A megawatt-level surface wave oscillator in Y-band with large oversized structure driven by annular relativistic electron beam

Quasi-Optical Theory of Relativistic Cherenkov Oscillators and Amplifiers with Oversized Electrodynamic Structures

Using the quasi-optical approach, we investigate wave propagation along the periodically corrugated surfaces and their interaction with rectilinear relativistic electron beams (REBs). At the periodical structure, the field can be expanded into a series of spatial harmonics, which, in the case of shallow corrugations, represent paraxial wavebeams with mutual coupling described within the method of effective surface magnetic currents. We present the dispersion equation for the normal waves. Two limit cases can be recognized: in the first one, the frequency is far from the Bragg resonance and the wave propagation can be described within the impedance approximation with the field presented as a sum of the fundamental slow wave and its spatial harmonics. In the interaction with a rectilinear REB, this corresponds to the convective instability of particles’ synchronism with the fundamental (0th) or higher spatial harmonics (TWT regime), or the absolute instability in the case of synchronism with the −1st harmonic of the backward wave (BWO regime). In the latter case, at the frequencies close to the Bragg resonance, the field is presented as two antiparallel quasi-optical wavebeams, leading to the absolute instability used in the surface-wave oscillators operating in the π-mode regime. Based on the developed theory, we determine the main characteristics of relativistic Cherenkov amplifiers and oscillators with oversized electrodynamical systems. We demonstrate the prospects for the practical implementation of relativistic surface-wave devices in submillimeter wavebands.

Self-Winding Helices as Slow-Wave Structures for Sub-Millimeter Traveling-Wave Tubes

We present a transformative route to obtain mass-producible helical slow-wave structures for operation in beam-wave interaction devices at THz frequencies. The approach relies on guided self-assembly of conductive nanomembranes. Our work coordinates simulations of cold helices (i.e., helices with no electron beam) and hot helices (i.e., helices that interact with an electron beam). The theoretical study determines electromagnetic fields, current distributions, and beam-wave interaction in a parameter space that has not been explored before. These parameters include microscale diameter, pitch, tape width, and nanoscale surface finish. Parametric simulations show that beam-wave interaction devices based on self-assembled and electroplated helices will potentially provide gain-bandwidth products higher than 2 dBTHz at 1 THz. Informed by the simulation results, we fabricate prototype helices for operation as slow-wave structures at THz frequencies, using metal nanomembranes. Single and intertwined double helices, as well as helices with one or two chiralities, are obtained by self-assembly of stressed metal bilayers. The nanomembrane stiffness and built-in stress control the diameter of the helices. The in-plane geometry of the nanomembrane determines the pitch, the chirality, and the formation of single vs intertwined double helices.

Design of uniform permanent magnet electronic optical system for 220 GHz sheet electron beam traveling wave tube

The sheet electron beam (SEB), for which is the low current density and large current, is highly attractive in the region of millimeter wave and terahertz vacuum electronic devices (VEDs). A uniform permanent magnet (UPM) electronic optical system (EOS) driven by a SEB for 220 GHz traveling wave tube (TWT) is designed in present work, in which the voltage and current for SEB is 17 kV and 0.3 A, respectively. For obtaining the stable high transmission rate EOS, the characteristics of SEB in UPM EOS are studied, including the emittance, orbital angle, and beam trajectories, which are discussed through the CST simulation. The results show that the emittances in the x-direction are varied from 0.003 to 0.016 mm rad and in y-direction are various from 1 × 10⁻⁴ to 3 × 10⁻⁴ mm rad, respectively, keeping below than 2.5 × 10⁻⁴ mm rad during transmission, which guarantees the stability of SEB in y-direction. For the design of complete EOS, the normal rectangular collector is used, in which the SEB is uniformed scattering.

Relativistic Surface Wave Oscillator in Y-Band with Large Oversized Structures Modulated by Dual Reflectors

To increase the generation efficiency of the terahertz wave in the Y band, the idea of dual-reflector is introduced in the relativistic surface wave oscillator (SWO) with large oversized structures. The dual-reflector and the slow-wave structure (SWS) construct a resonator where the field strength of TM₀₁ mode inside is intensively enhanced and then the efficiency is increased. The pre-modulation on electron beam caused by the reflector is also helpful in improving the output power. Meanwhile, the reflector can reduce the loss of negatively going electrons. Through the particle-in-cell (PIC) simulations, the optimized structure is tested to be stable and little power is transmitting back to the diode area. The output power reaches 138 MW in the perfectly electrical conductivity condition and the frequency is 337.7 GHz with a pure spectrum. The device’s efficiency is increased from 10.7% to 16.2%, compared with the device without any reflectors. The performance of device with lossy material is also focused on. In the situation of copper device, the output power is about 41 MW under the same input conditions and the corresponding efficiency is about 4.8%.