Final Report for Award DE-FG02-99ER54554 Kinetics of Electron Fluxes in Low-Pressure Nonthermal Plasmas Page: 2 of 6
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(a) total energy = 5eV (b) total energy = 12 eV
P 1 1 t t / / /
L 17 -
4 r -
0 5 10 15 0 5 10 15
Radius (cm) Radius (cm)
Figure 1: Electron flux density in an Argon ICP at (a) 5 eV and (b) 12 eV total energy.
particularly simple with Langmuir probes, since the negative probe voltage scale is equal to the to-
tal energy up to an additive constant. From two-dimensional measurements of the EDF at constant
probe voltage and computations of the gradients of the measured profiles, we were able to derive
the electron flux patterns in coordinate space at constant total energy.
Figure 1 shows two electron flux density patterns which were measured for the same plasma condi-
tions at different total energies. Again, the vector arrows only represent the direction of the electron
flux. It can be seen that at the lower total energy of 5 eV the electron flux density is directed from
the central region of the discharge, which corresponds to the maximum plasma potential or the
"bottom" of the potential well, towards the periphery. At the higher total energy of 12 eV, the
maximum of the EDF has shifted closely towards the coil position inducing an electron flux to-
wards the discharge center. This flux pattern bears close similarity to the "convection cell" in con-
figuration-total energy space observed in the positive column simulation.
The electrons experience the strongest heating in the region close to the induction coil, where the
RF electric field is strongest. The region close to the induction coil can thus be considered as a
source of energetic electrons. For the above experiment, in which the plasma potential is maximum
in the discharge center, the space charge electric field preferentially draws the electrons towards the
center. Hence for given total energy, the maximum of kinetic energy is at the center. Due to the
monotonous increase of most excitation and ionization cross sections with kinetic energy close to
their thresholds, the discharge center also is the location of the maximum of ionization and excita-
tion processes. This region thus corresponds to a sink for energetic electrons and a source for low
energy electrons. The spatial separation of the maximum of inelastic processes and the maximum
of the electric field results in the observed flux pattern of electrons: Low-energy electrons are pro-
duced in the central region of the discharge and diffuse towards the periphery. Close to the coil,
they experience strong heating and are "lifted" to higher energies. For these high energy electrons
the sink region in the discharge center induces the inward directed electron flux. There, the "con-
vection cell" is closed by inelastic collisions that transform high-energy into low-energy electrons.
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Kortshagen, Uwe. Final Report for Award DE-FG02-99ER54554 Kinetics of Electron Fluxes in Low-Pressure Nonthermal Plasmas, report, December 13, 2004; United States. (digital.library.unt.edu/ark:/67531/metadc778896/m1/2/: accessed September 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.