GHz Gyrotron Research Articles

Progress using collective Thomson scattering of microwave radiation to detect fast ions and plasma instabilities at the GDT open magnetic trap

For the big mirror magnetic trap (Gas-Dynamic Trap, Budker Institute, Russia), a system for recording of collective Thomson scattering spectra of microwave radiation has been developed to focus on the study of velocity distribution function of fast ions. A diagnostic complex includes a high-power 450 kW/54.5 GHz gyrotron as a source of probing radiation, two independent highly sensitive radiometers operating in the range of 54.47 ± 0.55 GHz for simultaneous registration of scattered radiation in two orthogonal geometries, and quasi-optical systems for focusing the probing and diagnostic microwave beams. We report the results of the last experimental campaign of 2023 with plasma heating by neutral beams, in which the scattering signals from the fast ions have been detected accompanied by high-frequency instability of plasma.

Divertor Tokamak Test facility project: status of design and implementation

Abstract An overview is presented of the progress since 2021 in the construction and scientific programme preparation of the Divertor Tokamak Test (DTT) facility. Licensing for building construction has been granted at the end of 2021. Licensing for Cat. A radiologic source has been also granted in 2022. The construction of the toroidal field magnet system is progressing. The prototype of the 170GHz gyrotron has been produced and it is now under test on the FALCON facility. The design of the vacuum vessel, the poloidal field coils and the civil infrastructures has been completed. The shape of the first DTT divertor has been agreed with EUROfusion to test different plasma and exhaust scenarios: single null, double null, XD divertor and negative triangularity plasmas. A detailed research plan is being elaborated with the involvement of the EUROfusion laboratories.

Experimental 35 GHz Gyrotron Efficiency Enhancement with Magnetic Field Taper in Fundamental Harmonic

Experimental 35 GHz Gyrotron Efficiency Enhancement with Magnetic Field Taper in Fundamental Harmonic

Resonant Ring with a Gain of 36 for Use with a 1 MW 110 GHz Gyrotron

A 110 GHz quasi-optical ring resonator, designed for use with a 1 MW pulsed gyrotron, has been built and successfully tested using a 100 mW solid-state source. A low reflectance (2.4%) input coupler and a low-loss, four-mirror ring demonstrated a compression ratio, defined as the ratio of output to input power, of 36. The 6 ns output pulses were generated from the 2 m length ring using a silicon laser-driven semiconductor switch (LDSS). The quasi-optical ring resonator was designed with large waist sizes so that input pulses of up to 1 MW will stay under the 35 kV/cm electric field limit for ionization in ambient air. Maximum compression gain was achieved by matching the input coupling fraction to the round trip loss in the ring, achieving close to critical coupling. The experimental output pulse shape obtained after firing the LDSS was modeled using the reflectance, transmittance, and absorptance of the switch vs. time and vs. laser pulse fluence, with good agreement found with theory. The timing for the peak energy efficiency of 32% was found and the main loss mechanism limiting that efficiency was found to be the absorptance in the silicon wafer.

Upgradation of 82.6GHz ECRH system for SST-1 and Aditya-U

In SST-1 and Aditya-U Tokamaks, two ECRH systems (42GHz-500kW and 82.6GHz-200k) are used to carry out various experiments related to ECRH assisted plasma start-up and heating at fundamental and second harmonic. The SST-1 tokamak is operated at 1.5T magnetic field and 42GHz ECRH system has been extensively for plasma start-up and heating. The 82.6GHz ECRH could not be used thoroughly due limitation in power as 200kW power is not enough for breakdown in SST-1 at second harmonic. The original 82.6GHz-200kW ECRH system is being upgraded in terms of ECRH power to carry out experiment at higher power. The 82.6GHz-200kW Gyrotron has been upgraded to 400kW power. The Gyrotron has been tested successfully at factory and delivers 400kW power at 48kV cathode voltage and draw around 20A cathode current, the efficiency of Gyrotron is around 42%. The 82.6GHz Gyrotron will be commissioned at IPR and the total ECRH power 0.9MW (0.5MW at 42GHz and 0.4MW at 82.6GHz) will facilitate to carry out ECR heating experiments simultaneously at fundamental and second harmonic. The paper presents the recent test results of 82.6GHz-400kW Gyrotron and discusses the experimental plan with upgraded ECRH system.

