STRUCTURE  OF  A  TRANSVERSE  CO₂  LASER

      The goal of a circulating gas laser is to remove a hot molecule from the discharge path as soon as possible after lasing, but there are, of course, limits to how fast gas can be pushed through such a structure. For extremely high flow rates a transverse- flow arrangement is used (sometimes called a gas transport laser), in which gas flows across the discharge electrodes in a manner similar to that of an excimer. Gas speeds are very high here in order to quickly replace warm gas (which does not lase well, due to a thermal population of lower energy levels) with cooled gas. Transverse CO₂ lasers operating at low pressures can produce enormous CW power outputs, but with long, transverse electrodes it is difficult to ensure that the discharge spreads out. Preionization may be employed (as it is with many excimer lasers) to sustain the discharge, or alternately segmented anodes may be used.

       Another use of a transverse arrangement, not specifically for cooling purposes but for the sake of electrode arrangement, is the TEA configuration. TEA lasers operate at very high pressures, usually several atmospheres, so allow the extraction of a large amount of energy from the lasing volume. These are strictly pulsed systems, and in such lasers, which are often built in a manner similar to excimer lasers, the need for transverse electrodes is apparent when considering that at such high tube pressures 20 kV can discharge only a few centimeters. In most cases, commercial excimer lasers can use a CO₂ gas mix with only a simple change in optics. Excimer lasers, too, employ transverse gas flow for cooling purposes, lending themselves well to CO₂ TEA laser use.