Periodic Permanent Magnet Focusing Research Articles

Investigation of Spindt Cold Cathode Electron Guns for Terahertz Traveling Wave Tubes

In this work, a Spindt cold cathode electron gun with a PPM (periodic permanent magnet) focusing system for a terahertz TWT (traveling wave tube) was designed and simulated based on the Pierce electron gun structure. More specifically, a new 3D (three dimensional) emission model was used, where the cathode radius of the electron gun was 1 mm and the cathode current was 30 mA, with an emitting half angle of about 28°. It was demonstrated that the electron beam was well focused with an electron beam radius of 0.3 mm and a filling ratio of 0.5 when the maximum value of the PPM field along with the axis was 0.122T. According to the simulation results, a planar cold cathode electron gun was developed. Measurements demonstrated that the I/V characteristics of the cold cathode gun were consistent with that of a cold cathode, revealing that the electrons emitted from the cathode are not intercepted when passing through the electron gun.

Design and Experimental Study of an E-Band CW Space TWT

This article presents a design and experimental study of an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E$</tex-math> </inline-formula> -band continuous wave (CW) space traveling wave tube (TWT) at 71–76 GHz. To ensure a high beam transmission, a double anode electron gun and periodic permanent magnetic (PPM) focusing system were designed, while linear tapered transition waveguides with diamond windows were used to form an energy coupling system as to increase output power. A four-stage depressed collector was also designed to improve overall efficiency. A prototype <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E$</tex-math> </inline-formula> -band CW space TWT was then fabricated and tested at a relatively lowered beam voltage of 14.8 kV and beam current of 73 mA. The testing results show that this tube is capable of providing over 87-W output power, 41-dB gain, and overall efficiency of over 31%, within a bandwidth of 5 GHz at 71–76 GHz, with a maximum saturated output power of 110 W, a corresponding gain of 49 dB, and an overall efficiency of 37%. Given the relatively lowered operating voltage and current, it is believed the overall performance of this <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$E$</tex-math> </inline-formula> -band TWT exhibits a substantial improvement as compared to those of the reported similar tubes.

Demonstration of a Modified Angular Log-Periodic Folded Waveguide Traveling Wave Tube at Ka-Band

This article demonstrates a Ka -band prototype traveling wave tube (TWT) with a single-stage gain of more than 35 dB, employing our previously proposed modified angular log-periodic folded waveguide (MALP-FW) slow wave structure (SWS). The basic operating principle of the MALP-FW SWS has been thoroughly investigated in our previous work, where its advantage of suppressing oscillations has been demonstrated by particle-in-cell (PIC) simulations. Guided by the principles, a prototype MALP-FW TWT operating at Ka -band is designed, fabricated, and tested. A 1600-G periodic permanent magnetic (PPM) focusing system is designed to focus the 12-kV and 100-mA electron beam. The measured maximum gain of the TWT is more than 35 dB at 32.5 GHz, and the output signal spectrum is noise-free. These results are in good agreement with the simulation results, which show that a maximum gain of 39.1 dB can be obtained at 32.4 GHz with an input power of 7 mW. This article validates the ability of MALP-FW SWS to suppress oscillations at the experimental level and its potential for future applications in terahertz TWTs.

Optimum Design of Electron Gun for 0.22-THz Traveling Wave Tubes

To improve the performance of a pencil beam electron gun for terahertz traveling wave tubes (TWT), a support vector regression (SVR) method combined with a goal attainment algorithm, which is used to optimize its structural parameters considering the periodic permanent magnetic (PPM) focusing system, is proposed in this article. Moreover, a parameter correction method to reduce the impact of the thermal expansion on the electron gun is proposed. An electron gun for 0.22-THz TWT gun is developed, in which a 30-mA/23.8-kV electron beam emitted by a curved barium–tungsten cathode with a beam loading of 4 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is achieved with the optimized parameters. To the high current density for THz beam–wave interaction, the area compression ratio of electron beam is used in beam tunnel and reaches 144. The experimental results are very similar to the simulation results, which indicate that the methods would be beneficial to the development of TWT.

