## 5 Conclusions and Perspectives  
In this work, injection schemes for plasma cavity of ECR ion sources were studied and compared, considering both conventional and innovative plasma chambers. In particular, an advanced antenna for the new chamber shape is studied. This new and groundbreaking antenna and cavity system is twisted so that the cavity volume resembles the electrons trajectories as they move under the influence of the confining magnetic field. The employed shape presents some advantages with respect to a classical cylindrical cavity usually employed in ion source setups. In particular, modes having an electric field profile with maximum near the cavity center, where the plasma usually generates, can be excited. Moreover, the electron-shaped geometry occupies less radial volume, providing more space for the magnetic system axial coils, which turns out to be advantageous for a fine tuning of the plasma-confining magnetic field. Such non-conventional cavity can be coupled to a radically innovative microwave injection. In view of this, a system based on a side-coupled slotted waveguide has been conceived and studied. The slotted waveguide has firstly been analyzed as an antenna operating in free space in order to test its behavior versus the fundamental tuning parameters. Subsequently, the slotted waveguide has been jointed to the two plasma cavity geometries and the two structures behavior has been studied in terms of S-parameters. Numerical results show that the new injection system greatly improves the excited modes inside the frequency band of interest, which could be useful in ion source frequency tuning operations. From the simulation, it has also been  
Figure 10: Electric field module plot on side and front slices for: (**a**) cylindrical cavity (\(f_{0}=14.388\) GHz) and (**b**) “plasma-shaped” cavity (\(f_{0}=14.466\) GHz) employing the side-coupled slotted waveguide injection system. Electric field has been normalized to the maximum value of \(10\) kV/m. found that the use of the \"plasma-shaped\" cavity in conjunction with the slotted waveguide injection system results in an axially-symmetric field distribution of the excited modes, and this could allow a more uniform power transfer to the plasma from the radiating slots. Furthermore, the use of the side-coupled slotted waveguide also allows more space on the cavity injection flange, employable for other ancillaries. The plasma-load effect will be predicted by our full-wave numerical tool considering the self-consistent interplay between the microwave field and a fully anisotropic 3D plasma filling the cavity [21; 22]. Ongoing simulations already show higher robustness of modal structures established in this kind of cavities. Finally, experimental characterization of the future slotted waveguide and plasma-shaped cavity prototypes will be performed, in terms of S-parameters and electric field measurements, in order to quantify the performance impact of internal surface roughness derived from the additive manufacturing versus standard realization technique. Preliminary vacuum tests on a pair of 3D metal printed containers samples have already been performed with positive and promising results. [PERSON] and [PERSON] addressed the conceptualization of the \"plasma-shaped\" cavity. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] are responsible for the methodology of the study, including analysis and development of the cavity and slotted waveguide microwave injection system. [PERSON], [PERSON] and [PERSON] carried out the cavity, antenna and final assembly numerical simulations and optimization. Writing-original draft preparation, G.S.M. ; writing-review and editing, [PERSON], [PERSON], [PERSON], [PERSON] and D.M. ; supervision and project administration, D.M. All authors have read and agreed to the published version of the manuscript. This research received no external funding. The authors wish to thank the 3 rd Nat. Comm. of INFN, under the PAN-DORA_Gr3, for the financial support.