The operation of a 250 W low-power Hall thruster called ISCT-200 has been studied using a two-dimensional hybrid model. Two different magnetic field topologies have been tested. One topology is called the unshielded configuration and corresponds to a standard magnetic configuration with a quasi-radial magnetic field, and the other is called the magnetic shielding (MS) configuration where the zone of maximum of magnetic field is shifted in the near field plume. In the MS configuration, close to the channel walls, magnetic lines are forced to be parallel to walls. In the shielded configuration, the ionization takes place very close to the exhaust region and the acceleration occurs downstream the exit plane in the near field plume. The MS configuration reduces the erosion very effectively since the cooling of the electron temperature inside the channel strongly diminishes the sheath potential drop and consequently the kinetic energy of ions impacting on channel walls. The shift of ionization and acceleration regions towards the near field plume also contributes to the reduction of erosion. Calculations show very similar performances for both magnetic field configurations, with a larger than measured thrust in the shielded version of the thruster. Also, thanks to a larger electron temperature a larger fraction of doubly charged ions is found in the shielded magnetic configuration of the ICST-200.