Most of the devices in this section switch by inducing an arcing process in a gaseous medium. I have included in the triggered spark gap section some mention of devices that actually use a liquid or solid substitute for the gaseous material that is the norm in triggered spark gaps.
The process of arc formation is actually quite complex physically, and it will not be gone into in any depth. Anyone who wishes to look more deeply into this aspect of device operation may contact the author for some suggestions as to suitable text books for use in such study.
Cold cathode trigger tubes are physically small devices designed to switch impulse currents and voltages of relatively small amplitude. Usually they are intended, as their name suggests, to trigger other larger devices.
Typically cold cathode trigger tubes are designed to switch pulses of a few hundred volts and a few hundred milliamperes. Most trigger tubes have three or four electrodes, anode, cathode (+ve and -ve terminals respectively), a trigger/control electrode and sometimes a priming electrode.
A trigger tube performs in a very simple manner akin to that of a triggered spark gap, excepting that usually the conduction is not by an arcing but glow discharge. The glow discharge is initiated when all of the following factors are present:
A sufficiently high voltage is present across the device (between anode and cathode)
A trigger pulse of sufficient amplitude is present at the trigger electrode.
The gas in the tube is primed.
Cold cathode trigger tubes rely upon some external or internal source to ionize the gas suitably for conduction to commence (This is called priming). This means that in theory some of these tubes will only switch a minute or so after the application of a suitable triggering voltage to the appropriate terminal of the device when some natural source of ionizing radiation ionizes the gas (forming a plasma) and hence causes conduction to commence. The triggering is basically random- it is subject to huge statistical variation even in apparently similar environments. Some devices incorporate a suitably ionizing source to reduce the maximum possible time delay after trigger application considerably. This source may be an electronic, radioactive or photon source of some form or other. However even the standard commercial devices often display a large variation (up to and above an order of magnitude different) between devices fired in sunlight and darkness, a standard commercial tube Z900T for instance displays a 20us delay in day light and a 250us delay in darkness.