A source plasma is generated in a multipole-type device in oxygen by a radio frequency discharge at 13.56 MHz. A SmCo magnet array is mounted between source and target plasma sections as a magnetic filter. In the target section, almost electron-free plasma is obtained. The device can be operated in the pressure range of 10(-3) to 10(-1) Torr. The plasma densities in source and target plasmas are 10(9)-10(10) cm-3 and 10(8)-10(9) cm-3, respectively.