Quasi-optical Launcher Research Articles

Progress of the electron cyclotron resonance heating system and the related experiments on J-TEXT

To augment the capabilities of the J-TEXT tokamak, efforts were undertaken in 2017 to commence the construction of an electron cyclotron resonance heating (ECRH) system. A significant milestone was achieved in 2019 when the successful operation took place using this 105 GHz ECRH system. The system consists of components including the gyrotron from GYCOM, a transmission line spanning approximately 30 meters, and a quasi-optical launcher equipped with an elliptical mirror and movable flat mirror. Another identical system was deployed in 2023 for experimental purposes. Notably, modifications were made to the launcher to support the injection of two beams. Various experiments have been conducted utilizing this system including assisted plasma start-up, control methods for tearing mode, and observations involving toroidal injection of electron cyclotron wave (ECW), etc.

I/O system of V-band second harmonic gyrotron for 100/200 kW operation

This paper presents the design studies of the common input and output system of a continuous wave (CW), V-band gyrotron at second harmonic for plasma diagnostics application. Two different designs are carried out with design 1 operating at TE7,3 mode having same geometrical parameters for dual power operation of 100 kW and 200 kW and design 2 operating at TE8,3 mode has same geometry that can support dual power operation of 100 kW and 200 kW respectively. The selection of the initial design parameters has been done using our in-house computer code Gyrotron Design Studio Second Harmonic Version 2020 (GDS2H-2020). The electron guns along with magnetic guidance systems are designed to generate electron beam with the required beam parameters using software package EGUN. The design and optimization of the Non-Linear Taper (NLT), Quasi-Optical Launcher (QOL) and RF window are carried out for both the designs. NLTs are optimized for maximum transmission efficiency of the desired modes using Computer Simulation tool (CST). QOLs are designed for converting the operating modes into Gaussian-like mode using commercial software packages Launcher Optimization Tool (LOT) and SURF3D. The designs of the single-disk RF window for high-power output beam extraction are performed using CST.

8.56-GHz quasi-optical launcher system with incident-mode selectivity on the QUEST spherical tokamak

An 8.56-GHz quasi-optical launcher system with incident-mode selectivity was developed for non-inductive electron cyclotron (EC) plasma ramp-up and long-pulse sustainment, and effective bulk-electron heating on the Q-shu University experiment with steady-state spherical tokamak (QUEST). A novel polarization-selective system was proposed to conduct fundamental and second harmonic EC heating and current drive experiments with ordinary and extra-ordinary mode waves, respectively. Selectivity of the incident modes or polarizations was confirmed using the whole developed system at a low power level. A large-sized mirror launcher was developed to inject the beam whose size was as small as possible near the QUEST major radius. The designed mirror performance was checked using a three-dimensional electromagnetic-wave simulator and a low-power test. Ultimately, 0.15-m sized beams for both the two polarizations were successfully attained near the QUEST major radius, even in low-frequency applications with long propagation.

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.

Output System of a 42 84 GHz 0.5 MW Dual Regime Gyrotron

In this paper, the design studies of the output system are carried out for a dual regime Gyrotron in the context of India’s requirement of clean energy. This design study consists of a dimpled wall quasi-optical launcher (QOL) and RF window. After proper coupling of energy from beam to RF wave, the amplified wave need to be down-convert in a Gaussian-like beam (much simplified lower order). The launcher is designed with a commercial software LOT/Surf3d. This Gaussian-like mode coming out from the gyrotron vacuum system through the RF window is coupled to the corrugated waveguide using Matching Optic Unit (MOU) section. The complete design of the RF window is carried out using Gyrotron Design Suite Version 4.0 (GDSv4.0 2016).

Design of a Launcher for the 105-GHz ECRH System on J-TEXT

To enhance parameters of the plasma and improve the magnetic confinement performance, a 105-GHz electron cyclotron resonance heating (ECRH) system is being built on the J-TEXT. We designed a quasi-optical launcher which mainly consists of a focusing mirror and a steering mirror. The focusing mirror is optimized by considering the localized deposition, and the steering mirror is close to the plasma boundary for flexible adjustment of the toroidal and poloidal injection angles of the wave. The toroidal and poloidal rotation angles of the steering mirror are independently controlled by the mechanical structure. For our design, the beam radius at the plasma center for perpendicular incidence is about 20 mm; the adjustable range of the toroidal and poloidal injection angles is ±14° and ±31°, respectively. These can meet the requirements for physics experiments on J-TEXT.