Results on the 1 MW CW 170 GHz gyrotron TH1509UA for ITER and DTT at the FALCON test stand

In this contribution, the tests of the pre-series gyrotron TH1509UA for the Divertor Tokamak Test facility (DTT) at the FALCON test facility are presented. This versatile test bed proves useful for testing continuous wave (CW) high-power gyrotrons, but also serves as a platform for testing components for the transmission line or the Upper Launcher of ITER and DTT. The gyrotron has demonstrated a power level of 1.02 MW at the gyrotron output window, corresponding to 980 kW at the output of the Matching Optics Unit (MOU) with a power variation during the pulse of < 2% after a stabilisation period. Additionally, an efficiency of 40% has been demonstrated during five consecutive 100 s pulses. Compared to the previous version, TH1509U, this gyrotron demonstrates the successful prevention of parasitic mode excitation over a wide range of parameters around the design operating point. The potential for even higher power performance has been shown in short pulses but not explored in long pulses yet, which instead focused on demonstrating compliance with the required specifications for the DTT project.

Compression of 20kW 170 GHz Gyrotron Output Radiation by Quasi-Optical Resonator with Laser Activated GaAs Switch

Compression of 20kW 170 GHz Gyrotron Output Radiation by Quasi-Optical Resonator with Laser Activated GaAs Switch

Design of the Collective Thomson Scattering System on HL-3 tokamak

A collective Thomson scattering (CTS) diagnostic system is being developed to measure fastion velocity distribution on HL-3 tokamak. A 140 GHz gyrotron belonging to electron cyclotron resonance heating (ECRH) system would be used to generate probe beam. The scattering spectra among HL-3 parameter ranges are calculated to assess diagnostic feasibility. Scattering signals will be detected by a heterodyne system, which is proposed to optimize diagnostic performance.

Adaptive design of the magnetron injection gun using constrained multiobjective optimization

The design of the magnetron injection gun (MIG) is an inverse problem with implicit objective functions and complex constraints. The computer-aided design using an optimization algorithm is a promising method to improve the performance of the MIG and reduce design complexity and time consumption. However, the dependence on the initial structure makes it difficult to realize the real sense of using algorithms to design. To address this issue, this paper proposes an adaptive design method for the MIG. The design problem is equivalent to a constrained multiobjective optimization problem. A pseudo-random structure generation algorithm is developed based on the geometric constraints and the first-order MIG design trade-off equations. The fully parameterized MIG structure is randomly generated by the decision variables composed of the scale coefficients ranging from (0,1) and the other structural parameters. This transformation can increase the effectiveness of the randomly generated MIG structures and make it easier to set the value range of decision variables. The proposed adaptive design method is further verified by successfully designing the MIG for an 800 GHz gyrotron via the co-simulation of MATLAB and CST.

Instability Entanglement in the Electron Cyclotron Maser

AbstractInvestigation of absolute instability (AI) and convective instability (CI) of the electron cyclotron maser has independently led to the birth of gyrotron oscillators and amplifiers. Here, it is demonstrated that these instabilities can form a cooperative relationship owing to the nonlinear behavior of the stimulated electron beam. The CI can be induced in a zero‐drive system with the assistance of AI, and the ohmic losses of all the excited waves inside the system are greatly reduced, which is called “instability entanglement” here. According to the theoretical and experimental study of a 167/330 GHz gyrotron, when instability entanglement occurs, the ohmic dissipation decreases to one‐ninth of the AI‐only condition, and the output power is enhanced by 20%. This discovery is promising for surpassing the ceiling of output power and frequency of gyrodevices placed by ohmic losses.

