The performance of a 0.45 J KrF excimer laser with an unstable cavity using a phase-unifying mirror as an output coupler is described. The output beam having a cross section of2.4×1.2 cm2exhibits a smooth far field profile with 86.5&percent; of the total near field energy contained within a far-field full angle&thgr;⩽0.05 mrad.The measurements for determining the beam quality factorM2and the beam angular stability are also reported. ©1997 American Institute of Physics.