By numerical simulation, we calculate the efficiency of a system to recover the energy carried away from a fusion reactor by the charged particles escaping through the magnetic mirrors. A beam of particles injected into a 2N collector electrostatic field configuration proposed by Post is studied, and the effect of varying the collector parameters (number of collector fins 2N, fin spacing, and electric field strength) as well as beam parameters (beam width, energy spread, and density at the entrance) is determined. We found that, for 2N ranging from 12 to 36, the optimum efficiency in the limit of low density, T0, ranges from 0.82 to 0.88, respectively, for a beam with a factor of 2 in energy spread. Increasing the energy spread to a factor of 4, more typical of a reactor situation, decreased T0 from 0.84 to 0.76 in the 24-fin system. We calculate that, for the same efficiency and applied electric fields, the 36-fin system can tolerate a maximum beam density greater by a factor of 2 than that for a 12-fin system. For a 24-fin system of reactor dimensions and energies this produces beam densities of 2.5 × l06 particles/cm3 or, equivalently, 2.8MW/m2.