Electron avalanches

Corona discharge is the simplest type of plasma generator. A feature of the corona discharge, which differentiates it from the other discharges, is that no dielectric is involved. Instead, an electron avalanche is initiated from a sharp metallic surface where the radius of curvature is small. The electric field has to be pulsed in order to prevent the plasma from going into the thermal mode and forming an arc. The electric field in corona reactors is about 50 kV/cm. 

If measurements are made in thin oxide films (of thickness less than 5 nm), at highly polished Al, within a small acceptance angle (a < 5°), well-defined additional maxima and minima in excitation (PL) and emission (PL and EL) spectra appear.322 This structure has been explained as a result of interference between monochromatic electromagnetic waves passing directly through the oxide film and EM waves reflected from the Al surface. In a series of papers,318-320 this effect has been explored as a means for precise determination of anodic oxide film thickness (or growth rate), refractive index, porosity, mean range of electron avalanches, transport numbers, etc. 

Every pulse is an electron avalanche generated by ionization of the counter gas as a result of the interaction with the incoming photon... 

Usually, there is an electrostatic potential of the order of 1 V across the electric double layer at tbe interface between a metal and an aqueous solution this potential produces an intense electric field of the order of 10 V cm in the compact layer 0.3 to 0.5 mn thick. Such an intense electric field can not be realized in any dielectrics of macroscopic size, because of dieleintense electric field can be sustained in a layer of several atomic thidmess where no electron avalanche can occur. 

Electron avalanching via (3) is the main process for producing A+ ions. However this requires the presence of priming electrons which can be... 

Figure 4.19 Time-dependent current i(f) and voltage U(t) signal produced by an electron avalanche in the channeltron/channelplate. The shaded area of the current pulse represents the total charge Q of the avalanche collected during the time rcoll on the detector s capacitance, thus producing l/mM on this capacitor.

From the function of a channeltron/channelplate detector it is obvious that high gains are desirable. However, ion feedback and space charge effects limit the gain with increasing charge of the electron avalanche, electron impact ionization with molecules of the residual gas or molecules desorbed under electron bombardment from the channel surface occurs more frequently. The ions produced are then accelerated towards the channel input. If such an ion hits the surface at the channel entrance, it may release an electron which again can start an avalanche of practically the same size, i.e., it causes after-pulses. 

Figure 4.24 Extraction of one-dimensional position information from a channelplate detector by using a discrete multianode with an RC line. For an explanation of the strips and the dashed circular area see the caption of Fig. 4.23. Each strip is connected to the RC line indicated by the resistor and capacitor symbols. The total charge Q of the electron avalanche (shaded area) incident on the multianode flows to both ends of the RC line, giving Qi and Q2 in amounts proportional to the distances (P, L) and (0, P), respectively. The two preamplifiers in which these charges are collected are indicated by triangles. From...

Intrinsic breakdown is explained as follows. When the field is applied the small number of electrons in thermal equilibrium in the conduction band gain kinetic energy. This energy may be sufficient to ionize constituent ions, thus increasing the number of electrons participating in the process. The result may be an electron avalanche and complete failure. Intrinsic breakdown strengths are typically of the order of 100 M V m 1. 

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