chemical physics of discharges - Argonne National Laboratory
chemical physics of discharges - Argonne National Laboratory chemical physics of discharges - Argonne National Laboratory
Again if this power gain is equated to the loss of energy of electrons in elastic collisions with the gas molecules, as would occur in the steady state, then qEo where a f -. m E 8 From (26) one would expect the electron "temDerature" for fixed Eo to diminish with increasing frequency, but for pressures of the order of one torr and gas masses of around 30 AMU the electron temperature will remain near the dc value for frequencies usually employed for gas discharge work. B. Elastic Collisions, and Diffusion - Since in either dc or ac discharges, the electrons will have high temperatures, one can expect that all the manifestations of a kinetic temperature will be present. Particularly important among these is diffusion through elastic collisions. If only electrons are present in a neutral gas they will tend to diffuse as would any other gaseous component with a current density where ne is the number density of the electrons and De is the diffusion coefficient of the electrons through the gas. will contain a mobility term as well so that I (25) If a field is also superposed, the current density \ , where the minus sign in the second term indicates that because of the negative charge 4 on the electrons, their motion will try to be in the direction opposite to that Of the applied field. pe is taken to be a positive number. Whether the electron motion is diffusion-dominated or field dominated depends on whether the first term l is larger '1
I 7 than or smaller than the second. In a gas discharge, the electrons will have produced ions and :these also will tend to diffuse through the neutral gas as well as respond to any electric fields. The current density of ions will be given therefore by a similar equation --3 s, = -9+ On, 4 “+F+ 9 From Eq. (4) the mobility of either type of particle is & Ifl[m(z>) and from kinetic theory, the diffusion coefficient is given by D e. KT/t-m). Since both these parameters have the mass appearing in the denominator the electron will diffuse and respond to an electric field much more rapidly than will the heavy ions. The case of major interest for discharge physics is that where the number density of ions is very nearly equal to that of the electrons. Clearly because the electron mass is very light these will try to diffuse awa:y from the ions. However, as soon as they do this, an electric field is set up between the electrons and ions so that the electrons are held back by the ions and the ions are dragged along by the electrons. We would thus expect that the current fluxes Se and S+ would be equal. If we set Se = S, 5 S, and ne = n+ t n, to indicate an electrically neutral plasma, Eqs. (28) and (29) can be combined to eliminate the electric field with the result that where Since and or and b z & re ,gel we can write (29)
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- Page 21 and 22: 21 region in w!iich large, nighlv l
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- Page 27 and 28: 27 (22) Franklin, J. L., Munson, M.
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- Page 41 and 42: ION-MOIECULE REACTION RATES MEASUFI
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7<br />
than or smaller than the second.<br />
In a gas discharge, the electrons will have produced ions and :these also will<br />
tend to diffuse through the neutral gas as well as respond to any electric fields.<br />
The current density <strong>of</strong> ions will be given therefore by a similar equation<br />
--3<br />
s, = -9+ On, 4 “+F+<br />
9<br />
From Eq. (4) the mobility <strong>of</strong> either type <strong>of</strong> particle is & Ifl[m(z>)<br />
and from kinetic theory, the diffusion coefficient is given by D e. KT/t-m).<br />
Since both these parameters have the mass appearing in the denominator the electron<br />
will diffuse and respond to an electric field much more rapidly than will the heavy ions.<br />
The case <strong>of</strong> major interest for discharge <strong>physics</strong> is that where the number density<br />
<strong>of</strong> ions is very nearly equal to that <strong>of</strong> the electrons. Clearly because the electron<br />
mass is very light these will try to diffuse awa:y from the ions. However, as soon as<br />
they do this, an electric field is set up between the electrons and ions so that the<br />
electrons are held back by the ions and the ions are dragged along by the electrons.<br />
We would thus expect that the current fluxes Se and S+ would be equal.<br />
If we set Se = S, 5 S, and ne = n+ t n, to indicate an electrically neutral<br />
plasma, Eqs. (28) and (29) can be combined to eliminate the electric field with the<br />
result that<br />
where<br />
Since<br />
and<br />
or<br />
and b z &<br />
re ,gel<br />
we can write<br />
(29)
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