Abstract: A plasma processing apparatus is disclosed, comprising: a plasma processing chamber; a substrate support disposed in the plasma processing chamber; a lower electrode disposed in the substrate support; at least one RF power source coupled to the lower electrode; an electromagnet unit disposed on or above the plasma processing chamber; and an upper electrode assembly, disposed above the substrate support, including: an insulating plate having a plasma exposed surface; a gas diffusion plate having a gas diffusion space; and an upper electrode plate, disposed between the insulating plate and the gas diffusion plate, including: a coolant inlet and a coolant outlet located at a location that is radially outward from the electromagnet unit; a coolant supply channel extending radially from the coolant inlet to a vicinity of the approximate center of the upper electrode plate; and a coolant return channel extending from the vicinity of the approximate center of the upper electrode plate to the coolant outlet.