Non-invasive measurement of plasma systems

A system for measuring a characteristic of a plasma within a plasma chamber, wherein the plasma chamber is a multi-frequency driven plasma system, and wherein the plasma chamber has a viewport, or a surface, which is permeable to electromagnetic radiation such at least a portion of the electromagnetic radiation emitted by the plasma in the plasma chamber passes through the viewport, the system comprising:

• a Radio Emission Spectroscopy, RES, system provided externally to the plasma chamber to absorb at least a portion of the electromagnetic radiation that has passed through the viewport, the RES being configured to:
• measure signals in near-field E- and B-field regions;
• measure a first value based on the signal measured by an antenna of the RES wherein the signal measured by the antenna is obtained from a plurality of powered RF electrodes configured to be independently modulated with a plurality of power sources, wherein at least two of the plurality of power sources operate at different RF frequencies simultaneously;
• and
• calculate a second value indicative of a change of magnitude of the characteristic based on a change of magnitude of the first value, wherein the characteristic is plasma power and/or plasma pressure.