Quelle Capri N400
the amount of mercury in the arc discharge tube of a Quelle Capri N400 sunlamp was limited to a few hundred milligrams
With sunlamps that operated at higher gas
pressures the number of ionisations would decrease
due to the shortening of the mean free path when
no other measures were taken. To avoid this, the
decrease in ionisations had to be compensated by
an increase of the supply voltage (and by
consequence also the field intensity) to a level
where the required ionisation voltage was reached
again within the shortened free path. Due to the
increased gas pressure the number and intensity of
the collisions would then increase, causing an
increase of the gas temperature too.
Because the gas pressure p is proportional to the
absolute temperature T according the relation p=nkT
(in which n is the number of gas atoms and k is the
Boltzmann factor) pressure and temperature will rise until a new thermal equilibrium is
reached. At higher temperatures the majority of the ionisations is caused by thermal
emission of electrons within the plasma column. The additional produced electrons
limit the need to increase the field intensity fully proportionally to the increase of the
gas pressure.
With arc discharges in an environment saturated with mercury vapour and under
low-pressure conditions the process of ionisation takes place over the entire
cross-cut of the tube. At higher pressures this process is more and more located in
the centre of the cross-cut, leaving a thermal isolating vacuum between the electrical
arc and the wall of the tube. This isolation allows the temperature of the electrical arc
to rise as high as 6000 degrees Celsius, much higher than the melting point of the
tube. In a high-pressure mercury vapour arc discharge tube as used in the Quelle's
Capri N400 sunlamp on display here for instance, the amount of mercury was limited
to no more than a few hundred milligrams and due to the high temperatures in the tube
all the mercury would vaporise rapidly.