Localized breaking of parameter uniformity by macroscopically-negative-permeability metamaterial in low-pressure microwave plasma

Abstract

Measurements of plasma parameters and transmitted microwaves are performed in a composite of double-split-ring resonators (DSRRs) making magnetic permeability negative and plasmas excited by a 2.45 GHz microwave. We launch the microwave at 100-200 W, modulated by a pulse wave at a low frequency and a low duty ratio to detect time evolutions of plasma parameters. We perform the measurements at different positions with varying distance from the supporting plate of the DSRRs, and find enhanced wave transmission in the composite with nonuniform profiles of electron temperature. To reinforce the measurements of electron temperature, we evaluate the electron energy distribution function (EEDF) by deriving the second deviation of electron current, upon which data smoothing methods are applied, and the estimated EEDF becomes spatially non-uniform with a similar profile to electron temperature. In terms of the validity of results on the enhanced wave transmission, one-dimensional electromagnetic particle simulations reveal sneaking microwaves inside the composite. In comparison with cases of the negative-permeability and the passive (positive-permeability) DSRRs, we confirm that the nonuniformity is attributed to the magnetic resonance of the DSRRs, which is a microscopic effect of the DSRRs to plasma generation, whereas their macroscopic effect makes permeability negative and enhances wave energy transfer inside the composite.