S cm 1 for the optirnum mixture i.ndicated above. The value of Yn for a flat
reaonator can be estimated by the formula [Ref. 2]
YII YgradnLy/(2DLa), ~1)
where Lp and Lg are the lengths of the resonaeor and the active medj.um respectively,
D ie rhe transverse dimension of the active zone. Under the canditions of our
~ 10
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experiments, typical values of the param-
prod~,~0"sc~r-` eters appearing in (1) were: Lp =1 m,
- ~ La = 0.8 m, D~ 2 cm; in this case,
~ g~ yn =2�10-3 cm'1. The value of Yn should
p ~ be somewhat lower for a planar-epherical
cavity, which has been experimentally con-
~ ~ f f irmed [Ref . 1 ] .
~ ~d
_y y~ The distribution of grad n observed in _
- ` Fi~. 2 was integrated with respect to the
-Y -l 0 / t~cM height of the chamber, and it was found
that the gas concentration varies with
Fig. 2. Change in static grad n respect to heig~?t by 7%, which correaponds
heightwise of the laser chamber: to a temperature difference of 10 K be-
' 1--Ar, 1.7 amagat units; 2--Ar, tween the top and bottom of the chamber.
3.8 smagat units; 3--Xe, 1.7 amagat Sources of such temperature inhomogeneity
units; 4--equivalent grad n for a are: a) the unsteady nature of chamber ~
concave spherical mirror with radius cooling, which is done by a short burst of _
af curvature of S m; 5--aum of curves liquid nitrogen into the cryostat; b) the _
2 and 4 presence of dielectric and metal parts in
the laser chamber, differi.ng considerably
in thermal conductivity; c) influx of heat through electric leads. (The design of
the laser and the cooling technique are descr.ibed in detail in Ref. 3).
To eliminate temperature inhomogeneities, plans for future experiments call for a
- change to steady-state cooling, use of a metal laser chamber and a considerable rE- -
duction o� heat inf lux through electrical leads,
The problem of the chamber material must be considered in more detail. The use of _
a dielectric chamber in previous experiments was dictated by the inadequate elec-
trical strength of ~he inert gases making up.the working mixture under conditiona
where the initial voltage drop acroae the discharge gap reaches 45 kV. To convert
to e matal chamber, a gas must be added to the working mixture Lhat has high elec-
trical strengCh, and r~imulGaneotir~7.y poasesaes the following properties: it is not
frozen out when cooled to 160 K, it does not absorb radiation in the pumping band
- of N20 (145-131 nm) and it does no~ quench atoms of 0(1Sp). These reqtiirements are
eatisfied by nitrogen foz which the rate constant of quenchiiig of 0(1Sp) is ex-
tremely emall (S�10"17 cm3/e) (Ref. 4]. -
Experiments were done to check out the operability o~ the Xe0 laser with nitrogen.
When 1 amagat unit of nitrogen was added to the working mixture, the breakdown dis-
- tance for 50 kV was leas than 10 cm. There was almoat no reduction in the effi-
ciency with which pumping was converted to induced emiasion inside the resonator.
These experiments sho*o the feasibility of using a metal laser chamber with diameter
of ~20 cm.
Dyrtamtic gr~adien~t af the i�ndex of refraetion. If static temperatiire inhomogeneities
are eliminaked, losses ~.n the active medium will be determine3 to a great extent by
the dynamic gradient of the index of reEraction that arises in photolysis of NZO
- due to nonunifnrm heating and change in the refractlon R of the active medium. Let
ua consider the loeses due to these two effects. -
1i
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Ite~racti.on is related to the refractive index of the medium by the expression
[Ref.. 5]
n� 1-{-1, 5 R N/A,
. whE~re N is gas concentration, A is Avagadro's number. Let us use Np to designate
the inilial ConcenGration of N20, and d(r,t) to denote the percenta~e of N20 mole-
cu~.es diseociated during photolysis. Then the change in the index of refraction
during photolyais is
en~(~, ~=1,5 ~R8 (r, t) NaA,
~ohere we have ~R =-2.6 cm3 for dissociation of N20 into r2 and 0[Ref. 6, 7], hence
- gradn (r, t)--0,65� 10-"Nagrad8(r, t). ~Z~
- As noted above, the second cauae of the change in n ie nonuniform 1-�eating of the
- gas as pumping radiat~!on is absorbed. The resultant presaure differential between
- regions with different temperatures ehould shift material from the high-temperature
- region near the source toward the periphery.
