Plasma Technology: Fundamentals and Applications

225 Pages·1992·5.91 MB·English

by  A. A. Rukhadze (auth.), Claudine Gorse (eds.)

PLASMA TECHNOLOGY
Fundamentals and Applications
PLASMA TECHNOLOYG
Fundamentals and Applications
Edited by
Mario Capitelli
and
Claudine Gorse
University of Bari
and Centro di Studio per la Chimica
dei Plasmi del CNR
Bari, Italy
SPRINGER SCIENCE+BUSINESS MEDIA , LLC
Librar y of Congress Catalog1ng-1n-PublIcatIo n Data
Plasma technolog y : fundamentals and application s / edite d by Mario
Cap1 t e1 11 and Claudlne Gorse.
p. cm.
Includes bibliographica l reference s and index.
ISBN 978-1-4613-6502-0 ISBN 978-1-4615-3400-6 (eBook)
DOI 10.1007/978-1-4615-3400-6
1. Plasma devices—Congresses. 2. Plasma (Ionize d gases)-
-Congresses. I . Cap 1te111, M. II . Gorse, Claudlne.
TA2030.P53 1992
621.044—dc20 92-13625
CIP
Proceedinsg of an international workshop on Plasma Technology
and Applications, held July 5-6, 1991, in II Ciocco (Lucca), Italy
ISBN 978-1-4613-65020-
© 1992 Springer Science+Busines sMedai New York
Originalyl published by Plenum Press, New York in 1992
All rights reserved
No part of this book may be reproduce,d stored in a retrieval system, or transmitted
in any form or by any means, electronic, mechanica, lphotocopying, microfilming,
recording, or otherwise, without written permission from the Publisher
PREFACE
The present book contains the proceedings of the workshop
"Plasma Technology and Applications" which was held at 11
Ciocco (Lucca-Italy) during 5-6 July 1991. The workshop was
organized just before ICPIG XX to emphasize the role of plasma
physics and plasma chemistry in different fields of
technology. Topics cover different applications such as lamps,
plasma treatment of materials (etching, deposition,
nitriding), plasma sources (microwave excitation, negative ion
sources) and plasma destruction of pollutants. Several
chapters deal with basic concepts in plasma physics, non
equilibrium plasma modeling and plasma diagnostics as well as
with laser interaction with solid targets.
The authors gratefully acknowledge the financial support
provided by university of Bari (Italy) and by CNR (Centro di
Studio per la Chimica dei Plasmi, Istituto di Fisica Atomica e
Molecolare (IFAM) and Progetto Finalizzato Materiali Speciali
per Tecnologie Avanzate) as well as the sponsorship of ENEA.
M. Capitelli
C. Gorse
v
CONTENTS
Plasmas in nature, laboratory and technology 1
A.M. Ignatov and A.A. Rukhadze
Laser diagnostics of plasmas 11
L. Pyatnitsky
Probe diagnostics of plasmas 27
G. Dilecce
Theory, properties and applications of non
equilibrium plasmas created by external
energy sources 45
E.Son
Non-Equilibrium plasma modeling 59
M. Capitel1i , R. Celiberto, G. Capriati,
C. Gorse and S. Longo
Gas discharge lamps 81
M. Koedam
Plasma etching processes and diagnostics 93
R. d'Agostino and F. Fracassi
Plasma deposition: processes and diagnostics 109
A. Koch
Correlations between active plasma species
and steel surface nitriding in microwave
post-discharge reactors 125
A. Ricard, J. Hubert and H. Michel
Simultaneous removal of NOx,SOx and soot in
diesel engine exhaust by plasma/oil
dynamics means 143
K. Fujii
DeNOx DeSOx process by gas energization 153
L. Civitano and E. Sani
Microwave excitation technology 167
P. Leprince and J. Marec
Negative ion source technology 185
H.J. Hopman and R.M.A. Heeren
vii
Quasi-stationary optical discharges on solid
targets 203
V.B. Fedorov
Index 223
viii
PLASMAS IN NATURE, LABORATORY AND TECHNOLOGY
A.M. Ignatov and A.A. Rukhadze
General Physics Institute
Moscow, USSR
WHAT IS A PLASMA?
A plasma as a state of matter has been known to people from times immemorial. The first in
Europe who realized that our Universe consisted of four roots - earth, water, air and fire, was a
Greek philosopher Empedocles (about 430 BC). Some time later, in 1879 (AD), William Crookes
distinguished the medium created in electrical discharges as the fourth state of matter which Irvine
Langmuir named a plasma in 1923. Nowadays we know a little more about the subject, and we have
to confess that Crookes was wrong. More than 99% of matter in the Universe is a plasma, and it is
rather to be called the first state of matter.
