JPRS ID: 10454 USSR REPORT PHYSICS AND MATHEMATICS

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• Content Type: REPORTS

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JPRS L/ 10454
13 April 1982
~JS~R Re ort
p
PHYSICS AND MATHEMATIC:S
cFOUO 3is2a
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~'PRS L/10~454
13 ~ April 19:82
- USSR REPORT
PHYSICS AND MATHEMATICS
(FOUO 3/82 ~
CONTENTS
- HIGH PRESSURE PHYSICS ~ '
Present State of High-Preseure Ph.ysics 1
LASERS AND MASERS ~
Promising Designs and Pumping Methods for Powerful C02
Process Lasers (Survey) 15
Change in Relaxation Rate of Upper Laser Levei With Prolonged
- Operatian of Cw Electron-Beam-Controlled C02 Process Laser,. 44
OPTICS AND SPECTROSCOPY
Holographic Measurements 48. ~
Feasibility of Controlling Gain of Enthalpy-Stimulated
~ L~.ght Scattering 52
Conversion of Optical Radiation Spatial Spectrum ir. ~arametric
Processes . 57
- - a- [I.II - USSR 21.H S&T k'OUO]
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HIGH PRESSURE PHYSICS
UD~ 53.092
PRESENT STATE OF HIGH-PRESSURE PHYSICS
Moscow VESTNIK AKADEMII NAUK SSSR in Russian No 9, Sep 81 pp 52-6~
[Article by S. M. Stishov, doctor of physical.-mathematical sciences]
[Text] Having arisen at the end of the 18th Century and the beginning of the 19th
Century, high-pressure physics as a ful.ly defined �ield of scientific research was
finally formulated in the first third o� our century. An outstanding role in this
was played by P. V. Bridgman. Evidently, it was he who first introduced the term
"High-pressure physics publishing a book with this title in 1931,* in which he
stated the results of his experimental research.
But neither P. Bridgman nor anyone else defined what would later be understood by
hi.gh-pressure physics. Clearly, that of R. Feynman must be considered one of the
best; it says, in essence: high-pressure physics is everything that ph5�sicists do
with the help of equipment for creating high pressures. Actually, pressure ~ip to
many millions of atmospheres does not change the properties of substances. One can-
not point to a single effect or phenomenon observed at around 1 million atmosph=:res
th~t could not be observed in some form at atmospheric pressure (thi.s means, first
of all, phase transitions, electronic transitions, metallization, etc.). Thus, in
experimentally achieved ranges of pressures, there i~ an absence of effects that
are unique ~or compressed matter. In this connection, the aituation in high-pressure
physics in some s~nse is opposite to the aituation in low-temperature physics,
where the unique eftects of quantum degeneration, auperfluidity, superconductivity,
and so forth are observed. However, it would be incorrect not to note that in the
presence of very high pressures excaeding hundreds and thousanda of atmospheres,
we will find unusual richeG o� physical phenomena, inc],ud~.ng full ionization of
matter, nuclear disintegration and nuclear reaction, neutronizat~m of matter, super-
conductivity, and super�luidity in proton and neutron systems** ~unfortunately, all
tha.s i.s unachievable directly by experLmental, etudy and conclusions can be only on
_ the basis of astrophysical data). Thus, expez'iment~l high-preseure physics is
nor.hing other than the "plain" continuation of th~ phyffiics of atmospheric pressure
in a new dimension, whe~e the possibilities for experimentatior? are substantially
increased and where the volume o� information is vastly growing.
~ For Russian translation, see P. V. Bridgman, "Fizika vysokikh davleniy" [High-
Pressure Physics], ONTI, 1935.
See D. A. Kirzhnits. "Uspekhi fiz. nauk" (Advances in Physical Sciences], 1571,
vo1. 104, p. 489.
