Axis-encircling Electron Beam Research Articles

Optimization of a Cusp Gun With a Grid for a Terahertz Gyrotron Traveling-Wave Amplifier

A magnetic cusp gun with a control grid was optimized through parametric simulations for a gyrotron traveling-wave amplifier with an output power of 10 kW and a central operating frequency of 220 GHz. The ON and OFF of the electron beam are controlled by applying appropriate biases to the grid. An axis-encircling electron beam of current 1.5 A, a pitch factor of 1.23, and a relative pitch-factor spread of ~3.5% were predicted when the gun was operated at 70 kV.

Design of a 1-THz Fourth-Harmonic Gyrotron Driven by Axis-Encircling Electron Beam

In this article, the design of a fourth-harmonic gyrotron with 1-kW-level output power at 1 terahertz (THz) is presented. A unique advantage of this design is that, with a well-optimized magnetic-cusp large-orbit electron gun, the designed gyrotron can operate from first-harmonic to fourth-harmonic in multiple discrete bands by varying the confining magnetic field strength. By carefully balancing the competing modes, the gyrotron can be tuned to operate at six candidate modes, including TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,2</sub> for fundamental harmonic, TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2,3</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2,4</sub> for second-harmonic, TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3,5</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3,6</sub> for third-harmonic, and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4,8</sub> for fourth-harmonic interactions. As the main operating mode, the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4,8</sub> can generate a peak output power of 1.68 kW, with an output efficiency of about 2.1%, and a magnetically controlled frequency tuning range of about 1.5 GHz around 1 THz. The impact of the longitudinal nonuniformity of the cavity at high-harmonic interaction was also studied. It shows that a radial tolerance of several micrometers will significantly elevate the start-oscillation current and deteriorate output performance. This design is to produce a THz source with ultra-wideband tuning capability ranging from the submillimeter-wave to 1-THz bands.

Development of a Magnetic Cusp Gun for a 1-THz Harmonic Gyrotron

High-quality axis-encircling electron beam is essential to drive a high-harmonic gyrotron. A magnetic cusp gun that can generate a low-spread axis-encircling electron beam at an extremely high magnetic-compression ratio was developed for a 1-THz fourth-harmonic gyrotron toward continuous-wave operation. The optimized gun could generate an axis-encircling electron beam with 80-kV voltage, 1-A current, and a nearly constant pitch factor of about 1.5 in a wide range of magnetic field strength. The transverse velocity spread was less than 2.1% and the guiding center radius was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$26.6 ~\mu \text{m}$ </tex-math></inline-formula> . The design was verified by using MAGIC, EGUN, and CST simulations and good agreements were achieved.

Universal Subterahertz Large-Orbit Gyrotron: Operation at the Second and Third Cyclotron Harmonics

Coherent subterahertz radiation in the continuous-wave regime was obtained in a large-orbit gyrotron (LOG) operated at a high harmonic of the cyclotron frequency. An electron–optical system containing a gun with a cusp followed by a section of adiabatic magnetic compression with a factor of 1000 ensures the formation of a 30 keV/0.7 A helical axis-encircling electron beam with acceptable velocity spread and pitch factor in a magnetic field of 5 T. Stable singlemode operation was obtained at frequencies of 0.394 and 0.267 THz with radiation powers of 0.37 and 0.9 kW at the third and second cyclotron harmonics, respectively. Tuning of the generation frequency in the gyrotron during its operation at the second cyclotron harmonic and complex regimes of competition of two waves excited at the second and third cyclotron harmonics are also studied.

Comparative analysis of terahertz fourth-harmonic gyro-TWAs and multiplying gyro-TWAs with axis-encircling electron beams

The harmonic operation in a terahertz gyrotron traveling-wave amplifier (gyro-TWA) permits a reduced magnetic field, whereas a harmonic multiplying gyro-TWA enables magnetic field reduction and frequency multiplication. This study comparatively analyzes the 400-GHz fourth-harmonic gyro-TWAs and fourth-harmonic multiplying gyro-TWAs with axis-encircling electron beams. This property of the gyro-TWAs with axis-encircling electron beams enables us avoid the appearance of the majority of the competing modes. According to the simulation results obtained using multi-mode codes, the attenuating severs suppress the remaining competing modes. In the case of a harmonic gyro-TWA containing a sever section, the copper-section start-oscillation length is significantly influenced by a decrease in the sever-section radius when compared with an increase in the sever-section length. Furthermore, a stable fourth-harmonic TE41-mode gyro-TWA containing sever sections is proposed. We propose fourth-harmonic TE41-mode multiplying gyro-TWAs, where the drive stages operate in the fundamental-harmonic TE11 mode to reduce the frequency of the drive wave. We subsequently develop a stable fourth-harmonic multiplying gyro-TWA with sever sections in the drive and amplified stages to avoid the competing modes and enhance the output power. The stable high-gain fourth-harmonic gyro-TWA can yield a peak output power of 2.7 kW at 400.6 GHz with a saturated gain of 75 dB and a bandwidth of 0.7 GHz for a 75-kV and 2-A electron beam with an axial velocity spread of 3%. Furthermore, a peak output power of 1.7 kW can be obtained by the fourth-harmonic multiplying gyro-TWA at 400.4 GHz with a saturated gain of 57 dB and a bandwidth of 0.4 GHz. Subsequently, we calculate the power and gain scaling for the harmonic gyro-TWA and harmonic multiplying gyro-TWA.

