Transversal and longitudinal mode selections in double-corrugation coaxial slow-wave devices

Abstract

To reduce the dimensions of relativistic backward wave oscillators (RBWOs) operating in the low frequency regime of less than 2 GHz, the theory of transversal and longitudinal mode selections are introduced in this paper. The transversal mode selection is achieved using the property of “surface wave” of the coaxial slow-wave structure (SWS) to excite the quasi transverse electromagnetic (quasi-TEM) mode without the higher transverse magnetic (TM) modes and it is proved that the coaxial SWS may decrease the transversal dimension of the SWS sections. In addition, the S-parameter method is employed to investigate the longitudinal resonant characteristic of the finite-length SWS, and the scheme of longitudinal mode selection is put forward. It is proposed that the introduction of a well-designed coaxial extractor to slow-wave devices can help to achieve the longitudinal mode selection and reduce the period number of the SWS, which not only can make the devices more compact, but also can avoid the destructive competition between various longitudinal modes, therefore can enhance the efficiency and stabilize the frequency. To sum up, the physical mechanisms of transversal and longitudinal mode selections ensure that the microwave is produced with a single mode and a narrow band. Based on the above discussion, a compact L-band coaxial RBWO is investigated and optimized in detail with the particle-in-cell KARAT code (V. P. Tarakanov, Berkeley Research Associates, Inc., 1992). In simulation, the L-band coaxial RBWO, driven by a 700 kV, 11 kA electron beam, comes to a nonlinear steady state in 20 ns. High-power microwave of quasi-TEM mode is generated with an average power of 2.66 GW, a frequency of 1.6 GHz, and power conversion efficiency of 34.5% in durations of 30–60 ns.

A high-efficiency L-band coaxial three-period relativistic Cherenkov oscillator

“…The coaxial SWSs are investigated systematically in our previous work 23 . Under the boundary conditions at the wall radii of the inner and outer SWSs and at the radii of the electron beam, we can obtain a homogeneous matrix equation by solving the Maxwell’s equations, the continuity equation and the motion equation with the field matching method.…”

“…In our previous work, the S-parameter method is employed to investigate the axial resonant characteristics of the finite-length coaxial SWSs 23 . To illustrate the characteristics of the coaxial extractor, the comparative investigations on the three-period coaxial SWSs with and without a coaxial extractor are conducted by the S-parameter method.…”

Experimental study of a compact P-band coaxial relativistic backward wave oscillator with three periods slow wave structure

“…[20][21][22][23] Compared with the conventional cylindrical SWS, the coaxial SWS, in which the lowest passband is in frequency range from 0 to its p-mode frequency, could decrease the transversal dimension of the RBWO largely for the same operating frequency. This approach has been successfully used to design the L-band RBWO, and important example is the device developed by Ge et al 24,25 This L-band RBWO possesses a coaxial SWS with a ripple only on the inner surface of the outer conductor, whereas the outer surface of the inner conductor is smooth.…”

Theory and design of microwave photonic free-electron lasers

“…These codes have been successfully applied in a huge variety of situations. Examples are found in astrophysics, particle acceleration and FELs [69,83,[142][143][144][145][146]. For FELs in particular, these codes allow the investigation of multidimensional effects and highly nonlinear phenomena, which is very complex using other techniques [144].…”