Development Of Gyrotrons Research Articles

Evaluation of a newly developed low reflection dummy load for high power millimeter waves

A low reflection dummy load (LRD) has been newly developed for gyrotron development and operation. This LRD is designed for 0.5 s of high power (∼1 MW) millimeter wave injection with a low reflection. In order to reduce the reflection, the LRD consists of an inner cylinder, a main chamber, and a conical mirror, which results in the efficient absorption by long pass length between the inner cylinder and the main chamber. The characterization of low reflection has been demonstrated by the water temperature rise of the DC break (DCB) section inside the gyrotron, which works as an indicator of the reflected stray millimeter wave from the dummy load (DL). The water temperature rise of DCB with LRD was decreased by 1%, 15% and 17% at three tested frequencies of 82 GHz, 110 GHz and 138 GHz, respectively, compared with the conventional DLs, and the power absorption of LRD is 4–7% larger than the conventional DLs. A thermal analysis by a finite element method (FEM) with experimentally obtained heat load distribution and cooling time constant showed that the maximum temperature at 0.5 MJ injection is below the melting point of the material but it is not acceptable for the cycle operation considering the allowable stress of the aluminum-alloy used in the present design. FEM analysis suggested that changing its material to CuCrZr enable to use for 0.5 MJ injection. The result of FEM analysis also shows that the CW injection with less than 0.15 MW is acceptable in the present design.

Experimental Studies of Microwave Tubes with Components of Electron–Optical and Electrodynamic Systems Implemented Using Novel 3D Additive Technology

Novel additive technology of the Chemical Metallization of Photopolymer-based Structures (CMPS) is under active elaboration currently at the IAP RAS (Nizhny Novgorod). The use of this technology has made it possible to implement components of electron–optical and electrodynamic systems for high-power microwave vacuum tubes, such as a gyrotron and a relativistic Cherenkov maser, the design and experimental studies of which are described in this paper. Within the framework of the gyrotron developments, we carried out a simulation of the distribution of the heat load on the collector of high-power technological gyrotron taking into account secondary emission. The prospect of a significant reduction in the maximum power density of the deposited electron beam was shown. The experimental study of the gyrotron collector module manufactured using CMPS technology demonstrated high potential for its further implementation. Recent results of theoretical and experimental studies of a spatially extended Ka-band Cherenkov maser are presented. In this oscillator, the 2D-periodical slow-wave structure made by the proposed technology was applied and a narrow-band generation regime was observed with a sub-GW power level. The design and simulations of a novel selective electrodynamic system for a high-harmonic gyrotron with the planned application of the CMPS technology are discussed.

Synthesis of two quasi-optical polarizers for the multi-frequency high-power millimeter wave system.

Millimeter wave systems based on powerful gyrotron can deliver megawatt microwave power, which is an important auxiliary heating method for magnetic confinement fusion devices. Reflective gratings are normally used as quasi-optical polarizers for polarization control of the wave in such systems. Based on the coordinate transformation method, we developed a numerical code to study the broadband polarization strategy. By considering the synthesis of two polarizers, we designed a pair of polarizers in the W band. Calculation results indicated that almost arbitrary polarization can be achieved in a wide frequency range. To verify the design, we set up a low-power test platform and experiments were carried out. The performance of the polarizers was tested from 80 to 105GHz with a step of 5GHz. The test results agreed well with the numerical results, indicating that the design is reasonable. With the development of multi-frequency gyrotrons, the study in this paper can be used as a reference for the design of broadband polarizers of multi-frequency millimeter wave systems.

Development of High-Current Relativistic Gyrotrons with the Operating TM Mode

Development of High-Current Relativistic Gyrotrons with the Operating TM Mode

Development of a high-harmonics gyrotron based on large-orbit gun

Development of a high-harmonics gyrotron based on large-orbit gun

Development of High-Current Relativistic Gyrotrons With the Operating TM Mode

Development of High-Current Relativistic Gyrotrons With the Operating TM Mode

Parasitic excitation of fundamental-cyclotron-harmonic waves in high-harmonic gyrotrons

A typical problem of the development of high-harmonic short-wavelength gyrotrons, namely, the competition between the operating wave and a parasitic fundamental-harmonic wave, is studied. A specific case of the excitation of a far-from-cutoff parasitic wave in the backward-wave oscillator regime is considered.

