Quasi-optical Mirrors Research Articles

Preparatory study of feasibility for a vertical viewing electron cyclotron emission diagnostic for the JT-60SA tokamak.

A preparatory study is underway to investigate the feasibility study of high-energetic, non-thermal electron distribution function measurements using a vertical-viewing electron cyclotron emission (ECE) diagnostic on JT-60SA. The system is designed to detect broad ECE spectra (70-260GHz) due to the second, third, and fourth harmonics using a focusing optics system with four quasi-optical mirrors in the upper port of JT-60SA. A Gaussian beam optics design is performed in vacuum, and ray tracing calculations are performed in plasma using the TRAVIS code to investigate density characteristics. A ceramic viewing dump is also designed to reduce the effects of multiple reflections from the opposing vacuum vessel surface.

Pre-conceptual design of the steering mirror for the DEMO electron cyclotron heating system

An Electron Cyclotron (EC) Heating and Current Drive (HCD) system will be installed in the EU DEMO Tokamak to provide plasma heating and stabilization by coupling up to 130 MW of mm-wave power into specific regions of the plasma. The mm-waves will be generated by gyrotrons and propagated through a quasi-optical multi-beam mirror system (MBMS), and subsequently through corrugated waveguides, to launchers located at dedicated Equatorial Ports (EPs). The present design of the launcher features two mid steering antennas (MSAs) for plasma stabilization each consisting of three-beam layouts and steering mirrors positioned at the upper and lower locations of the EP. The proposed concept for the steering mirrors operates on the pantograph principle, a mechanical assembly of parallel linkages, which translates the movement produced by pneumatic actuators into an exact angular rotation of the mirror around an offset virtual axis. This design uses flexures with specific structural compliance as mechanical joints resulting in the desired movement of the linkage system while avoiding the tribological difficulties associated with mechanisms operating under vacuum. This paper describes the present status of the DEMO EC HCD steering mirror as well as the analyses performed to validate this design.

Thermo-mechanical and quasi-optical analysis of the ECRH equatorial launcher steerable mirror towards the CFETR

This study reports a prototype design of the front steerable mirror (SM) in the Electron Cyclotron Resonance Heating (ECRH) system, which is used to validate the manufacturing technology of the large-scale quasi-optical mirror in the China Fusion Engineering Test Reactor (CFETR). As a plasma-oriented component, the SM will experience more neutron heat and plasma radiation heat than the remote focusing mirrors, and the simulation results indicate that the peak temperature of the SM would then reach 292.5 °C, the maximum deformation and the maximum equivalent stress would be 1.16 mm and 446 MPa, respectively. The beam exhibits little distortion because the thermal deformation gradient around the wave center is small, and the quasi-optical analysis shows that the position of the wave heating plasma should be shifted by -6 mm along the local Z-axis, while the amplitude of the central electrical field would increase by 0.3%, from 294.7 kV/m to 295.5 kV/m.

Preconceptual Design of the Port Cell Section for the EU DEMO Equatorial EC System

The EU demonstration power plant (DEMO) Tokamak will be equipped with an electron cyclotron (EC) system for plasma heating and magnetohydrodynamic (MHD) control. Up to six launchers will be installed into equatorial ports with the aim to inject maximum 130-MW millimeter-wave (mm-wave) power at dedicated positions into the plasma. The mm-waves will be generated in gyrotrons placed in a distinct building at distant location. From this gyrotron hall, a combined transmission line (TL) system of quasi-optical multibeam mirrors and individual waveguides (WGs) will propagate the mm-waves into the tokamak building. That followed, an optical system, composed of miter bends, diamond windows, valves, and microwave bellows, connects the TL through the gallery and the port cell with the EC launchers. This article presents the preconceptual computer-aided design (CAD) of this latter section of the EC system. Based on its general scheme and the given WG trajectories, the model takes into account the available space in the port cell with respect to required clearance for supply systems, assembly, and maintenance. At the preconceptual state, the design includes CAD models of all relevant mm-wave components in realistic dimensions at a level of details, appropriate to demonstrate the feasibility of the concept.

