Quasi-optical Transmission Line Research Articles

Conceptual design of a modular EC heating system for EU-DEMO

The European DEMO (EU-DEMO) reactor studies within EUROfusion aim to develop a fusion power plant concept. The large tokamak device needs an auxiliary heating power which, at the present stage, is provided by the Electron Cyclotron (EC) heating system with up to 130 MW foreseen to reach different regions of plasma for heating, suppression of instabilities and the possibility to support ramp-up and ramp-down phases. The present conceptual design of the system is based on 2 MW coaxial-cavity gyrotron sources, a transmission line (TL) using both circular corrugated waveguides and quasi-optical evacuated multi-beam TLs, and mirror antennas located in the Equatorial Port. In order to create a modular system, the sources are grouped in ‘clusters’, whose powers are combined in the quasi-optical TL, up to the tokamak building, where they are split and routed as single waveguides. In the launcher, they are combined together again on the launching mirrors, to save space for the apertures in the Breeding Blanket. The present EC heating system has a certain flexibility to adapt to changing design guidelines. The development status of the system is presented.

First Experimental Results on Gyrotron Backward-Wave Oscillator With Zigzag Quasi-Optical Transmission Line

First Experimental Results on Gyrotron Backward-Wave Oscillator With Zigzag Quasi-Optical Transmission Line

Development of the electron cyclotron resonance heating system for Divertor Tokamak Test

The Divertor Tokamak Test (DTT) facility, whose construction has started in Frascati (Italy), will be equipped with an ECRH (electron cyclotron resonance heating) system including 32 gyrotrons as microwave power sources. The procurement of the first batch of sources with 16 MW total power, based on 170 GHz/≥ 1 MW/100 s vacuum tubes, is in progress and will be available for the first DTT plasma. The system is organized into four clusters of 8 gyrotrons each. The power is transmitted from the Gyrotron Hall to the Torus Hall Building (THB) by a quasioptical transmission line (TL), mainly composed of large mirrors shared by eight beams coming from eight different gyrotrons and designed for up to 1.5 MW power per single beam, similar to the TL installed at the stellarator W7-X. One of novelties introduced in the DTT system is that the mirrors of the TLs are embodied in a vacuum enclosure, using large metal seals, mainly to avoid air absorption and risk of arcs. The main reason is to reduce the risk of air breakdown, maintaining a pressure of 10−5 mbar far away from the Paschen minimum. The TL estimated volume is between ∼70 and ∼85 m3. The direct connection of the TL to the tokamak vacuum vessel has been evaluated, and different solutions have been proposed in order to prevent a possible impact on DTT operations. The microwave power is injected into the tokamak using independent single-beam front-steering launchers, real-time controlled and located in the equatorial and upper ports of four DTT sectors. In-vessel piezoelectric walking drives are the most promising candidates for the launcher mirror movement considering their compactness and capability to operate in an environment with strong magnetic field under ultra-high vacuum. The DTT ECRH system design, presented here, is based mainly on existing and assessed solutions, although the challenging adaptations to the DTT case are considered.

Conceptual design of the DTT ECRH quasi-optical transmission line

The Divertor Tokamak Test (DTT) facility is being realised in Frascati, Italy for the study of the power exhaust issues in view of DEMO tokamak. A multi-MW Electron Cyclotron Resonance Heating (ECRH) system is foreseen with a first set of 16 gyrotrons at 1 MW and 170 GHz available for the first stage of operation; additional 16 gyrotrons with power from 1 MW to 1.2 MW are foreseen in a later stage for a total of up to 33.6 MW of ECRH at plasma. ECRH system itself is part of the 45 MW of external additional heating coupled to the plasma, provided also by Ion Cyclotron Resonance Heating and Neutral Beam Injection. The Transmission Line (TL) is fully quasi-optical, between 84 m and 138 m long and will transfer power up to 1.5 MW per beam from the building hosting the ECRH sources to the Torus Hall. The main section runs in a straight elevated corridor at a few meters above the ground level. In order to have low power losses and a simpler and manageable system, the long TL run is made by four quasi-optical Multi-Beam (MB) lines each transmitting 8 beams via shared mirrors, similar to the W7-X Stellarator TL. The system is thus organised in “clusters” each one made of the 8 gyrotron sources and the respective transmission line and launchers. Avoidance of losses in air and microwave leaks is assured by a vacuum enclosure of the whole line with the use of metallic gaskets. Single-Beam (SB) transmission lines are foreseen at the beginning and end of the MB line, to cover the distance from the gyrotron to a beam-combining mirror and from a splitting mirror to the launcher. The optical design has to cope with space constraints in the building and inherent conversion losses at the reflection on metallic mirrors, whose number was minimised. First evaluations with the electromagnetic modeling tool GRASP and first concepts for the MB mirror cooling are reported.

