Not yet reviewed by Pith; the record is open.

Using quantum electrodynamics particle-in-cell simulations, we optimize the gamma flare ($\gamma$-flare) generation scheme from interaction of high power petawatt-class laser pulse with tailored cryogenic hydrogen target having extended preplasma corona. We show that it is possible to generate an energetic flare of photons with energies in the GeV range and total flare energy being on a kilojoule level with an efficient conversion of the laser pulse energy to $\gamma$-photons. We discuss how the target engineering and laser pulse parameters influence the $\gamma$-flare generation efficiency. This type of experimental setup for laser-based $\gamma$ source would be feasible for the upcoming high power laser facilities. Applications of high intensity $\gamma$ ray beams are also discussed.