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We have demonstrated the feasibility of cooling high-power solid-state lasers with diamond windows, whose thermal conductivity is about two orders of magnitude higher than that of sapphire. An output power of 200 W was achieved from a single Nd:YVO4 slab in a zigzag configuration when pumped with 600-W diodes at 808 nm. The maximum output power previously reported in the literature using conventional cooling schemes is only about 100 W. A 2.3×4×24 mm3 slab was pumped from its broad side (4×24 mm2) through a 0.3-mm-thick optical diamond window placed in close contact with the lasing crystal. The diamond window, held in a water-cooled copper housing, acted as a heat conductor. The other broad side of the crystal was cooled directly by its water-cooled copper housing. Since pumping and cooling were along the same axis, a Cartesian thermal gradient was achieved, while the zigzag scheme was used to minimize thermal lensing. By using a BBO Q-switch, 70-W average power was obtained at 20 kHz with a pulse width of 19 ns and with a beam quality of 3 and 12 times the diffraction limit in the zigzag and transverse directions, respectively. The output of a two-head configuration was 295 W. In addition, a cavity was designed to achieve increased beam quality and 133 W was accomplished with a beam quality of 2 and 7.5 times diffraction-limited on the zigzag and nonzigzag axes, respectively. Operating this cavity with an RTP Q-switch produced 114 W with a beam quality of 1.5 and 9.5 on the respective two axes.