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A few wo~-ds t~:ust be said about pressure as a physical parameter. First, let us
= stress that the numerical signifi~ance of pressure, however large, says nothing
about its effect on matter. The degree of such effect, obviously, must be con-
- sidered the ratio of the work of compression which is determined bq the integral
fPdV to the size of the thermodynzmic potential of the system P=0. Thus, the effect
- of pressure on inatter is closely relzted to the response nf the system to the applied
effECt, in the given instance, to compressibility. Because of this, a pressure of
~ a million atmospheres t~irns out to have much less effect on less compressible matter
(such as iron, for example) than several thousand atmospheres on extremely compress-
= ible matt~r such as helium. Hence comes the rule: in research on any phenomena,
select objects that possess the highest degree of compressibility, other conditions
being equal. This provide~ the possibility for investigating a phenomenon over a
broad range of densities under relatively l~w pressures, and this is significant for
conducting precision measureme~?ts.
The need for achieving very high pressures uaually arises in the solution of
specifi_c scientific or applied proUlems. However, it is obvious that it is these
specific problems that stimulate development of the technology and methods for hig'~-
� pressures. A vivid example is the many year history of attempts to synthesize
diatnonds, which was brilliantly accomplished in 1955. As the result, diamonds and
borazon (cubic nitride o� boron) were sqnthesized, and high-pressure equipment was
created that permits achieving pressures of about 100,000 atm in combination with
temperature of up to 2000�C and high~r. In turn, the new technology yuickly found
application in physics, chemistry, and technology. Especially striking results
were achievPd in geophysics in research on olivine-spinel transformation and poly-
morphism of silica. These results lie at the base of modern understanding of the
structure and composition o~ layera of the earth. It must be noted that the aspz.ra-
tion to solve the puzzle of the earth's core is obviously one of the pmwerful incen-
tives that will determi.ne the development of nigh-pressure technology ir. the near
_ futiir~ (the pressure at the earth's center ie around 4 million atm).
CREATION OF HIGH PRESSURES
As follow~ from the equation of state ~(p,V,~)=0, to create increased pressure,
- it is necessary in ~ corresponding manner to change the volume or temperatur? of *ne
substance. Both means are used in practice.
Thermal means for creati.ng pressure are fully effective for gasses. Multistage
designs for gas thermal compressors are well known for creat~.;;g pressures up to
10,000 atm. Autoclave technology is also widely used in science and industry.
However, the direct compression of a substance is the most convenient me~ns ror
_ creating pressure. In this case, it is necessary to have a strong vessel, which
has at leas~ one movable wall. The simplest and most popular design for such a type
~ is the cylinder-piston type (fig. l~a).
_ As can be seen in the figure, in reali.zing this method, a strong,
often many-layered, fu11 cyl~nd~r is supplied with a piston and a ~
, plug. The cylinder ic filled with liquid or previously compressed
gas, and the pi~ton and plug are correspondingly compressed.
Usually, the plug serves as the pl,ace for locating electrical lead-
ins, which permit introducing detectors of various sizes into the
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~ ~ ~
~ , , I .
i ~ ; ; ~ . ~ ~ j~:
;~i ;~j;; . . ~ Q ,
~ ii~,; ~ . ~
. . . ~ , ~
.
,v ~
^ .
ry r / i~ ~ i;/ '
- fi lJ i~~/j~ ~ ~
~a)~ n ~b) ,~8,.~ ~
V ~7 (c) (d)
Il~li ~j V ~1
ri
~r = ' : r
~i, ~ ~ ' ; r . ~ , j
`--ti' - - - / ~ q , \ , p
!l ~ ~ ~ i5%,`4 i,
- ~ ~ ~~~j. ~ ~i~
~
, ~ ~ I ~ Q
::'IIII I ~ / .
a ~ @ L1 H! 3~ n v
(e) (f) (g) (h)
- Figure 1. Sketches of Hig~z-Pressure Devices
_ a. Cylinder-piston type chamber
b. The Bridgman anvil (a thin layer of aubstance is compressed
between two conic pistons)
c. The "Belt" type device.
d. The multiplunger tetrahedral device (four piatons located
in the space along the diagonal of the tetrahedron �
simultaneously compress the specimen)
e. Multiplunger cubi~ device
f. Device with sliding anvils (limitations in ehe degree of
compression are absent)
g. Spherical multip~.unger d~vice; it differs from d and e in
that the loading of the plungers is produced by loading the
whole device in a high hydrostatic pressure chamber
h. A two-stage spherical multiplunger device.
