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Reverse Magnetic System of Multibeam Klystron Amplifier


S. V. Kozlov, V. M. Pikunov

At designing of MBK devices two-dimensional program complexes in which longitudinal magnetic field component is calculated on the symmetry axis of magnetic system are commonly used. However, in this paper it will be shown that magnetic tips have the final sizes in reverse magnetic focusing systems, therefore transverse magnetic field component doesn't fade in apertures due to boundary effects. In the two-dimensional approximation investigated three-reversible magnetic system of a powerful broadband klystron amplifier, containing a 18-beam electronic gun, is reduced to an axially symmetric and examined by using software package Poisson 2D. Magnetic with apertures is replaced by a washer with varying thickness on two rows of flying channels. Reversible magnetic system becomes fully axisymmetric. The received results have shown that the transverse magnetic field component at the periphery of the magnetic system can be compared with the focusing component. Hence, during design of multibeam systems it is desirable after a two-dimensional trajectory analysis to conduct a three-dimensional modeling of physical processes for optimization of devices. In the next step a three-dimensional software package CST Studio Suite 2006 was used for more accurate analysis of reverse magnetic system. The transit channels of both rows are characterized by Non-uniform distribution of the transverse components of the magnetic field at observation points. Its amplitude reaches the minimum values at the center of apertures and increase at the edges of the apertures of the magnetic circuit. The presence of a magnetic field in the screen areas can affect the forming of the electron beams in electron-optical system of the gun and the deposition of the electron beam in the collector region of the multibeam klystron. The area of reverse magnetic field is not only within the magnetic circuit, but also it is outside of magnetic at a distance of radius of the aperture of the flying channel. Due to boundary effect transverse component of the magnetic field does not fall sharply to zero in the vacuum and inside the magnetic circuit due to its finite thickness. If the thickness of the magnetic circuit was much larger than the diameter of the aperture, the transverse component is headed for zero. Consequently, the electrons located closer to the edges of the drift tube will experience a greater deviation than closer to the center. Action of defocusing force will be maximum at the inlet and outlet of apertures in the magnetic circuit. Since the amplitude of the transverse magnetic field component increases with rows of flying channels, so current falling of beams from external row must be greater than from internal. The size of the reversal area in the longitudinal direction is about twice the radius of the aperture plus the thickness of the magnetic core and it increases with the size of the first. Thus, the results of the analytical analysis which is fair for infinite – thick magnetic, will be agreed numerical results only at the centers of magnetic cores apertures. The necessity of three-dimensional study of magnetic systems MBK, where there is a transverse magnetic field component, which must be included in trajectory analysis is shown. Transverse component determines low current flow in these systems, and reduction of its amplitude is an important task for developers.
June 24, 2020
May 29, 2020

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