In culculating the displacement of material due to the action of this effect, which
we wi11 call the gasdynamic effect, we can disregard the influence of thermal dif- -
- fu~ion. To determine the gasdynamic deneity gradient of the working medium and the
aseociated gradient oF the index of refraction at an arbitrary point af the active
medj.um and at an arbitrary inatant of time, we use the following system of equations
that inc.ludea equations of continuity, motion and the first law of thermodynamics
written with conei.deration of amallness of the final change in density ap, rate of
displacement of materisl u and Che inequality
ulr 1 are satisfied for each of the oxide films:
niAo + 2xiK ~~ix~ - ~K~- sin2 ~~2x~12~~ sin ~~~x~~/K~ Ci ~14~
A ~zi~ x~) � D~ D~ ~
s
where _
K =1- (1- ni/nsl sin' ~~sx~/21~ - .
C1= 2x~ n2 l f~iza -(1- 2 x~n~ sin~ ~ 2~l sin ~Qx~l ;
~ s/ L 1 : ~ / J
' D ffi ~-sin ~-�~-cos ~lX! sin ~'"1 cos ~'z' ,
1 n, 2 2 2 '
Da= ni ~cas S2' cos ~21 _ nl sin ~2' sin ~2' I; ~~,z = 4"~,.. .
~ /
71
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,q ~ Fig. 1. Absorptivity A(xz, xz) of a two-layer
- ~~e i~- " system (f.or isothermal oxidation T= 1100 K) as a -
~ function of thickness x2 of oxide Cu20, as plotted
Qd ; ~ from formulas (10), (12) at nl = 2.57, n2= 1.5, ~
kl = 0.05, K? = 0.0005, a ffi 1.06 um, Ap = 0.12,
0,4 xl/x2 = 0.23 '
.
_ ,
, . _
QP ~ -
~
_ p q,5 1,0 1,5 2,0 X,MA'M Fig. 2. Dependence A(t) for a copper
ps 2Bd 666 BG6 !0/0 fOB3 target in the case of heating by a
_ q T, �C YAG laser (a = 1.06 um) : optical con-
r~ ~ stants are the same as on Fig. 1;
0,6 ~ ~ dl =10-6 cm2/s; d2 = 4�10-2 cm2/s;
/ T1 = 9000 K; T2 = 19,000 K; U= 0.524;
a'~ j m=8U m~; c=0.382 J/g�deg; n=
~ 1.5�10' W/cm2�deg; Qp = 0.415; s=
0~ - 0. 25 cm, P= 10 T~T
s
p 4 B !7 16 t, ~
Tx ~ Xr;~,Fig. 3. Time dependences of temperature
- '""M and thickness of layers.of oxides when a
~Z~ copper target is heated by YAG laser
4
emission (values of parameters are ehe
~ same as in Fig. 2)
~00 x10 2
_ 000 1 X ~
~ 8 0
~ p 6 f0 !S t,c
dx~~dt,H~F Fig. 4. Time dependences of rates of formation of
~e oxide layers of Cu0 (a) and Cu20 (b} when a copper
~ 40 target is heated by YAG laser emission (parameters
~
a the eame as in Fig. 2)
. ?0 g /
~
0
f/X=~Qt~MKN~ ~
~a
- O,d 6
- b ~
i �
~ ~
- i~
Q s
5 1p 1S t, c
72
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Fig. 1 showa how the absorptivity A(xl, x2) behavea with a change in thickness of
one of the oxidea under isothermal conditions (the thickness of the second oxide
being determined by the Valenci formula (10)).
