Guard Ring Cathode Information

We manufacture specialized custom cathodes using electron emission defining (guard ring style) and stability enhancing techniques. Primarily, hexaborides (LaB6 and CeBix®) have been used for cathode materials, but we have also used hafnium carbide for particular uses where low currents and background atmospheres preclude the use of hexaborides. A common form of emission suppression is to embed an oriented single crystal in graphite to suppress side emission and to help shape the electric field. Single planar discs of 50 microns in diameter have been tested for use in high brightness, stable, and long life thermal sources. Line sources have also been developed with line width/lengths to 100-500 microns. Emission tests performed have shown that instabilities can originate from mounting methods. Improved mounting techniques can yield emission with short term beam current stability <0.05%.


For particular low current and extreme operating conditions and environments, we fabricate embedded cathodes from HfC (310). This is an extremely robust cathode material capable of operating in poor vacuum conditions and able to withstand ion back streaming successfully.


Several thermionic electron sources are shown that have unique properties, including a defined emission area without edge effects. These are beneficial for a range of applications including x-ray generation, electron microscopy, lithography, and microwave devices. The source can be a LaB6 (310) cylinder mounted flush in a specially shaped carbon guard ring.  We have made these carbon guard ring structures from graphite which is mounted in a conventional mini-Vogel type system, using separate pyrolytic carbon blocks as the heating element (see Fig. 1). These Mini Vogel Mounts (MVM) can also be configured with shunts (SMVM). The work function of the carbon guard ring is >2.5 eV higher, thereby, suppressing emission from its surface and limiting the emission to the LaB6 emitter. Cathodes have been built with source diameters from 50 μm to 1 mm and guard ring diameters from 100 μm to 2 mm.

Fig. 1:  (a) Vogel mount showing graphite guard ring and 100 µm HfC (100) cathode.  (b) View of a 0.25mm radius HfC (100) cathode surrounded by graphite guard ring and centered in Pierce type housing.

For line sources, a crystal cylinder is flattened from two opposite sides to form a nearly rectangular solid with a width equal to the diameter of the crystal. This is then embedded into a slot machined into pure graphite (shown in Fig. 2), the emitter surface polished, and again Vogel mounted. View our Cathode Guide to see the different customizations available for a guard ring cathode.

Fig. 2:  HfC (210) line cathode embedded in carbon for emission suppression; top hat style and standard geometry on right.



Transition Metal


Refractory Metals


Top Hat

Guard Ring

Etched Tip

Cathode Guide

Application Guide

Rebuilding Services

Refractory Metals

Source Types

Cathode Dimensions


Services & Parts


© 2019 Applied Physics Technologies, Inc.

1600 NE Miller St. McMinnville, OR 97128


Follow Us!