Gyrotron Development at High Order Modes

The present work has been characterized by higher order modes in the cavities of the Gyrotron; they are capable of producing RF plasma by developments of it. It uses for fusion systems. We choose the TE31,8 mode in our study. The main problem of gyrotron is the device of the thermal cavity loading. The problem of the thermal loading is solved when any parasitic modes suppress, absence of desired modes; the thermal loading is increased when the high power tube of gyrotron operation is unstable. The mathematical interaction model contains equations that describe the electron motion and the field profiles of the transferred electric modes of the resonator, these are interacting with electrons based on the finite difference method that has been designed to study the starting current, the frequency, quality factor and calculates the roots of the Bessel function by the program we designed in Fortran language. They are used to calculate the operation frequency. Good agreement is between our results and the previous published results both confirm the accuracy of the performance of the designed program.

Calibration of the KIT test setup for the cooling tests of a gyrotron cavity full-size mock-up equipped with mini-channels

In high-power fusion gyrotrons, the maximum heat-load on the wall of the interaction section is in the order of 2 kW/cm2, which is the major limiting technological factor for output power and pulse-length of the tube. The ongoing gyrotron development demands a very effective cavity cooling system for optimum gyrotron operation. In this work, the experimental investigation of a mini-channel cavity cooling using a mock-up test set-up is described. The mock-up test set-up will be used to experimentally validate the predictive simulation results and verify the mini-channel cooling performance. It is crucial for validation of the mini-channel cooling properties to determine the amount of the heat load introduced in the cavity wall by an induction heater. In order to estimate that heat load, full 3D electromagnetic simulations have been performed using the CST Studio Suite® software. A suitable calibration factor for the load deposited in the mock-up inner wall is identified after numerical investigation by a 3D thermal model. Calorimetry measurements are performed and the experimental results are compared with the simulation results obtained with a 3D thermal-hydraulic model, using the commercial software STAR-CCM+. When the calibration factor is applied, the experimental calorimetry is well reproduced by the simulations.

The Nonlinear Designs on a Frequency-Tunable THz Gyrotron With Three Frequency Regimes at 140, 250, and 263 GHz

Gyrotron, as a most likely practical high-power terahertz (THz) radiation source, has been widely applied to THz dynamic nuclear polarization-enhanced nuclear magnetic resonance (DNP-NMR) spectroscopy. In this article, an iris cavity with high-quality factor is theoretically designed for a frequency-tunable THz gyrotron, which could be tuned at three frequency regimes: 140, 250, and 263 GHz, respectively. Based on the self-developed nonlinear code, the beam wave interactions in the designed cavity are studied in detail when the corresponding high-order axial modes (HOAMs) are ${\mathrm {TE}}_{-\mathrm {6.1.q}}$ , ${\mathrm {TE}}_{\mathrm {0.4.}\mathrm {q}^{\prime }}$ , and ${\mathrm {TE}}_{\mathrm {8.2.q}''}$ . By tuning the applied magnetic field under the condition that the beam voltage U0 is 20 kV and beam current I0 is 0.10 A, the output power and frequency tuning ranges could achieve 0.75 kW and 1.15 GHz, 0.63 kW and 3.10 GHz, and 0.59 kW and 2.54 GHz. Meanwhile the calculation results are compared with the 3-D particle software CST, which have confirmed that the frequency and power tuning ranges of ${{\mathrm {TE}}}_{-\mathrm {6.1.q}}$ , ${\mathrm {TE}}_{\mathrm {0.4.q}'}$ , and ${\mathrm {TE}}_{\mathrm {8.2.q''}}$ could reach 0.46 kW and 1.07 GHz, 0.37 kW and 2.24 GHz, and 0.34 kW and 1.96 GHz. These results could be beneficial for the development of frequency-tunable gyrotron toward some up-and-coming novel THz applications.

