A 5-yr study of beam direct energy conversion was performed at the Kyoto University Institute of Atomic Energy to clarify the essential features of direct energy recovery from monoenergetic ion beams so that the performance characteristics of energy recovery can be predicted reasonably well by numerical calculations. The study used an improved version of an electrostatically electron-suppressed beam direct converter originally proposed by Lawrence Livermore National Laboratory. Secondary electron suppressor grids were added, and a helium ion beam was used with typical parameters of 15.4 keV, 90 mA, and 100 ms. By adopting negatively biased secondary electron suppressor grids, the energy recovery efficiency increased from 72 ± 4 to 87 ± 6% even at relatively high pressures of 10−2 Pa, based on three independent measurements of the incident ion current, including a newly developed “in situ” measurement by a Rogowski coil sensor. The operational region could also be extended to more high-pressure regions. A comparison of experimental results with numerical results by the two-dimensional Kyoto University Advanced DART (KUAD) code, including evaluation of atomic processes, shows excellent agreement. Adoption of the mesh-type electron suppressor instead of the solid suppressor results in further improvements in the beam direct energy recovery.