Rami Arieli: "The
Laser Adventure" Chapter 6, Helium-Neon Lasers, page 3a
Plasma Tube of He-Ne Laser:
The thin inner tube has a diameter
of about 2 [mm] and length of tens of centimeters. The inner tube is surrounded
by thick outer tube with diameter of
about 2.5 [cm] and is sealed from the outside. The purposes of the outer
tubes are:
-
To make a stable structure which protects
the inner tube and the laser mirrors from movements.
-
To act as a large gas reservoir which
refreshes the Neon gas that has been absorbed by the cathode.
The lasing process, which creates the electromagnetic radiation, is confined
to the inside of the inner tube which is filled with the gas mixture.
The gas mixture
is 85-90% Helium gas, and 10-15% Neon gas, a ratio of 1:6 to 1:10.
The gas pressure
is 0.01 Atmosphere (» 10 [torr]). At the
end of the tube, the electrodes are attached to a high voltage power supply
(AC or DC).