_ cylinder, first of a11 for temperatuxe and pressure. To create
pressure, the piston pushes down into the vessel with the sid of
a screw or hydraulic press, but the limit in the presaure achieved
in such a system is not at all determined by the power of the press,
~ut the laad-carrying capacity of the piston and cylinder. The
strength o� modern steel and hard alloys, in principle, permits
using the cylinder-piston~type system to achieve a pressure of
60,000 atm (the strength under pressure of the best instrument
steel is about 350 kgf/mm2, and hard alloys of~the tungsten- ~
carbide type cemented with cobalt, ~bout 600 kgf/n.m2. However,
much before this, many problems ariae ~*ith the sealing of
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compressed liquid or gas and, therefore, the field of hydro-
static pressure, that is, pressure that can be obtained by
compressed gas or liquid, is limited to 30,000 or 40,000 atm.
In recent years, industry in a number of countries began to produce strong 'and
flexi.ble tubing of alloys with low heat conductivity having outside diameter of
1 to 3 mm and able to contain pressure of up to 20,000 to 25,000 atm. It became
possible to separaL�e spatial].y the pressure being create~ for iehe pi~ton system and
the vessel in which the research is being performEd. This permits the creation of
various speci.alized interchangeahle chambers (optical, nonmagnetic, and others)
and to 'ocate them in a thermostat or cryostat, be~:ween magnetic poles, in an opti-
cal device, or a combination of these.
Using the piston system, it is possi.ble to simp].ify the experiiuent and, in place of
liquid or gas; to place inside the high-pressure chamber a plastic-solid body as
the means of transferring pressure. Thus, the sealing problem is made simple, but
complications arise in measurement and control of parameter~, and nonhydrostatic
components of stress appear. Z`he pressure limits in such devices ar.e 60,000 tn
65,000 atm.
It turne~i out, howeve~, that to achieve quasihydrostatic pressure (that is,
achieved by compressing so].ids), nther devic~s were more effective. Among them must
be noted the Bridgman anvil, the "Belt" device, and multiplunger devices. The
devices shown in Fi~;ure 1 b through h, with parts made of hard alloy, reach
pressures of up to 200,000 atm, depending OI1 design details. Recentl.y, F. Bundy,
af the General Electric laboratory, developed the "Belt" type of design, with a
- tip of syntheti.c poLycrystalline diamond, capable ~f generating pressures of up to
500,000 atm.
All ttiese devices can be adapted for conducting various physical, mineralogical,
technological, and other experiments, zncluding x-ray research on crystalline
structure of matter.
In recent years, the diamond-anvil me~had has received widespread use (fig. 2).
It began to develop intensivEly after the means was pr.oposed for measuring pressure
by the R-line shift of ruby 1~xminescence (the wave length of ruby R-line lumines-
cerice is virtually linearly dependent on pressure).
The essence of the method is as fol.lows. Two diamt~nds of
~ewe1 quality, polished in a specia'1 way (see fig. 2), are brought
together by means of an uncomplicated lever-screw or hydraulic
device. The specimen is placed directly between the working
sur�aces of the diamonds and, in ehis manner, a significant
pressure gradient ari.ses (see colored insert).* Improved tech-
nology provides for placing the specimen together with a small
piece of ruby i.n a washer made of strong and sufficiently plastic
mater.ial, as.shown in the i.nsert ~phc~tograph 4). A hole in the
washer is Fi.l.led with a mixti.~re of: methyl and ethyl al.cohol, which
p,rdvides hyclrostatic pr.essure up fio 100,0()0 atm. At the present
~ Photographs were made by V. N. Kachinslciy, as~ociate of the high-pressure
. sector of the Institute of Cryst~tl.lography imeni A. 4~. Shubnikov o� the USSR
Academy of Sciences.
