Generation of slowly rotating microwave plasma by amplitude-modulated resonant cavity

Slow rotation of microwave plasma at a rotational frequency of Omega/2 pi = 0.1-1000 Hz is realized to improve plasma uniformity by using a resonant cylindrical cavity and a solid-state microwave generator at a frequency of omega/2 pi = 2.4-2.5 GHz. The microwave at omega/2 pi is modulated in amplitude at Omega/2 pi and injected into the cavity from two orthogonal positions, exciting the TE111 mode. The cavity fields rotate either clockwise or anticlockwise at a frequency of Omega/2 pi when the phase differences, Delta phi at omega and Delta phi at Omega, between the input microwaves are properly set as calculated by a theoretical analysis and finite-difference time-domain simulation. Rotating plasmas are experimentally measured in the microwave discharges of argon at 0.1-20 Torr. When the rotational frequency is low (Omega/2 pi < 30 Hz), a plasma rotation is visible in the optical emission image; the azimuthal rotation of a local ion density is also confirmed by a rotatable Langmuir probe array. Conversely, when Omega/2 pi > 1000Hz, the electron density measurement by a curling probe reveals that the plasma rotation disappears in the downstream region. This observation is supported by a simplified analysis based on the diffusion equation, proving a characteristic distance of plasma rotation disappearance to be root D-a/Omega (D-a: ambipolar diffusion coefficient). (C) 2017 The Japan Society of Applied Physics