A laser comprising: a first electrode and a second electrode having respective first and second mutually facing surfaces (22, 24) that are spaced apart by a gap to form a slab, the gap being dimensioned to allow a plasma discharge to be formed by applying a radio frequency, RF, electrical drive signal across the gap; and a Z-fold resonator cavity including the slab and defined by an output mirror, a first fold mirror, a second fold mirror and a rear mirror, the resonator cavity comprising a round trip beam path that traverses the slab a first time from the output mirror to the first fold mirror, a second time from the first fold mirror to the second fold mirror and a third time from the second fold mirror to the rear mirror, and then back again with fourth, fifth and sixth traversals from the rear mirror to the second fold mirror, the second fold mirror to the first fold mirror, and from the first fold mirror to the output mirror respectively, wherein the second fold mirror is rotated by an adjustment angle (Δθ/2) away from the angle it would have if the mirrors were all plane mirrors and directed the round trip beam path by direct reflection, and wherein the rear mirror is rotated by an adjustment angle (Δθ) that is approximately twice the adjustment angle (Δθ/2) of the second fold mirror away from the angle it would have if the mirrors were all plane mirrors and directed the round trip beam path by direct reflection, thereby to suppress parasitic mode paths that would otherwise exist without the second fold mirror and the rear mirror being rotated away from direct reflection by said adjustment angles. a first electrode and a second electrode having respective first and second mutually facing surfaces (22, 24) that are spaced apart by a gap to form a slab, the gap being dimensioned to allow a plasma discharge to be formed by applying a radio frequency, RF, electrical drive signal across the gap; and a Z-fold resonator cavity including the slab and defined by an output mirror, a first fold mirror, a second fold mirror and a rear mirror, the resonator cavity comprising a round trip beam path that traverses the slab a first time from the output mirror to the first fold mirror, a second time from the first fold mirror to the second fold mirror and a third time from the second fold mirror to the rear mirror, and then back again with fourth, fifth and sixth traversals from the rear mirror to the second fold mirror, the second fold mirror to the first fold mirror, and from the first fold mirror to the output mirror respectively, wherein the second fold mirror is rotated by an adjustment angle (Δθ/2) away from the angle it would have if the mirrors were all plane mirrors and directed the round trip beam path by direct reflection, and wherein the rear mirror is rotated by an adjustment angle (Δθ) that is approximately twice the adjustment angle (Δθ/2) of the second fold mirror away from the angle it would have if the mirrors were all plane mirrors and directed the round trip beam path by direct reflection, thereby to suppress parasitic mode paths that would otherwise exist without the second fold mirror and the rear mirror being rotated away from direct reflection by said adjustment angles.