A wide-band structure for high-power traveling-wave tubes

In an effort to overcome the low power handling capabilities of the conventional wire helix considerable effort has been devoted to the development of filter-type slow-wave structures for use in high-power traveling-wave amplifiers. However, the shortcoming of the various filter-type structures such as the disk-on-rod and the apertured disk, is their inherently narrow bandwidth. For a filter-type structure operating at a center frequency of 3000 mc, the maximum attainable bandwidth is approximately 10 per cent or 300 mc. With the type of structure considered herein it is hoped to achieve a bandwidth of approximately 30 per cent at 3000 mc. The helical waveguide structure is composed of a helical tape wound inside a cylinder. The hole for the passage of the beam would normally be located along the axis of the helical tape. However, the field analysis indicates that stronger coupling might be obtained by placing the beam hole somewhat off center. In order to gain maximum coupling to the beam, several off-center electron beams would be located at a radius corresponding to that of the maximum of the electric field. The cavities are coupled continuously by the helical tape. The bandwidth, i.e., dispersion curve, should approach that of the conventional wire helix except in the dispersive region because the waveguide has a cutoff. The power handling capability clearly is greater than that of the wire helix. However, the impedance will be lower, resulting In a lower gain parameter C. Other factors such as a higher permissible value of (I
o
/V
o
) and a more nearly constant electric field along the spiral contribute to a higher gain parameter. In fact, the zero-order mode (β = O) will have no variation of electric field across the cavity. Because of the complexity of an exact mathematical analysis some approximations have been made. The assumption used in this analysis is that the pitch of the helical tape is considered small compared to its diameter. Each section of the waveguide is considered as a cylindrical cavity of height h, where h = 2 P and P = pitch of the tape. This is a valid assumption where the pitch is small, as the end faces of the cavities are essentially perpendicular to the cylindrical enclosure. The helical surface is considered to be a Riemann surface wherein φ is a periodic and single-valued coordinate; however, in the usual cylindrical coordinate system φ is a nonperiodic coordinate. Thus the fields are multi-valued functions of the coordinate φ. Under the above conditions and the generalization of the Bessel-function order n to its general parameter p, the familiar cylindrical wave-guide equations may be used. Under this transformation p is capable of assuming nonintegral values. On the basis of the theoretical analysis presented herein the helical waveguide structure seems best suited for high-frequency use, especially in the frequency range of 5–10 kmc. The bandwidth and gain approach those of the helix-type traveling-rave tube.