A radio frequency, RF, slab laser comprising: a first electrode and a second electrode having respective inwardly and outwardly facing surfaces (104, 110/ 106, 112), wherein their respective inwardly facing surfaces face each other and are spaced apart by a gap forming a slab waveguide of a thickness dimensioned to allow a plasma discharge to be formed by driving at least the first electrode with an RF drive signal, the first electrode being provided with at least one fluid circulation channel for distributing coolant fluid around the first electrode ; a vacuum housing enclosing at least the inwardly facing surfaces of the first and second electrodes inside a vacuum space configured to contain a gaseous lasing medium; and a plurality of hollow conductors connected to the outwardly facing surface of the first electrode to supply the RF drive signal to the first electrode and the coolant fluid to the at least one fluid circulation channel of the first electrode , characterized in that a portion of the outwardly facing surface of the first electrode is outside the vacuum space configured to contain the gaseous lasing medium, and wherein the hollow conductors connect to said portion, and wherein the vacuum housing is provided with an access aperture, and wherein a vacuum flange comprising or consisting of an electrically insulating connector extends from the access aperture, where it forms a vacuum-tight seal, to the outwardly facing surface of the first electrode, where it forms a further vacuum-tight seal, and defines said portion of the outwardly facing surface of the first electrode that is outside the vacuum space configured to contain the gaseous lasing medium. a first electrode and a second electrode having respective inwardly and outwardly facing surfaces (104, 110/ 106, 112), wherein their respective inwardly facing surfaces face each other and are spaced apart by a gap forming a slab waveguide of a thickness dimensioned to allow a plasma discharge to be formed by driving at least the first electrode with an RF drive signal, the first electrode being provided with at least one fluid circulation channel for distributing coolant fluid around the first electrode ; a vacuum housing enclosing at least the inwardly facing surfaces of the first and second electrodes inside a vacuum space configured to contain a gaseous lasing medium; and a plurality of hollow conductors connected to the outwardly facing surface of the first electrode to supply the RF drive signal to the first electrode and the coolant fluid to the at least one fluid circulation channel of the first electrode , characterized in that a portion of the outwardly facing surface of the first electrode is outside the vacuum space configured to contain the gaseous lasing medium, and wherein the hollow conductors connect to said portion, and wherein the vacuum housing is provided with an access aperture, and wherein a vacuum flange comprising or consisting of an electrically insulating connector extends from the access aperture, where it forms a vacuum-tight seal, to the outwardly facing surface of the first electrode, where it forms a further vacuum-tight seal, and defines said portion of the outwardly facing surface of the first electrode that is outside the vacuum space configured to contain the gaseous lasing medium. <EOS>