Plasma diagnostics via microwave absorption

An experimental investigation of cold plasma diagnostics via microwave absorption is the subject of this thesis, with the primary objective of estimating the electron number density of a low-pressure laboratory plasma. The work serves as a preliminary assessment of the microwave absorption technique for characterising plasma parameters, particularly focusing on identifying the frequency range within which the plasma frequency falls. The theoretical framework is established through an examination of plasma fundamentals, including the criteria for ionised gas to be classified as plasma, Debye shielding, and electron plasma oscillations. The interaction between electromagnetic waves and cold-plasmas is analysed using the cold plasma model, deriving the dispersion relations for both collisionless and collisional cases. Special attention is given to the condition of wave propagation, cutoff, and attenuation mechanisms. The experimental part employs a microwave diagnostic setup consisting of horn antennas operating in the 2-18 GHz range, a vacuum chamber with electrode-based plasma generation, and appropriate signal acquisition systems. The transmission factor, defined as the ratio between received microwave power with and without plasma, is measured across multiple frequencies. Two sets of measurements are conducted at different pressure levels: 1.4 mbar and 0.5 mbar. The data analysis uses a MATLAB script that implements a fitting algorithm based on the attenuation coefficient, allowing for the estimation of plasma frequency and thus electron density. Results yield electron number densities of 1.35 × 10^16 m−3 at 1.4 mbar and 1.67 × 10^16 m−3 at 0.5 mbar. Non-monotonic transmission behaviour observed in some measurements is discussed in terms of experimental limitations including wave interference and plasma instability.