A design study for a high-power, high-efficiency, high-growth-rate wideband traveling wave tube (TWT) in W-band using a staggered double-vane slow-wave structure (SWS) combined with three plan alignment pencil beams is described in this paper. The electromagnetic characteristic simulation shows that it has a wide bandwidth, high interaction impedance (about two to three times higher than those of the same structures with the sheet beam scheme), and a more simply designed input/output coupler. 3-D particle-in-cell simulations predict that the TWT can produce over 2000 W of output power from 91 to 95 GHz just using a 52-period two section SWS with a total length of 70.3 mm when the voltage and current of three pencil beams are set to 22 kV and 140 × 3 mA, respectively. The maximum peak output power is about 2256 W with a corresponding gain of 43.5 dB and an electronic efficiency of 12.2% at 94 GHz. The 3-dB bandwidth can be achieved at about 15 GHz with an instantaneous relative bandwidth of about 15.9%. Finally, the comparisons of sheet beam, multiple beam, and single pencil beam staggered double-vane TWT are presented and analyzed.

A theoretical model is developed for calculating the eigenmodes of the multi-gap resonant cavity. The structure of concern is a kind of ladder-type circuit, offering the advantages of easy fabrication, high characteristic impedance (R/Q), and thermal capacity in the millimeter wave to THz regime. The eigenfunction expansion method is used to establish the field expressions for the gaps and the coupling region. Then, the match conditions at the interface are employed, which leads to a group of complicate boundary equations in the form of an infinite series. To facilitate the mathematical treatments and perform a highly efficient calculation, these boundary equations are transformed into the algebraic forms through the matrix representations. Finally, the concise dispersion equation is obtained. The roots of the dispersion equation include both the axial modes in the gaps, which include the fundamental and the high-order modes, and the cavity modes in the coupling region. Extensive numerical results are presented and the behaviors of the multi-gap resonant cavity are examined.

To study the nonlinear beam-wave interaction for sheet beam klystron (SBK), a 2-D simulation code of SBK2D, based on the 2-D rod macroparticle model, has been designed and realized thoroughly in this paper. In our physical model, the sheet beam is simulated by a series of rods with nonzero thickness and length in y and z directions, respectively (y is the direction of sheet beam's narrow dimension, and z is the direction of beam motion). Then, the space-charge forces between the rod macroparticles are calculated by Green's functions approach, and the port-approximation methods are employed to simulate high-frequency fields for each cavity. Furthermore, the relativistic Lorentz motion equation is solved to obtain the parameters for further macroparticle motion, and the specific criterions are introduced to deal with the sheet beam interception problem by the tunnel and cavities. To verify our simulation code, an eight-cavity W-band SBK has been designed, calculated, and analyzed by SBK2D in detail. The good consistency between SBK2D and 3-dimensional Particle in Cell (3-D PIC) program indicates the reliability of the physical model and simulation code for our program. In addition, the calculation time of SBK2D is only about 1% of that of the 3-D PIC, which makes it more feasible and efficient to calculate and optimize the initial physical parameters for the design of complicated SBK.

In this paper, the cold-fluid model theory of an intense sheet electron beam is developed to investigate the diocotron instability during its transport in a uniform magnetic field. The model shows that if the magnetic field strength and the beam filling factor are increased separately, the diocotron instability will be suppressed, which extends the sheet-beam transport distance effectively. To verify the above conclusion, a set of complicated instruments, the electron beam measuring and analyzing system (EBMAS), was developed to measure the beam cross section, beam current density, and the 3-D trajectory. The sheet electron beam transport process in a uniform magnetic field with its diocotron instability is investigated on the basis of EBMAS measurements. The measured results agree very well with theoretical calculation and numerical simulation. A sheet-electron-beam tube based on a uniform magnetic field was then manufactured to drive a future W-band sheet-beam klystron (WSBK). Tuning the sheet-beam parameters over a broad range, including a cathode voltage range of 20-82 kV, current range of 0.5-4.27 A, and with a beam cross section of about 10 mm × 0.5 mm, the experimental beam transmission rate is above 98% through a beam tunnel of 100 mm in length.

In this paper, a multigap extended output cavity was designed by 3-D simulations, which served as the output cavity for a W-band sheet-beam extended interaction klystron (SBEIK). In our numerical design, the circuit dimensions were systematically optimized by parametric finite-difference time-domain simulations, and the equivalent circuit for the output cavity was also analyzed. The proper external loading was selected by using a region of loss in 3-D, and the output power was optimized. The results were verified by using the coupler and the waveguide. The 2π mode of the optimized five-gap extended output cavities had an ohmic Q (Q 0 ) of 1343.5, an external Q (Q e ) of 501.6, and a loaded Q (Q L ) of 365.2 at 94.5 GHz. The 3-D particle-in-cell simulations predict that the output cavity of the SBEIK (75 kV and 4 A) can stably produce more than 50 kW of output power using a prebunched beam.