Ion implanter and ion implantation method

An ion implanter includes a plasma shower device configured to supply electrons to an ion beam with which a wafer is irradiated. The plasma shower device includes a plasma generating chamber provided with an extraction opening, a first electrode which is provided with an opening communicating with the extraction opening and to which a first voltage is applied with respect to an electric potential of the plasma generating chamber, a second electrode which is disposed at a position facing the first electrode such that the ion beam is interposed between the first and second electrodes and to which a second voltage is applied with respect to the electric potential of the plasma generating chamber, and a controller configured to independently control the first voltage and the second voltage to switch operation modes of the plasma shower device.