High-Harmonic Gyrotrons with Axis-Encircling Electron Beams at IAP RAS

The Institute of Applied Physics of the Russian Academy of Sciences has for many years been developing sub-terahertz and terahertz large-orbit gyrotrons that permit selective oscillation at higher cyclotron harmonics than is possible in the conventional gyrotrons. Currently, experimental studies are conducted at two specialized facilities. A prototype universal sub-terahertz source for magnetic resonance spectroscopy is studied using a facility that generates long-pulse and continuous electron beams with particle energies of up to 30 KeV. Continuous selective oscillation at the second and third cyclotron harmonics with frequencies of 0.267 and 0.394 THz was obtained for a radiation power of 900 and 370 W, respectively. New resonators with periodic phase correctors have been developed to increase the efficiency of third-harmonic oscillation and obtain fourth-harmonic oscillation with frequencies of up to 0.65 THz. Using a facility with an electron energy of up to 80 KeV, we study the possibilities of increasing the pulse generation power at the third harmonic at frequencies close to 1 THz to employ in experiments on obtaining a gas discharge in a focused terahertz wave beam and generating high-power extreme ultraviolet radiation.

Axial modes in terahertz high-harmonic large-orbit gyrotrons

Terahertz (THz) gyrotrons can operate with a lower applied magnetic field in harmonic operation, but the weakened harmonic interactions in harmonic gyrotrons can introduce serious challenges when mode competition occurs. The use of an axis-encircling electron beam can greatly alleviate mode competition in a harmonic gyrotron. In this paper, we study axial modes for third-harmonic [Formula: see text]-mode large-orbit gyrotrons. Simulation results reveal that the minimum current for oscillation to begin in each axial mode in the gyrotron regime is associated with a specific range of applied magnetic field. To avoid mode competition, tapered applied magnetic fields and waveguide radii are employed to enhance the high-order axial modes and suppress the low-order axial modes. Furthermore, spurious transverse modes in a THz gyrotron are discussed below. A stable third-harmonic [Formula: see text]-mode large-orbit gyrotron at the third-order axial mode is predicted to yield peak output power of 6.5 kW at 768.1 GHz with an efficiency of 10% for a 75-kV, 0.85-A electron beam with an axial velocity spread of 3%.

Powerful continuous-wave sub-terahertz electron maser operating at the 3rd cyclotron harmonic

Coherent subterahertz radiation is achieved in an electron cyclotron maser (large-orbit gyrotron configuration) operating at a higher cyclotron harmonic in the continuous-wave generation regime. This auto-oscillator is based on the use of a gun with a magnetic field cusp and a section for adiabatic magnetic compression with a factor of 1000, forming a 30 keV/0.7 A axis-encircling electron beam in a magnetic field of 5 T. A stable single-mode generation is achieved in the case of operation at the third and second cyclotron harmonics at frequencies of 0.394 THz and 0.267 THz with radiation powers of 0.37 kW and 0.9 kW, respectively.

Influence of Perturbations in the Axial Symmetry on Formation of Helical Electron Beams in a System with the Reversed Magnetic Field

We perform three-dimensional numerical simulation of the electron-optical system (EOS) with the reversed magnetic field, which is designed for a large-orbit gyrotron (GBO) operated in a wide frequency range of 0.26–0.65 GHz, within the framework of a static model and analyze the influence of perturbations in the axial symmetry of electrodes and the magnetic field on the parameters of the axis-encircling electron beam. It is shown that the most significant perturbation of the parameters of the helical electron beam (HEB) is determined by the radial displacement of the cathode and the tilt of the cathode coil, which ensures the reversal of the magnetic field. Analytical estimations are presented, which allow determining the corresponding margins for the development of such systems.

