A plasma cools off, the electrons move more slowly and are
recaptured, and the plasma is no more unless the energy loss to its
surroundings is replenished. A voltage imposed on a plasma
accelerates the conducting charges and can maintain a plasma
indefinitely. A welding arc is a plasma maintained between oppositely
charged electrodes. In the GTA process one electrode is a tungsten
rod; the other is the workpiece.
The arc column itself is hot, say 10,000 to 20,000 “C. A voltage drop
of around one volt per millimeter is typical for an arc column. Thus if
the arc is conducting a 100 amp current, about 100 watts of power is
needed to maintain a millimeter of arc column, around the same as a
light bulb. The really important voltage drops, through which the
electrodes are heated, occur at the electrodes. This will be discussed
below.