Development of a compact coaxial cusped periodic permanent magnet focusing system.

To achieve the application of a periodic permanent magnet in high power microwave, a compact coaxial cusped periodic permanent magnet (CPPM) focusing system is constructed. The system consists of permanent magnets with different magnetization directions and soft magnets. Taking the required magnetic field performance and the effect of demagnetization into account, NdFeB and FeCoV are selected as the permanent and soft magnet materials. After the system is constructed, the magnetic field is measured. The results show that the guiding magnetic field strength and period of the CPPM are about 0.29 T and 26 mm, respectively, and the magnetic field distribution of measurement shows good agreement with the simulation results. However, there are some differences between the measurement and simulation results, and the differences are compared and analyzed here.

Demonstration of W-Band 30-W Continuous-Wave Folded Waveguide Traveling Wave Tube with Lowered Operation Voltage and Improved Gain Flatness

We have demonstrated and developed a W-band 30 W continuous-wave folded waveguide traveling wave tube (CW FWTWT) with 13.5-kV operation voltage and 4.8 dB gain flatness. In this paper, we quantitatively study the relationship between operating voltage and the key geometric dimensions of a folded waveguide, which is the key to the design of the low-voltage TWT. In addition, problems of improving beam transmission and suppressing oscillations are also studied, and the related design methodologies of the long-range electron gun, periodic permanent magnet (PPM) focusing system, low-voltage standing wave ratio (VSWR) coupling system, and horseshoe attenuator are discussed. On the basis of the above analysis, we have designed and fabricated two types of W-band CW TWTs. The test results show that beam transmissions are over 98.5% at DC and better than 96.5% at the highest peak RF power. Measured saturated output powers are greater than 33.6 W with saturated gain more than 42.2 dB within frequency range of 91–99 GHz, and an improved gain flatness of 4.8 dB is achieved.

Point-Focusing of Shear-Horizontal Wave Using Fan-Shaped Periodic Permanent Magnet Focusing Coils EMAT for Plate Inspection

Ultrasonic guided wave testing method is very effective in safety monitoring of complex structures, such as large-scale tank floor and industrial metal plate. The fundamental shear horizontal (SH) wave mode with the Lorentz force mechanism has the advantage that the propagation velocity does not change with the frequency. However, it is hard to recognize the dispersed ultrasonic signal accurately due to the bidirectional propagation characteristics of the SH guided wave. Therefore, a new transducer structure with a fan-shaped periodic permanent magnet (PPM) and centripetal conductor has been designed to achieve guided wave focusing and signal enhancement. Compared with the traditional transducers, the new-designed transducer has been proved to be more effective in energy focusing on the simulation and experimental verification. For the new structure of the transducer, the aperture angle, focusing radius, and coil rotation angle are studied in detail in this paper. The results show that, as the aperture angle and the coil rotation angle decrease, the signal intensity becomes higher at the focusing position. Moreover, as the focal radius increases, the amplitude of the ultrasonic signals on the focusing side and the divergent side will decrease, while the difference between the two sides will increase exponentially. In the defect detection in this paper, the reflected signal intensity of the new-designed fan-shaped PPM focusing coils electromagnetic acoustic transducer (EMAT) is approximately two times that of the traditional parallel PPM parallel coils EMAT. Therefore, the newly designed EMAT is of great significance for the signal enhancement.

Development of a 140-GHz folded-waveguide traveling-wave tube in a relatively larger circular electron beam tunnel

A 140-GHz folded-waveguide traveling wave tube (FWG-TWT) with a relatively larger circular electron beam tunnel was designed and developed. Using simulation software, the high frequency characteristics, the transfer characteristics, and the beam-wave interaction of the TWT were studied in detail. The simulation predicted up to 98 W output power and about 43 dB gain of FWG-TWT at the center frequency. The input and output couplers compatible with the FWG-SWS and the periodic permanent magnet focusing structure were designed. The FWG-TWT thus designed was fabricated and tested. The experimental result shows the output power exceeding 1.6 W and the gain exceeding 25 dB at 140 GHz when the input power is 5 mW. The reasons for the departure of the experimental from the simulated results have been explained.

