Corona Discharge -Electron Generation

Several things happen very rapidly (in a matter of a millisecond) in the small area

around the discharge electrode. The applied voltage is increased until it produces a

corona discharge, which can be seen as a luminous blue glow around the discharge

electrode. The free electrons created by the corona are rapidly fleeing the negative

electric field, which repulses them. They move faster and faster away from the discharge

electrode. This acceleration causes them to literally crash into gas molecules,

bumping off electrons in the molecules. As a result of losing an electron (which is

negative), the gas molecules become positively charged, that is, they become positive

ions (Figure 1). So, this is the first thing that happens—gas molecules are ionized,

and electrons are liberated. All this activity occurs very close to the discharge electrode.

This process continues, creating more and more free electrons and more positive

ions. The name for all this electron generation activity is avalanche

multiplication (Figure 2).

The electrons bump into gas molecules and create additional ionized molecules. The

positive ions, on the other hand, are drawn back toward the negative discharge electrode.

The molecules are hundreds of times bigger than the tiny electrons and move

slowly, but they do pick up speed. In fact, many of them collide right into the metal

discharge electrode or the gas space around the wire causing additional electrons to be

knocked off. This is called secondary emission. So, this is the second thing that happens.

We still have positive ions and a large amount of free electrons.