RF Interaction Circuit Research Articles

Design and Efficiency Enhancement Studies of Periodically Dielectric Loaded W-Band Gyro-TWT Amplifier

In this article, design, simulation, and efficiency improvement of W-band gyrotron traveling wave tube (gyro-TWT) amplifier operating at fundamental TE01 mode have been presented. A triode-type magnetron injection gun (MIG) has been designed and optimized for the desired quality of electron beam with a minimum spread 2% with velocity ratio 1.12 to improve the electronic efficiency of gyro-TWT. Parasitic modes in gyro-TWT have been suppressed by employing a periodic dielectric loading in the RF interaction circuit. The simulated amplifier developed an output power of ~203 kW in fundamental TE01 mode with an electronic efficiency ~29% and power gain ~57 dB for a nonspread gyrating electron beam. To improve the overall efficiency of gyro-TWT, a single-stage depressed collector has been designed as a spent electron beam recovery system. The proposed collector has enhanced the overall efficiency of gyro-TWT to ~48%.

Design and PIC Simulation Studies of Millimeter-Wave-Tunable Gyrotron Using Metal PBG Cavity as its RF Interaction Circuit

In this article, a millimeter-wave gyrotron using a metal photonic band gap (PBG) cavity as its RF interaction circuit is studied for its tunability. A PBG cavity operating in a TE7,2-like mode is designed and analyzed for its mode selective property at ~260 GHz. Particle-in-cell (PIC) simulation of the PBG gyrotron predicted a continuous wave (CW) RF output of ~121 W in the desired TE7,2-like mode with an axial mode number, $q = 1$ . Furthermore, the tunability of a PBG gyrotron is studied by varying the magnetic field, which, in turn, varies with the axial mode number of the desired operating mode. A broad continuous tunable bandwidth of ~1.5 GHz is obtained for more than ~1.5 W of the output power. The designed PBG cavity is thermally analyzed to study the structural deformation of metal rods and its effect on the performance of the tunable gyrotron.

Simulation Investigations and Optimization of a Millimeter-Wave Gyrotron for Its Tunability Using Magnetic and Thermal Tuning Schemes

The present gyrotron is designed to operate in $\text{TE}_{\text {7,2}}$ mode at 260 GHz for studying its beam–wave interaction behavior and its tunability for dynamic nuclear polarization/nuclear magnetic resonance (DNP/NMR) application. The operating frequency of the gyrotron is very sensitive to the physical dimension of the RF interaction circuit and its shape can be deformed due to the ohmic loss led temperature. Therefore, the combined magnetic and thermal tuning schemes are used to control the deformation and enhance the tunability of millimeter-wave gyrotron. The 3-D particle-in-cell simulation technique is used to study the beam–wave interaction behavior of gyrotron and validated with a multimode nonlinear code. Further, the structural deformation and thermal studies are made in a finite-element method based ANSYS. The use of combined magnetic tuning and water coolant based thermal tuning schemes have improved the tunable bandwidth of the present gyrotron to ∼1.88 GHz, which is ∼44.60% higher than the one obtained by varying the magnetic field only.

Design and Stability Studies of Second-Harmonic Gyro-TWT Amplifier Using Wedge-Shaped Lossy Ceramic Rod-Loaded Mode Selective RF Interaction Circuit

In this paper, the design and stability analysis of a W-band second-harmonic Gyrotron Travelling Wave Tube (gyro-TWT) amplifier loaded with the wedge-shaped lossy ceramic rods, operating in the circular azimuthally symmetric TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> mode, has been presented. The stability of the gyroTWT has been investigated against the spurious oscillations, including the reflective oscillation, gyrotron backward oscillation (gyro-BWO), and absolute instability. The transmission loss for the operating mode has been calculated as ~19 dB/cm at 91.4 GHz for achieving the stability. Furthermore, the gyro-TWT using the distributed loaded RF section has been investigated for its nonlinear behavior using a self-consistent nonlinear analysis. The saturated RF output power of ~450 kW at 91.4 GHz with ~18% efficiency, ~30-dB saturated gain, and 3-dB bandwidth of ~3% has been achieved for an electron beam of 100 kV and 25 A.

Gyro-TWT Using a Metal PBG Waveguide as Its RF Circuit—Part II: PIC Simulation and Parametric Analysis

In this paper, 3-D electromagnetic particle-in-cell (PIC) simulation of gyro-TWT amplifier using a metal photonic bandgap (PBG) waveguide as its RF interaction circuit is presented. A commercial finite integration-based PIC simulation code CST Particle Studio has been used to study the beam-wave interaction mechanism. The mode selectivity of a PBG waveguide has been investigated in the absence of an electron beam, so as to obtain the single-mode operation in a gyro-TWT. The simulated PBG gyro-TWT produced a stable RF output power of ~90 kW at 35 GHz for a 72-keV, 10-A annular electron beam with a velocity ratio of 1.05. The instantaneous bandwidth has been obtained as ~14% with respect to the operating frequency. Furthermore, the performance of the simulated PBG gyro-TWT has also been investigated through the parametric variation of the velocity spread, beam voltage, beam current, axial magnetic field, beam pitch factor, and the RF input power to validate the conceptual design of the device.

Nonlinear Investigation and 3-D Particle Simulation of Second-Harmonic Gyro-TWT With a Mode Selective Circuit

The beam-wave interaction behavior of a second-harmonic Ku-band high-power gyro-TWT comprising a mode selective RF interaction circuit has been investigated using a 3-D particle simulation code. The mode-selective RF circuit has been cut axially into four slices to restrain the electromagnetic modes not having m = 2 symmetry. A self-consistent nonlinear large signal code has been developed to compute the field amplitude, power, energy, and phase of the gyrating beam. The amplifier gives a saturated peak power of ~230 kW at 15.8 GHz for beam parameters of 80 kV and 20 A having a pitch of 1.1. The large signal gain of the device has been calculated as ~16 dB for the beam spread of 14%. Further, the nonlinear findings have been validated against the Particle-in-Cell code that predicts the saturated output power of ~193 kW in a circular TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> mode at the desired operating frequency.