Quasi-optical Launcher Research Articles

A Method for Gyrotron Beam Coupling into a Corrugated Waveguide

In the radio-frequency (rf) power transmission system of an electron cyclotron heating and current drive (EC H&CD) system, the gyrotron power should couple with the fundamental mode of the corrugated waveguide (HE11 mode) because unwanted higher-order modes affect the beam radiation characteristics, which is a problem in the quasi-optical launcher design. To achieve high HE11 mode purity, a beam coupling method that measures the transmission mode in the waveguide was examined using a 170-GHz high-power gyrotron for the first time. In beam coupling, the offset and tilt angle of the input beam at the waveguide inlet were minimized by controlling the angles of the mirrors in the matching optical unit (MOU) to minimize unwanted LP11 modes in the waveguide. The rf field profile in free space after 1.3 m of the waveguide from the MOU was measured, and the transmission mode content was analyzed. According to the analyzed mode content, the HE11 mode content was optimized by remote adjustment of the mirror angles with a digital controller. The optimization procedure of beam coupling achieved 95% of HE11 mode purity at the entrance of transmission line, which is the first demonstration that meets the criteria of the ITER EC H&CD system.

The Real-Time system for MHD activity control in the FTU tokamak

The Real-Time system for the control of the magnetohydrodynamics instabilities in FTU tokamak is presented. It is based on both a-priori information derived from statistical treatment of a database and Real-Time elaboration of live diagnostics data. The analysis codes are executed in different time threads based on multi-processors machines. The actuator is the 2×0.4MW 140 GHz ECRH system equipped with the new fast quasi-optical steerable launcher.

Gyrotron and its Electron Beam Source: A Review

Microwave occupies a glorious position in the electromagnetic spectrum and in that there are a number of devices in this frequency regime which are capable of high power operations. Among them, gyrotron has proven to be an efficient source for radio frequency (RF) generation at high power level and up to very high frequency. The gyrotron consists of several components like electron beam source, interaction structure, quasi-optical launcher, collector, RF window, magnet system, etc. All the components have their distinct role in the function of the device. Among them, electron beam source also called magnetron injection gun (MIG) is the generator of electron beam and it is very essential that MIG should produce and provide electron beam suitable for the beam-wave interaction at the interaction structure for the effective power growth. The paper presents the introduction of a microwave tube, gyrotron and its components alongwith review of the previous work, the background and the applications. The functions of various components of a gyrotron are discussed with particular highlighting on the electron beam emission from the electron beam source and the beam-wave interaction for power growth in the device. A review on different types of gyrotron electron beam sources is also presented.

Analysis of a Quasi-Optical Launcher Toward a Step-Tunable 2-MW Coaxial-Cavity Gyrotron

A high-efficiency quasi-optical launcher for coaxial-cavity gyrotrons has been investigated with the mode-matching analysis method. The launcher designed for the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">34,19</sub> mode at 170 GHz is analyzed from 118 to 218 GHz with 67 modes. This investigation is a preliminary study of a launcher toward a step-tunable 2-MW coaxial-cavity gyrotron. Twenty-four modes with fundamental Gaussian mode content (FGMC) that is higher than 90% have been found. At last, nine modes of interest are selected for SURF3D simulations to investigate the transmission properties of millimeter-wave beams through the output window. The results reveal that the beams have deviations of their center points at the RF window within only 2.1 mm, and the FGMCs range from 92.67% to 96.67%. This result indicates that even the present launcher with specific deformations designed for only one mode would be capable of a step-tunable operation.

Nuclear-Safety-Related and Shielding Analyses of the ITER Quasi-Optical ECH Launcher

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Nuclear-safety-related and shielding analyses were performed for the quasi-optical design of the electron-cyclotron-heating launcher installed in the ITER upper port. The critical nuclear responses, affecting the nuclear safety and technical performance of the launcher and nearby components, were calculated using the MCNP5 3-D radiation transport code. The computer-aided design (CAD) interface program McCad was applied for the conversion of the CAD model into MCNP geometry description. The radiation effects of the launcher's halved internal shield were estimated on the chemical-vapor-deposit diamond windows, vacuum vessel, and toroidal field coils. It was revealed that the nuclear-safety requirements were satisfied. </para>

DESIGN OF A 60 GHz, 100 kW CW GYROTRON FOR PLASMA DIAGNOSTICS: GDS-V.01 SIMULATIONS

In this work, the design studies of a 60GHz, 100kW CW gyrotron have been presented. Mode selection is carefully studied with the aim of minimizing mode competition and to yield a perfect solid beam output through an RF window with a suitable dimpled-wall quasi-optical launcher. Cavity design and interaction computations are then carried out. In addition, preliminary design of the magnetron injection gun, magnetic guidance system, launcher, and RF window are presented. Thus, we present a feasibility study, which indicates that the operation of such a gyrotron is possible and can give a power in excess of 100kW at an e-ciency > 35%. As a part of this work, a complete Graphical User Interface package \GDS V.01 (Gyrotron Design Suit Ver.01) has been developed for the design and conceptualization of speciflc gyrotrons.

