Analytical, Simulation, and Experimental Study of the Generation of Multiple Beams for Application to a Traveling Wave Tube (TWT)

Khandakar Nusrat Islam, Doctoral Student, Electrical and Computer Engineering

Abstract

The traveling wave tube (TWT) has been a reliable conventional vacuum electron device (VED) since the 1940s. Researchers, beginning in the late 1980s, extended the TWT to the relativistic electron beam regime to generate 100s MW power in X-band. Since the mid-1990s there has been little advancement in the field. Recently, the linear theory of a multi-stream TWT was published that showed super-exponential amplification properties. This study describes a novel technique for producing multiple electron beams with energy difference of about 4-31% with comparable currents from a single cathode at a single potential for a multi-stream TWT. This work presents a new model of two nested cathodes where two annular electron beams are generated and propagate in a smooth cylindrical pipe immersed in a strong magnetic field. The two nested cathodes are magnetically insulated coaxial diodes (MICDs). The simulation results are obtained using the MAGIC particle-in-cell (PIC) code for the experimental vacuum diode geometry of the SINUS-6 high-current electron beam accelerator at the University of New Mexico (UNM). Results are obtained which are then (i) compared with earlier experimental results for a single beam, (ii) study the current-voltage characteristics of two electron beams powered by a single cathode at a single potential immersed in a strong magnetic field, and (iii) show 4-31% energy difference with comparable currents between two beams.

The analytically derived results are obtained by extending Fedosov’s solution for generating a hollow electron beam from an MICD on a cathode stalk in an infinite magnetic field. Two electron beams are generated and accelerated downstream assuming zero initial kinetic energy of the electrons from the cathodes. Results show both electron beam currents ranging from 66 A – 2.8 kA with an energy difference ranging from 4-31% depending on voltages applied from 100 – 600 kV and the geometry of the two MICDs. An optimal geometry is a crucial factor in achieving the maximum energy difference between the electron beams for comparable currents. The analytical and numerical simulation results show good agreement. Preliminary experimental results using the SINUS-6 electron beam accelerator to validate the analytical and simulation results will be presented. This technique is viable for pulsed power-driven, relativistic electron beams for a relativistic multi-stream TWT.