~
_ ~
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z,
- time, methods have been develo~ed that permit using solidified
_ inert gasses, including helium, as a means �or tsansmitting
pressure. .
- Characteristic dimensiores of the device are as follows: diameter
af the working surface~;~ 0.3 to 1 mm; dimension o� supporting
surfaces, about 3 to 5 mm; the weight of each diamond, about 0.3
to 0.1 carat; the original thickneas of the washer, 0.1 to 0.3 mm;
and.the diameter o� the hole in the washer, 0.15 to 0.3 mm. The
whole device wi11 fit into the palm of a hand.
~ _ _ . .
' a (a)
i i%~-i ~
' 2I ~ ~ .
i , ~ . ~ , i
---f
- I 2 6 ~b~` 3
l
~ ~ -i -i
_ ~ % ~ s~ ~ .
~ i
_ ~ 2 i
~ / ~ i
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ �
_ ~ I ~ ~,f ~ ~ ~ J
I 3
~ Figure 2. Diamond Anvils
1. Diamonds; 2. Metal washer; 3. Specimen;
~ a and b are two tqpes of diamond working surfaces:
a-- simple anvils, and b-- ~aith additional conic ~
- grinding of the working surfacE~ at a small angle, which
reduces the stress gradient and permits the achievement
of greater pressure. ~
With diamond anvils, it is possible to obtain extremely high pressures of up to
- 1.7 million atm.* The device is easy to use for conducting various optical, x-ray,
Mossbauer, and other experiments. 4 virtue of this device of no little importance
_ is the possibility of visual observation.
In recent years, Academician L. F. Veresh~hagin and hia associates at the Institute
of High--Pressure Physics of the USSR Academy of Sciences and, aubsequently, A. Ruoff
of Cornell.Universit;~ (United StaCes) proposed using, to achieve very high pressures,
'f~ See H. K. Mao and P. M. Bell. "Science," 1978, vol. 200, p. 1145.
See S. Blok and D. P'yermarini [S. Block and D. Piermarini]. "Uspekhi fiz. nauk"
~Advances in Physical Sciencesa], 1979, vol. 127, p 765.
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the indentor-plane type of device. It is clear that this device can develop very
high contact pressures. However, this dovic~ cart be used onl.y ta m~asur~
electrical resistance, and the value oF this typ~ of device �or phyeicgl research
is not very clear.
MEASUREMENT OF HIGH PRESSURES
There is a large numUer of instruments for measuring hydrostatic presasre. They
_ include tubular, diaphragm, resistance, capacitance, and other manometers. However,
~ all of them are secondary and need calibrating with calibration instruments. Such
an instrument is the wei~hted piston manometer, in which an unknown pressure of a
_ liquid is balanced with a weight, azded by the piston system. As a result, all
that has to be known is the weight of the wei.ght and the diameter of the piston.
_ However, for precise determination of pressure, ir is necea~ary to take into
consideration the deformation of the piston atid cylinder and the hydrodynamic
ef.fects arising in the flow of liquid through the gap, and so forth. Deformation
corrections become especially large under pressures of 15,000 to 20,000 kgf/cm2.
The precision class of weighted-piston manometers in this r�ange of pressures is no
greater than 0.2 to O.S. Thus, absolute accuracy of ineasuring pressure in this
range c�:~nnot be less ehan 30 to 100 kgf/cmZ, and, consequently, the accuracy of
pressure measurement by any secondary devices cannot be less than the magnitudes
mentioned.
