This paper is concerned with a detailed analysis of the dominant modes of a slotted helix loaded in an empty waveguide, with a view to improving the efficiency of plasma production using such structures. To this end the dispersion relation for a slotted helix loaded inside a cylindrical waveguide is first derived. As in the case of a tape‐helix‐loaded waveguide, the different dominant modes of the system can be identified by suitable asymptotic expansions of the exact dispersion relation. For large diameter helices (kvrh>1, wherekvis the free‐space wavelength of the wave andrhis the helix radius), the system has one slow wave and one fast wave as its dominant waves. The field patterns of these two modes are studied (for both slotted and tape helices) and the dominant field components corresponding to each type of helix and mode, identified from the point of view of ease in the excitation of the slow wave for plasma production. Since the fast and slow waves have overlapping dominant field components, methods are also discussed for suppressing the fast wave which can be excited more easily due to the presence of the waveguide. Finally, the state of polarization (left/right handedness) of the slow‐wave mode is discussed with relevance to the coupling of the microwave to the plasma electrons in an external magnetic field.