Prototype of a Quasi-Optical Launcher System of a 4 mm Round-Trip Interferometer for the QUEST Spherical Tokamak Experiments

Prototype of a Quasi-Optical Launcher System of a 4 mm Round-Trip Interferometer for the QUEST Spherical Tokamak Experiments

Research activities and progress on the long pulse ECRH launcher for EAST

A long pulse Electron Cyclotron Resonance Heating (ECRH) system is developed on EAST tokamak for plasma heating and current profile tailoring. The ECRH system is designed to operate at 140GHz and to inject 4MW CW power. With respect to the physical objectives of the newly built ECRH system, a quasi-optical launcher is designed to inject 4MW continuous wave into plasma through an equatorial port. Gaussian beams delivered from evacuated corrugation waveguides will be focused and reflected by high thermal conductive metal mirrors, and then steered by using push-rod steering mechanism with entire scanning range of ±25° toroidally and over 30° poloidally in plasma cross section. The mirrors are carefully designed with mega watts power handling capability and optimum optical characteristics. The performance of steering mechanism has been tested before installation, an open-loop control system for ECRH launcher has been implemented for required mirror movement and proper polarization between plasma discharges. This paper will present the overall design and progress of the launcher, along with the performance in EAST campaigns. Considerations and possible upgrade of the design features relevant to long pulse operation are discussed.

Studies towards an upgraded 1.5 MW gyrotron for W7-X

Studies towards a 1.5 MW, 140 GHz CW gyrotron, with the capability of MW-class operation also at 175 GHz, are ongoing at Karlsruhe Institute of Technology in view of a possible future upgrade of the ECRH system of the stellarator W7-X. The upgrade of the existing 1.0 MW, 140 GHz European gyrotron for W7-X has been chosen as a development path. Detailed designs of the cavity, the non-linear uptaper, and the quasi-optical launcher for the upgraded gyrotron have been obtained and have been validated numerically. In parallel, a mode generator, intended for low-power tests of the quasi-optical mode converter system of the upgraded gyrotron, has been designed, manufactured, and successfully tested.

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.

Plans for the electron cyclotron heating system on J-TEXT

A new 105 GHz/500 kW/1 s electron cyclotron resonance heating (ECRH) system has been designed and being constructed on J-TEXT Tokamak. This system mainly consists of a microwave source, a transmission line, a launcher and other auxiliary units. Based on corrugated waveguides, the wave from the gyrotron can be efficiently transmitted with HE11 mode to the steerable quasi-optical launcher for injection. The transmission efficiency is about 85%, and the injection angle of the wave can be adjusted by the flat mirror of the launcher. Commissioning of this electron cyclotron heating system is scheduled to be done at 2019.

V-band Doppler backscattering diagnostic in the TCV tokamak.

A variable configuration V-band heterodyne Doppler back-scattering diagnostic has been recently made operational in the tokamak configuration variable. This article describes the hardware setup options, flexible quasi-optical launcher antenna, data-analysis techniques, and first data. The diagnostic uses a fast arbitrary waveform generator as the main oscillator and commercial vector network analyzer extension modules as the main mm-wave hardware. It allows sweepable single or multi-frequency operation. A flexible quasi-optical launcher antenna allows 3D poloidal (10°-58°) and toroidal (-180° to 180°) steering of the beam with 0.2° accuracy. A pair of fast HE11 miter-bend polarizers allow flexible coupling to either O or X mode and programmable polarization changes during the shot. These have been used to measure the magnetic-field pitch angle in the edge of the plasma by monitoring the backscattered signal power. Ray-tracing simulations reveal an available k⊥ range between 3 and 16 cm-1 with a resolution of 2-4 cm-1. Perpendicular rotation velocity estimates compare well against ExB plasma poloidal rotation estimates from charge exchange recombination spectroscopy.

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.