- The most widely used secondary pressure sensor is the so-called manganin
manameter, which is rppresented by a coil oi manganin wire with resistance of about
= 100 ohms: The electrical resi_star.ce of the nianganin ri.ses almost linearly with
pressure and possesses little temperature dependence at room temperature. In the
- use of the cali.brating weighted piston manometer, secondary instruments can be
calibrated up to pressures on the nrder of 30,OOO.to 35,000 atm without any
siibstantial loss of accuracy and, consequently, it does not generate special
problems in measuring pressure in this area. Rst under pressures exceeding these
values, there are more than enough problems. Ttie scale of pressures in this area
is based on reference lines, for which phase transitions in certain elements and
compounds are selected. The selection of reference substances is conducted with
consideration for many factors, one of which is the ease of observation of phase
transitions, for exnmple, with th~ axd o� measur~ment uf ele:.trical resiscance.
The pressures of ghase transitions, which serve as reference points can be
ro~ighly determineci with accuracy of the order of several kilobars with the aid of
char~e CO? lasers according to cooling methads, we glve their classification
by ~asdynamic and electra-optical arrangements, working conditions of cavities,
and met.hods of gas-discharge pumping, as well as examining the most advisable
L-ields of appli~ation and the limiting characteristics of different laser sqstems..
ParCicular attention is given to descript~on and analysis of differ.ent gas-discharge
arra?igements used for excitation of the active medium of C02 lasers. Not only
tlie Pliysi.c_a' , t~.t also the technical asper_ts of implementing these arrangements
are discuss~:d.
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1. Designs o1~. ~02 gas-discharge lasers
In the C02 laser, stimulate.d emission takt~s place on the transition between vi-
brational energy levels QO�1 and 1Q�0 of tt?~~ C02 molecule [Ref. 1, 2]. The quantum
efficiency of t't5'1:'~ transition, corresponding to emission of quanta with wavelength
of 10.6 Um, is fairly high: nKB = 0.4. ~
Due to heat release upon lasing of the mixture (5-10% of the ~nergy released in
the discharge goes to heating), and collisional relaxation of the upper laser
level, the processes of pumping of the Iaser mixture and stimulated emission are
unavoidably accompanied by heating of the gas, the temperature Tr of the lasing
miYture in the steady state being practica~ly proportional'to the power of the .
ener~y release in the discharge.
In the absence of lasing, the population of the upper laser level is propo~ctior~al
to the intensity of energy release in the discharge, and consequently (disregarding
dependence of the cross section of collisional relaxation ~r on gas temperature),
to the gas temperature as well. This relationship can only be weakened by account-
ing for the dependence of ~r on Tr.
The population of the lower lasing level is determined by collisional processes,
and at radiation intensities that are not too great it corresponds to Boltzmann
law, i. e. it increases exponentially with increasing Tr. In this connection,
- ~ upon attainment of a certain critical temperature TKp ~ 500-600�C [Ref..22], in-
verse population of the laser mixture dissapears. The maximum inversion is at-
tained at a mixture temperature of ToPt~ 200-300�C.
Thus, one of the principal conditions of C02 laser operation.is inadmissib.ility
of overheating the laser. mixture beyoild Topt, i. e. availability of efficient
_ cooling. At the present time, classification of electric- ~
discharge lasers has been proposed in accordance with q Q �p
methocis of cooling the l~ser mixture and stabilizing the 1~~-4~~~q~ ~8-".
