###### Abstract  
Transformation of a Bessel beam by a lens results in the formation of a \"perfect\" vortex beam (PVB) in the focal plane of the lens. The PVB has a single-ring cross-section and carries an orbital angular momentum (OAM) equal to the OAM of the \"parent\" beam. PVBs have numerous applications based on the assumption of their ideal ring-type structure. For instance, we proposed using terahertz PVBs to excite vortex surface plasmon polaritons propagating along cylindrical conductors and the creation of plasmon multiplex communication lines in the future (Comput. Opt. 2019, 43, 992). Recently, we demonstrated the formation of PVBs in the terahertz range using a Bessel beam produced using a spiral binary silicon axiom (Phys. Rev. A 2017, 96, 023846). It was shown that, in that case, the PVB was not annular, but was split into nested spiral segments, which was obviously a consequence of the method of Bessel beam generation. The search for methods of producing perfect beams with characteristics approaching theoretically possible ones is a topical task. Since for the terahertz range, there are no devices like spatial modulators of light in the visible range, the main method for controlling the mode composition of beams is the use of diffractive optical elements. In this work, we investigated the characteristics of perfect beams, the parent beams being quasi-Bessel beams created by three types of diffractive phase axions made of high-resistivity silicon: binary, kinoform, and \"holographic\". The amplitude-phase distributions of the field in real perfect beams were calculated numerically in the approximation of the scalar diffraction theory. An analytical expression was obtained for the case of the binary axicon. It was shown that a distribution closest to an ideal vortex was obtained using a holographic axicon. The resulting distributions were compared with experimental and theoretical distributions of the evanescent field of a plasmon near the gold-zinc sulfide-air surface at different thicknesses of the dielectric layer, and recommendations for experiments were given. b 2021 11 717 11 1  
electromagnetic field of plasmons, their orbital angular momentum relative to the axis of the system will be nonzero. Surface plasmons of this kind have been obtained experimentally in the visible range in many works (see, for example, [4; 5; 6] and reviews [7; 8]). It is possible, however, to excite another type of vortex plasmons, which propagate along an axisymmetric conductor and simultaneously rotate around its axis. In this case, the Poynting vector of a plasmon moves helically over the conductor, and, therefore, the plasmon carries an orbital angular momentum. In Ref. [9], we proposed using a combination of vortex surface plasmon polaritons propagating along a metal wire to create a multiplex plasmon transmission line, which is similar to using vortex beams for multiplex communication in free space [10]. Since the propagation length of plasmons in the visible range is of the order of 10 \(\upmu\)m [11], in transmission lines of macroscopic dimensions, it is necessary to use terahertz or mid-infrared radiation. In [12], experiments performed at a wavelength of 141 \(\upmu\)m demonstrated for the first time the excitation of vortex plasmons on an axisymmetric conductor and their propagation over a distance of 100 mm. To excite plasmons on the cylinder, higher-order vortex beams illuminated its end face (the end-fire coupling technique [13]). In this excitation method, the spatial distribution of the complex amplitude of the electric field of the vortex beam and its overlap with the distribution of the vortex plasmon field play an important role. The vortex plasmon field distribution is determined both by the properties of the conductor material and the wavelength of the incident radiation, as well as by the topological charge of the exciting beam. When creating a multiplex line, it is necessary to simultaneously launch several vortex plasmons with different topological charges on a cylindrical conductor.