When an a. c. arc is maintained through a long series of cycles, this means re-ignition in reverse direction after every passage of the current through zero. Switching out does not mean killing the arc that carries a normal current, but preventing re-ignition after a normal current zero. It would be incorrect to call the arc annihilated in the moment of zero current since its plasma may still be hot and highly ionized, i. e., prone to carry current and quickly to recover to a true arc as soon as sufficient voltage reappears. Certainly, the thin regions of cathode and anode fall are de-ionized instantaneously. The positive ions which create the cathode fall diffuse into the cathode in about 10−8 sec, and electron emission becomes negligible when the cathode temperature sinks below about 2500 °C. But, the plasma cools and de-ionizes slower. If an electron density of more than 1015 to 1016 electrons per m3 is still in the plasma1 when a suitable high voltage reappears, the plasma will carry a current that may heat the plasma so efficiently as to deliver ions and electrons for again building up the cathode fall and for carrying current to the electrodes. Re-ignition that starts from remaining ionization in the plasma is called thermal re-ignition 2.
KeywordsBurning Convection Heat Content Cadmium Recombination
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