Numerical Simulation of Direct Energy Conversion

The authors have studied the electrostatic direct energy conversion scheme originally proposed by Post (1970). The two dimensional electrostatic numerical simulation approach follows the motion of an ion beam as it is focused, decelerated and collected in a high voltage direct converter structure. Both the self-consistent space charge field and the background field due to the periodic electrodes are included. Collection efficiency (η) results for low density mono-energetic ion beams show maxima in excess of 90% for parameters which minimize retrograde particles. The efficiency information was used to determine analytically optimum parameters which maximize η for an ion beam having a flat top energy distribution with a 3:1 energy spread. As the number density n_i is increased up to and above a critical value n_c (where space charge and focusing fields are equal) efficiency becomes significantly degraded. An increasing steep potential hill forms and persists near the entrance to the direct converter which effectively disperses the ion beam. This phenomena is in a sense a space charge caused dynamic instability.