M. Thottappan

Gyro-TWT Using a Metal PBG Waveguide as Its RF Circuit—Part I: Analysis and Design

A gyro-TWT using a metal photonic bandgap (PBG) waveguide as its RF circuit has been analyzed using a selfconsistent nonlinear analysis. The analysis focuses on single-mode operation of gyro-TWT by eliminating the mode competition problem existing in an overmoded structure using a mode-selective PBG circuit. A Ka-band, TE01 waveguide mode PBG gyro-TWT has been designed, and analytical results have been computed to demonstrate the beam-wave interaction mechanism of the device. The nonlinear analysis predicted ~91-kW RF output power with an electronic efficiency of ~13% and the saturated gain of ~40 dB from this 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 TE21 mode at the desired operating frequency.

Analysis and PIC Simulation of a Gyrotron Travelling Wave Tube Amplifier

The analysis of a Ka-band gyrotron Travelling Wave Tube (gyro-TWT) amplifier using a uniform cylindrical waveguide as its interaction circuit has been presented for the TE01 mode of operation a self-consistent nonlinear analysis in the large signal regime. The analysis predicts that the saturated peak output power of ~ 134 kW with a power conversion efficiency of ~ 22.7 %. The saturated gain has been calculated as ~ 41.3 dB for the amplifier driven by 72 kV, 8.2 A electron beam of a pitch factor 1.05. The critical interaction length of smooth wall uniform metal guide is found 10.7 cm for the stable amplifier operation. The form factor and the norm factor have also been estimated with the large signal non-linear code. Further, for 5% spread the amplifier develops the peak power of ~ 128 kW with an electronic efficiency of ~ 21.7 % and the gain of ~ 41.07 dB. These results are found to be good harmony when the beam wave interaction is studied with a commercial 3-D electromagnetic particle in cell (PIC) code. The behaviour of the beam all over the length of the interaction circuit has been monitored by calculating its energy, momentum, phase, etc. with the help of a commercial PIC code and which also in good agreement with analytical results with 1% deviation.

Particle-in-Cell simulation of gyro-TWT using a metal PBG circuit

The Particle-in-Cell (PIC) simulation of a Ka-band Gyrotron Traveling Wave Tube (gyro-TWT) amplifier using a Metal Photonic Band Gap (MPBG) waveguide as its RF interaction circuit is presented to study the electron beam-RF wave interaction behavior. The MPBG guide is chosen as an RF interaction circuit to accomplish the single mode operation in the amplifier by its mode selective property at short wavelengths. The technique for generating hollow annular gyrating electron beam is demonstrated using “CST particle studio” code. The energy transfer phenomenon from the gyrating hot electron beam to the propagating electromagnetic (EM) wave has been studied. The PIC code predicts that the peak output power in the MPBG loaded gyro-TWT as ~ 90 kW at 35 GHz with an electronic efficiency of ~ 13 % for a velocity ratio of 1.05. The saturated gain has been calculated as ~ 40 dB and the 3 dB instantaneous bandwidth is obtained ~ 14%.

3-D PIC Simulation of Gyrotwystron Amplifier Using MAGIC

In this paper, a 3-D electromagnetic (EM) and particle simulation of an X-band high power gyrotwystron has been reported. The EM (cold) and the electron beam and RF wave interaction (hot) behavior have been studied using a commercially available 3-D particle-in-cell code MAGIC. The EM field analysis guarantees that the RF circuit operation is in the desired TE011 at the input cavity and TE01 circular electric waveguide mode at the output with the desired frequency of operation of 9.8 GHz. The hot simulation demonstrates the particles phase space behavior along the interaction circuit and the energy transfer phenomena. The present gyrotwystron develops an output power ~20 MW with an electronic efficiency of ~21% and a bandwidth of 10% for ideal electron beam velocity.

Particle-in-cell (PIC) simulation of a 250GHz gyrotron

Summary form only given: In this paper, the design and PIC simulation studies of a sub-millimetre wave, continuous wave (CW) gyrotron for dynamic nuclear polarization/nuclear magnetic resonance (DNP/NMR) spectroscopy applications is reported. The dynamic nuclear polarization could enhance the sensitivity of nuclear magnetic resonance which is a very impressive technique to enhance the spin spectra of biological samples. This particular application requires few tens of watts of CW power in terahertz and sub-millimetre wave regime. Therefore, one of the possible candidates is the fast wave gyrotron oscillator. To achieve better resolution in NMR spectroscopy one has to use higher magnetic field which in turn increase the cyclotron frequency. This ultimately increases the operating or resonant frequency of the device. In the present work, the gyrotron is designed at 250 GHz to operate in higher order TE52 mode, because at high frequency radius of interaction circuit will be less. The radius and length of interaction cavity are calculated as 2.01 mm and 23 mm respectively. The Eigen mode cold analysis is made to calculate the loaded quality factor (Q) as 1.15 × 104 at 249.91 GHz. The required magnetic field has been calculated as 9.1 Tesla. The designed gyrotron, including the RF cavity, down taper and up taper, is modelled and simulated in “CST Particle Studio” for observing the beam-wave interaction in it. In the present simulation ~ 107 Watts of peak power at the resonating frequency of 249.9 GHz with few GHz bandwidth has been achieved for the weakly relativistic gyrating electron beam of 12 kV, 100 mA. This type of gyrotron operating in sub-millimetre wave region has important applications in the research areas including physics, chemistry, biology, etc..

