A slab laser includes two elongated electrodes arranged spaced apart and face-to-face. Either one or two slabs of a solid dielectric material extend along the length of the electrodes between the electrodes. A discharge gap is formed either between one of the electrodes and one dielectric slab, or between two dielectric slabs. The discharge gap is filled with lasing gas. A pair of mirrors is configured and arranged to define a laser resonator extending through the gap. An RF potential is applied across the electrodes creating a gas discharge in the gap, and causing laser radiation to circulate in the resonator. Inserting dielectric material between the electrodes increases the resistance-capacitance (RC) time constant of the discharge structure compared with the RC time constant in the absence of dielectric material. This hinders the formation of arcs in the discharge, which enables the laser to operate with higher excitation power, higher lasing gas pressure, and higher output power than would be possible without the dielectric inserts.