Abstract: A plasma density monitor is described for determining the ion current of a plasma flowing through a conduit (14). The monitor comprises a plate (18) having a face and an orifice extending from the face through the plate. The internal surface of the orifice conforms with the internal surface of the conduit when the face is adjacent the conduit. The monitor also comprises a detector circuit (22) for sensing the ion current collected by the internal surface of the plate (18). Another embodiment monitors plasma density by determining electrical conductivity in the afterglow of a microwave induced plasma. A resonant LC circuit (50, C3) has a multi-turn coil (C3) surrounding the conduit, an RF oscillator circuit (54, 56) for driving the LC circuit, and a circuit (64, 66, 68, 70, 72, 76) for measuring the decay time of the LC circuit. Plasma flowing through the conduit alters the resistivity of the coil and the time constant of the LC circuit proportional to the electrical conductivity of the flowing plasma.