Concerning an application of a cusp type direct energy converter (CUSPDEC) to high density plasma, subjects on charge separation and deceleration have been experimentally investigated, assuming that previously proposed two-stage deceleration scheme will be employed. As a fundamental measurements on two-stage deceleration scheme, the ions reflected by electric field due to the point cusp ion collector have been successfully detected by a sub-collector as a function of the collector voltage. For efficient charge separation and energy conversion, an introduction of a negatively biased grid has been examined, resulting that the assistance of charge separation to cusp field is effective. By employing the negatively biased grid, direct energy conversion by the point cusp collector also achieved with excluding electrons flowing into the collector.