## 5 Discussion  
The results of the analytical and numerical calculations of the characteristics of terahertz vortex beams created by the three types of axicons show that their characteristics depend substantially on the axicon type. Although all these axicons create quasi-Bessel beams, in which the amplitude-phase distributions of the field in the first ring are almost identical, the distributions of the field at the periphery differ--insignificantly, at first glance. However, when a lens focuses these beams, the main contribution into the formation of perfect beams, which are essentially the Fourier transform of a quasi-Bessel beam, is made by the beam periphery, in which the energy significantly exceeds the energy contained in the first ring. As a result, as seen in Figures 5-7 and Table 1, the shape of the annular PVB is highly dependent on the phase distribution profile of each axicon. The diffraction phenomena associated with the relatively small number of zones in the axicons, which should be rather wide in the terahertz range because of the large wavelength, may also play a certain role in the formation of the PVB profile. From this, we can draw two conclusions. First, perfect beams formed in different experiments always bear the imprint of their origin, and the properties of each of them are largely individual. The second concerns the Bessel beams themselves. It is very likely that their inherent self-recovery property after passing through inhomogeneities also depends on their origin, since self-recovery is provided by the arrival of converging plane waves from the beam periphery into the optical axis region. Apparently, this circumstance should be taken into account in propagation of Bessel beams in inhomogeneous media. Another issue to discuss is the dependence of PVB parameters on the magnitude of the topological charge. In [27], it was shown theoretically and experimentally that, in contrast to the conclusions of the simplified theory, the radius and width of the rings of PVBs created in the visible range using a spatial light modulator grow with the topological charge. The results of our calculations show that in the case of terahertz vortices created using diffraction axicons, we observe no strong dependence on the topological charge, although there is a slight upward trend. We can trust the accuracy of calculations by this program, since, in [26], it was demonstrated that the images of perfect beams obtained in the experiment coincided with the calculated ones. In conclusion, let us discuss the possibilities of using axicons to excite surface plasmon polaritons on cylindrical waveguides. A factor favorable for the excitation of terahertz plasmons is the very small change in the radius and width of the rings of perfect plasmons with a change in the topological charge. Apparently, radially polarized PVBs can be applied to the excitation of azimuth-uniform SPPs using a holographic axicon. In this case, the half-width of the PVB ring is approximately 400 um, which is in good agreement with the decay length of plasmons on a gold surface covered with a zinc sulfide layer that is about 0.5 um thick. When using other axicons, one can expect the separation of SPPs in azimuth into \(\ell\) or 2\(\ell\) plasmons moving in parallel, which, however, carry the same total orbital angular momentum. Since in this case, the half-width of the rings is of the order of 1 mm, they can be used at a dielectric coating thickness of about 0.1 um, at which the decay length is close to this value. With linear polarization of the PVB, plasmons will be excited on the opposite sides of the cylinder, where the wave electric field vector is normal to the surface. Finally, note that the limited aperture of the beam on the axicon, which causes diffraction of the beam, can also be beneficial in experiments. Figure 4 shows that near the