Magnetic Focusing System Research Articles

The development of W waveband diffraction radiation oscillator

This study presents the development of a continuous wave diffraction radiation oscillator utilizing a sheet electron beam. The oscillator comprises a slow-wave system formed by double-comb gratings, an open resonant cavity consisting of a spherically curved mirror and a cylindrically curved mirror, and a sheet electron beam generated by a diode gun. A permanent magnetic focusing system stabilizes the transmission of the sheet electron beam within the slow-wave system. Through a combination of mechanical and electronic tuning, the oscillator generates stable signal output in the TEM00q mode. The oscillator's frequency tuning range spans from 87 to 97 GHz, achieving a maximum output power of 13.5 W.

Compact E-Band Sheet Beam Folded Waveguide Traveling Wave Tube for High Data Rate Communication

The design of a compact <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{E}$</tex-math> </inline-formula> -band (71–76 GHz) sheet beam (SB) folded waveguide (FWG) traveling wave tube (TWT) for high data rate communications is presented in this article. The round beam tunnel for a pencil FWG slow wave structure (SWS) has been redesigned with a rectangular beam tunnel to be suitable for the SB with increased beam current. A novel periodic permanent magnet (PPM)–periodic quadrupolar magnet (PQM) focusing system has been developed to reduce the beam ripple and improve the transmission ratio. A multistep side-out coupler is designed to import and extract the RF power from the SWSs and be compatible with the planar periodic magnetic focusing system. Particle-in-cell (PIC) simulations are employed to evaluate the performance of the high-frequency circuit. Driven by a 1 W signal, the 30-period circuit is estimated to produce more than 100 W power and achieve a 21 dB gain with a less than 0.4 dB variation over 71–76 GHz. Moreover, phase and linearity analysis show a less than 6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> in-band phase distortion and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 30 dB third-order intermodulation distortion (C/3IM) at 30 W of output power, meeting the requirements of a communications-focused TWT. Over 5% circuit efficiency and 40% overall efficiency when equipped with a single-stage collector can be achieved by the compact TWT over the target band.

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.

Studies of a Ka-band high power klystron amplifier at INFN-LNF

In the framework of the Compact Light XLS project [1], a Ka-band linearizer with electric field ranging from 100 to 150 MV/m is requested [2, 3, 4]. In order to feed this structure, a proper Ka-band high power klystron amplifier with a high efficiency is needed. This paper reports a possible solution for a klystron amplifier operating on the TM010 mode at 36 GHz, the third harmonic of the 12 GHz linac frequency, with an efficiency of 44% and 10.6 MW radiofrequency output power. We discuss also here the high-power DC gun with the related magnetic focusing system, the RF beam dynamics and finally the multiphysics analysis of a high-power microwave window for a Ka-band klystron providing 16 MW of peak power.

Эффект Фарадея в знакопеременном магнитном поле: математическая модель

When creating magnetic focusing systems, for example, for traveling wave lamps, there are known problems associated with the need to adjust the electron flow moving in a given direction along the axis of the device, that is, to align its axis with the magnetic axis of the focusing system. The ideal adjustment of the device is very difficult to achieve due to many influencing factors, therefore, it becomes necessary to create automated complexes for simulating the alignment of the electron beam in the magnetic field formation system for focusing systems. The alignment problems can be solved using magneto-optical methods, since it is known that when exposed to a magnetic field, the light exhibits a number of similar properties (reaction). The aim of the work is to create a mathematical model for the passage of a light beam through an alternating magnetic system based on the Faraday effect. As a result, a mathematical model has been obtained, that should allow modeling the change in the plane of light polarization during the passage (movement of the magneto-optical sensor) along the axis of the magnetic system. The simulation of the passage of light according to the Faraday effect through a magnetic system of one and five magnets is considered.

Design of optically pumped cesium beam tube with hexapole magnetic system for longer lifetime and better SNR

In order to get a better tradeoff between the long lifetime and high performances of cesium beam atomic clocks, we proposed a design scheme of compact optically pumped cesium beam tube (OPCBT) based on hexapole magnetic focusing (HMF) system for the confinement of cesium. A practical optically pumped cesium beam tube without HMF system has been implemented and tested with high-stability laser by modulation transfer spectroscopy (MTS), and a signal-to-noise ratio (SNR) of 18,000 in 1 Hz bandwidth and Allan deviation of 2×10−12/τ were obtained. Based on this test, the simulation analysis results of the newly designed tube with HMF show that the predicted lifetime can reach over 20 years without degradation of the performances, and the expected frequency stability of the clock can be 6.5×10−13/τ with a lifetime of 10 years.

