Abstract: The present invention generally provides a diagnostic head assembly which preferably includes both optical and non-optical diagnostic devices which locates the diagnostic devices in proximity to the processing region of a plasma reactor to enable in situ process control. Such diagnostic head assembly may include a quartz crystal microbalance, an optical endpoint detector and a Langmuir probe. The invention also provides microprocessor control of a plasma process using the information gathered from the optical and non-optical diagnostic devices. The invention further provides a plasma reactor that includes both optical and non-optical diagnostic devices positionable in a single diagnostic port provided in a sidewall of the reactor.