PHYSICAL PROCESSES IN ARC
By their nature all gases are normally
good electrical insulators, but it is well known that
the application of a sufficiently high electric filed
may cause a breakdown of the insulating
properties, after which current may pass through
the gas as an electric discharge. The term arc is
usually applied only to stable or quasi-stable
discharges, and an arc may be regarded as the
ultimate form of discharge; it is defined as a
luminous electrical discharge flowing through a gas
between two electrodes. Electric discharges are
commonly known from natural phenomena like
sparks whose lengths can vary. Discharges can
occur not only in gases, but also in fluids or solids
or in almost any matter that can turn from a state of
low or vanishing conductivity to a state of high
conductivity, when a sufficiently strong field is
applied. According to [1-4], starting with a uniform
distribution of ions when the current and voltage
are zero, the increase in voltage will cause space
charge sheaths to form next to the electrodes and,
because the mobility of the electrons is much
greater than that of positive ions, most of the
applied voltage will be across the space charge
sheath at the anode as seen in Figure 1. The current
densities in this sheath are very small and in order
to „restrike‟ the arc, the space charge sheath must
be broken down. If there are no ionizing agents, the
breakdown must be ionization by collision; it will
therefore require a minimum of several hundred
volts. Under the action of the electric field strength,
electrons are emitted from the cathode spot. These
collide with neutral molecules, thereby ionizing
them electrically. The ions in the arc column fly
now under the effect of the field strength towards
both electrodes and heat them by impact to high
temperature. The negative electrons hit the anode,
and the positive ions hit the cathode. In this way
new electrons are liberated within the arc column
and at the electrodes, and the process starts again.
According to [7] the dynamic characteristics of arcs
may be represented as in Figures 2 and 3.