Gas-filled tube
The voltage required to initiate and sustain discharge is dependent on the pressure and composition of the fill gas and geometry of the tube.
Hydrogen is used in tubes used for very fast switching, e.g. some thyratrons, dekatrons, and krytrons, where very steep edges are required.
Hydrogen (and deuterium) can be stored in the tube in the form of a metal hydride, heated with an auxiliary filament; hydrogen by heating such storage element can be used to replenish cleaned-up gas, and even to adjust the pressure as needed for a thyratron operation at a given voltage.
[4] In special cases (e.g., high-voltage switches), gases with good dielectric properties and very high breakdown voltages are needed.
Highly electronegative elements, e.g., halogens, are favored as they rapidly recombine with the ions present in the discharge channel.
The fundamental mechanism is the Townsend discharge, which is the sustained multiplication of electron flow by ion impact when a critical value of electric field strength for the density of the gas is reached.
High-pressure lighting tubes can require a few kilovolts impulse for ignition when cold, when the gas pressure is low.
[1] Large rectifiers use saturated mercury vapor with a small amount of an inert gas.
The voltage drop in forward bias decreases from about 60 volts at 0 °C to somewhat above 10 volts at 50 °C and then stays constant; the reverse bias breakdown ("arc-back") voltage drops dramatically with temperature, from 36 kV at 60 °C to 12 kV at 80 °C to even less at higher temperatures.
Air leaking into the tube introduces oxygen, which is highly electronegative and inhibits the production of electron avalanches.
Traces of mercury vapors glow bluish, obscuring the original gas color.
To prevent outgassing of the tube components during operation, a bake-out is required before filling with gas and sealing.
Thorough degassing is required for high-quality tubes; even as little as 10−8 torr (≈1 μPa) of oxygen is sufficient for covering the electrodes with monomolecular oxide layer in few hours.
Since the ignition voltage depends on the ion concentration which may drop to zero after a long period of inactivity, many tubes are primed for ion availability: Some important examples include the thyratron, krytron, and ignitron tubes, which are used to switch high-voltage currents.
Thyratrons can also be used as triodes by operating them below their ignition voltage, allowing them to amplify analog signals as a self-quenching superregenerative detector in radio control receivers.
They were filled with a pure inert gas such as neon because mixtures made the output temperature-dependent.
One miniature thyratron found an additional use as a noise source, when operated as a diode in a transverse magnetic field.
A: random pulses by cosmic radiation
B: saturation current
C: avalanche Townsend discharge
D: self-sustained Townsend discharge
E: unstable region: corona discharge
F: sub-normal glow discharge
G: normal glow discharge
H: abnormal glow discharge
I: unstable region: glow-arc transition
J: electric arc
K: electric arc
The A-D region is called a dark discharge; there is some ionization, but the current is below 10 microamperes and there is no significant amount of radiation produced.
The D-G region exhibits a negative differential resistance
The F-H region is a region of glow discharge; the plasma emits a faint glow that occupies almost all the volume of the tube; most of the light is emitted by excited neutral atoms.
The I-K region is a region of arc discharge; the plasma is concentrated in a narrow channel along the center of the tube; a great amount of radiation is produced.