[en] Loss mechanisms of TWDEC for a D-³He fuelled FRC fusion reactor are studied with numerical analysis. (1) The self-excitation of a travelling wave has been attained with external electric circuits, which shows the conversion efficiency is about 70-73%. (2) Three-dimensional behavior has been studied by axisymmetric approximation, showing that the loss related to the radial non-uniformity effect without collision with grids is about 5%. (3) The loss related to the collision with grids is the most important, being about 11%. (4) Effects of secondary electrons produced by the collision with the grids are rather small and subrings can suppress the effects of secondary electrons. (5) The loss related to leaked 3.6 MeV α particles is about 45-60% of the total energy of leaked α particles. (6) The gross efficiency of TWDEC is estimated to be over 60%