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We have developed a prepulse-free short-pulse Nd:glass laser system of 0.9-PW peak power to heat a pre-imploded high-density plasma. An optical parametric chirped amplification system is introduced to reduce the prepulses to an amplitude (1.5×10-8) of that of the main pulse. The compressor is a double-path grating pair system 94 cm in diameter compressing the 50-cm-diameter laser beam to 470 fs. An off-axis parabolic mirror has focused the 420-J energy to an intensity of 2.5×1019 W·cm-2. Part of the front end of the chirped pulse is seeded into the preamplifier of the GEKKO XII laser, used to implode a pellet target, to enable the petawatt laser to irradiate the pre-imploded pellet during stagnation of a few tens of picoseconds.