A method for synthesis of quasi-optical transmission line for the 45 GHz/20 kW electron cyclotron resonance ion source.

In this paper, the method for synthesis of a quasi-optical transmission line (QOTL) for a 45-GHz/20-kW Electron Cyclotron Resonance (ECR) ion source is proposed. The QOTL consists of five quasi-optical elliptical mirrors which had successfully transmitted the microwave power of about 12.8m from the output of the matching optical unit to the vacuum window of the corrugated waveguide. The lengths of the major and minor axes of the five elliptical mirrors are optimized by adopting the Nelder-Mead simplex algorithm to achieve the maximum power transmission efficiency. Meanwhile, analysis of the electromagnetic fields in the QOTL system has been developed in terms of the vector diffraction theory. The model of the QOTL system is built using the Altair Feko software to simulate the electromagnetic wave propagating in free space. After the optimized design and simulation verification, the QOTL system has been fabricated, installed, and measured. The experimental results reveal that the power transmission efficiency of the QOTL system is 86.5%, including diffractive and ohmic losses. Moreover, the correlation coefficient reaches 96.53% between the field pattern of the theoretical value and the measured value at the vacuum window of the 45-GHz/20-kW ECR ion source.

Development of fast-ion collective Thomson scattering diagnostics for the GDT experiment

In this paper, we describe hardware developed for the collective Thomson scattering (CTS) diagnostic for the large open magnetic trap GDT (Budker Institute, Russia) used in fusion researches. The diagnostic utilizes 400 kW/54.5 GHz gyrotron as a source of probe radiation and is aimed at the reconstruction of fast ion distributions over transverse and longitudinal velocities driven by high-power neutral beam injection. Formation and manipulation of the probe beam and collection of the scattered microwave radiation are done with large-aperture quasi-optical mirror systems. The receiving complex consists of two identical channels for simultaneous detection of scattering in two complement geometries. Each channel includes a highly sensitive radiometer/spectrum analyzer with the band of 54.470 ± 0.550 GHz, the noise temperature of 0.35 eV, the sensitivity less than 0.025 eV, the frequency/time resolution of 5–50 MHz/10 μs, and narrow-band notch filter providing at least -40 dB rejection in the band of 25 MHz with a central frequency tunable in 54.470 ± 0.050 GHz for protection of the input RF circuit from a backlight of powerful gyrotron radiation.

Non-inductive plasma current ramp-up through oblique injection of harmonic electron cyclotron waves on the QUEST spherical tokamak

The plasma current is ramped up primarily by a 28 GHz electron cyclotron wave (ECW) in the Q-shu University experiment Steady-State Spherical Tokamak (QUEST), with multiple harmonic resonance layers from the second to the fourth stay in the plasma core. A steering antenna comprising two quasi-optical mirrors enhances the power density of ECWs. The ECW beam is injected obliquely from the low-field side where the parallel refractive index is N∥ = 0.75 at the second-harmonic resonance layer. Analysis of the resonance condition has found that energetic electrons moving forward along the magnetic field resonate more effectively than those moving backward. Such symmetry breaking is consistent with the results of the current ramp-up experiment. The peak plasma current reaches Ip>70 kA, constantly injecting a beam of radio frequency power of 100 kW. Ray-tracing by the TASK/WR code demonstrates that the power of the 28 GHz extraordinary mode is absorbed by energetic electrons via single-pass cyclotron absorption.

