High Power Gyrotron Research Articles

Преобразование частоты излучения мощных гиротронов в условиях обратного рамановского рассеяния на дополнительном электронном пучке

AbstractThe regime of Raman backscattering of a pumping wave that is cocurrent with an electron beam into the counterpropagating wave can be used for the radiation frequency conversion in high-power gyrotrons. The absolute instability developing in this system ensures generation of a scattered signal in the absence of external resonators, which allows the frequency of scattered radiation to be smoothly controlled within 20–40% by varying the electron energy in the case of high-power (megawatt) gyrotrons used as sources of pumping radiation.

Developments towards 1MW Gyrotron Test Facility at ITER-India

ITER-India, the Indian domestic agency for the ITER project, has the responsibility to supply a set of two high power Gyrotron sources (1 MW, 170 GHz, 3600 s) along with the auxiliary systems for Electron Cyclotron Heating & Current Drive applications. For such high power Gyrotron systems, one of the challenging areas is the system integration and establishment of reliable integrated system performance. ITER-India plans to establish the integrated Gyrotron system performance that essentially meets the ITER requirements in a Gyrotron Test Facility which is specifically being developed at ITER-India. This paper discusses about the recent updates towards the Test Facility development which includes, development of cost effective & modular Body Power Supply (BPS), Industrial grade prototype interlock & protection modules, a Gyrotron field simulator and cooling water distribution system.

Improved Simulation of Quasi-Optical Launchers for High Power Gyrotrons with Smoothing Algorithm

Numerically optimized launchers for the quasi-optical output coupler of high power gyrotrons have been developed to provide RF beams with high Gaussian mode content. Generally, the profiles of numerically optimized launchers are quite complicated, so the improvement of their tolerance to fabrication errors is very important. In order to reduce the stray radiation generated by launchers with quite complicated wall profile, and also to reduce their sensitivity to fabrication errors, a method for the smoothing of the numerically optimized launcher wall has been developed at KIT, which is based on the spectrum reconstruction approach. As an example, a launcher designed for the KIT TE34,19-mode, 2 MW CW coaxial-cavity gyrotron has been investigated.

Overview on recent progress in magnetron injection gun theory and design for high power gyrotrons

The magnetron injection gun (MIG) is one of the most critical subcomponents in gyrotrons. The electron beam, which has the primary role on the gyrotron operation, is generated and configured at this part of the tube. The electron beam properties determine the excitation mode in the cavity, the power of the generated microwaves and the gyrotron efficiency. The operation of MIGs could be influenced by several factors such as trapped electrons, manufacturing tolerances, roughness of the emitter ring, emitter temperature inhomogeneity, electron beam neutralization effect, etc. The influence of many of these factors on the electron beam quality has been systematically investigated during the last years. Several novelties have been proposed in order to limit the influence of these factors on the gyrotron operation. In particular, new design criteria have been proposed for the suppression of electron trapping mechanisms, a new type of the emitter ring has been proposed to minimize the influence of the manufacturing tolerances and edge effects on the beam quality, alternative MIG design approaches have been proposed, etc. An overview of all these works will be presented here.

New trends of gyrotron development at KIT: An overview on recent investigations

Since many years KIT is strongly involved in the development of high power gyrotrons for use in ECRH. KIT is pursuing two development lines: (i) the conventional, hollow cavity gyrotron and (ii) the coaxial cavity gyrotron. KIT is pushing conventional cavity gyrotrons from 1 MW to 1.5 MW in a common project with IPP Greifswald. Coaxial cavity technology having the advantage of higher power capability, in particular at higher frequency, is used for the 2 MW 170 GHz short-pulse prototype. This gyrotron is currently being upgraded to allow pulse extension up to approximately 100 ms and up to 1 s in a second step.For a future DEMOnstration fusion power plant two challenging trends with respect to gyrotron features are recognized: (a) the operating frequency will be above 200 GHz and (b) the requested total efficiency of the gyrotron should be higher than 60%. KIT is addressing these requirements by investigating both gyrotron technologies for their performance at a frequency well above 200 GHz. We started careful analysis of multi-staged-depressed collectors.

