Our original research into low temperature plasmas stems from a long interest in gas lasers. Professor Arthur Maitland (who died in 1994) established a laser group at St Andrews in 1964. His groups research on Argon-ion lasers included investigations of different materials to contain the gas. Low power lasers use glass or quartz to contain the gas. But high power lasers, such as the Argon-ion laser, need to be water cooled because they are very inefficient. Only a fraction of a percent of the electrical energy in the gas is converted to light, the rest ends up as heat which has to be removed, otherwise the tubes would overheat and break.
At St Andrews, laser tubes were developed that were made from short (a few centimetres) metal segments, separated by insulators. These lasers were cooled by flowing water directly through the metal and were capable of emitting about 50 watts of laser light - very impressive in their time. The use of metal segment lasers led to theoretical studies of the effects of metal walls on the gas discharge - the fact that the walls are conducting, rather than insulating, leads to a change in the electrical characteristics of the ionized gas.
Plasma switch Copper Vapour Lasers, which emit green and yellow light, are much more efficient than Argon-ion lasers. About one per cent of the electrical energy is converted to laser light. In these lasers copper is vaporised through the heat that is dissipated in a neon discharge. These lasers produce pulses of light rather than a continuous beam. However, because there are many pulses per second the pulsed nature of the beam cannot be seen by the human eye.
Because of the high temperatures required to vaporise the copper, alumina tubes are used rather than quartz. Copper Vapour lasers with metal walls have also been investigated. A segmented metal discharge tube was built to produce a continuous high current discharge in a mixture of argon and copper vapour as the basis of continuous copper vapor laser. Much of the Metal Vapour Laser research was done in collaboration with EEV, Chelmsford.
The gas discharges described above are produced by passing an electrical current through a gas. It is also possible to ionize a gas by applying a time varying electrical field, for instance, a microwave field. In this case, the electrodes used to apply the field can be placed outside the discharge tube.
Some devices, for instance the Copper Vapour laser, require a switch in the electrical circuit, which can be closed very rapidly, many times per second. The switch of choice is the Hydrogen thyratron, another gas discharge device. The thyratron has an electrode structure such that many kilovolts can be applied to the switch without it conducting. However if a much smaller voltage, the trigger, is applied to an intermediate electrode within the switch, a gas discharge is produced and the switch closes.
Our research has led to the development of a thyratron with an annular geometry. This thyratron is not triggered by applying a voltage to an electrode but by applying a microwave electric field to the gas. This thyratron is special because it closes very rapidly; in less than three billionth's of a second. This thyratron was developed in collaboration with DRA (Malvern). Microwave-triggered thyratrons with different properties have been investigated in a program spanning the last 10 years.
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.