Design and Experimental Evaluation of Dual-Anode Electron Gun and PPM Focusing of Helix TWT

A dual-anode convergent electron gun with a perveance of 0.14 $\mu \text{p}$ has been designed and developed for a high-efficiency helix traveling-wave tube (TWT), which would ultimately have application in a satellite communication system. The electron gun with an isolated beam forming electrode and an M-type dispenser cathode of 3.1-mm button diameter has been designed using EGUN, OMNITRAK-3-D, and OPERA-3-D codes. It consists of two isolated anodes, namely, control anode and ion-barrier anode, in addition to a ground accelerating anode to meet the requirement of long life and high reliability. Initially, in cathode selection, life considerations are ignored, and the gun is designed with the available cathode. Periodic permanent magnet (PPM) focusing has been designed using a Sm2Co17 high-intensity magnet in order to reduce the size and the weight of the overall TWT. Confined flow beam focusing has been adopted. A compact, small-size electron gun assembly has been designed using a ceramic insulator and a suitable support structure with due considerations for high-voltage and thermal requirements. A high-voltage analysis has been carried out using ESTAT code, and a thermal analysis has been carried out using ANSYS code. Designed electron gun and PPM focusing has been experimentally validated by developing and testing a beam-stick tube with the application of rated potentials at various electrodes in a gun and a collector. The design and experimental results for the electron gun and PPM focusing have been found in close agreement.

A comparative study of PPM and solenoid focusing in multibeam electron gun

This paper represents the comparison of periodic permanent magnet (PPM) and solenoid focusing for dual anode multi-beam electron gun using OPERA3D code. The electron gun has been operated at 6 kV having 75 mA beam current with 0.45 mm beam waist radius. The design has an additional feature of cathode protection from ion bombardment with the application of extra ion barrier anode.

Temperature stability of magnetic field for periodic permanent-magnet focusing system

In this study, finite element analysis based on an Ansoft Maxwell software was used to reveal the temperature stability of a magnet ring and the equivalent structural periodic permanent-magnet (PPM) focusing system. It is found that with the temperature increasing, the decrease rate of magnetic induction peak (B z)max of single magnet ring is greater than that of remanence B r of magnet in the range from room temperature to 200 °C, however, the PPM focusing system do have the same temperature characteristics of permanent-magnet materials. It indicates that the magnetic temperature properties of the PPM system can be effectively controlled by adjusting the temperature properties of the magnets. Moreover, the higher permeability of the magnets indicates the less H cb, giving rise to lower magnetic induction peak \( ( {B_{\text{z}} })^{\prime }_{\hbox{max} }. \) Finally, it should be noted that the magnetic orientation deviation angle θ (<15°) of permanent magnets has little effect on the focusing magnetic field of the PPM system at different temperatures and the temperature stability. The obtained results are beneficial to the design and selection of permanent magnets for PPM focusing system.

Particle Simulation of Slow-Wave System in Multi-Beam Traveling Wave Tube

A time-dependent particle code has been developed to simulate the motion and changing regularity of particles in multi-beam traveling wave tube. Magnetic focusing and interaction models of multi-beam traveling tube were established, and static magnetic field was calculated. The process of bunching particles by periodic permanent magnetic focusing system and beam-wave interaction was also simulated. The results show that particle simulation can really reflect the motion of particles in multi-beam traveling wave tube, which can provide some references for particle study of traveling wave tube.

An Analytical Approach for Obtaining User-Defined Axial Magnetic Field Profile of a PPM Focusing Structure

In this paper, a simple analytical approach for obtaining user-defined axial magnetic field profile of a periodic permanent magnet (PPM) structure used for focusing the electron beam in linear beam tubes is presented. Axially magnetized permanent ring magnet and single cell of a PPM structure are represented by equivalent solenoids, and their equivalent parameters are obtained from the dimensions and material properties of these magnet structures. Axial magnetic field for the finite-length PPM structure is obtained using the superposition of equivalent solenoids of all PPM cells. Finally, a methodology is presented to obtain a user-defined axial magnetic field profile in a PPM structure. Analytically computed axial magnetic field is compared with finite-element-method simulation and measured results and found to show a very good agreement.