Plasma-chemical processes under high-power gyrotron’s discharge in the mixtures of metal and dielectric powders

Synthesis of nano- and microstructured materials is a field of high activity in material science. Plasma-chemical processes are effective and versatile tools for this purpose. Plasma can be generated in different ways, in this work we use pulses of high-power gyrotron (75 GHz/550 kW) to initiate a discharge in mixtures of metal and dielectric powders. Depending on the starting composition of mixture it is possible to obtain new materials. It was shown earlier that MgAl2O4 can be prepared from Mg/Al2O3 mixture. Now we demonstrate that organic compounds can also be introduced into the mixture. Using melamine as a source of nitrogen we prepared aluminum oxynitrides (Al11O15N, Al27O39N) starting from Al/Al2O3. An addition of small amount of carborane C2B10H12 makes the process even more intensive, since in this case hydrogen is involved. Proposals on possible mechanism was made basing on observations during the process and composition and morphology of final products.

Frequency Tunable sub-THz Gyrotron for Direct Measurements of Positronium Hyperfine Structure

The feasibility of a high-power sub-THz gyrotron with smooth wideband frequency tuning suitable for direct measurement of the positronium hyperfine structure is demonstrated numerically using both averaged equations and PIC-code simulations. Analytical estimates show that the frequency-tunable powerful sub-THz radiation can be generated through the excitation of high-order axial modes in a gyrotron with short cavity driven by an electron beam with high current. Simulations show that an output power of 0.5–1 kW can be obtained at a frequency of about 0.2 THz within a 10-GHz band which are the parameters needed for testing of quantum electrodynamics predictions through the spectroscopy of positronium.

Numerical Investigation on Spent Beam Deceleration Schemes for Depressed Collector of a High-Power Gyrotron

Single-stage depressed collector systems are widely used in high-power gyrotrons in order to increase the overall efficiency. The position where the deceleration of the beam takes place influences the collector efficiency. In this paper, several deceleration schemes are numerically investigated for the short-pulse prototype of the 170-GHz, 1-MW European gyrotron for International Thermonuclear Experimental Reactor. The results of this paper are in a qualitative agreement with the results of dedicated experiments. A simple idea is also proposed for the significant increase of the overall gyrotron efficiency without an important technological modification.

Design and High-Power Test of an Internal Coupler to HE11 Mode in Corrugated Waveguide for High-Power Gyrotrons

Gyrotrons produce power in high-order, transverse electric modes that are typically converted to a Gaussian beam inside the tube. In fusion plasma heating applications, the power is transmitted through a diamond window to an external mirror optical unit where it is coupled into an HE11 mode in corrugated waveguide. A new coupler is described that couples the cavity RF power directly into corrugated waveguide inside the gyrotron. A high-power gyrotron with the improved coupler was tested using 1-MW, 3- $\mu \text{s}$ pulses and demonstrated equivalent power and efficiency as a traditional coupler with HE11 mode output in corrugated waveguide. In addition, higher HE11 mode purity was achieved at the gyrotron output.

High-Power Relativistic Milimeter-Wave Gyrotron Operating at the Second Cyclotron Harmonic

We present the results of the theoretical and experimental studies, which are aimed at developing pulsed gyrotrons with relativistic electron beams operating in the millimeter-wavelength range and retaining the multimegawatt power level. The use of the interaction at the second gyrofrequency harmonic, which is characterized by a sufficiently high efficiency and availability of the solution for the mode selection problem, allows one to double the generation frequency without increasing the magnetic field. In the corresponding experiment, the output radiation power exceeded 2 MW, and the efficiency was about 30% at a wavelength of 5.5 mm.

An Improved Diagnostic Device for Magnetron Injection Guns of High-Power Gyrotrons

The quality of the electron beam generated by a magnetron injection gun (MIG) is crucial for the performance of high-power high-frequency gyrotrons. The measurement of the electron properties would be very important for interpreting the experimental results and for improving the design of the gyrotron. Considering the increased power of the electron beam, this is a quite challenging procedure. In this paper, we study a diagnostic device based on the retarding field method for the measurement of the beam properties of an MIG. The influence of several factors on the measurement, such as the relativistic properties of the electrons, the magnetic field compression, and the voltage depression are theoretically and numerically investigated. In addition, the possibility to accurately extrapolate the properties of a high-power electron beam based on low-power beam measurements is studied in detail. In this way, an improved analytical methodology is proposed for the accurate estimation of beam quality from the experimental measurements.

