Time and space behaviour of a plasma potential induced by external ac voltage is studied experimentally. The characteristic frequency of the voltage lies in Ω ⪅ ωBi, and/or ωBi ⪅gwLH frequency regions. It is shown that either the potential oscillation, its frequency being close to ion plasma frequency in the sheath, or positive pulses of the potential develop in the plasma. In the latter case an additional plasma flux arises across the magnetic field. This flux contains ions which strike the wall with an energy of several hundred eV.The effects studied may be responsible for particle lifetime decrease and enhanced impurity influx in ICRF heating experiments.