We solved syetem of equations (9), (11)-(13) numerically on a computer for the case
of heating of a copper target by YAG laser emission. An example of calculation of
relation A(t) is shown in Fig. 2. It is clear that in the heating process a double
intarference pattern arises in the change of ab sorptivity high-frequency oscil- ~
lationa are modulated by low-frequency oscillations. The former are due to growth
- of the oxide Cu20, and the latter are caused by growth of CuO. The corresponding
dependences of T(t), xl(t) and x2(t) are shown in Fig. 3.
Besides the oscillations associated with interf erence phenomena, during the growth
of the oxide films a modulation is imposed on the dependence A(t) that is due to
' the nonmonotonic nature of change in xl and x2. These peculiarities in the varia-
tion of x~ and x2 can be seen most clearly on the time dependences of rates of -
oxidation dx1~2/dt (Fig. 4). The depth of the corresponding modulations of absorp-
tivity depends on the kinetic constants of the problem. .
Analysis has shown that the overall pattern of change in absorptivity of a copper
target when heated in ~ir by radiation with a= 1.06 um is ma.de up of interference
oseillations complicated by nonuniformity in the growth of oxide layers. On the
o[her hand, when ttie copper target is heated by C02 laser emission, calculation by
the model given above shows that at heating rates cloae to those shown in Fig. 3,
oxide Cu0 has practically no influence on heating dynamics since its maximum thick-
neas ie xmax"'~~2n1 � -
It can be readily understood that the number of interference oscillations of A(t)
; ehould increase (other parameters remaining unchanged) with decreasing a, and for ~
three-layer oxide films (for example with heating of iron), a triple interference
pattern may show up.
4. Kel~xal-ton uf Pert~~rhar.ion~ Ln the Case of ~ao-Layer OxidatLon
1t sho~i.ld be nated ttiat in tlie taser heating proc:ess, when oxidation conditlons are
not isothermal, formula (l0), strictly speaking, is not valid. However, it can be
shown thttt the deviation Erom thiA law in cr~ses of interest can be disregarded.
Actually, let relation (10) be violated at fi ime t= 0 at some temperature T. We
denote
xi~~)lX:~~)=u(~)=uo-{-8(t), h(1=0)=80 ~ Ibo I��uo), (15)
~ where 5(t) is the deviation of u(t) from up = up(T) assigned by the Valenci formula
(10).
Substituting (15) in (9), and linearizing the e quations with respect to d, we get a
solution that describes relaxation of th~ initial perturbation:
8 (t) = bo exp - Rd' ~T~ ~ 2d11 ' (16)
�uo O X2 ~t i j
73
,.r._.r,.r,. ,.,.r
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where R= ((�~-z)'-}-8�'z 1'~~,z=d1(77/d9(7~. Hence the logarithmic decrement is
y==Rda~T)/�uczzo~ (17)
whare x20 is the initial rhickness of oxi.de xp.
Thue the Valenci formula can be used even under nonisothermal conditions if the .
~ characterietic times of temperature change ~t during heating satisfy the condition -
~t� Y"1. Estimates show that in typical cases of laser heating of a copper target
this condition is met in the temperature range of practical interest where the oxide '
film that is formed hRS a noticeable effect on the optical properties of the target.~ ,
For example Fig. 3 shows that at T~ 990 K, x2 ~ 0.1 um, wt,ich gives Y~ 0.6 srl from
(17) .
u Satisfactory fulf illment of the Valenci
formula ia conf irmed by the graphs of u(T)
6 and up(T) given in Fig. 5 as calculated
on the basis of Fig. 3, and from formula
~ u u (10). It is of interest to note that the
� oscillations that show up on the graphs
1 of xl and x2 (Fig. 3) are practically
absent on the dependence of u(T).
uo
0 ~
B6Q l000 1160 1dq7 T,K Under isothermal conditions in accordance ~
' with (9) the oxide layers grow paraboli-
Fig. 5. Curvea for u(T) = xl/x2 and cally, but with altered constants,
. up ('T) : graph �or u(T) plotted from
data of Fig. 3, and graph for up(T) di�-d,(1-(�uo-z)/z)