Numerical Studies on the Influence of Cavity Thermal Expansion on the Performance of a High-Power Gyrotron

An iterative procedure is described, which models the influence of the thermal expansion of the gyrotron cavity on the expected gyrotron performance. It is a multiphysics simulation method, which involves electrodynamic, thermal-hydraulic, and thermo-mechanical simulations. The method is applied to the first European 170-GHz, 1-MW continuous wave prototype gyrotron for the ITER. According to the simulations, a performance reduction of ~15% is expected at nominal operating parameters, because of the thermal expansion of the cavity. Alternative operating points to mitigate this effect are proposed and numerically validated. The numerical results are discussed in light of experimental findings.

Design and Experiment of a High Power and Broadband Ku-Band TE11 Mode Gyro-TWT

The design and experiment of a high power and broadband Ku-band gyrotron traveling-wave tube (gyro-TWT) amplifier operating in the TE11 mode are presented in this paper. A new-type broadband rotating TE11 mode input coupler is applied to excite operating mode and a broadband three-disk beryllium oxide ceramic output window is used in the designed gyro-TWT. In order to suppress the potential oscillations, expand working bandwidth, and improve the heat dissipation capability of high frequency system, a periodic beryllium oxide and silicon carbide composite lossy ceramic rings loaded high-frequency circuit with a nonlinear small change output taper is adopted in the gyro-TWT. When the beam voltage is 70 kV and the beam current is 12 A, the experiment of this Ku-band gyro-TWT developed shows that the measured maximum output average power is ~20 kW at the frequency 16 GHz, corresponding to a maximum output pulse power of ~200 kW (10% duty ratio), a gain of 43.0 dB and a maximum efficiency of 23.8%, −3-dB saturation gain bandwidth of 2.6 GHz (~16.3%), and −1-dB saturation gain bandwidth of 1.5 GHz (~9.4%). Under the condition of 10-W constant input power driven, the measured −3-dB constant driven bandwidth is 1.4 GHz (~8.8%).

High-Field Liquid-State Dynamic Nuclear Polarization in Microliter Samples.

Nuclear hyperpolarization in the liquid state by dynamic nuclear polarization (DNP) has been of great interest because of its potential use in NMR spectroscopy of small samples of biological and chemical compounds in aqueous media. Liquid state DNP generally requires microwave resonators in order to generate an alternating magnetic field strong enough to saturate electron spins in the solution. As a consequence, the sample size is limited to dimensions of the order of the wavelength, and this restricts the sample volume to less than 100 nL for DNP at 9 T (∼260 GHz). We show here a new approach that overcomes this sample size limitation. Large saturation of electron spins was obtained with a high-power (∼150 W) gyrotron without microwave resonators. Since high power microwaves can cause serious dielectric heating in polar solutions, we designed a planar probe which effectively alleviates dielectric heating. A thin liquid sample of 100 μm of thickness is placed on a block of high thermal conductivity aluminum nitride, with a gold coating that serves both as a ground plane and as a heat sink. A meander or a coil were used for NMR. We performed 1H DNP at 9.2 T (∼260 GHz) and at room temperature with 10 μL of water, a volume that is more than 100× larger than reported so far. The 1H NMR signal is enhanced by a factor of about -10 with 70 W of microwave power. We also demonstrated the liquid state of 31P DNP in fluorobenzene containing triphenylphosphine and obtained an enhancement of ∼200.

Output System of A 220-/247.5-/275-GHz, 1.0-MW, Triple-Frequency Regime Gyrotron

In this paper, the design studies of an output system [comprising a nonlinear taper, a quasi-optical launcher (QOL), and an RF window] for 220-/247.5-/275-GHz, triple-frequency gyrotron are presented. The precisely chosen mode-triplet is TE39,14/TE43,16/TE47,18 to support adequate power transfer at their corresponding frequencies of operation. The nonlinear uptaper is optimized using our in-house code Gyrotron Design Suite version 4.0 (GDSv4.0 2016) [7] in order to reduce the mode conversion for all the frequencies. A common dimpled wall QOL is designed and simulated using commercially available launcher optimization tool. A chemical vapor-deposited-diamond RF window is recommended with a decent transmission bandwidth for all the operating frequencies. The knowledge of correct phase of the Gaussian-like beam coming out of the RF window of any high-power gyrotron is most important for scheming a correct matching optic unit. Hence, the theoretical phase of the beams at a window plane is retrieved using an iterative phase retrieval algorithm.

