Multi-channel bound states in the continuum in coaxial cylindrical waveguide

Bound states in the continuum (BICs) or embedded trapped modes are widely studied in different physical systems. The studies are restricted to a single open scattering channel. In the present paper we consider BICs embedded into several continua in a cylindrical resonator opened by two coaxially attached cylindrical waveguides with different radii. We demonstrate that engineering the BICs requires a degeneracy of three eigenmodeds of the closed resonator. That is achieved by variation of both the length and the radius of the resonator. (paper)