discharge [Ref. 20, 21]. The sense of this classification ' v-0 2
is illustrated by Fig. 1. ~ R
~ ~ 4iQ-= -
In lasers of the f irst~type (discharge tube coaxial with ~ 1
the optical system), the laser mixture is cooled and the m - ' p
discharge is stabilized by diffusion processes. The heat ~ 4+ 4~ p Y
- released is carried off to the cooled wall of the laser
tube via molecular diffusion, and ambipolar plasma dif- Fig. 1. Types of C02
fusion prevents discharge contraction. The presence of lasers: v--gas flow
gas flow in lasers of this type is not in principle neces- velocity; Q--heat flux;
- sary, and its role is reduced to preyenting poisoning q--charged particle
- of the active meditun as a result of various plasma- flow, P--quantum flux;
chemica]. reactions [Ref. 'l3]. 1--opaque mirror; 2--
~ output mirror
In lasers of the second type, differing from the first only in rapid circulation
~of gas through the discharge tube, discnarge stabiliz~tion is handled as before
by ambipolar diffusion; however, the temperature of the laser mixtvre is main-
tain~d on the admissible level by the rapid rate of gas circulation (convective
coolin~) .
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r~~x ~~~~ri~.aAL u~c vi~~.i
Finally, in lasers of the third type the gas is circulated in the directinn per-
pendicular to the optical axis. In this case, it is the rapid gas flow that
prevents both overheating of the mixture and discharge transition to arcing.
It should be noted that such a classification does not always reflect the princi-
ple of discharge stabilization or particulars of the electro-optical arrangement
- of the laser. The characteristic diffusional dimension is not always determined
by the radius of the glass tube or the size of the discharge zone. At high gas
velocities, charge diffusion due to flow t~irbulence may exceed ~unbipalar dif-
fusion and play an appreciable part in dl.scharge stab ilization. The discharge
may be stabilized r.ot only by dif�usion, but also by electronic means (combined
action of electric fields [Ref. 24], rotation of the electric field vector in
_ space [Ref. 25], etc.). Flow in the direction perpendicular to the optical axis
does not preclude diffusional discharge stabilization [Ref. 26]. Moreover, de-
pending on the degree of f low turbulence, the same electric diagram of a laser
may be classified as both type II and ty~e III [Ref. 26]. Therefore, hereafter
we will divide al1 lasers with resper_t to type of mixture cooling (diffusional,
- r_onvective).
One of the most important chara~teristics of the laser is its efficienGy. The
total efficiency r~ of the laser defined as the ratio of the laser emission power
to the total expended electric power can be represented as
_ _
_ _ _
~-~~cn~H'IOIIT'Ip'ICO~ ~i~
~ where nI~ is the vibrational efficiency of tlie puznping method, i. e. the fraction
of power released in the positive discharge columi~ that is expended on excitation
oE vibrational levels 00�1 of C02 and v= 1 of nitrogen; nonT is the optical effi-
ciency equal to the fraction of vibrational excitation coupled out of the reso-
nator cavity to molecules that make a transition to the ground state as a result
of stimulated emission processes; nP is the eFficiency of the discharge circuit,
which is equal to the ratio of electric power released in the positive column
to the power of discharge supply sources; nco is the efficiency of the coolin~;
system with consideration of the efficiency of supply sources, as well as power
~ expenditures on producing gas flow and maintaining working pressure. Most publi-
- cations on gas-discharge lasers represent experimental laboratory research and
: do not contain information on the optimum values of nco� Theref.ore the efficiency
of laser systems is frequently chara.cterized by the so-called electro-optical
eff:Lciency:
- ~ ao -~1 ~cn~l i~rl onT+ ~2~
clefined as the ratio of laser emission power to the electric power released in
the positive discharge column.
1.1, C0~ lasers w~~th dif:frision cooling of working.mixture
_ Sliown in Fi~. 2 is a typica]. diagram of a C02 laser with diffusional cooling of
the worlcing mixture (diffusion laser) tliat is the simplest among other laser sys-
tems. Usual.ly the diff.usion laser cons~sts oi a water-cooled discharge tube 1
within which a se].F-maintained discharge is kept alive by electrode system 2.
