It has been shown on tokamak CASTOR by using an emissive Langmuir probe that positive plasma 'biasing' takes place in a radially very narrow layer just at the grill mouth. The effect scales linearly with the power applied. Apparently, this positive biasing results from the charge separation due to the escaping electrons, accelerated by the near field lower hybrid wave effects in this region, along the magnetic field lines (Fuchs V et al 1996 Phys. Plasmas3 4023).