A proposed extended cavity for coaxial relativistic klystrons

The quality factor of a coaxial klystron cavity is relatively low due to the coupling to the TEM mode which can propagate in the annular channel. This leads to low efficiency, especially at high frequency, e.g., X-band. To mitigate this effect we replace the usual cavity with an extended length, active cavity. The cavity consists of either a coaxial structure partially loaded with dielectric material or with a periodic structure. The interaction of the electrons is with the TM01 mode within the cavity, which extends over several wavelengths. Outside the loaded section only the TEM mode can exist. At the boundaries the TM01 mode and the TEM mode are coupled therefore part of the power gained by the former mode is transferred to the TEM mode in the channel. One of the advantages of this system is the fact that the interaction length is larger than the wavelength, the local electric field intensity lower and therefore the breakdown probability is reduced. A theoretical analysis of the device is discussed including analytical and simulation work. Typical results for a 7 kA, 400 kV beam show an equivalent gain of 1 dB/cm can be expected at 9 GHz.