The criteria governing the choice of cavity gas for light-ion-beam fusion reactors have been investigated. Possible mechanisms of laser initiation of plasma discharge channels and the effects of cavity gas choice on one of the most promising mechanisms are discussed. The shock overpressure and the thermal heat flux experienced by the first wall are studied for a variety of cavity gases. Small amounts of alkali metal vapors are found to be useful in both limiting the thermal heat flux and initiating the plasma channels. A 50-Torr argon cavity gas with a 0.2% sodium impurity has been found to allow both efficient laser channel initiation and first-wall survivability.