Final Report for Award DE-FG02-99ER54554 Kinetics of Electron Fluxes in Low-Pressure Nonthermal Plasmas Page: 3 of 6
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With our two-dimensional model based on the solution of the nonaveraged Boltzmann equation
[J2] the exact same flux pattern could be found in the simulations. The model indicates that for our
experimental conditions, a typical electron goes through the "convection loop" more than ten times
before reaching the discharge wall. This can be concluded from the electronic excitation frequency
being more than ten times higher than the ionization frequency, which equals the electron loss fre-
quency in a steady-state plasma.
2) Observation of the "diffusive cooling" in experiments
A second focus of our studies was the investigation of the time-evolution of the electron distribu-
tion function in the afterglow of a pulsed plasma [J2,J3]. For this purpose, we performed a system-
atic study of time-resolved probe meas-
urements in the afterglow of an induc-
1011 tively coupled Argon plasma. Our dis-
free trapped charge system used a Pyrex chamber with
25 28 cm diameter, and a height of 10 cm.
1010 30 2015 Pressures were between 5 and 70 Pa.
10 5 4 3 2 1 s Probe measurements were performed
with a 5 mm long, 0.254 mm diameter
10s -' Tungsten cylindrical probe. Our meas-
E urement procedure consisted of taking
10$ time-resolved current-time samples I,(t)
p0 at constant probe voltage, repeating this
W /procedure for various probe voltages, and
10' -then cross-converting the data set into a
kinetic ene y. complete time series of current-voltage
106 I - characteristics I,(V,) for the entire af-
-6 -4 -2 0 2 4 6 8 10 12 14 16 terglow. From the probe characteristics,
Probe Potential (V) the EDFs were determined using the
well-known Druyvesteyn method and
Figure 2: Measured EDFs at different times in the after- numerical double differentiation.
glow of a 15 mTorr Argon plasma.nueiadobeifrntto.
Figure 2 shows a set of measured EDFs
at different times in the afterglow. For physical clarity, we displayed the EDF as functions of the
probe voltage rather than the more commonly used form of kinetic energy. The reason for this
choice is that the wall potential, which plays the role of dividing the EDF into trapped and free
electrons, remains fixed in this representation. Since we referenced our probe to a grounded metal-
lic wall, the wall potential for all times in the afterglow is at 0 V. The zero of kinetic energy of
electrons is given by the right-hand zero crossing of the EDF, i.e. at 14.8 V for the lps EDF. From
this voltage, the kinetic energy is increasing towards the left-the more negative voltages-with
the range between the zero crossing and OV representing the trapped electrons, and the range of
voltages less than OV representing the free electrons. It is clearly seen that the EDFs at all times in
the afterglow show a significant change in slope at the boundary between free and trapped elec-
trons. In the free electron range, the EDF drops much faster than in the trapped electron range, indi-
cating that the wall loss of electrons leads to a rapid depletion of the EDF at higher electron ener-
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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/m1/3/: accessed February 18, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.