Terahertz Large-Orbit High-Harmonic Gyrotrons at IAP RAS: Recent Experiments and New Designs

We describe new versions of high-harmonic gyrotrons with axis-encircling electron beams developed on the basis of two (80-keV pulsed and 30-keV continuous wave) experimental setups. Selective operation at the second (0.267 THz) and at the third (0.394 THz) cyclotron harmonics was observed in the 30-keV gyrotron. Cavities with periodic phase correctors, where a far-from-cutoff axial mode with a decreased diffraction Q-factor is excited in a gyrotron-like regime, are designed to improve the operation at the third harmonic, as well as to achieve the fourth-harmonic operation at frequencies up to 0.65 THz. At the pulsed gyrotron setup, a sectioned cavity with a decreased diffraction Q-factor was experimentally tested.

Amplification of Frequency-Swept Signals in a -Band Gyrotron Travelling Wave Amplifier

The frequency agility of a ${W}$ -band gyrotron travelling wave amplifier (gyro-TWA) was demonstrated by the amplification of a frequency-swept input signal (chirp signal). The gyro-TWA was developed to provide high output power and wide bandwidth by using a helically corrugated interaction region. In the experiment a ~0.5-W signal of linearly swept frequency bandwidth of 1 GHz was amplified and a gain of over 30 dB was measured when driven by a 40 keV, 1.5 A, and axis-encircling electron beam.

Cusp Guns for Helical-Waveguide Gyro-TWTs of a High-Gain High-Power W-Band Amplifier Cascade

The evaluation, design, and simulations of two different electron guns generating the beams for W-band second cyclotron harmonic gyro-TWTs forming a high-gain powerful amplifier cascade are presented. The optimum configurations of the systems creating nearly axis-encircling electron beams having velocity pitch-factor up to 1.5, voltage/current of 40 kV/0.5 A, and 100 kV/13 A with acceptable velocity spreads have been found and are presented.

Characteristics of a KA-band third-harmonic peniotron driven by a high-quality linear axis-encircling electron beam

We employ a high-quality linear axis-encircling electron beam generated by a Cuccia coupler to drive a Ka-band third-harmonic peniotron and develop a self-consistent nonlinear calculation code to numerically analyze the characteristics of the designed peniotron. It is demonstrated that through a Cuccia coupler, a 6 kV, 0.5 A pencil beam and an input microwave power of 16 kW at 10 GHz can generate a 37 kV, 0.5 A linear axis-encircling beam, and it is characterized by a very low velocity spread. Moreover, the electron beam guiding center deviation can be adjusted easily. Driven by such a beam, a 30 GHz, Ka-band third-harmonic peniotron is predicted to achieve a conversion efficiency of 51.0% and a microwave output power of 9.44 kW; the results are in good agreement with the Magic3D simulation. Using this code, we studied the factors influencing the peniotron performance, and it can provide some guidelines for the design of a Ka-band third-harmonic peniotron driven by a linear electron beam and can promote the application of high-harmonic peniotrons in practice.

Broadband Amplification of Low-Terahertz Signals Using Axis-Encircling Electrons in a Helically Corrugated Interaction Region.

Experimental results are presented of a broadband, high power, gyrotron traveling wave amplifier (gyro-TWA) operating in the (75-110)-GHz frequency band and based on a helically corrugated interaction region. The second harmonic cyclotron mode of a 55-keV, 1.5-A, axis-encircling electron beam is used to resonantly interact with a traveling TE_{21}-like eigenwave achieving broadband amplification. The gyro-TWA demonstrates a 3-dB gain bandwidth of at least 5.5GHz in the experimental measurement with 9GHz predicted for a wideband drive source with a measured unsaturated output power of 3.4kW and gain of 36-38dB. The approach may allow a gyro-TWA to operate at 1THz.

The design of a multi-harmonic step-tunable gyrotron

The theoretical study of a step-tunable gyrotron controlled by successive excitation of multi-harmonic modes is presented in this paper. An axis-encircling electron beam is employed to eliminate the harmonic mode competition. Physics images are depicted to elaborate the multi-harmonic interaction mechanism in determining the operating parameters at which arbitrary harmonic tuning can be realized by magnetic field sweeping to achieve controlled multiband frequencies' radiation. An important principle is revealed that a weak coupling coefficient under a high-harmonic interaction can be compensated by a high Q-factor. To some extent, the complementation between the high Q-factor and weak coupling coefficient makes the high-harmonic mode potential to achieve high efficiency. Based on a previous optimized magnetic cusp gun, the multi-harmonic step-tunable gyrotron is feasible by using harmonic tuning of first-to-fourth harmonic modes. Multimode simulation shows that the multi-harmonic gyrotron can operate on the 34 GHz first-harmonic TE11 mode, 54 GHz second-harmonic TE21 mode, 74 GHz third-harmonic TE31 mode, and 94 GHz fourth-harmonic TE41 mode, corresponding to peak efficiencies of 28.6%, 35.7%, 17.1%, and 11.4%, respectively. The multi-harmonic step-tunable gyrotron provides new possibilities in millimeter–terahertz source development especially for advanced terahertz applications.

