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We proposed an oversized coaxial multi-beam relativistic klystron amplifier (OCMB-RKA) with a new input coupler of lesser complexity giving enhanced input microwave power absorption rate for generating high power millimeter-wave radiation in Ka-band. The high-frequency structure of the OCMB-RKA was designed, fabricated, and cold-tested for results that agreed well with the simulated results. Furthermore, using the 3D particle-in-cell (PIC) code, the beam-wave interaction of the OCMB-RKA was simulated, considering two different materials (stainless steel and copper) making the interaction structure. When a 500 kV, 3 kA electron beam, and a focusing magnetic flux intensity of 1 T were adopted in the PIC simulation, a five-cavity OCMB-RKA yielded the output power of 610 MW at 30.525 GHz with the electronic efficiency of 40.7% and the saturated gain of 55.3 dB. Experimentally, the OCMB-RKA demonstrated its capability of generating high power Ka-band radiation typically of 390 MW power with the pulse width of 27 ns.

A miniaturized, high electronic efficiency, S-band metamaterial extended interaction klystron (MEIK) is proposed. The metamaterial-based extended interaction resonant cavities were designed, fabricated, and tested, the assembled and cold-tested results confirm the feasibility of miniaturization in MEIK. Furthermore, the particle-in-cell (PIC) simulation predicts that, typically, when the beam voltage and current are 30 kV and 3 A, respectively, the focusing magnetic field is 1000 G, a 3-cavity MEIK yields the output power of 56 kW at 2.453 GHz, with the saturated gain of 47 dB and the electronic efficiency of 62%, and, importantly, with the cavity diameter of its interaction structure being ~1/2 of a conventional extended interaction klystron. As a power amplifier, the emerging MEIK with its miniaturization and high electronic efficiency has potential applications such as in future accelerators and microwave heating systems.

A simple metamaterial loading scheme for a helical slow-wave structure was proposed in lieu of a conventional metal vane loading. The proposed structure consists of a helix in a metal envelope supported by rectangular dielectric rods each with I-shaped metal-strip epsilon-negative metamaterial printed on both of its radial faces. Our study, based on CST simulation, showed that the proposed simple structure gives controlled dispersion as obtained by the conventional vane-loaded helix with large interaction impedance values even higher than those of the latter.

We proposed a miniaturized S-band extended interaction klystron (EIK) assisted by metamaterial. The interaction structure of the EIK consists of a cylindrical resonant cavity filled with a metamaterial (complementary electric split-ring resonator) array. The study based on CST Eigenmode Solver predicted that the transverse cavity dimension of the proposed metamaterial assisted EIK is 1/3-1/2 times smaller than that of its conventional counterparts, while the device delivers typically 122 kW power, 53.5 dB gain and 43.5% electronic efficiency with an input drive of 0.27 W for typical beam and magnetic field parameters. The proposed metamaterial assisted EIK has potential applications in radar, industrial heating, accelerator, and satellite communications.

Forward- and backward-wave interaction analyses of a plasma-filled helix traveling-wave tube are developed following the Eulerian hydrodynamic approach and taking into account the effects of the following three factors: 1) the thermal velocity of plasma electrons; 2) collisions between the plasma electrons; and 3) the finite diameter of the electron beam and its proximity to a metallic boundary. The electron beam is treated as a moving charged fluid, and the plasma is treated as a stationary quasineutral fluid. The coupled circuit and the electronic equations are developed, and a Pierce-type hot (beam-present) dispersion relation is obtained, which is then interpreted for the forward-wave gain-frequency response and for the backward-wave oscillation criterion of the device. In the absence of the slow-wave structure, the dispersion relation reduces to that of a beam-plasma amplifier, whereas in the absence of the plasma, the dispersion relation passes on to that of a conventional vacuum traveling-wave tube. The analysis is further validated with an existing forward-wave interaction analysis of a plasma-assisted helix traveling-wave tube that treats the beam-wave interaction in the device as a boundary-value problem by considering the beam-plasma system as a propagating medium of an equivalent relative permittivity tensor. Interestingly, similar to a beam-plasma amplifier, a plasma-filled TWT supports no backward-wave oscillation in the reactive-plasma convective instability.

Simple closed-form formulas for the estimation of the π-mode stopband and the stopband attenuation in an azimuthally asymmetric helical slow-wave structure (SWS) are developed, following the coupled-mode analysis of multiple reflections. The formulas are simple and amenable to easy computation, and also allow the use of the dispersion characteristics of the structure obtainable from any standard electromagnetic modeling, thereby accruing the accuracy of 3-D electromagnetic analysis. The analysis is benchmarked against published results with close agreement. Based on the analysis, SWS configurations that would reduce the deleterious effects of asymmetry and also would improve TWT efficiency are suggested.

A simple equivalent circuit analysis of the ring-bar slow-wave structure of a traveling-wave tube is developed, using the coupled transmission-line approach, for the dispersion and interaction-impedance characteristics of the structure. The effects of the ring and the bar widths of the structure are included in the analysis. The analysis is validated, with respect to 1) an X-band structure, against both measurement and simulation, as well as 2) a Q-band structure, against simulation.

The concept of broadbanding of a helical slow-wave structure using ldquomultidispersionrdquo phase velocity taper has been analytically explored, following Pierce's small-signal analysis in order to form a physical basis behind this approach. Furthermore, large-signal analysis has been used to make out the efficacy of the physical basis arrived at from the small-signal analysis. Useful inferences are also drawn on the flattening of gain- and power-frequency responses and the reduction of second-harmonic content, using multidispersion phase velocity taper.

This paper presents a simple self consistent large-signal formulation for computation of saturated gain, power and saturation length of a gyro-TWT. A computer code has been developed in C++ language. The computed results were found to be matching with the reported results obtained from other formulation. Using this code, a gyro-TWT interaction structure has been designed at 35 GHz with beam voltage of 50 kV.