Gyrotron Backward-wave Oscillator Research Articles

First Experimental Results on Gyrotron Backward-Wave Oscillator with Zigzag Quasi-Optical Transmission Line

First Experimental Results on Gyrotron Backward-Wave Oscillator with Zigzag Quasi-Optical Transmission Line

Sectioned Gyrotron Backward-Wave Oscillator with a Zigzag-Shaped Quasioptical System: Spatiotemporal Theory and Some of Its Results

Sectioned Gyrotron Backward-Wave Oscillator with a Zigzag-Shaped Quasioptical System: Spatiotemporal Theory and Some of Its Results

Nonlinear and self-consistent single-mode formulation for TM-mode gyrotrons.

This work develops a nonlinear and self-consistent framework for the single-mode simulation of TM-mode gyrotrons. Unlike TE modes, a nonlinear TM wave equation is derived by considering the additional axial modulation on the electron beam due to the interaction with the axial electric field. Together with the electrons' equations of motion, particle tracing simulation is conducted to model TM-mode oscillation. For a uniform structure, the electron-beam efficiency of the TM_{11}-mode gyrotron at the W band can achieve 30% over broad parameter space. Its beam-current, beam-voltage, and pitch-factor tuning properties are investigated under different magnetic fields. By optimizing the interaction structure of the proposed gyrotron backward-wave oscillator (gyro-BWO), the maximum interaction efficiency is higher than 30% with a frequency tuning range of more than 6 GHz at the pitch factor of 1.5. The peak efficiency can remain high of 32% at low beam voltage (10 kV) and low magnetic field (32.8 kG), indicating additional operating conditions. These special features may facilitate the development of low-cost and compact gyrotron systems and show great potential in the applications for TM-mode gyro-BWOs.

High-Power Tunable Source of Chaotic Radiation Based on a Ka-Band Helical Gyro-BWO

We present results of experiments where Ka-band chaotic oscillation regimes with a bandwidth of about 1 GHz have been obtained in a pulsed gyrotron backward wave oscillator (gyro-BWO) using a helically corrugated waveguide. The averaged radiated power reached 40 kW with a pulse duration of about 120 μs and a repetition rate of up to 10 Hz. The variation of magnetic field allows for tuning of the central frequency of the radiation in the 32.5-35.5 GHz range.

Linear and Nonlinear Characteristics of a Low-Voltage Gyrotron Backward-Wave Oscillator in the Subterahertz Range

Gyrotrons operating at low voltage are highly attractive for subterahertz applications because such an operation can greatly reduce the size and cost of gyrotron systems. Among the oscillator versions of gyrotrons, the gyrotron backward-wave oscillator (gyro-BWO) features continuous frequency tuning in a broadband. This study investigates the characteristics of the beam-wave interaction in a low-voltage gyro-BWO in the subterahertz wave range. Operation at lower voltages has the advantage of a broader frequency tuning range and the suppression of negative $k_{z}$ competing modes. The phenomenon of a sharp increase in the start-oscillation current near the lower limit of the operating voltage is investigated. In addition to the linear characteristics, the dependence of the efficiency and tuning bandwidth on voltage is analyzed. The potential performance of a low-voltage gyro-BWO is predicted using the existing beam parameters: beam voltage of 2 kV, beam current of 0.2 A with a velocity ratio of 1.0, and axial velocity spread of 20%. Although the gyro-BWO operating at 2 kV exhibits the problem of low efficiency (an efficiency of 5.6%), it has a broad frequency tuning range of 3.0 GHz. The efficiency can be enhanced to 15% (a peak power of 60 W) by tapering the waveguide radius. Possible competing modes in low-voltage operation are considered and a stability analysis reveals stable operation.