Double-mode and double-beam staggered double-vane traveling-wave tube with high-power and broadband at terahertz band

In this paper, a 220 GHz broadband and high-power staggered double-vane traveling-wave tube has been designed and verified. Firstly, a planar double-beam staggered double-vane slow-wave structure is proposed. By using the double-mode operation scheme, the transmission performance and bandwidth have been almost increased with twice as the single mode. Second, to satisfy the high output power requirement and improve the stability of the traveling-wave tube, a double pencil-beam electron optical system has been designed, the 20–21 kV driven voltage and the 2 × 80 mA current are set as the design target. By using the mask portion and the control electrode in the double beam gun, the two pencil beams can be focused with a compression ratio of 7 along their respective centers with narrow distances of about 0.18 mm with good stability. The uniform magnetic focusing system has also been optimized. The stable transmission distance of planar double electron beams could reach 45 mm with the focusing magnetic field of 0.6 T, which was long enough to cover the whole high-frequency system (HFS). Then, to verify the availability of the electron optical system and the performance of the slow-wave structure, particle-in-cell (PIC) simulation has also been carried out with the whole HFS. Results demonstrate that the beam-wave interaction system can get a nearly 310 W peak output power at 220 GHz with a 20.6 kV optimized beam voltage and beam current of 2 × 80 mA, the gain is of 38 dB with the 3-dB bandwidth over 35 dB about 70 GHz. Finally, the high precision microstructures fabrication has been carried out to verify the performance of the HFS, the results show that the bandwidth and the transmission properties are in good agreement with simulation result. Thus, the proposed scheme in this paper is expected to develop the high-power and ultra-wideband terahertz band radiation source with potential applications in the future.

An Approach to Focus the Sheet Electron Beam in the Planar Microstrip Line Slow Wave Structure

Planar microstrip line slow wave structures (PML-SWSs) have great prospects in the millimeter and terahertz wave fields. However, the development of the devices employing PML-SWSs is severely limited due to the focusing difficulties. Compared with the conventional sheet electron beam (SEB) SWS, the asymmetric boundary condition of PML-SWS and high local electric field further increase the instabilities of the SEB transport in PML-SWS. A new approach, named the uniform magnetic focusing system with the matching electric field (UMFS-MEF), is presented in this article, which can effectively solve these issues. The simulation results illustrate that the expansion rate of the SEB in UMFS-MEF is significantly lower than that in the conventional uniform magnetic focusing system (CUMFS) with the same magnetic field strength <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> . Moreover, the present calculation results show that the UMFS-MEF requires only 10.6% of the magnetic field strength of CUMFS for maintaining the same expansion rate. This means that the UMFS-MEF can effectively suppress the instabilities, and employ a very low magnetic field to maintain the stabilities of the SEB transport. Finally, we give several other schemes to realize the method.

Design and Experiment of a Hollow Beam Electron Optics System for Ka-Band Extended Interaction Klystrons

In this article, the design and experiment of a hollow electron beam (HEB) electron optics system (EOS) for Ka-band extended interaction klystrons (EIKs) are presented. To realize the fast switch of the current emission at a low cutoff voltage, an electron gun with two focus electrodes (FEs) is chosen. The emitting surface of cathode is a ringed spherical surface. The first focus electrode (FE1) is around the cathode, and the second focus electrode (FE2) is set in the central hole of the cathode. The current emitted from the cathode surface is about 2A, when the operating voltage is 25 kV. Three simulation codes, Egun, Superfish, and CST, are used for designing the electron gun and the permanent magnet focusing system (PMFS). Some sensitivity analyses with respect to the PMFS’s critical parameters are presented. The simulation shows the dc beam transmission is 100%. The electron beam, confined by 6000 Gauss uniform field, propagates through the drift tube of 37 mm with good laminarity and small ripple. The emission current can be suppressed when the voltage of the FEs is −3.4 kV. According to the results of simulation, the Ka-band EIK had been constructed and the hot-test experiments had been fulfilled in succession. The experimental results exhibit that the dc beam transmission is about 99% and the beam can sustain 98% transmission at high frequency (HF) operation. In addition, the output power is higher than 8 kW in the bandwidth of over 100 MHz.

Electron-Optic System With High Compression of a Multiple Elliptic Electron Beam for a Miniaturized THz-Band Vacuum Electron Device

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.

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.