Towards a 1.5 MW, 140 GHz gyrotron for the upgraded ECRH system at W7-X

• 1.5 MW gyrotron development for the ECRH system upgrade at the stellarator W7-X. • Numerically verified complete design of the 1.5 MW 140 GHz CW gyrotron. • Low-power tests of the gyrotron quasi-optical output coupler for theTE 28,10 mode. For the required upgrades of the Electron Cyclotron Resonance Heating system at the stellarator Wendelstein 7-X, the development of a 1.5 MW 140 GHz Continuous Wave (CW) prototype gyrotron has started. KIT has been responsible to deliver the scientific design of the tube (i.e. the electron optics design and the RF design), with contributions from NKUA and IPP. The prototype gyrotron has been ordered at the industrial partner, Thales, France, and is expected to be delivered in 2021. In parallel, a short-pulse pre-prototype gyrotron has been developed at KIT, to provide the means for a first experimental validation of the scientific design in ms pulses, prior to the construction of the CW prototype. This paper reports on the status of the 1.5 MW CW gyrotron development, focusing on the scientific design and its numerical and experimental validation.

Russian Gyrotrons: Achievements and Trends

The last decade has contributed to the rapid progress in the gyrotron development. Megawatt-class, continuous wave gyrotrons are employed as high-power millimeter (mm)-wave sources for electron cyclotron heating (ECH) and current drive in the tokamaks and stellarators. The progress in gyrotron development pushes ECH from a minor to a major heating method. Also gyrotron based technological complexes successfully applied in electron cyclotron resonance ion sources, for microwave ceramic sintering and diamond disk production. The paper describes the main features of high frequency gyrotrons. Some data about pulsed and CW tubes, working in the terahertz frequency range, are given. These gyrotrons operate (in some specific combinations) at very low voltage and beam current, demonstrate an extremely narrow frequency spectrum or wide frequency tuning. Although in comparison with the classical microwave tubes the gyrotrons are characterized by greater volume and weight due to the presence of bulky parts (such as superconducting magnets and massive collectors where the energy of the spent electron beam is dissipated) they can easily be embedded in a sophisticated laboratory equipment (e.g., spectrometers, technological systems, etc.). All these advantageous features have opened the road to many novel and prospective applications of gyrotrons.

Magnetron-Injection Gun with Increased Current for Frequency Tunable Medium Power Sub-THz Gyrotron

A magnetron-injection gun able to provide very high ratio tj≈ 0.5 of the beam current to the Langmuir current has been designed, while maintaining good enough quality and stability of a formed helical electron beam. The specific feature of the gun is the shape of the cathode with two singular points of the electric field distribution which ensure close to uniform electric field profile within the emitter ring for wide range of operating currents. The gun provides the smoothed dependence of the beam parameters for currents up to 1.5–2 A and is suitable for wide (~ 10 GHz) frequency range gyrotron operation with the central frequency of 0.2 THz and output power ~ 1 kW within the whole tuning band. Such approach for the magnetron-injection gun design can be used for development of other promising high power sub-THz gyrotrons.

Experimental Study of Multi-Mode Dynamics of THz-Band Pulsed Magnetic Field Gyrotron

The multi-mode dynamics of pulsed magnetic field gyrotron due to excitation of different modes at the fronts of high voltage pulse was investigated. Detailed and accurate measurements of start-up scenario and excitation of the operating and spurious modes in the 0.5 THz pulsed magnetic field gyrotron were made for the first time. The performed numerical simulations of multi-mode dynamics of the gyrotron showed a good qualitative agreement with the experimental results. The presented information looks useful for future development of high frequency pulsed gyrotrons.

Theoretical and Experimental Investigations of Terahertz-Range Gyrotrons with Frequency and Spectrum Control

We present the results of recent IAP RAS investigations, which were aimed at controlling the radiation frequency and spectrum of THz-range medium-power gyrotrons. Problems of extending the smooth frequency tuning band and providing high-frequency stability in CW regimes are discussed. The development of gyrotrons with stated parameters are of interest for many modern applications, including DNP/NMR and RAD spectroscopy, direct measurements of positronium hyperfine structure, and diagnostic of various media.

