A systematic study, including computer simulations and experiments, has been done in order to highlight the field emission behaviour within different microlens geometries. Results about the energy distribution and electron trajectories within electron optics approach, for example, are presented for different kind of field emission sources: Spindt-type microtips, regular microtips and nanotips. Applications appropriate to the different sources are proposed.

1E09

Diamond Cold Cathodes

M.W. Geis, J.C. Twichell, T.M. Lyszczarz, K.E. Krohn and N.N. Efremow

Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 021 73-9108

Substantial progress has been made in understanding the physics of diamond cold cathodes. Four distinct regions: the metal contact, bulk diamond, vacuum interface, and free space, exhibit distinct physical processes affecting the behavior of the cathode. The contact region mediates the transfer of electrons and holes between the metal and the diamond. The bulk of the diamond limits the transport of carriers. Any dopants in this region will modify the fields throughout the diamond. The vacuum interface and the diamond termination at this interface control the emission into the vacuum, and impact the field in the diamond. Finally the electrons transit the vacuum region outside the diamond. Here, the electric field is set by the difference in potential between the anode and the diamond surface, which in turn depends on the charge distribution within the diamond. A number of diagnostics have been used to examine the characteristics of each region.

While 11-b p-type diamond field emitters are quite robust, their behavior is similar to metals. The back contact is a standard forward biased Schottky diode and electrons are emitted from the valence band. In contrast, type I-b diamonds are semi-insulating. Substitutional nitrogen plays the role of an n-type dopant at the electron injecting back contact. Optical DLTS measurements demonstrate the substantial positivc charge rcsides at this interface. Photocondition experiments confirm the contact character, and provide transport coefficients. While problems remain, much of the initial variability of diamond cold cathodes can now bc related to distinct physical processes.

*This work was sponsored by the Defense Advanced Research Projects Agency. Opinions, interpretations, con- clusions, and recommendations are those of the author and are not necessarily endorsed by the United States Government.

1E10

A Simplified FEA-based TWT model

K.L. Jensen, E. G. Zaidman, K. Nguyen, M.A.Kodis and M. Garven

Naval Research Laboratory, Washington, DC 20375

Knowledge of cmitter geometry and materials allows prediction of gain, cfficiency, powcr output, and circuit length of an emission gatcd TWT. The cxtension of a simplc analytical model" examines the basic physics involved, and gives a lower limit to the performance from an FED-based TWT.

Previous approaches to modeling an FEA cathode in an emission gated TWT (Twystrode) treated the FEA and tube physics separately, and joined analyses using the Fowler-Nordheim A and B parameters. These parameters were therefore independent. Here, A and B can be predicted on the basis of the geometry, materials, and uniformity of the array, allowing for a seamless relation of fabrication issues to tube performance.

The emitter parameters are tip and gate radius, tip height, cone angle, work function and the uniformity of the array (i.e., distribution in the B value). The performance of the simple TWT model is characterized by gain, electronic efficiency, rf power, helix length and taper. The input power is given from an analysis of the array, and the emitted current is modulated by sinusoidally varying the gate potential. The beam is accelerated into the helix circuit, where the output Q power is extracted. An analytical model is used to determine the portion of the electron kinetic energy converted to rf power.

We present the results of this analysis for obtainable emitters, as well as emitters expected in the near term. An estimation of the actual performance would require particle simulation modeling. Rather, the present study is a complementary study for such simulations, which are essential for thc determination of thc competitiveness of an emission grated rf amplifier. We will compare the analytical model to numerical simulations of the beam- circuit interaction,

This work supported by the Office of Naval Research. *K.Nguyen, et al., Monterey Power Tube Conf. 1996

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