Reflective Gyrotron Backward-Wave Oscillator With Piecewise Frequency Tunability

The gyrotron backward-wave oscillators (gyro-BWOs) feature a continuous frequency tunability in a single transverse mode, while the step-tunable gyrotrons are famous for their ultrawide frequency hopping between various transverse modes. This study proposed a reflective gyro-BWO with a piecewise-continuous and wide tunability. The frequency is tunable in one transverse mode and can be switched to another by changing the applied magnetic field or/and the beam voltage. Meanwhile, the high-order axial modes (HOAMs) inside a cavity with a low-Q factor could broaden the bandwidth of each transverse mode. The interaction structure is optimized for the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> modes that serve as the principal operating modes. The adjacent TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">51</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">61</sub> modes could be excited to broaden the overall tuning range by adjusting the operating parameters. The measured magnetic tunings show a wide bandwidth of 36 GHz from 182 GHz at 6.6 Tup to 218 GHz at 8.0 T, which agrees with the numerical simulation predicted from the Computer Simulation Technology (CST). Besides, the measured voltage tunings exhibit a bandwidth of 2 GHz for the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> mode with a low-beam voltage from 12 to 19 kV. This study demonstrated that the reflective gyro-BWO could generate piecewise frequency tuning in four transverse modes and will be an excellent, moderate-power, terahertz source for many applications.

Terahertz Broadband Whispering-Gallery Mode Gyrotron Backward-Wave Oscillator

Conventional gyrotron backward-wave oscillators (gyro-BWOs) operate in a low-order mode, e.g., TE $_{0,1}$ mode. As the operating frequency extends to the terahertz (THz) band, the transverse size of low-order mode cavity shrinks, and the power capability is reduced, consequently. A solution to adopt an overmoded interaction cavity with a significantly enlarged index of the operating mode is valid on the condition that the challenging problem of mode competition can be controlled during the broadband frequency tuning. In this paper, a high-order whispering-gallery mode (WGM) THz gyro-BWO with a cathode-end output circuit is investigated. A segment-tapered circuit is applied to suppress the ${Q}$ factors of competing modes and to obtain a the competition-free stable start-oscillation scenario. The theoretical result predicts that the effective frequency tuning range continuously covers between 252.3 and 260 GHz when the ${B}$ -field is changed from 9.41 to 9.96 T. Our studies are beneficial to the development of high-performance sources for THz biomedical and material science applications.

Electronic-Tuning Frequency Stabilization of a Terahertz Gyrotron Oscillator

Gyrotron oscillator, as one kind of efficient terahertz (THz) source, is developed with the capability of frequency and power tuning to meet diverse application demands. Due to system nonlinearity, it is quite difficult to achieve high frequency stabilization (FS) during power tuning without using external auxiliary components. This paper proposes an FS scheme with simultaneous power tuning in gyrotron backward-wave oscillator state. This is realized by compensating the detuning electron cyclotron frequency during the electronic tuning process. The required quasi-linear relationship between the electron-beam pitch factor and the accelerating voltage is numerically verified by using the SLAC EGUN code. The investigation focuses on a Watt-level open-cavity gyrotron oscillator. The tens-of-Watts-level output power can be tuned by over 40% at the FS point, and especially the self-adaptive FS reaches the accuracy of about 10-MHz level. The proposed scheme would be attractive for the high-stability THz-wave heating in material sciences and biomedicines.

Asymmetric linear efficiency and bunching mechanisms of TM modes for electron cyclotron maser

This study examines the transverse magnetic (TM) waveguide modes, which have long been considered as the unsuitable ones for the operation of the electron cyclotron maser. The beam-wave coupling strength of the TM modes, as expected, is found to be relatively weak as compared with that of the transverse electric (TE) waveguide modes. Unlike TE modes, surprisingly, the linear behavior of the TM modes depends on the sign of the wave number kz. The negative kz has a much stronger linear efficiency than that of the positive kz. The bunching mechanism analysis further exhibits that the azimuthal bunching and axial bunching do not compete but cooperate with each other for the backward-wave operation (negative kz). The current findings are encouraging and imply that TM modes might be advantageous to the gyrotron backward-wave oscillators.

A broadband gyrotron backward-wave oscillator with tapered interaction structure and magnetic field

The gyro-monotron and gyrotron backward-wave oscillator (gyro-BWO) are the two oscillator versions of gyrotrons. While serving different functions, they are also radically different in the RF field formation mechanisms. The gyro-monotron RF field profile is essentially fixed by the resonant interaction structure, while the gyro-BWO possesses an extra degree of freedom in that the axial RF field profile is self-determined by the beam-wave interaction in a waveguide structure. The present study examines ways to utilize the latter feature for bandwidth broadening with a tapered magnetic field, while also employing a tapered waveguide to enhance the interaction efficiency. We begin with a mode competition analysis, which suggests the theoretical feasibility of broadband frequency tuning in single-mode operation. It is then shown in theory that, by controlling the RF field profile with an up- or down-tapered magnetic field, the gyro-BWO is capable of efficient operation with a much improved tunable bandwidth.

