A plasma processing system is provided with diagnostic apparatus for making in-situ measurements of plasma properties. The diagnostic apparatus generally comprises a non-invasive sensor array disposed within a plasma processing chamber, an electrical circuit for stimulating the sensors, and means for recording and communicating sensor measurements for monitoring or control of the plasma process. In one form, the sensors are dynamically pulsed dual floating Langmuir probes that measure incident charged particle currents and electron temperatures in proximity to the plasma boundary or boundaries within the processing system. The plasma measurements may be used to monitor the condition of the processing plasma or furnished to a process system controller for use in controlling the plasma process.