Design and Experiment of a Quasi-Optical Mode Converter

Quasi-optical mode converter is an important component to realize the high-efficiency output for a high-power gyrotron oscillator. In this paper, design and experiments of a quasi-optical mode converter, consisting of a Denisov-type launcher and three quasi-optical mirrors, are carried out for the development of 140 GHz/TE28,8 mode gyrotron oscillator. Based on scalar diffraction method, the field distribution at the radiation aperture of the Denisov-type launcher is optimized to make the vector correlation of the aperture field with the ideal Gaussian field reach 96.2%. Based on the geometric optics method and the Gaussian beam matching method, the focusing mirror and the beam shaping mirrors were designed. A 3D full-wave analysis software Surf3D was used to obtain the field distribution on each mirror surface and the output window, which shows that the output beam with Gaussian mode content of 96.67% is obtained at the output window. The total power conversion efficiency of the quasi-optical mode converter is 93.98%. The design and experiments of the quasi-optical mode converter are performed to use the output signal of the self-developed TE28,8 mode generator as its input one under the conditions of the simulated demonstration for its conversion characteristics and the strict control for machining precision as well as the process of the converter assemble and testing. The tested results indicate that the design is in good agreement with the experiment, which is helpful for engineering design and experimental demonstration of the quasi-optical mode converter development.

Thermal management of a quasi-optical mirror in a millimetre-wave gyrotron

• A millimetre-wave mirror of gyrotron was thermally analysed considering three different surface heat loads on mirror surface. • The heat load on reflecting surface of the mirror were considered: i) over the full reflecting surface, ii) over the circular cross section similar to that of RF beam, and iii) over the circular cross section in Gaussian distribution. • Thermal images were captured under these three considerations. Values and locations of highest temperature attained on the mirror were observed. Maximum temperature reached in three considerations of heat load were 348.72, 330.53 and 391.64 °C, respectively. • Further, three cooling channel routings: i) cooling cavity, ii) rectangular serpentine, and iii) circular serpentine were implemented behind the mirror. The rectangular serpentine cooling channel was the most effective for the considered millimetre-wave mirror. • The maximum temperature reached under consideration of Gaussian heat load was below 400 °C (allowable maximum temperature). Therefore, the present cooling proposal would be suitable for the heat load of 5 kW on reflecting surface of the mirror. A quasi-optical mirror of a millimetre-wave gyrotron was thermally analysed considering three different surface heat loads on mirror surface. The heat load on reflecting surface of the mirror were considered: i) over the full reflecting surface, ii) over the circular cross section similar to that of RF beam, and iii) over the circular cross section in Gaussian distribution. Thermal images were captured under these three considerations. Values and locations of highest temperature attained on the mirror were observed. Maximum temperature reached in three considerations of heat load were 348.72, 330.53 and 391.64 °C, respectively. Further, three cooling channel routings: i) cooling cavity, ii) rectangular serpentine, and iii) circular serpentine were implemented behind the mirror. The rectangular serpentine cooling channel was the most effective for the considered millimetre-wave mirror. The maximum localised temperature reached under consideration of Gaussian heat load was below 400 °C (allowable maximum temperature). Therefore, the present cooling proposal would be suitable for the heat load of 5 kW on reflecting surface of the mirror.

Microwave System of Transverse Output for a High-Power ${W}$ -Band Gyro-TWT

A conceptual design and the results of concrete simulations of the output microwave system of a ${W}$ -band gyrotron traveling-wave tube (gyro-TWT) with a predicted output pulse power of more than 300 kW are presented. A distinctive feature of this system is the transverse (relative to the translational velocity of the electrons in the gyro-TWT) extraction of the output power. The designed microwave circuit includes a waveguide mode converter and a set of quasi-optical mirrors. An oversized hole in the first 45° inclined mirror enables a lossless transport of the electron beam to the collector. The ring-like distribution of microwave radiation on the mirror is formed by a profiled waveguide mode converter and provides minimal radiation diffraction loss. For the synthesis of the mode converter, particle swarm optimization (PSO) method was used. The system of external quasi-optical mirrors provides the transformation of the ring-like beam into the HE1,1 mode of a corrugated waveguide with an efficiency of more than 94% in the frequency range 92–98 GHz. The mirror profiles were synthesized using the Katsenelenbaum–Semenov iterative procedure.