Optimization of distributed-loss circuit of gyrotron traveling wave amplifier using multi-objective genetic algorithm

The resistance and thickness of distributed-loss circuit of gyrotron traveling wave amplifier are optimized by using the multi-objective genetic algorithm. Maximum gain of 15.9 dB is obtained by optimizing the thickness and resistivity of loss layer as 0.116 mm and 77 882 times the resistivity of copper, respectively. The accuracy is verified by the single-objective genetic algorithm, which makes the amplifier realize effective restraint of gyrotron backward wave oscillation and achieve the maximum gain.

Design and Simulation of a &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$Q$ &lt;/tex-math&gt;&lt;/inline-formula&gt;-Band Gyrotron Backward-Wave Oscillator With Distributed Loss

This paper presents the design and simulation of a $Q$ -band gyrotron backward-wave oscillator that is operated at fundamental TE01 mode. A tapered interaction circuit with distributed loss is employed to suppress both the transverse and axial mode competitions. A nonlinear code is used to study the start-oscillation current and interaction behavior. A Computer Simulation Technology (Particle In Cell) (CST-PIC) method is employed to verify the numerical calculation. A simulation result of 127-kW output power and 22.7% efficiency with a 3-dB bandwidth of 1.3 GHz driven by a beam of 70-kV voltage and 8-A current is predicted. The results of nonlinear code and CST-PIC calculations show good agreement.

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.

Theory and Experiment of a W-Band Tunable Gyrotron Oscillator

A gyrotron capable of both frequency and power tuning is a promising coherent millimeter-THz wave source. A self-consistent nonlinear theory is applied to investigate the electron cyclotron interaction between electron beam and wave modes of axial nonfixed profiles in an extended W-band <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">${\rm TE}_{01}$ </tex-math></inline-formula> mode cylindrical cavity. It is revealed that tuning the magnetic field strength can excite electron cyclotron resonances on forward wave, backward wave, and even simultaneous on both waves, which makes the system operate under distinctive states, namely the gyrotron backward wave oscillation state and the gyromonotron state. In this paper, a W-band prototype gyrotron oscillator based on an extended cylindrical waveguide cavity is built, and the experiment test indicates that the system starts oscillation in a relative wide range of the operation parameters. The measured frequency spectrum reveals the system iteratively switches between the lower order instability axial modes, and it operates under nonstationary oscillation states. The experimental measurement of highest output power <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">${\sim}{\rm 8}~{\rm kW}$ </tex-math></inline-formula> is consistent with the theoretical predictions. An optimized gyrotron circuit with efficiency exceeding 20% and tunable bandwidth over 10 GHz is also presented. The free oscillation behaviors revealed in this paper provide interesting guidance for developing tunable gyrotrons in millimeter-THz wave range.

Stable 0.3-THz Gyrotron Backward-Wave Oscillator With a Tapered Coaxial Interaction Waveguide

Mode competition is a severe problem in the development of a high-power gyrotron backward-wave oscillator (gyro-BWO) in terahertz (THz) region. To improve the stability of this device, this paper investigates the possibility of using a tapered coaxial waveguide as the interaction structure of a 0.3-THz gyro-BWO. The inner cylinder rarefies the mode density near the operating mode. Moreover, tapering the wall of the outer cylinder suppresses almost all the competing modes and shifts the optimum magnetic tuning range of the operating mode under stable operating conditions. The gyro-BWO is predicted to stably generate a peak power of 17.2 kW with 11.5% efficiency at 0.307 THz, and a continuous tuning bandwidth of 3.4 GHz $({\approx}{1.1\%})$ for a 30-kV, 5-A electron beam with $\alpha=1.0$ and an axial velocity spread of 5%. Compared with the uniform case, the tapered coaxial interaction waveguide enhances both the stability and the frequency tunability of the THz gyro-BWO.

