  We present ion backflow measurements in a Micromegas (MICRO-MEsh GASeous
detector) TPC device developed for the next high energy electron-positron
linear collider under study and a simple explanation for this backflow. A
Micromegas micro-mesh has the intrinsic property to naturally stop a large
fraction of the secondary positive ions created in the avalanche. It is shown
that under some workable conditions on the pitch of the mesh and on the gas
mixture, the ion feedback is equal to the field ratio (ratio of the drift
electric field to the amplification field). Measurements with an intense X-ray
source are in good agreement with calculations and simulations. The conclusion
is that in the electric field conditions foreseen for the Micromegas TPC (drift
and amplification fields respectively equal to 150-200 V/cm and 50-80 kV/cm)
the expected ion backflow will be of the order of $2 - 3 x 10^-3$. In addition,
measurements have been done in a 2T magnetic field: as expected the ion
backflow is not altered by the magnetic field.
