Chemistry Reference
In-Depth Information
Anode
+ + + +
+ +
+
Cathode
streamer
Initial avalanches
Cathode
Pre-breakdown phase (10
2
ns)
Decay (10
1
ns)
1-2 ns
FIGURE 8.3
Schematic illustration of the breakdown mechanism for the model of
cathode-
directed streamer, caused by a positive space discharge
. Time axis is directed from the left
to the right.
Initial avalanches
denote here a continuous process, including the electron drift,
ionization, and the secondary emission from the cathode.
streak-photography. To interpret their results, the authors [17,18] formulated the
basic ideas of the breakdown mechanism, schematically presented in Figure 8.3.
All the later investigations of the model under consideration have been carried
out by means of computer modeling. The authors [12] demonstrated that if the
development of the breakdown begins with a single electron emitted from the cathode,
then for a sinusoidal feeding voltage, the development of the MD proceeds according
to the model presented in Figure 8.3, but for the impulse voltage, the mechanism of
the
avalanche-to-streamer transition
can be realized. Such a transformation of the
MD mechanism caused by an increase of the voltage pulse steepness has been found
to result in considerable growth of the energetic yield of ozone due to the decrease
of ion losses. However, the latter conclusion disagrees with the modeling results
[9,13-15], according to which the fraction of ion losses appears to be very low even
in the case of a sinusoidal feeding voltage. According to the described model, the
development of the cathode-directed streamer is caused by the distortion of electric
field due to a positive space charge within the MD channel (Figure 8.3). Accumulation
of this charge occurs during the prebreakdown phase of MD development, which can
take up to 10
−
7
s [12,14]. The final phase of decay is characterized by the deposition
of the charge carriers (electrons and ions) onto a dielectric surface, resulting in a
monotonous decrease of electric field.
8.1.1.2.4 Accumulation of Negative Space Charge Followed
by the Cathode-Directed Streamer
This model appeared as a result of the attempts to give a reasonable interpretation to
certain experimental findings of the authors [20,21], in particular to the existence of a
weak but profound prebreakdown anode glow of microsecond duration. Some ideas
of the MD mechanism including an accumulation of the negative space charge during
the prebreakdown phase have been stated in [20], and a more detailed description
of the corresponding physical model (Figure 8.4) is presented in [22]. However, this
model has not been used in computer simulations as yet.
Contrary to the MD mechanisms described earlier, the scheme in Figure 8.4
takes into account surface charge left on the dielectric electrodes after a previous