Design and simulation studies of a metal PBG cavity for millimetre wave gyrotrons

Summary form only given. The design and simulation studies of a metal Photonic Band Gap (PBG) cavity operating in TE32 mode for 140 GHz gyrotron are presented. The radius of cylindrical PBG cavity at 140Hz has been calculated as 2.73 mm. This is realized by making a defect in a regular periodic PBG lattice that is approximately equal to radius of the conventional waveguide for TE32 mode at 140 GHz frequency. The Eigen mode analysis for the removal of both seven and nineteen rods has been studied. The operating for the PBG cavity is located in the global band gap diagram which is obtained through the electromagnetic mode analysis of regular PBG structure. In the present work seven rods are removed to have the desired radius of the cavity in TE32 mode at 140 GHz. The periodicity and radius of the rod are calculated as 2.03mm and 0.79mm respectively. The designed metal PBG cavity is modelled and simulated using “CST Microwave Studio”. The reflection and propagation characteristics of the PBG cavity are studied at 140 GHz using the time domain solver. The reflection (S11) and transmission (S21) coefficients are obtained as -22 dB and -0.86 dB respectively at 140 GHz. The mode confinement inside the PBG cavity has been studied using the Eigen mode solver. We observed that the designed TE32 mode has confined at 139.58 GHz that ensures the mode purity at millimetre wave operation. Further, the quality factor of cavity operating in TE32 mode has been calculated as 8890. The quality factor of other nearby competing modes also has been observed. The nearby modes including TE23, and TE13 are well generated inside the defect at 174.16 GHz and 149.68 GHz respectively. The quality factors of PBG cavity is compared with the quality factor of simple cylindrical cavity for the desired and competing modes. It was observed that the quality factor of TE32 mode in PBG cavity is greater than other competing modes as compared to simple cylindrical cavity. The metal PBG structures offer good mode purity; hence the mode competition problem in gyrotron is eliminated.

Design and PIC simulation a stagger tuned gyro-twystron

In this paper, the design and particle simulation of a Ka-band stagger tuned gyro-twystron amplifier is presented. The concept of stagger tuning in gyro-twystron improves the bandwidth along with high RF output power which is suitable for radar application. The complete amplifier structure is modelled in “CST particle studio” for studying the beam wave interaction behaviour. The PIC simulation predicted a peak output power of ~ 135kW for the beam parameters of 80 kV, 3A with a velocity spread of 5%. The electronic efficiency has been obtained as 56%, and the saturated gain of 57 dB. The stagger technique enhanced the bandwidth by ~ 2% about the centre frequency.

Design and PIC simulation of a mega-watt class gyro-twystron

In this paper, the design and Particle-in-Cell (PIC) simulation of an X-band high power gyrotwystron amplifier operating in TE01 mode is presented for studying its stability and beam wave interaction behaviour. The device includes an input cavity, field free drift tube, and an output waveguide section which is loaded with a lossy dielectric to improve its stability. The complete amplifier is modelled in “CST particle studio” for studying the beam-wave interaction behaviour. The PIC simulation of the amplifier predicted ~ 20MW at 9.87 GHz in the fundamental TE01 mode for the relativistic gyrating electron beam of 440 kV, 240A with a velocity spread of 5%. The amplifier shows saturated gain of ~ 24 dB and an electronic efficiency of ~ 19%.

Analytical and PIC simulation studies of Ka-band gyro-twystron amplifier

In this paper, a Ka band gyro-twystron has been designed and analyzed using a self-consistent nonlinear theory and validated through a commercial 3-D Particle-in-Cell code. The numerical analysis predicted around 157 kW at 35 GHz with a saturated gain of ∼25dB and an electronic efficiency of ∼30%. Further, the device is modeled using CST Particle Studio for studying its 3D beam-wave interaction behavior. The PIC simulation predicated ∼150 kW of RF peak output power with the gain of 24.5 dB and a power conversion efficiency of ∼29%.