The ECRH-Power Upgrade at the Wendelstein 7-X Stellarator

The existing ECRH system at W7-X consists of 10 gyrotrons, with output power levels ranging from 0.6 MW up to 1.0 MW each at a frequency of 140 GHz, quasi-optical transmission lines and microwave launchers at the plasma vessel. Compared to other large fusion experiments, W7-X has a relatively low power-to-volume ratio. However high heating power is particularly necessary for achieving high plasma beta values, where the improved confinement of fast ions, one of the optimization criteria of W7-X, can be examined. It is therefore necessary to expand the ECRH systems in several consecutive steps. It is planned to increase the number of gyrotron positions from 10 to 12 and at the same time to evolve the gyrotron output power in several development steps from 1 MW to nominal 1.5 MW and, finally, up to 2 MW. At the same time, the transmission lines will also be upgraded for 2 MW operation. A special effort is also made to improve the reliability of the system by the fast control system.

Design and Experimental Research of 45-GHz Quasi-optical Transmission Line for Melting Rock

Design and Experimental Research of 45-GHz Quasi-optical Transmission Line for Melting Rock

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.

Cyclotron Resonance Maser With Zigzag Quasi-Optical Transmission Line: Concept and Modeling

A microwave system in the form of a quasi-optical (QO) transmission line is proposed as an interaction circuit of an amplifier or an oscillator based on the cyclotron resonance maser (CRM) instability. The circuit consists of periodically spaced along the horizontal <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${z}$ </tex-math></inline-formula> -axis focusing mirrors, which transport the Gaussian wave beam along a zigzag path with vertical and inclined segments. A beam of electrons having sufficiently high perpendicular velocities is guided by a static B-field along the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${z}$ </tex-math></inline-formula> -axis so that the e-beam periodically crosses the wave beam. The CRM instability occurs in regions where an electromagnetic wave having a Bz-component, as in a gyrotron, propagates strictly across the e-beam resulting in the interaction with minimal sensitivity to particle velocity spread. Three-dimensional Particle-In-Cell (3-D PIC) simulations demonstrate the attractiveness of this “zigzag” CRM for the implementation of relatively broadband amplifiers or oscillators with unique (more than an octave bandwidth) frequency tunability at wavelengths of 1–2 mm.

Progress of DTT ECRH system design

The Divertor Tokamak Test (DTT) facility [1], whose construction is starting, will study a suitable solution for the power exhaust in conditions relevant for the future fusion device DEMO. DTT can achieve the value of 15 MW/m for the divertor figure of merit PSEP/R by employing 45 MW of auxiliary heating power to the plasma. To achieve this goal, the selected heating systems are Electron Cyclotron Resonance Heating (ECRH), Ion Cyclotron Resonance Heating (ICRH) and Negative (ion based) Neutral Beam Injector (NNBI).The ECRH system relies on up to 32 gyrotrons (operating each at 170 GHz to supply from a minimum of 1MW to a maximum of 1.2 MW for 100 s), a Quasi Optical (QO) transmission line (TL), consisting of multi-beam mirrors installed under vacuum to reduce the overall transmission losses below the target of 10% and independent (single-beam) front-steering mirrors capable to direct the beams individually in real-time for assisted plasma breakdown, control of neoclassical tearing modes and sawtooth, ECCD and main electron heating. Although the ECRH system design presented here will be based mainly on existing and assessed technologies, like the 170 GHz gyrotron type developed for ITER and the QO TL installed at W7-X, challenging adaptations to the DTT case have to be made. In particular, the design of a QO TL under vacuum is novel and needs detailed analysis of the stray radiation along the line in order to set the requirements for the mirror dimensions and/or the cooling of the vacuum chamber that encloses the mirrors. A further relevant question is the reliability of the ECRH system: the development of automatic algorithms to control such a large number of gyrotrons is foreseen to provide the required amount and distribution of power into the plasma.

Movable quasi-optical signal transmission line as part of a scanner for planar field measurements of millimeter wave range

Movable quasi-optical signal transmission line as part of a scanner for planar field measurements of millimeter wave range

КВАЗИОПТИЧЕСКАЯ ЛИНЗОВАЯ ЛИНИЯ ПЕРЕДАЧИ С ОПТИЧЕСКИМ КОНТРАСТОМ, БЛИЗКИМ К ЕДИНИЦЕ

The quasi-optical transmission line with optical contrast close to 1 is considered. Such devices can be used to study the effect of laser radiation on biological media, in microwave, terahertz, optical and X-ray wavelength ranges, as well as in acoustics. The use of composite refractive lenses, which are a set of identical spherical lenses arranged one after another on the same optical axis and with a refractive index close to one, reduces the loss of reflection in such a lens. Reducing the focal length in such refractive lenses is possible by increasing the number of refractive surfaces.

