The triggered spark gap is a simple device, a high voltage trigger pulse applied to a trigger electrode initiates an arc between anode and cathode. This trigger pulse may be utilized within the device in a variety of ways to initiate the main discharge. Different spark gaps are so designed to employ one particular method to create the main anode to cathode discharge. The different methods areas follows-
Field distortion: three electrodes; employs the point discharge (actually sharp edge) effect in the creation a conducting path
Irradiated: three electrodes; spark source creates an illuminating plasma that excites electrons between the anode and cathode.
Swinging cascade: three electrodes; trigger electrode nearer to one of the main electrodes than the other.
Mid plane: three electrodes; basic triggered spark gap with trigger electrode centrally positioned.
Trigatron: trigger to one electrode current forms plasma that spreads to encompass a path between anode and cathode.
Air
SF6
Argon
Oxygen
Often a mixture of the above materials is employed. However a few spark gaps actually employ liquid or even solid media fillings. Solid filled devices are often designed for single shot use (they are only used once- then they are destroyed) Some solid filled devices are designed to switch powers of 10TW (10 000 000 000 000 Watts) such as are encountered in extremely powerful capacitor bank discharges. Except (obviously) in the case of solid filled devices, the media is usually pumped through the spark gap. Some smaller gaps do not use this system though.
Usually Gas filled spark gaps operate in the 20-100kV / 20 to 100kA range though much higher power devices are available. I have one spec for a Maxwell gas filled device that can handle 3 MA - that's 3 Million Amperes! But then it is the size of a small car!! More commonly gas filled devices have dimensions of a few inches. Packages are often shaped like large ice pucks though biconical, tubular and box like structures are also seen.
Sparkgaps are often designed for use in a certain external environment(eg. they might be immersed in oil). A system for transmitting the media to the appropriate part of the device may sometimes be included.
Common environments used are:
Air
SF6
Oil
Typical spark gap device no.'s are: TG7, TG113, TG 114 etc. etc.
Spark gaps are damaged by repeated heavy discharge. This is an inevitable consequence of such high discharge currents. Electrode pitting being the most common form of damage. Between 1 and 10 thousand shots per device is usually about what is permissible before damage begins to severely degrade performance.
EG&G make miniature triggered spark gaps specially designed for defense applications. these devices are physically much smaller than normal spark gaps (few cm typical dimensions) and designed for use with exploding foil slapper type detonators.
Laser switching of spark gaps. The fastest way to switch a triggered spark gap is with an intense pulse of Laser light which creates a plasma between the electrodes with extreme rapidity. There have been quite a few designs employing this method, chiefly in the plasma research area.
Triggered spark gaps tend to have long delay times than Thyratrons (their chief competitor, at least at lower energies) However once conduction has started it reaches a peak value exceptionally rapidly (couple of nanoseconds commutation.)