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Belmag J200-U2 sunlamp
MEAN-FREE-PATH
Apart from voltage and current, the physical units
gas pressure, temperature and field intensity are of
crucial importance for the nature- and the
effectiveness of a gas discharge. When the gas
pressure is too low, there are not enough gas atoms
or -molecules available to produce a significant
quantity of ultraviolet radiation or to start a
discharge at all. When on the other hand the gas
pressure is too high, free electrons are not able to
gain enough speed before they collide with an atom
and as a result mainly elastic collisions will occur.
An important property in this context is the mean
free path L that indicates the average distance an
electron can travel in the longitudinal direction of an
electric field without a collision with a gas atom. For
example: The ionisation energy of mercury is about
10 eV. The average field intensity F is the potential
difference UL between the cathode and the anode,
divided by the distance d between cathode and anode. With UL=500 volt and d=2 cm,
F equals 500/0,02 = 25.000 V/m. To allow an electron to ionise an atom of mercury,
a mean free path L of (10 V)/(25.000 V/m) or 0,4 mm would then be required.
Measured on the scale of an atom this is a long distance that can only be realised in
an atmosphere that contains a low quantity of atoms so at a low gas pressure. In
practice the circumstances are a little better since the voltage drop between the
cathode and the anode is not linear. The largest drop in voltage is located just after
the cathode in an area that measures only one or a few millimeters. This is the area
where the majority of the ionisations take place and from here most ions travel
towards the cathode to free additional electrons. In a quiet glow discharge a specific
light effect may be noticed in this ionisation area, caused by recombinations between
electrons that just left their energy in their firsts collision and atoms that just have
been ionised and did not speed up in the direction of the cathode yet. In an arc
discharge as for instance developed by a Belmag J200-U2 sunlamp on display here,
this phenomena disappeared in the violence of the thermal emission of the cathode.
In the area after the first ionisation zone and just before the anode, the voltage drops
only little and the field intensity is just enough to form a column of hot gas (plasma) in
which in a delicate equilibrium a continuous process of ionisation takes place. Finally
it is this column that is responsible for the emission of the desired ultraviolet