Concentric Split-Ring Resonator Microwave Microplasma Generation at Off-Resonant Frequencies

Abstract

The driving frequency of a microstrip concentric split-ring resonator (CSRR) was varied to determine how the generated microplasma changes as a function of frequency, specifically at off-resonant frequencies. The microplasmas were generated in argon at 1 torr, using a forward microwave power level of approximately 10 W. The resonant frequency was experimentally found to be 796 MHz, and measurements were obtained at this frequency and ±10 MHz from resonance. Double Langmuir probe measurements were performed on a 2-D grid, parallel to the surface of the device, to obtain plasma density and electron temperature maps. The results show multiple peaks in plasma properties due to electromagnetic coupling with the nested ring. The electron temperature above the primary discharge gap is 8.5 eV at 786 MHz, 9.5 eV at 796 MHz, and 7.6 eV at 806 MHz. A secondary discharge is also observed above the secondary nested ring gap, and the plasma in this region is generally of lower temperature. The off-resonant operation resulted in lower temperatures and densities, with larger decreases in the 806 MHz case. The electromagnetic response of the device was modeled to understand the electric field in the discharge gap. The modeling revealed that the electric field vectors at the bounds of the discharge gap point in the opposite directions for the 786 and 796 MHz cases, increasing the electric field in the gap, but these vectors point in the same direction for the 806 MHz case, decreasing the electric field.