Electron Gun and Output Coupling System for a 220-/251.5-GHz, 2-MW Triangular Corrugated Coaxial Cavity Gyrotron

In this paper, design studies of a common electron gun and output coupling system for the dual regime operation of a 2-MW coaxial cavity gyrotron are presented. The operating frequencies (220/251.5 GHz) are selected, such that the proposed gyrotron can be used for plasma heating applications in the future experimental fusion reactors. A coaxial magnetron injection gun (MIG) is designed for the generation of hollow electron beam with the desired parameters. Initial geometry of the electron gun is determined through our in-house code Gyrotron Design Suite version 4.1 2016 (GDSv4.1 2016). The MIG is further optimized and simulated through the beam trajectory code ESRAY. A nonlinear taper (NLT) is designed with maximum transmission efficiency at both the operating frequencies. A quasi-optical launcher (QOL) is designed and numerically optimized through the commercial software, launcher optimization tool. The designed launcher efficiently converts both the operating modes into Gaussian-like mode. A single-disk RF window is designed for the extraction of high-power output beam from the gyrotron at their respective operating frequencies. The design of the NLT and the RF window are carried out through GDSv4.1 2016.

First results from protective ECRH diagnostics for Wendelstein 7-X

Wendelstein 7-X (W7-X) is a steady state capable optimised stellarator. The main heating system is electron cyclotron resonance heating (ECRH) operating at 140 GHz providing up to 9 MW microwave power. The power is launched into the machine by front steerable quasi-optical launchers in X- or O-mode. While in X-mode the first pass absorption is 99%, it is only 40... 70% in O-mode. O2-mode heating is forseen for high density operation above the X2 cutoff density of m−3. A set of diagnostics has been developed to protect the machine from non absorbed ECRH power which can easily damage in vessel components. The non absorbed power hitting the inner wall is measured by waveguides embedded in the first wall (ECA diagnostic). In order to prevent the inner wall from overheating or arcing, a near-infra red sensitive video diagnostic with a dynamic range of 450...1200 °C was integrated in the ECRH launchers. Thermal calculations for the carbon tiles predict a temperature increase above the detection threshold for scenarios of plasma start-up failure or poor absorption on a time scale of 50 ms. However, the temperature increase measured by an IR camera in experiments with failed break down, i.e. no ECRH absorption for up to 50 ms, was only C. In discharges with 5% transmission the measured temperature increase was comparable. The stray radiation level inside the machine is measured by so called sniffer probes resembling microwave diode detectors which were designed to collect all radiation approaching the probing surface independent of incident angle and polarization. Five sniffer probes are installed at different toroidal positions. They were integrated in the ECRH interlock system. During the first operational phase of W7-X this was the only available plasma interlock system. The signal quality proofed to be high enough for a reliable termination in case of poor absorption. After a breakdown phase of 10 ms, the sniffer probe signals dropped by more than an order of magnitude. Especially in the very first days of operation, most discharges died by a radiative collapse due to impurity influx. In this case the heating power was reliably switched off due to the increased level of stray radiation. Moreover, ECRH bolometers with a slower response time in the launcher ports and an empty diagnostic port were used to estimate the stray radiation level in the ports. In the launcher ports it could be shown that the stray radiation could lead to an overheating of the bellows in long discharges. Possible counter measures are discussed.

RF Behavior and Launcher Design for a Fast Frequency Step-tunable 236 GHz Gyrotron for DEMO

AbstractAs part of the EUROfusion project, the conceptual design of a 1 MW 236 GHz hollow-cavity gyrotron is ongoing at IHM, KIT for a DEMOnstration Power Plant (DEMO), along with a 2 MW coaxial-cavity design concept. Fast frequency-tunable gyrotrons (tuning within a few seconds) are recommended for plasma stabilization using a non-steerable antenna. In this work, the mode-selection approach for such a frequency-tunable gyrotron is presented and suitable operating modes for fast frequency tunability are suggested. Magnetic field tuning has been studied as an effective technique to tune the gyrotron operating frequency. The step-tunability of the 236 GHz gyrotron within the frequency range of ±10 GHz in steps of 2–3 GHz is demonstrated in numerical simulations. A hybrid-type Quasi-Optical Launcher (QOL) has been designed for a step-frequency tunable gyrotron with sufficiently high Fundamental Gaussian Mode Content (FGMC).