Primarily, a plasma was defined by Langmuir as a gas consisting of electrons, several types
of ions and neutral atoms and molecules. Nowadays the range of application of thiswordismuch larger.
Thus, we can speak about the plasma of metals and semiconductors, the plasma of electrolytes, the
quark-gluon plasma, etc. Anyway, unlike mathematics, it is not very wise to give any definitions in
physics: the best way to understand something is make out its features. Therefore,we should rather
regard a plasma as some mixture of charged and, perhaps, neutral particles and study its main
properties.
First of all, we have to introduce the main plasma parameters. They are:
1. The particle density, i.e. the number of particles in one cubic centimeter, is usually
denoted as n with some subscripts indicating the species of particles( e.g. n e for electrons,
n i for ions, n 0 for neutrals). Usually, the plasma on the whole has no electric charge, i.e. if there is
only one type of ions with the charge - e , their total number is equal to the number of electrons with
the charge e, although the local densities of the species may vary in time and space. The typical
=
value of the electron density in the laboratory plasma is; n e 105 + 1015 cm -3, sometimes up
to 1020 cm -3, and in some astrophysical objects it may be up to 1032 cm -3.
An important parameter is the plasma ionization rate
ne
r=-- (1)
ne+no
characterizing the relative number of charged particles. For the weakly ionized plasma this quantity is
less than 10-2, the plasma is fully ionized if it is of the order of unity.
Plasma Technology, Edited by M. Capitelli and C. Gorse
Plenum Press, New York, 1992
2. The plasma temperature is the average kinetic energy of particles. It is usually measured in
electron-volts ( 1 eV = 11604 deg Kelvin). If two particles have a large mass ratio then the energy
exchange between them is reduced and slowed down. That is why the groups of particles with
different masses may have different temperatures; the ratio of electron and ion temperatures may
exceed one thousand.
The average distance between particles is of the order of n -1/3, i.e. the average potential
energy of the particle with the charge e is e i n1 /3 ( Here we use the CGS units). The ratio of the
average potential energy and the temperature
(2)
is another important dimensionless number which is called a gas parameter. If it is small, rJ«I, and
in most applications it really is, the particles move relatively free and the plasma behaves like a gas,
while for large rJ» 1 the plasma looks rather like a fluid or even a solid state. In this lecture we
discuss the properties of gaseous, or ideal plasmas with 'YJ < < 1 only. The plasma temperature varies
from about 0.1 e V (flame) to 10 ke V desired for the thermonuclear fusion, and, as usual, much larger
magnitudes may be found in the sky.
3. Some external parameters are also to be added to this list. The most important of them is
the magnitude of the external magnetic field, BO, which may reach the value of 100 kG for laboratory
plasmas.
Of course, there are many other plasma parameters which are relevant for its numerous
applications, but, in fact, most of physical phenomena may be understood with the help of these
three quantities.
Although the plasma physics may seem to be a very complicated matter, it is based on a
few simple phenomena. Here we discuss briefly three of them.
Pebye screenjn~
Suppose that we have immersed some charged object, e.g. a positively charged plate, into the
neutral plasma ( ne = nj ). It creates an electric field acting upon the charged particles of a plasma,
i.e. it attracts electrons and repulses ions. Therefore, any charged object causes the distortion of
the charge distribution in its vicinity yielding in turn to some distortion of the electric field. We can
describe this process in a following simple way.
First, let us ignore the ion motion - they are massive and cannot be shifted so easily. The
distortion of the electron density, CJne ,may be described with the help of the Boltzmann formula:
ne + CJne (x) = 11e exp (-e<p (x) I Te) , where tp(x) is the electric potential, x is the distance to a plate,
ne is the unperturbed electron density and e is the charge of the electron ( e<O ). If the potential
tp (x) is small enough we can expand the exponent and approximately express the density distortion
as
= _
CJne enep(x)
(3)
Te
Recollect now that the potential itself depends on the charge distribution according to the
Poisson equation
d2,,,(x\ =
~ -41teCJne(X) (4)
Solving it together with Eq.(3) yields to the desired result
tp (x) =1{kJ exp (-x / rD) (5)
2
where f{J 0 is the potential of the plate and
T ) 1/ 2
rD= ( e (6)
43re2 ne
The equation (5) expresses a very important fact: the static electric field cannot penetrate into
a plasma deeper than few rD ; as they say, it is screened. The intrinsic scale rD (6) is called the
Debye length in honor of Peter Debye who studied this phenomenon in electrolytes.