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~ The nirrors of the opt~c31 cavity are placed at the
ends of the diacharge i.ube: opaque mirror 3, and
working
coolant myXture ~emi-transparent (or beam-hole) inirror 4. Stabi ity
� ~ of the laser output characteristics over a prolonged
i~--=~--=~=+i P time is maintained by weak circulation of the l.as~r
~ r--_ ~ mixture, or by placing a regenerating element inside ~
z Ra w the sealed-off laser [Ref. 23]. As a rule, diffusion
+ lasers use mixtures of C02:N2:He = 1:1:3 or 1:1.6 (or
close to them) at overall pressure p-10-20 mm Hg.
Fig. 2. Construction of
laser with diffusional Limiting discharge characteristics of the diffusion
cooling of working mixture ~aser are due to the efficiency of cooling the work-
- ing mixture (Topt.~ 250�), and also the discharge stability. Ma.ximLUn values of
- the volumetric energy input due to rate of cooling of the mixture o~ can .
be evaluated from the stationary equation of heat balance in the discharge
.
. _
~IE)ozn~x(T~p~-T~T~~~a, 3~ ' .
where j and E are the current density and electric field strength in the dis-
charge, r,= cppD is the hPat conduction of the gas mixture (cp, p and D are the .
specific heat, density and coefficient of diffusion), T~T is the temperature of
- the cooled wall, l1= R/2.4 is the characteristic dimension that determines heat
transfer in a cylindrical tube of radius R.
As a rule, pinching of the discharge in the tube is ionization-thermal in nature
[Re�. 17, 27]. The increment of development of this instability can be repre-
sented as
~ y~Anp
_ and gasdynamic parameters ot the flow or geometric characteristics of the gas-
discharge chamber. Therefore the np necessary for calculating convective
lasers must be taken from experiment.
28
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APPROVED FOR RELEASE: 2007/02/09: CIA-RDP82-00850R000500050030-8
t
In the case of~'a~transverse discharge,�it has been shown by ex~eriments [Ref. 5,
11, 12, 34, 43] that np changes in the following way: 1) increases with pres-
sure up to p~ 20-40 mm Hg, and then saturates or begins to decrease; 2) increases
with gas velocity up to v~ 50-100 m/s, and then saturates or begins to fall; 3)
decreases witYi,~:ncreasing.height H of the discb?rge gap; 4) beginning at a ce'r-
tain length, decreases with increasing Z(there is no additivity of the contri-
bution in the direction of flow); 5) increases with addition of He to the mixture;
6) decreases with increasing C02 content and H20 in the working mixture. Specific
mass energy inputs wg i.n a gas-discharge chamber with self-maintained dc discharge
decrease with increasing pressure, and increase with increasing Z[Ref.12, 34,43].
Typica]. characteristics of gas-discharge chambers with self~naintained dc discharge
are summarized in Table 2.
Most gas-dischar~e chambers with transverse discharge use ~nixtures that contain
he:titun; at partial pressure of nitrogen not exceeding 30 mm Hg, and~total pres-
sure p of the mixture of the order of 50 mm Hg or less, flowrate of 30-100 m/s,
H= 3-6 cm and Z~ 20-40 cm, we can get np ~ 2-5 W/cm3 at wg ~ 250-300 J/g in .
he.l_ium mixtures, and 150-250 J/g in helium-free mixtures. Because of the high
cost of helium, the use of helium-free mixtures is of particular interest for
lasers with partial renewal of the working mixture.
~ Substituting typical values of np in (15), we get Z~ v/vr for a gas-discharge '
chamUer with self-maintained dc discharge, and therefore high n3O can be achieved ~
only by using a combined cavity scheme. Si,nce the diameter of mirrors used in
_ the laser D~ H< Z, resonators of convective lasers with ptunping by self-maintained
dc discharge are nearly always multipass [Ref. 5, 11, 12].
At the present time the characteristics of longitudinal self-maintained dc dis-
char~;es have not been adequatel~s studied. For. l~aer mi.xtur~s we have on?;~ is~lated
values ot-