Breakdown of the heavy noble gases in a focused beam of powerful sub-THz gyrotron

The results of experimental investigations of heavy noble gas breakdown thresholds in a focused beam of powerful subterahertz radiation are presented. The gas discharge, maintained by the powerful radiation of the terahertz frequency band, is a new specific object of gas discharge physics. This paper presents the experiments with pulsed gyrotrons capable of generating a power of 250 kW at a frequency of 250 GHz. The gyrotron wave beam was focused by means of quasioptical mirrors into the spot with a diameter less than 3 mm that provided intensity into the focal spot up to 3.5 MW/cm2. The achieved electrical field intensity was enough for gas breakdown into the range of pressure values of 1–1500 Torr for heavy noble gases (argon and krypton). The boundary values of field intensity for discharge existence were measured. Experimental data were compared to the analytical model of Raizer and Vyskrebentsev for monatomic gases.

Quasi-optical polarizer system for ECHCD experiments in the QUEST

A new transmission line from 28 GHz gyrotron to QUEST spherical tokamak (ST) has been developed for the highly efficient electron cyclotron heating and current drive (ECHCD) experiments. The ECHCD effect depends on the incident mode or polarization states of the injected beam. Although two corrugated-polarizer miter bends were developed and assembled to transmission line in the QUEST, arcing events were frequently detected at the polarizer miter bend when the high-power millimeter-wave were transmitted. A new quasi-optical (QO) concept for the polarizer system composed of two corrugated plate and a QO mirror was proposed to avoid the arcing at polarizer. The QO mirror was designed for coupling of input and output HE11 modes at the polarizer, its coupling property is discussed with Kirchhoff integral. Polarization-setting performance of the developed QO polarizer was examined in a low-power test system, and is discussed with grating-polarizer calculation.

28-GHz ECHCD system with beam focusing launcher on the QUEST spherical tokamak

New polarizer and launcher systems on a 28-GHz electron-cyclotron (EC) heating and current drive (HCD) system have been developed for non-inductive second-harmonic EC-plasma-current ramp-up in the QUEST spherical tokamak. A launcher system with two quasi-optical mirrors providing beam steering capability was designed to focus the incident beam to a small-sized waist of ∼0.05 m at the second-harmonic EC resonance layer. A relatively large focusing mirror was designed based on a Kirchhoff integral code developed to derive wave solutions. The focusing property of the launched beam was first confirmed with a 3-dimensional (3D) electromagnetic-wave simulator. Focusing characteristics were also checked at low-power test facilities, together with the steering capability. The performance of this launcher system was demonstrated to work as designed, and assembled in the QUEST device. The system was applied to the non-inductive second-harmonic EC plasma ramp-up experiments with no optimization required regarding the incident polarization. The results obtained for the non-inductive plasma ramp-up are also presented.

Preliminary conceptual design of the DTT EC heating system

The Divertor Tokamak Test (DTT) facility has been proposed in the European roadmap to study solutions to mitigate the issue of power exhaust in conditions relevant for DEMO. The Italian DTT tokamak [1] is being designed to reproduce the optimal divertor magnetic configuration and test a prototype under reactor relevant power flow in the scrape off layer (PSEP/R>15 MW/m). A mix of three heating systems will equip the machine to reach the target value of 45 MW delivered at plasma: Electron Cyclotron Heating (ECH), Ion Cyclotron Heating (ICH) and Negative Neutron Beam Injector (NNBI). The present reference design considers a capability of 20–30 MW of ECH power at plasma to support and assist different tasks.The gyrotron sources (1 MW/170 GHz/100 s) will be based on the depressed collector technology with >40% efficiency and will exploit the experience gained from the solutions developed for ITER. A Transmission Line (TL) with 90% efficiency and >1 MW/beam power handling is being considered, using Quasi-Optical (QO) multi-beam mirrors. The conceptual design of equatorial and upper launchers based on the front steering concept is being developed to reach the required deposition locations for the given application. The EC wave absorption efficiency has been investigated and is presented here with dedicated beam tracing calculations using the GRAY code [2].

