Laser apparatus

A laser apparatus comprises a gaseous lasing medium having an active region (12) within which lasing occurs, the active region (12) defining, in part, a flow path for gas. The apparatus also includes excitation means for supplying energy to the active region including an electrode arrangement (14, 16), and a barrier member (38; 48) arranged adjacent to the active region, such that net momentum is imparted to gas in the active region (12) upon an electrical discharge being supplied to the active region (12) of gas. As a result, a substantially unidirectional, circulatory gas flow is generated within the flow path to aid heat removal from the gas. The barrier member (38; 48) defines a plurality of flow passages (39) for gas flow, each of which presents substantially the same impedance to gas flow in opposing flow directions.