The European High Power laser Energy Research (HiPER) project aims at demonstrating the feasibility of high gain inertial confinement fusion using the fast ignitor approach. A baseline target has been recently developed by Atzeni et al (2007 Phys. Plasmas14 052702). We study here the robustness of this target during the compression phase and define pulse shape tolerances for a successful fuel assembly. The comparison between a standard and a relaxation pulse shows that the latter allows one to reduce both the laser power contrast and the growth of perturbations due to Rayleigh–Taylor instability. We have found that with 95 kJ of absorbed laser energy one can assemble the fuel with a peak density around 500 g cm−2 and a peak areal density of 1.2 g cm−2. This implies a total target gain of about 60.