I/O System for A 77/154-GHz, 0.5-MW Dual Regime Gyrotron

This paper contributes to the design of a common input/output (I/O) system to support a dual regime gyrotron operation. I/O system comprises of different subassemblies such as magnetron injection gun (MIG), quasi-optical launcher, and RF window. The operating frequencies selected are 77/154 GHz with the desired mode-pair TE15,4/TE28,8, respectively. The necessary selection criteria of such unified design for these subassemblies has been briefly discussed. The initial design parameters have been selected using our in-house code Gyrotron Design Suite version 3.0 2014 (GDSv3.0 2014). Final optimization of MIG to generate electron beam with proper parameters to support the maximum power transfer has been carried out using a particle trajectory code ESRAY. A dimpled wall, helical cut launcher has been designed to convert the complex higher order modes to a simple Gaussian-like beam and has been optimized with a commercial software launcher optimization tool. The design and optimization of RF window has been done using GDSv3.0, which will allow the chosen frequency beam to be extracted out from the gyrotron.

Reconstruction Progress of the COMPASS-D ECRH System on J-TEXT

To make the J-TEXT tokamak more flexible for physics experiments, an electron cyclotron resonance heating (ECRH) system is being developed. The main parts of the J-TEXT ECRH system are donated by the Culham Science Center from the former COMPASS-D, including six 60 GHz/200 kW/5 s (0.5 s) gyrotrons, six magnets, and auxiliary power supplies for the gyrotrons, transmission parts, and so on. For the ECRH system, the output mode of the gyrotron from the former Varian Associates, Inc. is TE $_{{\mathrm {02}}}$ mode and converted to TE $_{{\mathrm {01}}}$ mode for low loss transmission. The TE $_{{\mathrm {01}}}$ –TE $_{{\mathrm {11}}}$ –HE $_{{\mathrm {11}}}$ mode converter is used at the end of the transmission lines for efficient coupling. Then, the high-power wave is injected to the desired area of the plasma by a rotatable quasi-optical launcher. A 100 kV/60 A power supply based on a pulse step modulation technology has been developed to feed all the gyrotrons. The control system based on the compact reconfigurable I/O—9068 platform of National Instruments for monitoring, timing, and protection is being designed. Other units, such as the cooling loops, the launcher, and a spared auxiliary power supply, are on the way. We hope that four subsystems can be finally used on J-TEXT.

Design of 170 GHz, 1.5-MW Conventional Cavity Gyrotron for Plasma Heating

In this paper, an overall conceptual design of a 170 GHz, 1.5 MW, continuous wave (CW) conventional cavity gyrotron is presented for plasma heating applications in thermonuclear fusion reactors. The operating mode is carefully selected with due consideration of design constraints/goals and mode competition. The TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">36</sub> ,10 mode is selected as operating mode for the present study. A weakly tapered conventional cavity resonator is considered for the study of the RF-behavior. Single mode and multimode time dependence self-consistence calculations are carried out for power and efficiencies. In addition, the design studies of a triode type magnetic injection gun, magnetic guidance system, output system that consists of an optimized nonlinear taper, a highly efficient dimpled-wall quasi-optical launcher and a single disk Chemical Vapor Deposition diamond window are also reported. Results obtained support an output power of 1.5-MW CW power at 170 GHz with a conventional cavity gyrotron with 35% efficiency without single stage depressed collector. This device is intended to serve as heating source for international thermonuclear experimental reactorlike machines.

Improved method for analyzing quasi-optical launchers

An improved method for analyzing the radiation characteristic of the quasi-optical launcher is presented. The launcher is decomposed into an open-ended waveguide and a helical cut according to the proposed method. The radiation from the open-ended waveguide is obtained by using the Stratton—Chu formulation. The helical cut's radiation is calculated based on its current distribution, gained by several iterative computations, which helps to improve the calculation accuracy since the diffraction effect introduced by the helical cut is considered during the calculation. The proposed method is used to study different launches, and the results are compared with the existing results. Good agreement is achieved between the results obtained from our proposed method and the reported results. The present results provide an alternative for analysis and synthesis of the optical-mode converter.