Products > Custom Cathodes > Guard Ring Cathodes

We describe a simple thermionic electron source that has several unique properties including a defined emission area without edge effects, and when mounted in certain configurations, has a flat electric field across the emitting surface. These features can benefit a range of applications including x-ray generation, electron microscopy, and microwave devices. The source is an HfC cylinder with the (100) crystallographic plane on-axis mounted flush in a carbon guard ring. Applied Physics Technologies has made three basic types of carbon guard ring structures. One type can be made of pyrolytic carbon, which is also used as the heating mechanism when current flows through the lower portion. The second type is fabricated from graphite and is mounted in a conventional mini Vogel type system, which uses separate pyrolytic carbon blocks as the heating element (see Fig. 1a), and the third type utilizes a colloidal carbon coating. The work function of HfC(100) is ~3.4 eV whereas the pyrolytic carbon guard ring is ~1 eV higher. Therefore, even at elevated temperatures virtually all the electrons are produced from the HfC surface. In these cases, the guard ring, shown in Fig. 1b, functions to suppress unwanted off-axis emission that is typically limited electronically by a Wehnelt or physically by a downstream aperture. Cathodes have been built with HfC(100) source diameters from 50 μm to 1 mm and guard ring diameters from 1-3 mm.

Figure 1(a) : Vogel mount showing graphite guard ring and 100 µ m HfC(100) cathode.

As one example, Applied Physics Technologies explored placing this emitter in a Pierce style electron gun where the electric field is shaped by surrounding the cathode with an electrode at the same electrical potential. In this geometry there is a drawback with conventional planar cathodes since there is always a gap between the cathode and the Pierce electrode. The electric field lines dip into this gap (see Fig. 2a) enhancing the field at the edge of the cathode causing the electrons emitted from this region to follow a diverging trajectory. When APTech's guard ring cathode is mounted in a Pierce electrode there is still a gap, but the resulting field enhancement is not on an emitting surface and the field lines remain uniformly flat across the emitting surface. If retrofitted into an existing un-apertured Pierce style optical system, the reduced emitting area can result in a reduction of the spot size of the optical system. In this configuration, the radial position of the guard ring cathode within the Pierce electrode is also less critical due to the lack of edge effects (see Fig. 2b). These are electric field models and were done using Lorentz 2D-RS V6.2.

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

For small source sizes (e.g. < 50 μm) we have explored using the carbon coatings on more standard thermionic source geometries. Here Applied Physics Technologies shaped the HfC(100) crystal similar to 3(b) 3(a) a LaB 6 Vogel mounted emitter, which utilizes a 90° cone and an apex truncation of between 5-50 µm. The cone portion is coated with carbon to suppress emission as shown in Fig. 3.

fffffffffffffffffffffffffffffffffffFig. 2(a) fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffFig 2(b)

Figure 2: Voltage equipotential lines from (a) simple thermionic cathode without a guard ring showing edge effects, and (b) from a cathode with a guard ring. Note parallel field lines above the emiting surface.

Figure 3 : Truncated thermionic cathode with carbon coated guard ring shown here after extended operation.