Electron Gun Design

Introduction

TWI can design, test and build electron guns for a variety of industrial and experimental applications, including welding, 3D printing, surfacing and X-ray generation.

Beams are generated by electron guns. Careful design can give very high intensity beams that can be projected several metres to the work piece in a vacuum chamber. Beams are focused with magnetic lenses for our applications, and deflected with magnetic deflection coils.

Electron guns comprise a cathode, where the electrons are produced. The cathode is at a high negative potential, typically in the range -30kV to -150kV. There is a vacuum gap between the cathode and an anode, which is at ground potential. The anode has a hole in it, so the electrons are accelerated towards it and then pass through the hole. They then travel at a constant speed (usually a third or more of the speed of light) until they impact on the work piece or target, where they release their kinetic energy as heat and X-rays.

Electron Guns

Around the cathode there is an electrode that shapes the electric field to provide an electron beam with the required optical properties. If this electrode is at an even higher negative potential than the cathode it is called a grid, bias cup or Wehnelt electrode and the gun is called a triode. In this case, the potential on the grid controls the beam current produced.

If the electrode around the cathode is at the same potential as the cathode, the gun is called a diode and the beam current is controlled by the varying the emissivity of the cathode. Many cathodes are thermionic – i.e. they are heated and then some of the electrons gain sufficient energy to escape from the material surface. Cathodes can also be made from plasmas or, for low power applications, may be photo emitters or field emitters.

Modelling

Electron guns are designed using specialised modelling software that has been found to accurately predict the characteristics of a beam generated by an electron gun design. The software models the electric field produced both by the gun electrodes and by the distribution of beam current. It models how the electrons are produced at the cathode, given the properties of the cathode material.

Output from the software is generally in the form of ray diagrams or trajectory plots that show how beamlets making up a small part of the beam originate on the cathode surface and are focused and deflected by the electric and magnetic fields in the gun column.

Recently, a new way of visualising the electron beam has been investigated at TWI, where bunches of electrons, which are emitted from the cathode at the same time, are tracked as they progress through the gun column.

Trajectory Plot – Leaving the Cathode

In the example illustrated (Figure 1) from a 3D simulation of a triode gun with a tungsten ribbon cathode, electrons leave the cathode and their paths converge to a crossover a short distance in front of the cathode face. The convergent angle of the electrons is due to the strong curvature of the electric field introduced by the geometry of the grid cup – i.e. a small hole close to the cathode. It can also be seen that electrons leave the filament legs but remain trapped behind the grid cup.

Cathode Animation

The same information can be viewed as an animation of bunches of electrons leaving the cathode and following the same trajectory (Figure 2). It takes about 1 nanosecond for the electrons to travel their first 100mm.

Viewing the Gun in 2D

As the gun is axi-symmetric we can also model the field and plot the electron paths in two dimensions (Figure 3). Some of the electrons accelerate to 60keV more quickly than their neighbours leaving the cathode at the same time.

The Gun Electric Field

The cathode is at a potential of -60kV in this example (Figure 4) and the anode is ground potential. If we plot the potential in the z-direction we gain a contour. The electrons descend this potential surface after leaving the cathode, accelerating towards the anode.

Trajectory Plot - Lenses

A magnetic lens is essentially a solenoid wound around the beam path. Plotting a set of trajectories through a magnetic lens field illustrates how the beam is rotated and squeezed by the field (Figure 5).

Lens Animation

If we continue to follow the electrons that left the cathode at the same time, it can be seen how the group is twisted and converged initially by the lens field (Figure 6).

Summary

Design of electron guns is assisted by use of specialised modelling software
Viewing the outputs of the model analysis in new ways gives insight into how electron guns and electron optics affect the beam
TWI has provided design, build and test of bespoke electron beam generators for Industrial Member companies

For more information about how we can assist you with bespoke electron beam design, build and testing, please contact us.

Colin Ribton Technology Fellow - Electronic Beam Processing and Technology

Colin Ribton graduated from The University of Nottingham with a joint honours degree in Pure and Applied Physics and was awarded a PhD by Brunel University London for his research into electron guns.

His role at TWI is now as Technology Fellow and involves leading teams developing new electron beam systems for applications in welding, additive manufacture and X-ray generation. In particular, this has required the development of electron optics including generators, lenses and deflection coils, using magnetic and electrostatic modelling. He presents regularly at international conferences, has some 40 publications including a number of patents and has contributed to ISO standards on electron beam welding. He is active in promoting EB technology for new applications and has worked extensively on electron beam characterisation systems, which are used to assess the manufacturing readiness of welding and additive manufacturing equipment. He has been the industrial supervisor for three successful PhD students.

}