PURPOSE:To enable a troidal electric field to be produced even in a narrow space without causing any strain by installing concentric arc-like electrodes extending along the equipotential surface of a troidal electric field to be produced on each of facing two surfaces. CONSTITUTION:Base plates 2 and 2' made of an insulating material are respectively attached to the facing surfaces of magnets 1 and 1' which are located parallel to each other at a given interval with spacers (Sa) and (Sb) interposed. The base plates 2 and 2' have an arc-like shape extending along the central path (0) for ions. Concentric arc-like electrodes (A1-An) or (B1-Bn) are buried at equal intervals (D) in each of the facing surfaces of the base plates 2 and 2'. They are buried in the base plate 2 or 2' at such an angle as to extend along the equipotential surface of a troidal electric field to be produced in the area surrounded by the base plates 2 and 2'. Information about an electric potential to be applied to each electrode to produce a troidal electric field is stored in a memory 4. Voltage based on the above information is applied to each electrode by supplying the information read by a reading-controlling circuit to a power supply 3.
