Vacuum electronic devices for microwave generation (e.g., klystron, traveling wave tube,...) require high-current-density electron beams matched to a focusing solenoid. The standard approach is to use a gun outside the field that generates a converging beam. The motivation is to allow relatively low current density at the cathode, consistent with operation at moderate temperature for long lifetime. The beam reaches its target current density at the waist where is enters the magnetic field. The goal is to achieve a Brillouin-type equilibrium with minimal transverse energy. This condition has the advantages of low axial energy spread and minimal focusing field.
We have posted a detailed case study of an electron gun and focusing solenoid for a microwave research application. The gun generates a beam with 120 keV kinetic energy and 2.0 A current. The challenge is that the beam radius must be less than 0.25 mm through the focusing system. The report touches on a broad spectrum of design issues:
- Estimating gun parameters from analytic expressions for beam compression.
- Determining electrode geometries to generate a converging beam with minimal emittance.
- Representing effects of the gap between the cathode and focusing electrode.
- Mapping electric field stress.
- Designing a pulsed solenoid with high field uniformity.
- Ensuring magnetic shielding of the cathode is a surface for zero canonical angular momentum.
- Tuning the transition region at the field entrance to minimize envelope oscillations.
Download the complete document: egundesign.pdf
Figure 1. Converging beam electron gun design.
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