Thermal characteristics of a split‐disk amplifier system with 50 mm diam aperture were measured by a Schlieren method with a ball stopper of 88 &mgr;m diam under repetitively pulsed operation for a pump loading of up to 1 kW(1 kJ/ Hz). The thermal beam divergence reached 100 &mgr;rad at an average input power of 200 W(1 kJ/0.2 Hz) under 300l/min(0.73 m/s) N2gas flow. The thermal limitations of this amplifier system are discussed under repetitive operation.