Thomson microwave scattering for diagnostics of small plasma objects enclosed within glass tubes

Abstract

In this work, coherent microwave scattering in the Thomson regime was demonstrated for small-scale plasmas enclosed within a glass tube and validated using a well-known hairpin resonator probe technique. The experiments were conducted in a DC discharge tube with a diameter of 1.5 cm and a length of 7 cm. Thomson microwave scattering (TMS) diagnostics yielded electron number densities of about 5.9 × 1010 cm−3, 2.8 × 1010 cm−3, and 1.8 × 1010 cm−3 for air pressures in the discharge tube of 0.2, 0.5, and 2.5 Torr, respectively. Measurements using the TMS technique were consistent across the tested microwave frequencies of 3–3.9 GHz within the margin of error associated with non-idealities of the IQ mixer utilized in the circuit. The corresponding densities measured with the hairpin resonator probe were 4.8 × 1010, 3.8 × 1010, and 2.6 × 1010 cm−3. Discrepancies between the two techniques were within 30% and can be attributed to inaccuracies in the sheath thickness estimation required for correct interpretation of the hairpin resonator probe results.