Nonadiabatic Effects on Beam-Quality Parameters for Frequency-Tunable Gyrotrons

We propose an unconventional electron gun structure in which the emitter is located on a concave cathode surface with a non-uniform electric field. Such a design violates the intuition that an emitter should place close to a uniform electric field to reduce the velocity spread. The commonly employed design guide based on the adiabatic condition predicts a huge velocity spread of 24%, but the simulation using EGUN code and verified with CST particle studio shows a very low spread of 2.8%. Examining the magnetic moment and the kinetic energy of the beam reveals that the electrons experience a relatively long acceleration process due to the much weak electric field. That's why the non-adiabatic effect matters. In addition to the cyclotron compression and the E$\times$B drift, the "resonant" polarization drift plays a crucial role in reducing the overall velocity spread.Simulations show a decent beam quality with the pitch factor of 1.5 and the transverse velocity spread of 2.8% over a wide range of the magnetic field (7.4-8.0 T) and the beam voltage (12-22 kV) with a high structural tolerance. The promising results with the wide working range enable the development of continuous frequency-tunable gyrotrons.

To the Theory of Gyrotrons with Wide Emitters

The main trends in gyrotron development are escalation of the radiated power and increasing the frequency of coherent radiation. For both trends, it is beneficial to develop gyrotrons with wide emitters because this allows one to use cryomagnets with smaller inner bore sizes. For analyzing and optimizing the operation of gyrotrons with wide emitters, it is proposed to represent such emitters as a superposition of thin rings and analyze the properties of electron beams emitted by each of these rings. The present paper consists of two parts. In the first part, the peak values of the orbital velocities and their spread are determined in all fractions of an electron beam in a gyrotron with the standard and widened emitters; also, the effect of profiling the anode on characteristics of these electron beam fractions is considered. In the second part, the interaction efficiency of electron beams produced by thin emitter rings is described and the relationship between these efficiencies and orbital-to-axial velocity ratios in these beams is discussed.

Development of a High-Power Continuous-Wave Gyrotron for High-Efficiency Technological K-Band Microwave Complexes

We analyze the possibility to create new generation continuous-wave gyrotrons operating in the Kband (at frequencies of 28-30 GHz) at the fundamental cyclotron resonance with an output power of 25–30 kW and more. For a gyrotron with a magnetically shielded solenoid, the electron-optical and electrodynamic systems have been optimized, which ensures generation with high efficiency (about 50%) at acceptable thermal loads on the key device elements. The proposed design of the gyrotron ensures long-term continuous-wave operation of the technological complex with a total system efficiency of about 35%, which is significantly superior to the existing analogs.

Development of the first ITER gyrotron in QST**The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

This paper presents a summary of recent progress pertaining to the manufacturing and inspection of ITER gyrotrons and the operation system in QST. Major achievements are as follows. (i) The final design of the ITER gyrotron was accomplished and manufacturing of two ITER gyrotrons was completed. Then operation test in QST prior the shipment to ITER has started with ITER relevant high voltage power supply configuration. The 1st ITER gyrotron has achieved 1.05 MW operation for 300 s with 51% efficiency. Measured cooling channel waveform of 300 s pulse demonstrated thermally stable condition representing sufficient cooling performance for 1 MW CW operation; (ii) 5 kHz modulation operation was demonstrated up to 200 s at with >0.8 MW at flat top of pulses; (iii) 300 s/1 MW operation was repeated for 20 shots with successful 19 shots which demonstrating >95% of reliability requirement. These results lead to success of ITER EC H&CD system construction and commissioning toward first plasma.

Experiment of a High-Power Sub-THz Gyrotron Operating in High-Order Axial Modes

To explore the approach for generating high-power frequency-tunable subterahertz (sub-THz) to THz radiation, a 220-GHz high-order axial modes (HOAMs) gyrotron is developed, and the proof-of-principle experiment is presented in this paper. Experimental results demonstrate that driven by an electron beam of high voltage and high current, HOAMs are successfully excited in a long cylindrical cavity. By tuning the operating magnetic field, a mode transition between the first four axial modes was experimentally observed with a stepwise frequency tuning range of 0.79 GHz around 219 GHz, while a 0.58-GHz frequency tuning range via beam voltage tuning by operating in the first three axial modes. During the process of frequency tuning, the output power was kept not less than 0.45 kW, while the maximum output power was 3.80 kW with an output efficiency of 6%. These experimental results of mode transition and mode competition between axial modes in high-power millimeter-wave gyrotron could be beneficial for the development of frequency-tunable gyrotron toward some up-and-coming novel THz applications.