Structure and magnetic field of periodic permanent magnetic focusing system with open magnetic rings

The magnetic field along the central axis for an axially magnetized permanent magnetic ring was investigated by analytical and finite element methods. For open magnetic rings, both calculated and measured results show that the existence of the radial magnetic field creates a remarkable cosine distribution field along the central axis. A new structure of periodic permanent magnet focusing system with open magnetic rings is proposed. The structure provides a satisfactory magnetic field with a stable peak value of 120 mT for a traveling wave tube system.

带开口磁环的周期永磁聚焦系统的轴向磁场

带开口磁环的周期永磁聚焦系统的轴向磁场

X波段耦合腔行波管周期永磁聚焦系统

X波段耦合腔行波管周期永磁聚焦系统

An Improved Analysis of PPM Focusing Structures Including the Effect of Magnetic Saturation in the Iron Pole Pieces

In this paper, an improved analysis of the periodic permanent magnet (PPM) focusing structure for linear beam tubes is presented. An estimation of critical dimensions for the iron pole pieces to avoid magnetic saturation is made. The method is based on the permeance calculation of all possible flux paths using magnetic circuit theory. The analytically calculated axial magnetic field using the present approach is compared with measured and simulated results obtained by using the finite-element-method-based commercial software ANSYS. The agreement is very good.

Application of high-temperature permanent magnets Sm(Co, Fe, Cu, Zr) Z in PPM focusing system

Rare earth permanent magnets Sm(Co, Fe, Cu, Zr) Z with outstanding performance and high-temperature thermal stability were fabricated. Optimized by Fe content and process, Sm(Co 0.72Fe 0.15Cu 0.1Zr 0.03) 7.5 magnet with B r>0.75 T and H ci>1300 kA/m at 300 °C can be obtained. According to the performance data of Sm(Co 0.72Fe 0.15Cu 0.1Zr 0.03) 7.5, the magnetic field along central axis B Z in periodic permanent magnet (PPM) focusing system was simulated using electromagnetic field analysis software Maxwell 2D/3D. The B Z exhibited typical cosine curve along central axis, and the peak value of B Z was high enough to meet the demand of PPM focusing system at room temperature even at 200±20 °C. Additionally, a kind of simple cooling structure for PPM focusing system was designed by setting cooling pipe between polepieces. Simulated results showed that smooth cosine curve of B Z was successfully achieved with good control of the thickness of cooling pipe.

Periodic permanent magnet focusing system with high peak field

In this study, hybrid periodic permanent magnet (PPM) system is studied, which has high axial magnetic field and low magnetic leakage. By simulation computation, some laws of magnetic field distribution vs. structure dimensions were obtained. A hybrid PPM is designed and constructed whose peak field reaches 0.6 T. The factors inducing discrepancies between computational results and practical measurements are analyzed. The magnetic field distribution is very sensitive to the variations of constructional parameters. Construction accuracy greatly influences the magnetic field distribution. Research results obtained here are potentially valuable for future work.

Transport of electron beams in periodic permanent-magnet focusing systems with nonharmonic distribution of the magnetic field

The results of the study of the transport of electron beams in periodic magnetic fields are presented for the case when the distribution of the magnetic field is essentially different from a sinusoidal distribution in different parts of the drift channel. An easy-to-implement nonharmonic distribution of the magnetic field is found. This distribution ensures a small ripple of the envelope of a high-intensity electron beam. The results of application of combined periodic permanent-magnet (PPM) focusing systems in high-powered pulse travelingwave tubes (TWT) are presented for the TWTs having a slow-wave structure that is superposed with the pole pieces of the PPM focusing systems.