## 1 Introduction  
Microwave plasma sources (MPSs) are driven by applied frequencies of 1-100 GHz and lead to electron heating and excitation of a discharge from the electromagnetic waves that either penetrate the plasma or exist along the surface [1, 2]. With weakly ionized MPS used for material processing, waves are important to transfer energy from the excited surface of the waveguiding structure to the bulk plasma, where the wave energy is absorbed [3]. The main element of the MPS, for material processing, is the microwave-to-plasma applicator because it determines the structure of the electromagnetic field in the plasma as well as the absorption energy efficiency of plasma [4]. The structure of the electromagnetic field in plasma is determined by the correlation of the internal plasma parameters (electron density, plasma frequency, etc.) and the external source parameters (waveguide structure, size of source, frequency, etc.). The energy absorption efficiency of MPS is determined by the wave penetration depth in the plasma, wave field strength at the plasma surface, and waveguiding structure. Among the MPS for material processing, the sources generally have a form in which waves propagate along the transmission lines. The plasma is generated by the waves emitted from the dielectric surface which replaces a part of the waveguiding structure. Depending on the size and structure of the dielectric surface, plasma sources using the transmission line field can be classified in the following ways: (i) plasma is a part of the waveguiding structure (transmission line applicators) and (ii) plasma is sustained by the field leaking out of the structure (antenna applicators) [5]. For the transmission line applicator, the cross section of the plasma-dielectric contact is wide, whereas for the antenna applicator, the plasma-dielectric contact cross section is narrow. So, in the case of antenna applicator, the process stability is high since the wave propagation mode and the resonant cavity in the waveguiding structure are less affected by plasma generation. Hence, in material processing, which requires stable plasma operation, antenna applicator type MPSs are preferred. Among the antenna applicator type MPS, LMPSs are mainly used for large area processing [6]. Erstwhile LMPS models [7] were either in the form of discharge tubes placed inside the guiding structure or in the form of discharge tubes across the wide waveguide wall parallel to the electric field [8; 9]. However, in terms of productivity improvement and large area material processing, the shape of the discharge tubes inside the guiding structure is not suited for the application, so the shape of the discharge outside the guiding structure is demanded. A SW sustained source called the duo-plasma line source, which uses the quasi-coaxial line TEM wave mode, was invented according to the shape of the discharge outside the tube with a waveguiding structure [10; 11]. Due to the high scalability with a large processing area [12; 13], duo-plasma line source has been applied to many plasma enhanced chemical vapor deposition (PECVD) processes such as hard coating, thin film, synthesis of graphene, and display panels, which require a large area, an operation pressure of 0.2\(\sim\)3 Torr, and high speed processes [14; 15; 16; 17]. In the quasi-coaxial line TEM wave mode, the isotropically emitted transverse wave field from the inner conductor is perpendicular to the tangential plane of the window surface in all radial directions, and the dielectric surface in all radial directions is exposed to interact with the plasma generation. Therefore, it is easy to generate a uniform plasma in the radially omnidirectional plasma-dielectric window surfaces. However, it is difficult to maximize the process productivity by concentrating the plasma density in a one-directional selected deposition region.