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A compact, efficient water-cooled heatsink has been developed for >400 W continuous-wave (CW), diode edge- pumped microchip lasers. A vertical, water jet-impingement cooling system enabled the compact heatsink of 10-mm diameter to be suitable for a close arrangement of the focusing lenses around a microchip. Comparative experimental studies and simulations by finite element analysis show that thermal resistance of the heatsink with a microfin structure on the water-impinged back surface drops to less than 40% that of the heatsink with a conventional flat back surface. The average heat-transfer coefficients over the fin contact area with cooling water were estimated to be 62 and 80 kW/m2. K at water flow rates of 1 and 2 L/min, respectively. The thermal resistance of 0.24degC/W was obtained in an area with a 5-mm diameter of the heatsink with a 140-mum-wide microfin.