Forming of high current density electron beam for terahertz vacuum O-type amplifiers

The analysis of low-perveance electron beams with high density inside the drift space with a diameter lower than 0,3 mm for THz devices are presented in this work. In these studies the values of perveance of the beam are in range of (0,14÷0,35)10^-7 A/V^3/2. In this case the quantity of forming longitudinal low-perveance electron beams is determined by the influence of the initial thermal transverse velocities of electrons. Using the software Lorentz-¹ the three-dimensional analysis of electron beams structure were carried out for the cases of focusing in electron optical system with magnetic-shielded cathode and cathode in uniform magnetic field. The result of electron optical system simulation with magnetic-shielded thermionic cathode is presented where ripple factor of electron beam drifted in magnetic field (1.2 T) of 0,13. The design of electron optical systems with array of field-emission cathode with the low-voltage grid control and with a density of cells N=10⁷-10⁸ cm^-2 is also performed. Modeling of field emission was carried out in papers^{2, 3} by using the various technique of the initial transverse velocity. In our report we prefer the two-stage modeling as simulation the trajectories of the particles for one cathode-grid cell in magnetic field and assessment of the potential which corresponds to the maximum transverse velocity of electrons. For electron gun specifies the number of particles corresponds to the density of cells and the value of U_max. The calculation of the potential of transverse velocity was carried out according to the formula: U_max = 8,8-10^-4(r₀B)² where r₀-the radius of the cyclotron orbits (mm) defined for one cell in a magnetic field B (Gs). Based on the analysis we note that planar structures with micro-sized cathode have a significant drawback associated with a spread of the transverse- velocity of the electrons, due to the lens effect in the holes of grid.