Generation of 2.45-GHz microwave plasma filament in sub-atmospheric pressure

Plasma photonic crystals (PPCs), which are periodically arranged plasmas, are expected to have engineering applications in electromagnetic wave control devices. Our objective is to construct filamentary structures using 2.45-GHz microwave discharge plasma that can generate high-density plasma and to understand their plasma characteristics while keeping in mind PPCs for controlling electromagnetic waves near 10 GHz. This work explores the pressure and power characteristics of plasma filaments produced by helium quasi-atmospheric microwave discharges. We measured the length characteristics of plasma filaments within a power range of 150–900 W and the pressure range of 200–900 hPa. In the transition region, the filament length is proportional to power and increases from 5 mm to the saturation value regardless of pressure. The filament length in the saturated region decreases from approximately 50 mm to approximately 40 mm in the pressure range of 200–900 hPa. According to spectroscopic measurements, the electron density of the filament plasma was 4.81×10⁹–4.02×10¹⁰ cm^-3.