# Non-Adiabatic Effects on Electron Beam Quality for Frequency-Tunable Gyrotrons

## TL;DR

This paper demonstrates that a non-uniform electric field in a novel electron gun design significantly reduces velocity spread in electron beams, enabling improved frequency-tunable gyrotrons despite non-adiabatic effects.

## Contribution

The study introduces a non-uniform electric field electron gun that defies traditional adiabatic design rules, achieving low velocity spread through non-adiabatic effects and resonant polarization drift.

## Key findings

- Velocity spread reduced to 2.8% in simulations
- Effective over a wide magnetic field and voltage range
- Supports development of frequency-tunable gyrotrons

## Abstract

We propose an unconventional electron gun structure in which the emitter is located on a concave cathode surface with a non-uniform electric field. Such a design violates the intuition that an emitter should place close to a uniform electric field to reduce the velocity spread. The commonly employed design guide based on the adiabatic condition predicts a huge velocity spread of 24%, but the simulation using EGUN code and verified with CST particle studio shows a very low spread of 2.8%. Examining the magnetic moment and the kinetic energy of the beam reveals that the electrons experience a relatively long acceleration process due to the much weak electric field. That's why the non-adiabatic effect matters. In addition to the cyclotron compression and the E$\times$B drift, the "resonant" polarization drift plays a crucial role in reducing the overall velocity spread.Simulations show a decent beam quality with the pitch factor of 1.5 and the transverse velocity spread of 2.8% over a wide range of the magnetic field (7.4-8.0 T) and the beam voltage (12-22 kV) with a high structural tolerance. The promising results with the wide working range enable the development of continuous frequency-tunable gyrotrons.