Progress on the conceptual design of the antennas for the DTT ECRH system

One of the main goals of the Divertor Tokamak Test (DTT) facility is to reach a ratio of power crossing the separatrix over the major radius of about 15 MW m − 1, as the one expected in DEMO. For this purpose, up to 45 MW of external additional heating power shall be coupled to the plasma, provided by Electron Cyclotron Resonance Heating (ECRH), Ion Cyclotron Resonance Heating and Neutral Beam Injection. The foreseen total ECRH power installed at full power shall be 32 MW, generated using 1 MW/170 GHz gyrotrons, for 100 s. The present ECRH system foresees two launching antennas per DTT sector, based on the front-steering concept. The equatorial antenna is dedicated to plasma heating and current drive and the upper antenna to the stabilization of MHD instabilities. This paper focuses on the latest design concept for these two antennas and on the definition of the ex-vessel matching optics unit of the last section of the evacuated Transmission Line (TL). The design has been updated to be compatible with the insertion of CVD diamond windows, to separate the vacuum environment of DTT from the one of the TL. This choice requires adding corrugated waveguide sections between the last mirror of the TL and the first mirror inside the port, requiring some adaptation of the in-vessel optics and of the supporting structure. The possibility to modify the steering range for the launching mirror has been also investigated to be compatible with the new design of the first wall and for the upper antenna, to reach the q = 2 surface in the new plasma scenario.

Design and experimental results of a 28 GHz, 400 kW gyrotron for electron cyclotron resonance heating

A high-power 28 GHz gyrotron has been successfully developed at the Institute of Applied Electronics, China Academy of Engineering Physics. This gyrotron was designed for electron cyclotron resonance heating (ECRH) in the spherical tokamak XL-50. A diode magnetron injection gun was designed to produce the required gyrating electron beam. The gyrotron operates in the TE8,3 mode in a cylindrical open cavity. An internal quasi-optical mode converter was designed to convert the operating mode into a fundamental Gaussian wave beam and separate the spent electron beam from the outgoing microwave power. A tube has been built and successfully tested. The operational frequency of the tube is 28.1 GHz. For beam parameters at an accelerating voltage of 71 kV and beam current of 16 A, the gyrotron has delivered an output power of 400 kW, with a pulse length of 5 s. The output efficiency is about 50% with a single-stage depressed collector. The gyrotron has been installed on the XL-50 and has played an important role in the ECRH experiments.

Control and Manipulation of Microwave Polarization and Power of a Frequency-Agile 198 GHz Gyrotron for Magnetic Resonance

The measurement and manipulation of the microwave polarization emitted from a frequency-agile 198 GHz gyrotron for dynamic nuclear polarization (DNP) are demonstrated. In general, gyrotrons emit linearly polarized radiation, yet in this case elliptical polarization is observed from the 198 GHz gyrotron window. Indeed, half of the microwave power is circularly polarized while the other half is linearly polarized with a polarization of 60∘ with respect to the horizontal plane. For optimal use of microwave power for DNP experiments, the elliptical polarization from the gyrotron is converted into circular polarization with a Martin-Puplett interferometer (MPI). The dependence of the DNP enhancement on the microwave polarization was investigated by modifying the microwave polarization with the MPI. In addition, the MPI can generate a linearly polarized beam, which holds promise for future development of induction-mode electron spin detected experiments.

Copper coil magnetic system study in a 60 GHz gyrotron design

This paper presents the study of a gyrotron magnetic system with a field intensity of 2.45 T and a triode magnetron injection gun system to achieve electron trajectories into the cavity operating zone to generate a 60 GHz radiofrequency used in electron cyclotron resonance heating applications. The operating mode of the gyrotron is TE4,3. A copper coil system arrangement of four solenoids with a stationary current of 750 A and a magnetic compression of 16.53 with a magnetic field homogeneity of 0.10% over a length of 56 mm in the operating zone was designed. A very good approximation of the magnetic field profile of the copper coil system was obtained using a novel formulation derived from the geometry of the coils. The magnetron injection gun parameters were obtained applying the trade-off equations, resulting in a cathode radius of 13.5 mm. The helicoidal and laminar trajectories of the electrons with a transverse-velocity of 1.175 are obtained from the magnetic and gun systems. The resulting trajectories are generated using the 2-D EGUN code. The study represents an effort to create an Experimental Theoretical Fusion Platform of the High Magnetic Field Program at the Autonomous University of Nuevo León (UANL).