Subthreshold self-sustained discharge initiated by a microwave beam in a large volume of high-pressure gas

The presented results are based on the experimental studies of generating a long plasma column in gas (or gas mixtures) at atmospheric pressure both in free space and in a closed chamber. The microwave generator GYROTRON was used as an energy source. Its parameters were as follows: the microwave pulse power was in the range of 200 ≤ P ≤ 600 kW, the wavelength was λ ≈ 0.4 cm, and the pulse duration was in the range of 0.5 ≤ τi ≤ 20 ms. Under strong subthreshold conditions, a plasma column with a length of up to L ≤ 50 cm was created using a microwave beam that was formed using a quasi-optical mirror system. The discharge initiation system had an original design. Based on the discharge structure, expansion dynamics and typical discharge plasma parameters, this discharge can be attributed to the type of microwave discharges that are known as self/non-self-sustained discharges. The discharge properties and advantages of using this discharge as a basis of a plasma-chemical reactor are discussed.

The Experimental Verification of Gaussian Beam Coupling for ECH Transmission Line at 400 GHz

We design a quasi-optical transmission line system for a 400 GHz gyrotron beam. The 400GHz Gaussian beam is injected to a corrugated waveguide bounced from a quasi-optical mirror. From detailed 2D field patterns of the output beam emitted from the corrugated waveguide, we analyze the mode contents and the source of non-ideal beam expansion

Validation experiment of a numerically processed millimeter-wave interferometer in a laboratory.

We propose a new interferometer system for density profile measurements. This system produces multiple measurement chords by a leaky-wave antenna driven by multiple frequency inputs. The proposed system was validated in laboratory evaluation experiments. We confirmed that the interferometer generates a clear image of a Teflon plate as well as the phase shift corresponding to the plate thickness. In another experiment, we confirmed that quasi-optical mirrors can produce multiple measurement chords; however, the finite spot size of the probe beam degrades the sharpness of the resulting image.

Quasi-Optical High Purity HE<sub>11</sub>-Mode Exciter for Oversized Corrugated Waveguide Transmission

Oversized corrugated waveguide transmissions are used to transmit high power millimeter wave for electron cyclotron heating in ITER. The HE11 mode purity of the waveguide components is a critical issue not only in the high power operation but also in the low power operation to evaluate the components. Conventional single Gaussian beam system causes edge diffraction and excites higher order modes at the waveguide aperture. A proposed HE11-mode exciter uses beam interference between two Gaussian beams interference and serves the high purity HE11 mode without the edge diffraction. The HE11-mode exciter consists of a scalar feed horn antenna, quasi-optical mirrors and a beam splitter (combiner). Phase matched mirrors correct the Gaussian-like beam excited from the horn antenna to pure Gaussian beam. Authors report the experimental study of the exciter.

Optics Design for Microwave Imaging Reflectometry in LHD

An optics system for microwave imaging reflectometry (MIR) in the Large Helical Device (LHD) was newly developed to optimize the performance of the two-dimensional microwave receiver array. Reflected microwaves from the plasma and the first local oscillator (LO) wave are transmitted to the receiver array via the optics from the front. Finite-difference time-domain (FDTD) calculation was used to design the ellipsoidal or hyperboloidal shapes of the quasi-optical mirrors. It is confirmed that the LO beam in the constructed system covers the receiver antenna aperture area as intended. The S/N ratios of the signals are improved with this optimized optics system from those in the previous system.

Investigation of first mirror heating for the collective Thomson scattering diagnostic in ITER

Collective Thomson scattering (CTS) has the capabilities to measure phase space densities of fast ion populations in ITER resolved in configuration space, in velocity space, and in time. In the CTS system proposed for ITER, probing radiation at 60 GHz generated by two 1 MW gyrotrons is scattered in the plasma and collected by arrays of receivers. The transmission lines from the gyrotrons to the plasma and from the plasma to the receivers contain several quasioptical mirrors among other components. These are designed to produce astigmatic beam patterns in the plasma where the beam shapes will have a direct impact on the signal strength of the diagnostic, the spatial resolution, and the robustness of probe and receiver beam overlap against density excursions. The first mirror has a line of sight to the plasma and is thus exposed to severe neutron streaming. The present neutronics and thermomechanical modeling of a first mirror on the high field side indicates that the mirror curvature may warp due to heating. This may alter the beam quality, and therefore, thermal effects have to be accounted for during the design of the mirror. The modeling further demonstrates that thin mirrors are superior to thick mirrors from a thermomechanical point of view.