Collective Thomson scattering diagnostic at Wendelstein 7-X.

A Collective Thomson Scattering (CTS) diagnostic is installed at Wendelstein 7-X for ion temperature measurements in the plasma core. The diagnostic utilizes 140 GHz gyrotrons usually used for electron cyclotron resonance heating (ECRH) as a source of probing radiation. The CTS diagnostic uses a quasi-optical transmission line covering a distance of over 40 m. The transmission line is shared between the ECRH system and the CTS diagnostic. Here we elaborate on the design, installation, and alignment of the CTS diagnostic and present the first measurements at Wendelstein 7-X.

A Subthreshold High-Pressure Discharge Excited by a Microwave Beam: Physical Basics and Applications

A new form of discharge excited by a microwave beam in a high-pressure (up to atmospheric and higher) gas in free space and in a closed chamber is discussed. For the first time, the discharge was implemented by means of a gyrotron with a pulse power of 200 ≤ P ≤ 600 kW, a pulse duration of 0.5 ≤ τ ≤ 20 ms, and a wavelength of λ = 0.4 cm. Under deeply subthreshold conditions in atmospheric-pressure air, a plasma column with a length of L = 50 cm was generated by a microwave beam formed with the help of a quasi-optical transmission line. With the use of the MIG-3 gyrotron complex with the above parameters, generation of a plasma column with a length of several meters is possible in principle. The parameters and structure of the formation of the plasma investigated make it possible to class it as a self-non-self-sustained (SNSS) discharge, discovered and described for the first time at the Prokhorov General Physics Institute, Russian Academy of Sciences. One of the important applications of this type of discharge is plasmachemical cleaning of the urban air environment of hazardous contaminants.

Improving the Efficiency of Quasi-Optical Bragg Reflectors

In the analysis of propagation of wave beams in quasi-optical transmission lines, the loss rate and transformation of modes by various elements of the transmission line are of great importance. The effects occurring when a spatially bounded, obliquely incident wave beam is reflected from a Bragg resonator being a periodic, plane-layered medium with varied dielectric permittivity are considered. Such a reflector can be used as a frequency-selective element of the transmission line. Since the reflection coefficient of a plane wave from the Bragg reflector depends on its incidence angle, there occurs a transformation of the reflected beam. Analytical expressions for the beam displacement, the angle of deflection of the beam from the mirror reflection direction, and the wavefront curvature variation have been obtained. The influence of the mentioned effects on characteristics of the reflected wave beam in the main band of the Bragg reflector is numerically analyzed. The possibility of compensation for these distortions is considered.

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

System Design and Subsystem Technology for a Future Spaceborne Cloud Radar

The 2007 Decadal Survey for the National Aeronautics and Space Administration recommended an Aerosol/Cloud/Ecosystems (ACE) mission with a new advanced cloud profiling radar. In this letter, we describe ACERAD, a radar design that could meet the goals for the ACE mission. ACERAD has 35 GHz (Ka-band) and 94 GHz (W-band) channels. It would scan at Ka-band and be nadir looking at W-band. The scanning antenna geometry is a classical Dragonian illuminated by a feed array; a reduced-size prototype of the antenna has been successfully fabricated and tested. ACERAD also uses a quasi-optical transmission line at W-band for low-loss routing of the signal from the transmitter to the antenna and from the antenna to the receiver. This letter briefly describes the ACERAD instrument design and test results for some of the major technologies needed for ACERAD implementation.

Коммутационные управляемые устройства на p—i—n-диодах миллиметрового диапазона длин волн

The paper presents the results of research and development of concentrated type p—i—n-diodes switches providing the switching time at the level of a few nanoseconds. To increase lock losses of (~40 dB) the authors use a cascade connection of diodes into waveguide and microstrip transmission line of q = p/2 electric length. Investigation results of creation of switching devices using longitudinally and transversely-distributed p—i—n-structures in the shortwave part of the millimeter wavelength range (f = 300 GHz) are presented. When developing switching devices intended to control the level of microwave power in the millimeter wavelength range, a number of special features arise limiting the achievement of optimal parameters. The dimensions of the metal ceramic packaged p—i—n-diodes and the mounting elements of semiconductor structures become comparable with the wavelength. As a result, package cannot be considered in the calculations and in the design as the capacity of the concentrated type. In our case the diode package is considered in the form of radial line which is able to transform the input impedance of the transmission line to the terminals of the diode structures, and realize high-impedance state (parallel resonance) in the device circuit in the mode of microwave power transmission. Engineering calculations for the given parameters of the silicon mesostructures showed the possibility of creating high-speed devices for switching microwave power with good characteristics. For diode assembling industrial clockwork ruby jewels with high quality of surface finish, strength and appropriate dimensions are applied as dielectric bushings. Suffice it to say that in the frequency range of 150 GHz, we used the bushings with dimensions of external diameter D = 0.4 mm and a height of h = 0.15 mm. The switches created provide transient units of nanoseconds, isolation more than 40 dB at relative frequency bandwidth of 30-40%. Evaluating progress in the development of millimeter wavelength devices, it should be noted that at frequencies greater than 200 GHz, the application of the concentrated type diodes is problematic. In this range it would seem to be promising to use bulk semiconductors. In this case surface-oriented p—i—n-structures can be applied as unified elements of the control devices in the shortwave part of the millimeter range, as well as in quasi-optical transmission lines.