A Wideband Electron-Optical System of a Subterahertz Large-Orbit Gyrotron

Specific features of the electron-optical system, which forms the helical axis-encircling electron beam (HAEEB) in an electron gun with the moderate accelerating voltage $U_{o}$ (30 kV) and the cusp magnetic field, are considered. The optimal electrode geometry allowing one to obtain HAEEB without electrons reflected from the magnetic mirror for a pitch factor of up to 2 is found. The developed electron gun can be used in a set of pulsed and continuous-wave large-orbit gyrotrons employing high cyclotron harmonics (second, third, or fourth harmonics) in a wide frequency range of 0.26–0.52 THz (the magnetic field intensity B being 3 to 5 T). The frequency range can be extended even to 0.65 THz, if B is increased up to 6.3 T and $U_{o}$ , up to 45 kV, simultaneously.

Conformal Cross-Flow Axis-Encircling Electron Beam for Driving THz Harmonic Gyrotron

In a magnetic cusp gun, the canonical-angular- momentum (CAM) spread of the initially emitted electrons is crucial in generating substantial beam velocity spread. A new method called electron beam flow-feature compensation is proposed to build an axis-encircling electron beam with zero velocity spread by optimizing the cross-flow trajectories to compensate for the initial CAM spread. This method provides an effective solution to the velocity-spread problem in terahertz gyrotrons and increases the emission current to a level that is several times higher than the level that can be obtained using current technology.

Terahertz electron cyclotron maser interactions with an axis-encircling electron beam

To generate terahertz radiation via the electron cyclotron maser instability, harmonic interactions are essential in order to reduce the required magnetic field to a practical value. Also, high-order mode operation is required to avoid excessive Ohmic losses. The weaker harmonic interaction and mode competition associated with an over-moded structure present challenging problems to overcome. The axis-encircling electron beam is a well-known recipe for both problems. It strengthens the harmonic interaction, as well as minimizing the competing modes. Here, we examine these advantages through a broad data base obtained for a low-power, step-tunable, gyrotron oscillator. Linear results indicate far more higher-harmonic modes can be excited with an axis-encircling electron beam than with an off-axis electron beam. However, multi-mode, time-dependent simulations reveal an intrinsic tendency for a higher-harmonic mode to switch over to a lower-harmonic mode at a high beam current or upon a rapid current rise. Methods are presented to identify the narrow windows in the parameter space for stable harmonic interactions.

PERMANENT MAGNET SYSTEM AND ELECTRON GUN DESIGN FOR A 3RD HARMONIC PENIOTRON

This study discusses the operating characteristics of a large-orbit electron gun and a corresponding permanent magnet system of a 3rd harmonic peniotron. After optimization, a novel axis-encircling electron beam with axial velocity spread 4.48%, guiding centre deviation ratio 6.97% and high velocity ratio 2.03 is obtained. Driven by the electron gun, an output power of 35.4 kW is obtained, and the device efficiency is up to 56.0%, which is an attractive result in laboratories. The main advantages of such a peniotron are its compact size and low cost, which can meet the needs of vehicle, airborne and other mobile devices. The numerical analysis reveals that the relative axial position between the electrode system and magnet system has a great influence on the device performance, which needs careful control and precise adjustment.

Theoretical Study of a Fourth-Harmonic 400-GHz Gyrotron Backward-Wave Oscillator

The requirement of strong magnetic field is one of the major difficulties for terahertz gyrotrons. A plausible solution is to operate at higher cyclotron harmonic denoted as s, in which the magnetic field strength is reduced to 1/s of the value for the fundamental harmonic operation. This paper presents a systematic theoretical investigation of a fourth-harmonic 400-GHz gyrotron backward-wave oscillator with relatively high efficiency. An axis-encircling electron beam is employed to suppress the mode competition. The operating mode is the TE41 mode. The efficiency and bandwidth are optimized for the magnetic field tuning. Simulations suggest that the fourth-harmonic circuit is capable of achieving highest interaction efficiency ~6.5%, and tunable bandwidth 2.8 GHz at 400 GHz. The weak beam-wave coupling and serious Ohm loss on the circuit wall limit the overall performance.