Design of a Gridded Electron Gun for Traveling-Wave Tubes: an EGUN case study César Candido Xavier 1 and Cláudio C. Motta 2 1 Instituto de Pesquisas Energéticas e Nucleares/CNEN-SP 2 University of Sao Paulo 1 cesarcx@usp.br , 2 ccmotta@usp.br , Av. Lineu Prestes 2242 – Cid. Universitária - Sao Paulo – Brazil - 05508-000 Abstract: A design procedure of a 0.7Perv electron gun, with grid and shadow grid, under space charge-limited flow to be used in traveling-wave-tube (TWT) is presented. The EGUN was used to determine the self-consistent electric fields for the electron beam and, as a result, it was obtained a current/perveance growth as the grid voltage increased and the cathode-to-anode spacing decreased. This behavior was observed when grid voltages and cathode-to-anode spacing were varied from 0-600 V and 0.8-1.2mm, respectively. Keywords: TWT; EGUN; electron gun; space-charge- limited flow. Introduction The proprietary design of TWTs is of high commercial value since it represents over 50% of all sales of microwave vacuum electronic devices [1] such as in satellite transmitters and in high-power radar systems. These applications require the use of grids, which ensure the beam flow control that can be pulsed whenever it is needed. The electron gun is one of the critical components of a TWT, and computer simulations of the gun model are crucial for the success of the design, ensuring not only lower cost and reduced time but also the ability to develop highly complex geometries. Many codes have been developed in the last two decades, such as [2]-[4]. The EGUN, a seminal software program, was used to simulate the guns. The three major characteristics of EGUN are: user friendly; ability to work under space-charge- limited flow conditions; and solve electron trajectory equations using guns modeled even with grids and shadow grids. Deviation from EGUN results and the theoretical one of the standard Pierce diode for a sheet electron beam is about 1%. In this work, in order to obtain a 0.7Perv electron gun, with grid and shadow grid, under space-charge-limited flow, the gun’s behavior was evaluated by changing the following parameters (Figure 1): cathode radius disc r k ; electrode focus angle θ; cathode-to-anode distance d; grid- to-cathode distance g d ; and grid voltages V gd . The first part of this work presents the main characteristics of the gun model used in simulations. The second part describes the most relevant results obtained in the design of an electron gun working at 30 kV – 3.5A. Figure 1 – Gun parameters changed within EGUN to evaluate its response behavior: cathode radius disc r k ; electrode focus angle θ; cathode-to-anode distance d; grid-to-cathode distance g d ; and grid voltages. The gun model The simulations were conducted, and the gun’s parameters varied as indicated in Table 1. Table 1 – Range of values used in EGUN simulations Variable Initial Value Final Value Step r k (mm) 11.9 13.1 0.3 θ(degrees) 6.2 39.2 3 d(mm) 8.5 9.25 0.125 g d (mm) 0.9 1.1 0.1 V gd (Volts) 0 600 100 The anode voltage was set to 30kV in all simulations. In view of the EGUN’s limitations, a scale factor of 8 was used since, when the mesh is created, there must be at least one mesh unit of separation between the different elements. As the EGUN is a two-dimensional (2D) code that supports either rectangular or cylindrical symmetry, the original grids were approximated as five concentric rings due to the axisymmetric model.