  The ion backflow is the main limiting factor for operating time projection
chambers at high event rates. A significant effort is invested by many
experimental groups to solve this problem. This paper explores a solution based
on operating a passive bi-polar wire grid. In the presence of the magnetic
field, the grid more effectively attenuates the ion current than the electron
current going through it. Transparencies of the grid to electrons and ions are
measured for different gas mixtures and magnitudes of the magnetic field. The
results suggest that in a sufficiently strong magnetic field, the bi-polar wire
grid can be used as an effective and independent device to suppress the ion
backflow in time projection chambers.
