Thermionic Cathodes

Thermionic cathodes use heat to expel electrons from a solid. They are in contrast to cold (field emission) cathodes, which use high electric fields to tear the electrons out of the solid.

Thermionic cathodes come in various categories. Those with the lowest temperatures are cathodes containing a thick film layer of mostly barium oxide on nickel. They operate at 750º to 800º centigrade. Slightly higher in temperature are cathodes with barium oxide on tungsten. These mostly manifest themselves as coated tungsten wire cathodes. We offer on a custom basis both kinds of oxide cathode. Because of their low temperature, oxide cathodes can be made large - 12 inches or more in diameter. They are a quick, inexpensive way to generate lots of electrons for research projects or other uses. A major drawback of oxide cathodes is that average current density cannot exceed about 1 Amp/cm2. However, pulsed emission currents easily exceed 20 Amps/cm2.

  Diagram of Reservoir Cathode  

For higher average emission current density, we offer dispenser cathodes. These come in two varieties - impregnated and reservoir. Both employ a porous tungsten matrix. Impregnated cathodes have barium compound in the pores of the matrix. When the cathode is heated, this barium compound interacts with the tungsten and evolves free barium, which coats the emission surface. Barium on tungsten is a low work function configuration and the further addition of osmium to the matrix lowers it even more. We can typically get 4-5 Amps/cm2 from an osmium-coated impregnated cathode operating at 980º centigrade. This loading can be pushed to over 20 Amps/cm2 if the temperature is raised. These cathodes have longer life than oxide-coated cathodes. They can operate 60,000 hours at 4 Amps/cm2.

Because the temperature is higher on these cathodes than on oxide-coated nickel cathodes, construction is more difficult and expensive. Also, they are limited in size. However, there are many devices where dispenser cathodes are required.

One drawback of oxide-coated nickel is that it is one use only. Once the system is let up to air, the cathode must be replaced. This is a problem in demountable systems where test elements within the system must be changed occasionally. But we have reused dispenser cathodes after the vacuum is broken - sometimes 4-5 times.

Reservoir cathodes are even fussier to build than impregnated, but they last even longer and can be pushed to even higher emission currents. Current densities of 100 Amps/cm2 have been achieved. In the case of reservoir cathodes, we do not impregnate the pores of the tungsten matrix. Rather, we build a little box or reservoir behind the matrix and fill it with barium emission material. When heated, the barium comes out of the reservoir, infiltrates through the matrix and coats the forward surface. Also, these cathodes are very long life, over 100,000 hours. We have an ongoing project to build the world's first miniature reservoir cathode. It is only .060 inches in diameter.

Another thermionic cathode is lanthanum hexaboride, which operates at about 1450º centigrade. It is often used in scanning electron microscopes and lithography systems for semiconductors, and in systems that must be opened to air many times. They do not degrade when vacuum is removed.

Finally, there is the venerable bare tungsten wire cathode, which operates at 2100º centigrade. If thorium is added to the surface, this temperature can be lowered by about 100 -200º C.

In addition to offering all the cathodes discussed above, we also offer a variety of services associated with them. For example, we can provide you with testing services on our computerized life test station. It is capable of simultaneously life testing 48 cathodes. Almost any performance characterization you can imagine is within its reach - including Richardson plots, Schottky plots, IV characteristic plots and standard activity curves.

Another service we often perform is the construction of cathode test vehicles. These are vacuum assemblies made of glass in which one or more cathodes are housed. We can assemble the cathodes in a close-spaced diode configuration, or in more sophisticated arrangements including Pierce structures such as are used in traveling wave tubes, or in cross-over gun structures such as are used in cathode ray tubes. Because we use mass-produced CRT components and glass envelopes and stems, we can build testers quickly and cheaply.

We can also provide design services in which we determine the type of cathode and the mounting configuration that best answers a stated objective.

Copyright 2005, ebeam, Inc.