Nonstationary oscillation of gyrotron backward wave oscillators with cylindrical interaction structure

The nonstationary oscillation of the gyrotron backward wave oscillator (gyro-BWO) with cylindrical interaction structure was studied utilizing both steady-state analyses and time-dependent simulations. Comparisons of the numerical results reveal that the gyro-BWO becomes nonstationary when the trailing field structure completely forms due to the dephasing energetic electrons. The backward propagation of radiated waves with a lower resonant frequency from the trailing field structure interferes with the main internal feedback loop, thereby inducing the nonstationary oscillation of the gyro-BWO. The nonstationary gyro-BWO exhibits the same spectral pattern of modulated oscillations with a constant frequency separation between the central frequency and sidebands throughout the whole system. The frequency separation is found to be scaled with the square root of the maximum field amplitude, thus further demonstrating that the nonstationary oscillation of the gyro-BWO is associated with the beam-wave resonance detuning.

High Power Wideband Gyrotron Backward Wave Oscillator Operating towards the Terahertz Region

Experimental results are presented of the first successful gyrotron backward wave oscillator (gyro-BWO) with continuous frequency tuning near the low-terahertz region. A helically corrugated interaction region was used to allow efficient interaction over a wide frequency band at the second harmonic of the electron cyclotron frequency without parasitic output. The gyro-BWO generated a maximum output power of 12kW when driven by a 40kV, 1.5A, annular-shaped large-orbit electron beam and achieved a frequency tuning band of 88-102.5GHz by adjusting the cavity magnetic field. The performance of the gyro-BWO is consistent with 3D particle-in-cell numerical simulations.

Stability Analysis of a Second Harmonic Coaxial-Waveguide Gyrotron Backward-Wave Oscillator

This study analyzes the stability of a Ka-band second harmonic gyrotron backward-wave oscillator (gyro-BWO) with a coaxial interaction waveguide. All of the possible competing modes in the frequency tuning range are considered. To suppress various competing modes, the downstream part of the coaxial interaction waveguide is loaded with distributed losses. Although the competing modes have different kinds of transverse field distributions, simulation results show that the losses of the outer cylinder and those of the inner cylinder serve as complementary means of suppressing the competing modes. The losses can stabilize the competing modes while having minor effects on the start-oscillation current of the operating mode. Detailed investigations were performed involving the dependence of the start-oscillation currents on the parameters of the lossy inner cylinder and the lossy outer cylinder, including the resistivity and the length of the lossy section. Moreover, under stable operating conditions, the performances of the second harmonic coaxial gyro-BWO with different sets of circuit parameters are predicted and compared.

Harmonic mode competition in a terahertz gyrotron backward-wave oscillator

Electron cyclotron maser interactions at terahertz (THz) frequencies require a high-order-mode structure to reduce the wall loss to a tolerable level. To generate THz radiation, it is also essential to employ cyclotron harmonic resonances to reduce the required magnetic field strength to a value within the capability of the superconducting magnets. However, much weaker harmonic interactions in a high-order-mode structure lead to serious mode competition problems. The current paper addresses harmonic mode competition in the gyrotron backward wave oscillator (gyro-BWO). We begin with a comparative study of the mode formation and oscillation thresholds in the gyro-BWO and gyromonotron. Differences in linear features result in far fewer “windows” for harmonic operation of the gyro-BWO. Nonlinear consequences of these differences are examined in particle simulations of the multimode competition processes in the gyro-BWO, which shed light on the competition criteria between modes of different as well as the same cyclotron harmonic numbers. The viability of a harmonic gyro-BWO is assessed on the basis of the results obtained.

Selective mode suppression in a W-band second harmonic coaxial-waveguide gyrotron backward-wave oscillator

A gyrotron backward-wave oscillator (gyro-BWO) encounters increasingly severe mode competition problems during development toward the goal of higher power at high frequencies. A coaxial interaction waveguide with distributed losses is proposed to enhance the stability and frequency tunability of a W-band second harmonic gyro-BWO. The losses of the inner and outer cylinders complement each other and effectively stabilize all of the competing modes while having minor effects on the operating mode. Under stable operating conditions, the W-band second harmonic coaxial gyro-BWO has a predicted peak output power of 71 kW with a magnetic tuning bandwidth of 1.0 GHz.