Physical Review Special Topics-Accelerators and Beams
الملخص العربي
The back bombardment (BB) effect limits wide usage of thermionic rf guns. The BB effect induces not only ramping-up of a cathode’s temperature and beam current, but also degradation of cavity voltage and beam energy during a macropulse. This paper presents a comparison of the BB effect for the case of dispenser tungsten-base (DC) and lanthanum hexaboride (LaB6) thermionic rf gun cathodes. For each, particle simulation codes are used to simulate the BB effect and electron beam dynamics in a thermionic rf gun cathode. A semiempirical equation is also used to investigate the stopping range and deposited heat power of BB electrons in the cathode material. A numerical simulation method is used to calculate the change of the cathode temperature and current density during a single macropulse. This is done by solving two differential equations for the rf gun cavity equivalent circuit and one-dimensional thermal diffusion equation. High electron emission and small beam size are required for generation of a high-brightness electron beam, and so in this work the emission properties of the cathode are taken into account. Simulations of the BB effect show that, for a pulse of 6 μs duration, the DC cathode experiences a large change in the temperature compared with LaB6, and a change in current density 6 times higher. Validation of the simulation results is performed using experimental data for beam current beyond the gun exit. The experimental data is well reproduced using the simulation method.
محمود عبد العظيم بكر | Mahmoud Abdel Aziem Bakr | 4253