Modification of the collective Thomson scattering radiometer in the search for parametric decay on TEXTOR

Strong scattering of high-power millimeter waves at 140 GHz has been shown to take place in heating and current-drive experiments at TEXTOR when a tearing mode is present in the plasma. The scattering signal is at present supposed to be generated by the parametric decay instability. Here we describe the heterodyne detection system used to characterize the newly discovered signal measured at TEXTOR, and we present spectral shapes in which the signal can appear under different conditions. The radiation is collected by the receiver through a quasi-optical transmission line that is independent of the electron cyclotron resonance heating transmission line, and so the scattering geometry is variable. The signal is detected with 42 frequency channels ranging from 136 to 142 GHz. We demonstrate that the large signal does not originate from gyrotron spurious radiation. The measured signal agrees well with independent backscattering radiometer data.

Architecture of central control system for the 10 MW ECRH-plant at W7-X

Electron Cyclotron Resonance Heating (ECRH) is the main heating method for the Wendelstein 7-X stellarator (W7-X) which is presently under construction at IPP Greifswald. The mission of W7-X is to demonstrate the inherent steady state capability of stellarators at reactor relevant plasma parameters. A modular 10 MW ECRH-plant at 140 GHz with 1 MW CW-capability power for each module is also under construction to support the scientific objectives. The commissioning of the ECRH-plant is well under way; three gyrotrons are operational. The strict modular design allows to operate each gyrotron separately and independent from all others. The ECRH-plant consists of many devices such as gyrotrons and high voltage power supplies, superconductive magnets, collector sweep coils, gyrotron cooling systems with many water circuits and last but not least the quasi-optical transmission line for microwaves with remote controlled mirrors and further water cooled circuits. All these devices are essential for a CW operation. A steady state ECRH has specific requirements on the stellarator machine itself, on the microwave sources, transmission elements and in particular on the central control system. The quasi steady state operation (up to 30 min) asks for real time microwave power adjustment during the different segments of one stellarator discharge. Therefore, the ECRH-plant must operate with a maximum reliability and availability. A capable central control system is an important condition to achieve this goal. The central control system for the 10 MW ECRH-plant at W7-X comprises three main parts. In detail these are the voltage and current regulation of each gyrotron, the interlock system to prevent the gyrotrons from damages and the remote control system based on a hierarchy set of PLCs and computers. The architecture of this central control system is presented.

EU Megawatt-Class 140-GHz CW Gyrotron

The first series tube of the gyrotrons for the 10-MW electron cyclotron resonance heating system of the stellarator W7-X was tested at Forschungszentrum Karlsruhe (FZK) and yielded a total output power of 0.98 MW, with an efficiency of 31% (without a single-stage depressed collector) in short-pulse operation and of 0.92 MW in pulses of 180 s (efficiency of almost 45% at a depression voltage of 29 kV). The Gaussian mode output power was 0.91 MW. The pulselength at full power (1 MW) is limited at FZK by the available power supply. At a reduced electron beam current, it is possible to operate at longer pulselengths. At an output power of 0.57 MW (electron beam current of 29 A), the pulselength was increased to 1893 s. There was no physical reason for a limitation of this pulse: The pressure increase during the pulse was less than a factor of two and ended up at a very low value in the 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> mbar range. The tube was delivered to Max-Planck-Institut fuumlr Plasmaphysik Greifswald for tests at full power and up to 30-min pulselength. The Gaussian mode RF output power, measured in a calorimetric load after a 25-m-long quasi-optical transmission line (seven mirrors), was 0.87 MW at a total output power of 0.92 MW in 30-min pulses. Again, no indications for a limitation in pulselength were found. The second series tube was tested in short-pulse operation and showed a strange behavior concerning a mode hopping which has not yet been understood. The third series gyrotron delivers up to now 0.65 MW at a pulse duration of 180 s. Preliminary operation of the prototype tube as a two-frequency gyrotron delivered 0.41 MW in 10-s pulses at 103.8 GHz (TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21,6</sub> mode)