High power mm-wave loss measurements of ITER ex-vessel waveguide components at the FALCON test facility at the Swiss Plasma Center

Many future fusion devices will rely heavily, if not solely, on electron cyclotron (EC) heating subsystems to provide bulk heating, instability control (neoclassical tearing mode (NTM) stabilization), and thermal instability control. Efficient use of the installed heating power (gyrotrons) requires low-loss transmission of the power over 100s of meters since the mm-wave sources need to be installed where the stray magnetic field has a small amplitude. Transmission lines are used to propagate the mm-wave power over this long distance. Quasi-optical techniques (mirrors) are used at W7X and are planned for DTT, for example. Guided components are installed at DIII-D, TCV and elsewhere and are planned at JT60SA and ITER. High power test facilities exist to evaluate the power transmission of assemblies of guided components (transmission lines). The European test facility FALCON was setup by Switzerland and Fusion for Energy (F4E) in Lausanne Switzerland at the Swiss Plasma Center (SPC) in the Ecole Polytechnique Fédérale de Lausanne (EPFL). Operations are funded through a framework contract with F4E. SPC operates the facility. Two ITER-class 170GHz gyrotrons are housed within the facility and used to evaluate the thermal behaviour of components provided by various ITER partners. Loss measurements are presented for miter bends and waveguides of several materials at two different diameters. The results are used to model the expected losses in the ITER ex-vessel waveguides (EW) of all five EC launchers.

Rapid microwave sintering of functional electroceramic materials

Rapid microwave sintering processes with heating rates of up to 300 °C/min and zero isothermal hold have been implemented using a 5 kW 24 GHz gyrotron system for high-temperature microwave processing of materials. ZnO-based varistor ceramics, BaTiO3/SrTiO3 dielectric ceramics and Gd:CeO2 ceramics for solid electrolyte applications have been sintered to densities of up to 96% of the theoretical value. Using in situ optical dilatometry, correlation between the development of thermal instability under intense volumetric microwave heating and the early onset of densification has been revealed. The influence of the absorbed microwave power on densification and grain growth has been studied by comparing direct and susceptor-assisted microwave heating processes. The possibility of tailoring the microstructure and functional properties of the obtained materials by choosing optimal regimes of rapid microwave sintering is discussed.

ITER collective Thomson scattering-Preparing to diagnose fusion-born alpha particles (invited).

The ITER Collective Thomson scattering (CTS) diagnostic will measure the dynamics of fusion-born alpha particles in the burning ITER plasma by scattering a 1MW 60GHz gyrotron beam off fast-ion induced fluctuations in the plasma. The diagnostic will have seven measurement volumes across the ITER cross section and will resolve the alpha particle energies in the range from 300keV to 3.5MeV; importantly, the CTS diagnostic is the only diagnostic capable of measuring confined alpha particles for energies below ∼1.7MeV and will also be sensitive to the other fast-ion populations. The temporal resolution is 100ms, allowing the capture of dynamics on that timescale, and the typical spatial resolution is 10-50cm. The development and design of the in-vessel and primary parts of the CTS diagnostic has been completed. This marks the beginning of a new phase of preparation to maximize the scientific benefit of the diagnostic, e.g., by investigating the capability to contribute to the determination of the fuel-ion ratio and the bulk ion temperature as well as integrating data analysis with other fast-ion and bulk-ion diagnostics.

