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Plasma Technology: Fundamentals and Applications
Name: Plasma Technology: Fundamentals and Applications
Author: a. a. rukhadze auth.
Pages: 225
Year: 1992
Language: English
File Size: 5.91 MB
Downloads: 0
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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

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CONTENTS Plasmas in nature, laboratory and technology A.M. Ignatov and A.A. Rukhadze Laser diagnostics of plasmas L. Pyatnitsky Probe diagnostics of plasmas G. Dilecce Theory, properties and applications of non equilibrium plasmas created by external energy sources E.Son Non Equilibrium plasma modeling M. Capitel1i , R. Celiberto, G. Capriati, C. Gorse and S. Longo Gas discharge lamps M. Koedam Plasma etching processes and diagnostics R. d'Agostino and F. Fracassi Plasma deposition: processes and diagnostics A. Koch Correlations between active plasma species and steel surface nitriding in microwave post discharge reactors A. Ricard, J. Hubert and H. Michel Simultaneous removal of NOx,SOx and soot in diesel engine exhaust by plasma/oil dynamics means K. Fujii DeNO x DeSO x process by gas energization L. Civitano and E. Sani Microwave excitation technology P. Leprince and J. Marec Negative ion source technology H.J. Hopman and R.M.A. Heeren 1 11 27 45 59 81 93 109 125 143 153 167 185 vii

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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 = 10 5 + 10 15 cm 3, sometimes up to 10 20 cm 3, and in some astrophysical objects it may be up to 10 32 cm 3. An important parameter is the plasma ionization rate ne r= ne+no (1) 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

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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 n 1 /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 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) Te (3) 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

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where f{J 0 is the potential of the plate and rD= e ( T ) 1/ 2 43re 2 ne (6) 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 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 d v (t) m at = 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

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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 e(w)=1+ 4JriG =l_ are. w w (w + LV) (11) where ( ) 1/2 wp = 4 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 = Wp 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 the average number of collisions of various species per second. By the order of magnitude Vei =rr 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: n 2 = 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

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