High-power sub-terahertz source with a record frequency stability at up to 1\u2009Hz

Many state-of-the-art fundamental and industrial projects need the use of terahertz radiation with high power and small linewidth. Gyrotrons as radiation sources provide the desired level of power in the sub-THz and THz frequency range, but have substantial free-running frequency fluctuations of the order of 10−4. Here, we demonstrate that the precise frequency stability of a high-power sub-THz gyrotron can be achieved by a phase-lock loop in the anode voltage control. The relative width of the frequency spectrum and the frequency stability obtained for a 0.263 THz/100 W gyrotron are 4 × 10−12 and 10−10, respectively, and these parameters are better than those demonstrated so far with high-power sources by almost three orders of magnitude. This approach confirms its potential for ultra-high precision spectroscopy, the development of sources with large-scale radiating apertures, and other new projects.

Transformation of High-Power Gyrotron Output Radiation Frequency under Conditions of Raman Scattering on Additional Electron Beam

Transformation of High-Power Gyrotron Output Radiation Frequency under Conditions of Raman Scattering on Additional Electron Beam

Synthesis of oxide and nitride ceramics in high-power gyrotron discharge

Synthesis of oxides, nitrides, and oxynitrides of silicon and aluminium by a pulsed microwave discharge in the mixtures of metal and dielectric powders is described. The microwave pulses were generated by high-power gyrotron (frequency 75 GHz, power up to 550 kW, pulse duration from 0.1 to 15ms). SiO2 + β-Si3N4 (1:1 by molar) and α-Al2O3 + AlN (2:1 by molar) mixtures with Mg (1 and 5wt%) were treated in air with microwave pulses with power of 250÷400 kW and duration of 2÷8 ms. It was found that the discharge cannot be initiated for both mixtures in absence of Mg at any pulse power and duration. When 1% of Mg was added, the discharge was observed for both mixtures under 8 ms pulses of 400 kW; however, the amounts of materials produced were not enough for analysis. With 5% of Mg the discharge was observed for both mixtures under 8 ms pulses of 350 kW, and products of the plasma-chemical processes in the Al2O3 + AlN mixture were analyzed.

Initiation of dusty structures in chain reactions under the action of gyrotron radiation on a mixture of metal and dielectric powders with an open boundary

A dusty plasma formed in chain exothermal reactions initiated by radiation of a high-power gyrotron in mixtures of metal and dielectric powders has been described. An oscillatory character of such chain reactions, as well as the appearance of dust particles at the first (explosive) stage, has been detected. The tracks, velocities, and sizes of dust particles have been measured. It has been revealed that ensembles of dust particles appear in a reactor after switching-off of the gyrotron against the background of development of chemical reactions. The time of existence of these ensembles is three or four orders of magnitude larger than the duration of a microwave radiation pulse. The quasistationary state of the low-temperature plasma with charged macroparticles appears because of both the chemical heating of the mixture in the reactor and thermophoresis. It has been shown that dust particles are necessary as crystallization nuclei for the creation (or deposition) of complex composites of nano- and micromaterials produced in secondary plasma chemical synthesis.

Strong yellow emission of high-conductivity bulk ZnO single crystals irradiated with high-power gyrotron beam

We report the strong yellow emission of bulk ZnO single crystals irradiated with the high-power gyrotron beam. Hydrothermally grown bulk crystals with high conductivity are irradiated at room temperature with up to 60-W output of a sub-terahertz gyrotron wave source. During gyrotron irradiation, the high-conductivity crystals exhibit intense emissions with a peak of around 2 eV (600 nm) and a longer-wavelength tail. The sample temperatures were also elevated from room temperature to above 1000 K by irradiation. However, when heated up to 1250 K using a heater without irradiation, the ZnO crystals do not exhibit similar visible emissions. We then use the generalized Planck's radiation in non-equilibrium states as an explanation of our experimental observations. The emission peak intensity can be enhanced by the gyrotron-induced non-equilibrium states, and the emission peak position can be related to the Urbach energy. With high intensities in the visible wavelengths, the emissions of the irradiated crystals can be readily observed with our bare eyes or with inexpensive digital cameras. As the spatial distribution of the yellow emission reflects the gyrotron beam pattern, the bulk ZnO single crystals can then be utilized for the quick diagnosis of gyrotron beam patterns and positions.