Planar Distributed Three-Beam Electron Optics System With Narrow Beam Separation for Fundamental-Mode TWT in W-Band

A planar distributed three-beam electron optics system with narrow beam separation is investigated in this article for W-band fundamental-mode staggered double vane traveling wave tube (TWT). An integrated electron gun with control electrode for planar distributed three-beam array has been constructed based on the grid loaded method, which is a general method to obtain planar multibeam array by 1-D compression. Then, the uniform magnetic focusing system is constructed for planar distributed three-beam array, where the peak value of axial magnetic field <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> can be achieved 4600 Gs. The pole piece with novel stepped hole is considered in this magnet system, which can reduce the transverse magnetic field disturbance effectively. The optimization of the magnet structure and magnetic saturation analysis of the pole piece is fully considered to ensure the transmission stability of the three-beam array and reduce the volume of the whole system. The simulation results show that the three-beam array can transmit 70 mm with the transmission rate of 100% under the uniform magnetic focusing system. The whole system stability is verified, including the assembly error, working voltage fluctuations, and variations of magnetic field. The designed electron optics system can be used for the fundamental-mode staggered double vane traveling wave tube in W-band to improve the high output power in future.

Design of Magnetic Focusing system for a Compact Ka band Helix TWT

In this paper we discuss the design of Magnetic focusing system (MFS) for a compact helix travelling wave tube (TWT) operating in Ka-band. Issues related to the design of the magnetic focusing system have been discussed in detail along with practical measurement results. The key design parameters considered for this TWT are: the cathode voltage is around 9.3kV, beam current is 200mA and total length of the tube not more than 6 inch with minimal weight.

Power Enhancement in W-Band Pulsed Folded Waveguide TWT

To obtain higher power in ${W}$ -band traveling-wave tubes (TWTs), a modified folded waveguide (FWG) with non-concentric bends is introduced and demonstrated, which has higher coupling impedance compared with the normal FWG structure and is utilized to eliminate the band-edge instability of TWT. Phase velocity tapering (PVT) is also used to improve the interaction efficiency between the beam and the circuit. Furthermore, a higher beam current is used and the corresponding magnetic focusing system is designed to confine the beam. A prototype tube was then fabricated and tested at a beam current of 220 mA and beam voltage of 21.9 kV. The testing data show that this ${W}$ -band TWT is capable of delivering an output power of over 450 W with a 4.5-GHz bandwidth at a duty cycle of 10%, with a maximum power of 647 W and electronic efficiency of 13.4%. That is the highest output power and electronic efficiency test results of ${W}$ -band TWTs reported so far.

Electron-optical system for dual radial sheet beams for Ka-band cascaded angular log-periodic strip-line traveling wave tube

A novel electron-optical system is proposed for dual azimuthally stacked radial sheet electron beams (ARSEBs). The two sheet beams are emitted from curved surfaces of two cathodes and are focused by a focusing electrode, which is carefully designed to produce a symmetric field distribution for the two beams and to avoid interference between the beams. A common tunable periodic cusped magnetic focusing system is also designed to focus the two ARSEBs. The dual sheet beams are used in a traveling wave tube (TWT) consisting of a cascade of two azimuthally supported angular log-periodic strip-line (ASALS) slow-wave structures (SWSs). The transmission efficiency in the presence of beam–wave interaction is 93%. The particle-in-cell simulation results show that for a beam voltage of 4400 V and a current of 0.39 A, the proposed cascaded TWT has a maximum gain of 32.5 dB at 35 GHz and a maximum output power of 180 W; this is a significant gain improvement over a TWT using a single ASALS SWS.

Stability Analysis of 300-GHz Sheet Electron Beam Transport in a Periodic Rectangular Quadrupole

A novel periodic magnetic focusing system is proposed. The stability of elliptical sheet beam transport in the magnetic field of a periodic rectangular quadrupole (PRQ) is studied and compared with that in the magnetic field of periodically cusped magnets (PCMs). Theoretically predicted matching conditions are verified by numerical simulations. An 11.7- kV, 35-mA, 0.32 mm $\times $ 0.06 mm elliptical sheet beam is confined in a 0.53 mm $\times $ 0.08 mm beam tunnel and transported over a distance of 30 mm. Both the PCM and PRQ have similar characteristics of electron beam distribution change, beam emittance, and beam transmission. The results of the initial stability analysis indicate that PCMs are preferable to PRQs because of their ability to confine the beam with a long magnet period.

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.

A powerful pulsed “point-like” neutron source based on the high-current ECR ion source

The paper presents recent results of a "pointlike" neutron source development based on a D-D fusion in a D-loaded target caused by its bombardment with a sharply focused deuterium ion beam. These developments are undergoing at the Institute of Applied Physics of Russian Academy of Sciences in order to study a possibility to create an effective and compact device for fast-neutron radiography. The last experiments with a beam produced by a gasdynamic high-current ECR ion source and its focusing with a magnetic lens demonstrated that 60 mA of deuterium ions may be constricted to a transversal size of ∼1 mm at the focal plane. With a purpose to improve this result in terms of the beam current and its size, a combined electrostatic and magnetic focusing system is proposed and analyzed. It is shown that the combined system may enhance the total beam current and reduce its footprint down to 0.13 mm. All numerical analysis was performed using the IBSimu code.