an apparatus for plasma processing of a wafer includes an annular structure including a magnet , where the structure is concentric with the wafer holder ; the magnet generates a magnetic field for deflecting charged particles incident on the structure , thereby preventing damage to the structure by those particles . accordingly , the structure may be of a material susceptible to erosion during the plasma processing , so that the magnetic field reduces that erosion . the cost of consumable parts in the apparatus is thus reduced . the annular structure may be characterized as a ring having a groove formed therein , with the magnet disposed in the groove . the magnet may be either a permanent magnet or an electromagnet .