Microwave plasma generation by the fast rotation and slow pulsation of resonant fields in a cylindrical cavity

Abstract

A digitally controlled solid-state microwave generator allowing variable frequency operation and precise phase control is adopted for plasma generation. In this study, a resonant cylindrical cavity is used as a microwave applicator in place of conventional waveguides. In order to improve the plasma uniformity, the TE111 mode is agitated by injecting microwaves into the cavity from two spatially orthogonal directions, with a temporal phase difference ϕ. Theoretical analyses and finite-difference time-domain simulations derive the following effects of the phase control. In the case of ϕ = ±π/2, fast rotation of the cavity field takes place with a rotational frequency of ω/2π (= 2.4–2.5 GHz), where ω denotes the microwave angular frequency. On the other hand, when ϕ is linearly modulated in time with a low frequency of Ω/2π (= 0.1–1000 Hz), slow pulsation takes place, in which the cavity field alternately excites a circular rotation and a standing oscillation at the modulation frequency. These effects are experimentally confirmed in microwave discharges in argon at 0.1–20 Torr with total injection powers from 50 to 800 W. Two-dimensional images of the optical emission from the generated plasma show that both the fast rotation and slow pulsation improve azimuthal plasma uniformity.

Uniformity enhancement of a microwave surface-wave plasma by a field agitation

“…In this work, we present a different approach to achieve uniformity in a high-density, large-area SWP based on the modulation of TM mode microwave field distribution. There are two major differences between the work in the previous studies 11,12) and this work. Firstly, the cylindrical cavity used in this work is designed to resonate in higher-order mode, namely TM .…”

“…The numbers for the mode (m = 1, and n = 3) are determined from the operation frequency of a microwave generator source (2.458 GHz) and the diameter of the cylindrical cavity (395 mm). 10,12,15,16) Secondly, the method for re-distribution of the electric field inside the chamber is different to some degree; the phase difference between the two microwave inputs (f) was either fixed (fast rotation) or changed gradually (slow pulsation) in the previous studies, but in this work the f is controlled by toggling its value between 0°and 180°. By setting the f to be 180°, the field would have a distribution rotated by 90°anticlockwise compared to the f = 0°case.…”

“…The non-uniformity of the large-area microwave plasma is attributed to the fact that the typical chamber diameter is significantly longer than the wavelength of the microwave (∼12.2 cm for 2.458 GHz). [6][7][8][9][10] Previous studies 11,12) have proposed rotating microwave fields with TE 111 mode to enhance the plasma uniformity by imposing the phase difference between the two orthogonally placed microwave sources or by modulating the field amplitude of the microwave sources.…”

“…Hence our system is anticipated to exhibit distinctive characteristics such as: (1) higher plasma density from a higher power transfer efficiency than the TE 111 mode-based system used in the previous studies. 11,12) (2) Enhanced uniformity in azimuthal direction for a large-area SWP. Such characteristics are observed from the plasma emission pattern image as well as the ion density from the Langmuir probe.…”

Effects of a Cylindrical Metallic Cavity on the Radiation of Longitudinally Polarized Limited-Diffractive Bessel Beams

“…Similarly, the spatially distributed field obtained by multiple Bessel beam reflections inside metallic cavities paves the way to the use of such structures for microwave heating. Moreover, the compression of plasmas inside metallic cylindrical cavities together with their interaction with RF and microwave fields, with RF and microwave fields, allows exciting them in plasma ignition systems [26], [27], [28]. Thus, the presence of metallic cavities needs to be accurately modeled.…”