A comparative study of a variety of electromagnetic (EM) structures designed for suppressing a gyro-BWO instability in the W7-X gyrotron beam-duct is presented. The EM fields are calculated using the finite element code HFSS. It is shown that among dielectric-loaded ring-structures the one having random azimuthal indentations on the copper ring is efficient and so are conical dielectric-loaded structures without copper rings. © 2011 IEEE.