Multi-harmonic electron cyclotron heating and current drive scenarios for non-inductive start-up and ramp-up in high field ST-40 spherical tokamak

Non-inductive start-up and ramp-up is an important topic for spherical tokamak reactor design as the central solenoid implementation is highly restrictive particularly for the low-aspect-ratio tokamak configuration. In the high field spherical tokamak (ST), ST-40 with B T0 ⩽ 3 T, a preparation is underway for high power ECH and ECCD current start-up/ramp-up experiments utilizing two MW-class 140/105 GHz gyrotrons. Here, we explored various ECH/ECCD scenarios for a low-field-side (LFS) launch-angle steerable waveguide launcher placed near the mid-plane region. Due to the large toroidal field variation of ST configuration, multiple cyclotron harmonic resonance layers could exist within the plasma. In this start-up and ramp-up regime, both fundamental and second harmonic ECH resonances must be considered. We find that even with the presence of X-II resonance layer in the plasma, an efficient X-I ECH and ECCD regime can be accessed for the low electron temperature T e0 as low as 200 eV which is a typical starting temperature of ECH heated plasmas in an open-field-line configuration. The presence of X-II resonance could become significant at higher T e0 as X-II absorption increases with T e0 which could reduce the current ramp-up efficiency as the power reaching X-I is reduced. Finally for the pure X-I regime where the 2Ωe resonance is moved outside the plasma with B T0 ∼ 3.4 T, we find that it is possible to reach the full current of I p ∼ 1 MA fully non-inductively with the ECH power of ∼1 MW at n e0 ∼ 1.0 × 1019 m−3 using 105 GHz frequency gyrotron. By reducing the outer limiter position R L ∼ 78 cm to 70 cm, the pure X-I regime is recovered at the rated ST-40 magnetic field of B T0 ∼ 3.0 T. This X-I regime is accessible with a relatively broad range of launched n II or the launching angles. A survey of X-mode X-II ECH and ECCD at higher density regimes is also shown for completeness.

Optimized Design of a 170-GHz Gyrotron Magnet With Thermally Insulated Structures

An electron cyclotron is used to heat plasma by driving current in a fusion system. This system requires a strong and accurate magnetic field distribution for the interaction between the electron beam and the radio frequency wave energy [1]. A superconducting magnet is designed for a 170 GHz gyrotron system here. The magnet is composed of conduction cooled NbTi coils, a GM cryocooler with thermal links, supporters, and a cryostat. The magnet was designed using an optimization method that considered the normal stresses generated by electromagnetic forces, the critical current margin, the axial field profile, and field uniformity and losses under AC operation. A quench analysis was also carried out to ensure the safe operation of the gyrotron magnet. The inner bore of the designed superconducting coil is 200 mm to obtain a proper resonance region. A bobbin and structures for thermal insulation were also designed considering the heat transfer and cooling capacities of the cryocooler. The gyrotron magnet will be manufactured using an automatic winding machine based on these design results.

Solenoid-free current drive via ECRH in EXL-50 spherical torus plasmas

As a new spherical tokamak designed to simplify the engineering requirements of a possible future fusion power source, the EXL-50 experiment features a low aspect ratio vacuum vessel (VV), encircling a central post assembly containing the toroidal field coil conductors without a central solenoid. Multiple electron cyclotron resonance heating (ECRH) resonances are located within the VV to improve current drive effectiveness. Copious energetic electrons are produced and measured with hard x-ray detectors, carry the bulk of the plasma current ranging from 50–150 kA, which is maintained for more than 1 s duration. It is observed that over one ampere current can be sustained per watt of ECRH power issued from the 28 GHz gyrotrons. The plasma current reaches I p > 80 kA for high density (>5 × 1018 m−2) discharge with 150 kW ECRH. An analysis was carried out combining reconstructed multi-fluid equilibrium, guiding-center orbits of energetic electrons, and resonant heating mechanisms. It is verified that in EXL-50 a broadly distributed current of energetic electrons creates a smaller closed magnetic-flux surface of low aspect ratio that in turn confines the thermal plasma electrons and ions and participate in maintaining the equilibrium force balance.