A new 915 MHz coaxial-line-based microwave plasma source

Abstract Microwave plasma is known for its versatility in providing tailored operating conditions (pressure, working gas composition and residence time of reagents) for specific applications. Microwave plasma sources (MPSs) are vital in modern applications, demanding continuous improvement. This work introduces a coaxial-line-based nozzleless MPS that operates at atmospheric pressure at an unique frequency of 915 MHz. The measured electrodynamic characteristics in nitrogen of the MPS highlighted the need for improved energy efficiency of the device. The main novelty of this work lies in improving an energy efficiency of the presented MPS, which led to an advanced new version of the device. To achieve this, a dual strategy is employed. Firstly, numerical simulations are used to design a construction modifications to the MPS, which should increase the efficiency of transferring microwave energy from the microwave source to the generated plasma. In this step, a standard model for homogeneous plasma and a two-port equivalent method were used. Then, the theoretical results were experimentally validated by manufacturing a new energy improved version of the MPS. In the new MPS the achieved reflected microwave power (losses) was less than 3% of incident microwave power in the tested range of nitrogen flow rate (50–100 Nl/min). Compared to the MPS before improvement, this means a two-fold decreasing the reflected microwave power. To test the new MPS, the electrodynamic characteristics of the new device version and properties of the microwave plasma generated in nitrogen, using optical emission spectroscopy (OES), were investigated. The OES was used to determine the vibrational T vib and rotational T rot temperatures of nitrogen molecules and molecular ions. In this work, the estimated T vib and T rot temperatures for nitrogen molecules ranged from 4000 to 5300 K, depending on discharge conditions, while for nitrogen molecular ions, the temperatures changed between 4700 and 6100 K, respectively. Both the T vib and T rot temperatures decrease linearly along the plasma flame.