>50 MW peak power, high brightness Nd:YAG/Cr4+:YAG microchip laser with unstable resonator.

We demonstrated a flat-convex unstable cavity Nd:YAG/Cr4+:YAG ceramic air-cooled microchip laser (MCL) generating a record 37.6 and 59.2 MW peak power pulses with an energy of 17.0 and 24.1 mJ and a width of 452 and 407 ps at 20 Hz by using a uniform power square and hexagon pump, respectively. For hexagon pump, the near field hexagon donut beam was changed in to a Bessel-like beam in far field, whose beam quality was estimated as 2nd moment M2 of 7.67. The brightness scale of unstable resonator MCL was achieved up to 88.9 TW/(sr·cm2) in contrast with flat-flat cavity MCL. However, the high intense center part of Bessel-like beam increased its brightness effectively more than 8 times, up to 736 TW/(sr·cm2).