Electron avalanche



An electron avalanche is a process in which a number of ultraviolet radiation emitted by the excited medium's atoms in the aft-tip region.

Analysis

A plasma begins with a rare natural 'background' ionization event of a neutral air molecule, perhaps as the result of electrode depending on its polarity, whereas the electron will be accelerated in the opposite direction. Because of the huge mass difference, electrons are accelerated to a much higher velocity than ions.

High-velocity electrons often collide with neutral atoms inelastically, sometimes ionizing them. In a STP, free electrons exist for only about 11 nanoseconds before being captured. Captured electrons are effectively removed from play — they can no longer contribute to the avalanche process. If electrons are being created at a rate greater than they are being lost to capture, their number rapidly multiplies, a process characterized by exponential growth. The degree of multiplication that this process can provide is huge, up to several million-fold depending on the situation. The multiplication factor M is given by

M = \frac{1}{1-\int_{X_1}^{X_2} \alpha\, dx}

Where X1 and X2 are the positions that the multiplication is being measured between, and α is the ionization constant. In other words, one free electron at position X1 will result in M free electrons at position X2. If the voltage gradients are substitiuted into this equation the result is

M = \frac{1}{1-|\frac{V}{V_\mathrm{BR}}|^n}

Where V is the applied voltage, VBR is the breakdown voltage and n is an empirically derived value between 2 and 6. As you can see from this formula, the multiplication factor is very highly dependent on the applied voltage, and as the voltage nears the breakdown voltage of the material, the multiplication factor approaches infinity and the limiting factor becomes the availability of charge carriers.

Avalanche sustenance requires a reservoir of charge to sustain the applied voltage, as well as a continual source of triggering events. A number of mechanisms can sustain this process, creating avalanche after avalanche, to create a avalanche breakdown can occur, culminating in an electrical spark that bridges the gap.

See also

References

  • Breakdown effects in semiconductors
 
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