An Approach to Electron Optical System Design for Dual-Pencil-Beam Traveling Wave Tubes

In this paper, a stacked dual beam electron gun and the associated electron optical system are proposed. The stacked dual beam electron gun includes two compact focusing electrodes which help to achieve dual sheet beams. As an application of this dual beam electron gun, a 340 GHz integrated dual beam traveling wave tube (TWT) based on the staggered dual vane slow-wave structure (SWS) is also put forward. In order to reduce the length of the TWT, a novel input/output coupler is introduced. The overall transmission characteristics of the SWS structure together with the input/output couplers show a wide bandwidth covering a frequency range of 306 GHz to 360 GHz. Based on the parameters obtained for the integrated TWT, a stacked dual-beam electron gun with dual focusing electrodes is designed to achieve a beam current of 43 mA, a beam voltage of 21.4 kV, and a cross-sectional size of each beam of 0.3 mm × 0.08 mm. A uniform magnetic field of 0.52 T is utilized to focus the dual electron beams, and a beam transmission efficiency of 97.1% is achieved over a length of 50 mm. Finally, particle in cell simulation results show that the integrated dual-beam TWT can generate an output power of 5 W over the frequency range of 315 GHz to 350 GHz, with the maximum output power of 24.5 W at 330 GHz.

The long-distance and stable transmission of multiple sheet electron beams is one of the most critical challenges hindering the development of integrated multiple sheet electron beam devices. A uniform magnetic focusing structure is an effective scheme to focus multiple sheet electron beams, but it significantly increases the size and weight. To address this issue, this letter proposes a novel weak pole offset periodic cusped magnetic (WPO-PCM) focusing structure with a more compact geometry. In this structure, both the horizontal and vertical sides of each sheet electron beam are primarily confined by a large <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${B}_{z}$ </tex-math></inline-formula>, while a small <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${B}_{y}$ </tex-math></inline-formula> is used for fine-tuning. The theoretical analysis, simulation, fabrication, and measurement of this WPO-PCM focusing structure are presented. The measured magnetic fields coincide well with their simulated counterparts, indicating that the structure could provide a peak <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${B}_{\text {z}}$ </tex-math></inline-formula> of 0.8 T. The effectiveness of the methodology for this WPO-PCM is further verified through the co-simulation of the dual-sheet beam electron optic system and the beam-wave interaction of the entire traveling wave tube.

We report a new application of high magnetic fields to structural biology. We usually design and fabricate a magnet to achieve uniform magnetic field as well as uniform magnetic field gradient. In this new application we adopt a uniform magnetic force field. It has been found that the growth of protein crystals is affected by the presence of magnetic force. Development of uniform magnetic force field magnets is now in progress at the Tsukuba Magnet Laboratory of the National Research Institute for Metals. These magnets are superconducting magnets because they must be continuously run for several days to grow protein crystals. The first magnet wound with NbTi is now under installation. This magnet is designed to generate a uniform force field of 240 T/sup 2//m in a cylindrical space of 10 mm in diameter and 10 mm in height, and the magnetic force field fluctuation along z-direction is better than 0.4%. In liquid helium, it could achieve the design current.

The electron optical system (EOS) plays a vital role in vacuum electronic devices (VEDs). A 19.3-mA, 19.4-kV electron gun with a multistage focus electrode is designed, and the multistage focus electrode advantage is analyzed to generate a high-quality beam that meets the requirement of traveling-wave tubes (TWTs) operating higher than 0.6 THz. The tunnel and beam waist radii are 50 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$35~\mu \text{m}$ </tex-math></inline-formula> , respectively, and the current density in the waist reaches as high as 501 A/cm2. A novel dynamic-matched focusing magnetic field upswing from 0.35 to 0.5 T at the circuit tail, produced by an open-side asymmetrical magnetic focusing system, is proposed to balance the increasing divergence force of the rise of space charge focus. Two different asymmetrical magnet design methods are analyzed. Compared to the uniform magnet (UM) and inclined magnet (IM), the inclined dynamic-matched magnet (IDMM) performs optimally in beam transmission and output power, verified by particle-in-cell (PIC) simulation. The proposed dynamic-matched focusing magnetic field can benefit the performance of terahertz TWTs by improving the beam transmission rate and beam–wave interaction efficiency.

