A fast-axial-flow carbon dioxide laser is operated with a rectangular cross section, radio frequency excited, slab discharge. The narrow electrode gap and the series capacitance of the discharge tube walls produce excellent discharge stability at higher input power density and pressure than in conventional fast-axial-flow lasers. For a single-section test discharge, the mass flow per unit cross-sectional area is improved by high-pressure operation and the mass flow choking effect at sonic gas exit velocity is avoided. For CW operation, the discharge power density has been increased to 200 W·cm-3 and laser operation demonstrated at a pressure of 500 torr. In pulsed operation at pressures of 300-400 torr, repetition frequencies of 15 kHz have been achieved with the pulse duration reduces to 15-25 µs, suitable for materials processing of low thermal conductivity materials.