According to the Eq(3) the electron density deviation is proportional to the potential. It
means that the charge neutrality of a plasma cannot be violated at the scale larger than the Debye
length. Anyattempt to separate electrons and ions causes the electric field attracting them to each
other. This is the essence of one of the most important ideas of plasma physics, the self-consistent field
concept: we cannot regard particles and fields in a plasma as independent, they have to be considered
as a single object.
It should be noticed that the expressions (3) and (6) are valid in the gaseous plasma only.
Inequality 17< <1 (see Eq. (2» means that many particles are to be in the volume of the size of the
Debye length. Otherwise we would obtain the nonsense : the field is screened by less than one particle.
Waye propa~tion
What we have described is the plasma response to the static electric field. Its response to the
alternating field is also of great interest. To illustrate it let us consider the equation of motion of a single
electron
m -d- v- a(tt) = eE(t) -vv(t) (7)
where E (t) is the electric field and v (t) is the electron velocity. The second term in the
right-hand side of Eq.(7) represents the frictIonal force acting upon an electron moving through a
plasma and the coefficient V is the so-called effective collisional frequency. This term roughly
describes the short-range interactions between particles and depends on what particles the electron is
colliding with; we shall discuss its value a little later.
We can always consider the reaction of a plasma to the monochromatic wave, i.e. represent
the electric field as E (t) = Eexp (-iw t + ikx). Substituting this into the Eq(7) we obtain for the
velocity
(8)
Now we can calculate the current density:
. iinE
J =en v = m (W+iV) =aE (9)
where
a= m (W+iV) (10)
is a complex conductivity of a plasma.
3
It should be stressed that the conductivity is a frequency dependent complex quantity
consisting both of real (active) and imaginary (reactive) parts. As usual, the dielectric
permittivity, e (w), of a medium is connected with its conductivity by the relation
are.
e(w)=1+4Jwri G=l_ w (w + LV) (11)
where
1/2
wp = ( 4 Jr~2 n ) (12)
The last quantity has the dimension of the frequency and, respectively, is called the plasma frequency.
It gives us a natural time scale for various plasma processes.
To make out its physical meaning recollect that in any medium the electric displacement, D,
for not too strong fields is proportional to E , namely, D = e E .. If there are no external sources then
=
D 0 . In most dielectric media it means that E = 0 , but in a plasma, as may be easily seen from the
Eq.(ll), the dielectric permittivity may be zero for some frequency. Suppose for a moment that the
frequency w in Eq.(ll) is large compared to the collisional frequency and put V =0. Then the
solution of the equation e (w) E = 0 may be written either as E = 0 fo r th e arbitrary
w oras W =W p for the arbitrary E ¢() •. It means that the electric field in a plasma can sustain itself
and oscillate with the frequency Wp without any external sources. These intrinsic oscillations were
discovered by I.Langmuir and now are called after his name.
Let us look now for the solution to the equation
=
e (w) 0 for v¢() . Suppose that Wp>>V (for most cases of interest it is really so), then the frequency
of the Langmuir oscillations is
(13)
As we see there appears a small negative imaginary part of the frequency signifying the temporal
damping of the oscillations which is caused by the collisions (short-range interactions) between
particles. Sometimes this collisional damping is irrelevant due to a very high frequency of the
oscillations or a very short duration of any other process we are interested in. In this case they speak
about a collisionless plasma, although we have to remember that this term is pretty conventional.
Exact expressions for the collisional frequency are obtained by the kinetic theory of a
plasma. For our purposes it is sufficient to notice that v may roughly be estimated as a maximum
of the electron-ion collisional frequency, Vei , and electron-neutral collisional frequency, Ven ,
each of them mea=nirnr~ the average number of collisions of various species per second. By the order
of magnitude Vei 2 Wp, i.e. it is small compared to Wp i n gaseous plasma, and
=
ven=d lIT no, where a is a radius of a neutral atom and lIT (Telme)1I2 is the electron thermal
velocity.
Finally, notice that the frequency of the Langmuir oscillations we have obtained does not
depend on the wavelength. It is because we have ignored the thermal motion of the particles.
Taking the latter into account yields to some corrections to the plasma frequency depending on the
wavenumber.
We can regard the propagation of the electromagnetic waves nearly in the same manner.
Actually, we already have everything to do it. The index of refraction of a media , n, ( by
definition, n = c k/ W , C being the speed of light in vacuum) is related to the dielectric permittivity:
n2 = e (w). The necessary condition for the propagation of the electromagnetic wave is n = 1, i.e. the
dispersion relation determining the dependence of the wave frequency, w, on its wavenumber, k, is
(14)
4