In this article, novel square-ring vertex-bar (SRVB) and rectangular-ring vertex-bar (RRVB) slow wave structures (SWSs) are proposed to develop high-efficiency wide bandwidth traveling wave tubes (TWTs). High-frequency characteristics including dispersion, interaction impedance, and transmission of these novel slow wave circuits are analyzed. It is shown that both the SRVB and RRVB possess the advantages of flatter dispersion and higher interaction impedance of the fundamental space harmonic of the forward wave in comparison with the traditional circular ring-bar (RB) SWS. Beam–wave interaction study of SRVB TWT with a pencil beam and RRVB TWT with a sheet beam is carried out. The particle-in-cell (PIC) simulations predict that with the beam voltage of 15.7 kV and beam current of 0.23 A, the maximum output power of the SRVB TWT and the RRVB TWT can reach 616 and 695.6 W, respectively. Both the SRVB TWT and RRVB TWT can produce larger output power, higher differential small-signal gain, and wider 3-dB bandwidth than the RB counterpart. The SRVB TWT is superior to the RRVB TWT for its stability against backward-wave oscillations but with relatively small output power.

: This project studies the viability of a high-power traveling wave tube (TWT) using a novel disk-on-rod slow-wave structure (SWS), which admits a large diameter, high current, annular electron beam. The annular electron beam carries a much higher current than a pencil beam, and the use of SWS allows moderate bandwidth. The gain and bandwidth were studied using analytic theory and simulation. The cold-tube as well as the hot-tube dispersion relation were constructed, from which the Pierce gain parameter, C, and the space charge parameter, QC, are extracted. Two very different methods were used to validate the calculation of C and they yield identical results. The gain was then calculated using Pierce classical theory of TWT based on C and QC, and was spot-checked against simulation results from the ICEPIC, MAGIC and CHRISTINE codes. Fair agreement was observed. The preliminary conclusion is that the disk-on-rod TWT is a viable, high-power extension to the conventional TWT which uses a pencil beam. The most important issue appears to be the excitation of unwanted modes.

In the terahertz (THz) band, electron beam quality significantly influences beam-focused transmission and beam-wave interaction, and the compressive electric field distribution determines it. This article introduces a stepped-conical (SC) electron gun optimized to improve beam laminarity. Compared with the traditional single-stage (SS) gun, the SC gun reduces velocity dispersion by 36.9%, resulting in a 55.3% reduction in population velocity standard deviation (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sigma $</tex-math> </inline-formula>). Under a 0.67-T uniform magnetic (UM) field focusing, both beams achieve transmissions over 99% within a 40-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula>m-radius, 80-mm-length beam tunnel. Verified through CST particle-in-cell (PIC) simulations, the hot transmission of the SC gun-based beam could maintain an over 99% transmission ratio, while the SS gun decreased to 95.6%. Under the 1-mW input signal, the SS gun-based beam amplifies the signal by 40 dB in the slow wave structure (SWS) circuit, yielding an output power of 400 mW. In contrast, the SC gun-based beam achieves a gain of 48 dB and an output power of 900mW, representing a 112.8% improvement. The high-quality electron beam generated by the SC electron gun not only enhances the transmission efficiency of ultrafine electron beams but also significantly improves the gain and power output of the THz traveling-wave tubes (TWTs).

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 $140\times 3$ 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.

In a practical helix traveling-wave tube (TWT), there are always magnetic focusing for constraining the electron beam as it passes through the interaction region and attenuator or sever for suppressing the oscillations, including backward-wave oscillation (BWO) and improving the output power. In the event that the attenuator (sever) and magnetic focusing were considered, a linear theory is employed to analyze BWO start oscillation condition. Numerical results show that the start oscillation length of the TWT with attenuator is much larger than that of the TWT without attenuator, that it is possible to increase the start oscillation length by tailoring the parameters of the magnetic focusing. So, in order to predict the BWO accurately, we should take into the attenuator (sever) and magnetic focusing account.

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.

