DESIGN IMPROVEMENTS FOR MODE AND POLARIZATION CONVERTERS FOR 60 GHZ GYROTRONS

Gyrotrons currently in use typically have circular-waveguide microwave outputs in the TE(,01) or TE(,02) modes which are not efficient for plasma heating. Periodic perturbation type mode converters have been successfully used to convert these outputs to a TE(,11) or HE(,11) mode which have more efficient plasma heating characteristics. Methods are discussed by which the efficiency and power handling capability of those mode converters can be increased while their length is simultaneously reduced. The mode converter's bandwidth is also usually increased. These improvements are accomplished by (1) changing the perturbation period slightly from the beat wavelength between the input and the desired output mode to account for the effect of the presence of unwanted modes, (2) inserting sections of straight waveguide to adjust the relative phases of the important modes so that coupling to unwanted modes is minimized, (3) varying the amplitude of the perturbation from period to period to maximize the converter efficiency, and (4) choosing the profile of the perturbation period to further reduce the coupling to unwanted modes.;The theory and design for a polarization converter which transforms a linearly polarized (stationary) TE(,11) mode to a circularly polarized (rotating) TE(,11) mode is also presented. The design consists of an elliptical section of waveguide between two smoothly varying tapered sections.;A mode converter system which can produce any output "polarization" of a TE(,11) or HE(,11) mode from linear to left- or right-hand polarized using the polarization converter is also discussed.;Generally the output power of a gyrotron is not purely in a single mode but may contain a few percent of its output power in other modes. If the first mode converter following the gyrotron has been optimized for a single input, the presence of other modes will normally decrease the converter's efficiency. In some cases, the efficiency can be increased to single mode levels by adjusting the relative phase between the input modes and adjusting the lengths of the flat-section inserts. In other cases, it is more effective to insert a short single-cycle converter near the gyrotron output to convert its unwanted mode components to the nominal output mode.