A direct energy converter (DEC) designed for thermal ions escaping from a fusion reactor consists of a cusp magnetic field and one or two stage decelerating electrodes. The electrons are deflected along the field lines and consequently separated from thermal ions that are not fully magnetized. The ions are led to the electrodes to produce DC power. For basic investigation of this type of DEC, the CUSPDEC, a small-scale experimental device is constructed and its characteristics have been investigated by using a low-energy plasma source. In this paper, the device is applied to the GAMMA 10 tandem mirror in order to investigate the capability of separation of charged particles as well as to demonstrate energy conversion from ions in much more reactor-relevant environment. The separation of electrons and ions with energies of the order of keV is achieved by using a slanted cusp magnetic field for the first time. It is also found that the separated ions are decelerated by the electric field in front of ion collectors and flow into the collectors at a high potential to produce DC power.