CHARACTERISTICS OF PULSED DISCHARGE SOURCE-

PROTOTYPE FOR GAS ANALYSIS BY PLASMA ELECTRON SPECTROSCOPY (PLES) METHOD

K.D. Kapustin(1), A.A. Kudryavtsev(1,*), P.M. Pramatarov(2), M.S. Stefanova(2)

(1) St.Petersburg State University, St.Petersburg, Russia

(2 Institute of Solid State Physics, Bulgarian Academy of Sciences, Sofia, Bulgaria (*) akud@ak2138.spb.edu

A plasma electron spectroscopy (PLES) method for investigation of penning and associative ionization processes was developed in [1]. It is based on the probe measurements of electron energy spectra e in plasma, which are created by collisions of excited and neutral particles:

* { }pA B A B e e++ ® + + , (1)

near energy of penning electrons *p ie e e» - (where, *e is the ecxitation energy of excited

(metastable) particles *A and ie is the ionization energy of particles B ). When electron energy

distribution function (EEDF) is formed in nonlocal regime (see [2] for details), i.e. electron energy relaxation length el is greater than the characteristic discharge dimension L - Lel > ,

the electrons move in discharge volume with conservation of their full energy ( )w e re j= +

(kinetic plus potential energy). If the energy of penning electrons is p wee > F (where wF is the

potential between axis and discharge wall), these electrons reach the wall in a free diffusion regime. In the absence of electric field (e.g., in the afterglow) or when it has a small value (e.g., negative glow plasma, beam plasma), when eT , and hence weF are small, the EEDF of penning

electrons has a shape of sharp peak near energies pe e= , reproducing the spectrum of reactions

( )R e of type (1) [1].

Using the mesuared electron spectra ( )R e and the well known constants for reaction (1) one can identify the presence of inpurities and determine their concentrations [3]. It is convenient to use helium metastable atoms as *A particles in (1) because their high excitation energy (19.8 eV) is sufficient to ionize any other atom or molecule (except Ne) and to produce characteristic electrons having kinetic energy of several eV. According to (1): *

p ie e e» - .

To demonstrate the ability of PLES method proposed, a prototype of miniature plasma source for inpurities identification is constructed. The source-prototype is a short pulsed glow discharge in helium, consisting of cathode fall and nonlocal plasma of negative glow (without positive coloumn). The electrodes are made of 4 mm diameter Wo rods and inter electrode gap is 4 mm. The measured voltage-current characteristics (VAC) for discharge are shown in figure 1. They have positive slopes, as it should be for short discharges.

One- and two-dimensional self-consistent fluid models were are used for simulations. The plasma-chemical model includes five atomic and two molecular excited levels of helium and more than 80 reactions between them. The rates of plasma-chemical reactions with participation

Topic number: 10.

20th ESCAMPIG, 13-17 July 2010, Novi Sad, Serbia P3.33

of electrons and their transport coefficients were determined by solving the kinetic Boltzmann equation. The self-consistent electric field was calculated using the Poisson’s equation.

The results from the simulations in the active phase of the discharge at 40 Torr He pressure, 15 mA pulsed current and 300 V applied voltage (see Fig. 1) are presented in Figs. 2-4. Spatial profiles of the densities of electrons ( en ), ions ( in ) and helium metastable atoms

( mn ) at the end of the active discharge phase are presented on the Fig. 2. It is clear that the

discharge consists only of cathode layer of 0.3 mm thickness and of negative glow plasma. In Figs. 3, 4 the temporal evolution of electron, ion, helium metastable atom and molecule densities and electron temperature in the afterglow are presented. After a rapid fall of the electron temperature (some tenths of microseconds), the metastable atom density remains relatively high for a long time (hundreds of microseconds).

The results demonstrate that the small pulsed plasma source makes it possible to obtain relatively high densities of metastable and charged particles and low electron temperature necessary for PLES realization (implementation).

Fig. 1. Experimental VAC. Fig. 2. Axial distributions of en , in and mn

Fig. 3. Temporal decay of en , in and mn Fig. 4. Temporal decay of electron temperature

References [1] A.B. Blagoev, N.B.Kolokolov and A.A. Kudrjavtsev, 1994 Physica Scripta, 50 371 [2] L.D.Tsendin, 1995 Plasma Sources, Sci. Technol. 4 200 [3] A.A.Kudryavtsev and A.B.Tsyganov, December 18, 2007 US Patent 7309992

20th ESCAMPIG, 13-17 July 2010, Novi Sad, Serbia P3.33