The study of three types of magnetic focusing structures for terahertz band sheet beam traveling wave tubes is presented in this article. Although there are a lot of reported literatures regarding the study of magnetic focusing structures, most of them just focused on a single type of focusing structure and lack of systematic research (for example, only simulation study), with a limited discourse on the comparative performance across different types of focusing structures. In addition, many reported simulations regarding the focusing structures are normally based on the ideal electron beam, resulting in reduced simulation accuracy to some extent. In this article, three types of focusing structures (uniform magnetic (UM) / pole offset periodic cusped magnetic (PO-PCM)/PCM-tunable quadrupolar magnet focusing structures) are studied in detail by theoretical analysis and simulation design. The PO-PCM focusing structure is experimentally studied as a representative. Simulation results based on the electron beam emitted from the designed sheet beam electron gun show that a stable long-distance transmission over 112 mm and 85 mm can be achieved with the UM focusing structure and the PCM focusing structures, respectively. To verify the design, the microfabrication and test of the PO-PCM focusing structure is carried out as a representative. The measured magnetic field matches well with its simulated and theoretically calculated counterparts. The beam-wave interaction cosimulation results based on different focusing structures have a discrepancy among them.

We present the results of studies aimed at development of multiple-sheet-beam microfabricated traveling-wave tubes (TWT) at sub- THz frequency band. The double-tunnel TWT with a meander-line slow-wave structure (SWS) at V -band and the triple-beam TWT with grating SWS at G-band are designed. Technologies for microfabrication of the SWSs are discussed. The results of SWS fabrication and characterization are presented. Multiple-beam electron-optic system is designed and beam transportation in a uniform magnetic field is studied. The results of 3-D PIC calculation of small-signal and large-signal gain are presented.

Electron-optic systems (EOSs) with high compression are required for THz-band vacuum-tube electron devices to reduce the cathode load, which is necessary to increase lifetime and enable operation in a continuous-wave (CW) mode. However, it is difficult to achieve high compression of multiple electron beams. In this article, we present the design and simulation of EOS with triple elliptic electron beam for a 0.2-THz traveling-wave tube. The triode electron gun with planar electrodes provides <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times 0.06$ </tex-math></inline-formula> -A electron beam with compression factor of 16. At the cathode, the current density is 26.31 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> while in the beam tunnel it is higher than 400 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The magnetic focusing system with 1.34-T axial magnetic field consisting of Nd–Fe–B permanent magnets and pole pieces is also designed. The numerical simulation predicts stable beam transmission at 25-mm distance.

The design of a wideband W-Band travelling wave tube using the staggered double-vane slow-wave structure combined with plan alignment three pencil beams is described in this paper. A more simple new type input and output coupler is brought forward with high transmission characteristics and short beam-wave interaction length for the easily beam transport and structures fabrication later. Particle in cell (PIC) simulations predict that when the beam voltage is of 22kV and beam current of 3×140 mA respectively, the travelling wave tube can produce over 2.25 kW of peak power within the total length of 70mm, also with a corresponding gain of 43 dB and an electronic efficiency of 12% at 94 GHz. And the 3 dB bandwidth can be achieved about 15 GHz.

We investigated the attainable stability of the monofilar-helix traveling wave tube (TWT) with respect to the parasitic backward-wave oscillation under periodic magnetic focusing. We accounted for the effect of the electron beam rotation in the nonhomogeneous focusing magnetic field, magnetic field amplitude and periodicity, and the specific coupling of the electron beam with the synchronous harmonics of the electromagnetic wave existing on the slow-wave circuit. This specific coupling takes place in the monofilar-helix TWT only and decreases its stability with respect to the parasitic backward-wave oscillation in comparison to other types of helix TWT. For the purpose of this analysis, we developed further the linear two-dimensional (2-D) multiwave theory of the helix TWT described in our previous work . In this theory, we account for space charge fields by means of waveguide excitation model based on the direct solution of Maxwell equations. Differently from , the present version of our theory allows for accounting of the coupling of the synchronous 0th spatial harmonic (forward employed for the amplification) and -1st (parasitic backward) harmonic of the circuit wave with the electron beam in the monofilar-helix TWT. By numerical modeling, we obtained quantitative data on the starting conditions for the parasitic backward-wave oscillation in the typical monofilar-helix TWT in the intersection point of the dispersion curves of these two harmonics. The described coupling mechanism does not exist in the bifilar-helix TWT. Therefore, the TWT with the bifilar helix is more stable with respect to the parasitic backward-wave oscillation than the TWT having the monofilar helix. These theoretical results correlate with the existing experimental evidence. The results of the present study, in particular, explain why the backward-wave oscillation suppression by tailoring of the focusing magnetic field has a significant effect in the bifilar-helix TWT but does not work in the monofilar-helix TWT.