A passively Q-switched Nd:YAG laser with a master-oscillator power-amplifier configuration based on Brillouin and Raman pulse compression has been developed. The laser operates at 100 Hz repetition rate, producing 50 mJ pulses of approximately 30 ps duration at 1530 nm wavelength with near-diffraction-limited beam quality (M2≤1.2). The effect of spherical aberration in thermally loaded Nd:YAG rods was studied, and efficient aberration compensation was achieved by use of a specially designed aspheric element.