Progress in the 10-MW 140-GHz ECH System for the Stellarator W7-X

During the last years, electron cyclotron heating (ECH) was proven to be one of the most attractive heating schemes for stellarators because it provides net-current-free plasma startup and heating. Both the stellarator Wendelstein 7-X (W7-X), which is under construction at the Max-Planck-Institut fu umlr Plasmaphysik, Greifswald, Germany, and the International Thermonuclear Experimental Reactor (ITER) tokamak, which will be built in Cadarache, France, will be equipped with a strong ECH and current-drive system. Both systems are comparable in frequency and have continuous-wave capability (140 GHz, 10 MW for W7-X and 170 GHz, 24 MW for ITER). The commissioning of the ECH plant for W7-X is well underway; the status of the project and the first integrated full-power test results from two modules are reported and may provide valuable input for the ITER plant. The ten gyrotrons at W7-X will be arranged in two subgroups symmetrically to a central beam duct in the ECH hall. The RF wave of each subgroup will be combined and transmitted by a purely optical multibeam-waveguide (MBWG) transmission line from the gyrotrons to the torus. The combination of five 1 MW gyrotron beams to one beam line with a power of 5 MW reduces the complexity of the system considerably. The single- and MBWG mirrors and the polarizers have been manufactured. Cold tests of a full-size uncooled prototype line delivered an efficiency exceeding 90%. The microwave power will be launched to the plasma through ten chemical-vapor-deposited-diamond barrier windows and in-vessel quasi-optical plug-in launchers, allowing each 1-MW RF beam to be steered independently. The polarization, as well as the poloidal and toroidal launch angles, will be adjusted individually to provide optimum conditions for different heating and current-drive scenarios.

Detailed Beam-Pattern Calculations for the ITER Electron Cyclotron Heating and Current Drive Upper Launcher

Electron cyclotron resonance heating (ECRH) and electron cyclotron current drive systems in fusion-grade devices meet the severe requirements (in terms of high power handling capability, extended steering range, and room availability) that guide the design of complex multiple-mirror quasi-optical launchers. A valuable step in this process is a beam-pattern calculation in vacuum including relevant electromagnetic effects not easily included in analytical evaluations. In fact, the analytical approach is a means to study the design layout at a first order and is able to derive the relevant quantities as a function of the steering angle and of the beam path in a form suitable to interface with most of the currently available beam-tracing codes. On the other hand, electromagnetic calculations using physical optics tools provide a complete description of the resulting full beam pattern, including the effects of aberration, beam truncation, thermal deformation of the mirrors, and the surrounding structures. Moreover, numerical calculation with reliable and benchmarked codes is a very efficient way to test subsequent updates of a given launcher model, once the basic geometry has been implemented. In this paper, we discuss in particular the application of the GRASP® code to the case of the remote steering option for the ITER ECRH upper launcher.

Optimization of Quasi-Optical Launchers for Multifrequency Gyrotrons

Recent techniques that were developed for the synthesis of mode converters in quasi-optical launchers for single-frequency gyrotrons have been applied to the optimization of launchers for multifrequency gyrotrons. Using those techniques, a significant performance enhancement of a multifrequency launcher can be obtained.

General Astigmatic Beam Propagation in Complex Quasi-Optical Launchers: Application to the ITER ECRH Remote Steering Upper Launcher

Launching systems that use in sequence more than one mirror to direct and focus electron cyclotron (EC) waves with a sufficient steering capability in relevant absorption regions of fusion plasmas may produce general astigmatic beams. The double curvature of a generic reflecting surface, even induced by deformation effects in quasi-optical systems that handle high power, is an additional source of general astigmatism. Describing the propagation of general astigmatic Gaussian beams is a necessary step in the optimization phase of a complex EC resonance heating (ECRH) launcher, since simple astigmatism treatment does not reproduce the main feature of these beams, whose spot and phase ellipse orientation changes with propagation even in free space. The correct orientation for both spot ellipse and phase ellipse is one of the input key parameters to perform realistic calculations with beam-tracing codes, which aim to characterize a launching system in terms of localized heating and current drive efficiency. In this work we describe the influence of double-curvature effects and deformations on beam propagation in terms of beam dimensions and directions. In particular, we present an application of the theory of general astigmatic Gaussian beam propagation in vacuum to the case of the remote steering option for the ITER ECRH upper launcher. In this option beams are found to be strongly astigmatic, with a major/minor axis ratio in relevant absorption regions ranging from 2.3 to 4.4 in the cases examined. Furthermore, the major axis of the resulting spot ellipses presents an orientation angle variation (from the last mirror to the expected absorption regions) ranging from 9.1 to 22.8 deg in the cases investigated. The final orientation is close to a vertical direction with respect to the equatorial plane of ITER.

Sub-THz Beam-Shaping Mirror System Designs for Quasi-optical Mode Converters in High-power Gyrotrons

Methods, algorithms and theories required in sub-THz beam-shaping mirror designs for Quasi-Optical (QO) mode converters (launchers) in high-power gyrotrons are presented in this article. The state-of-the-art design procedure highlights many novel ideas like the Iterative Phase Retrieval (IPR) technique, the Taylor-Interpolation FFT (TI-FFT) algorithm and the iterative Geometrical Optical Phase Correction (GOPC) method, which ensure fast, reliable, flexible and accurate sub-THz beam-shaping mirror system designs in the high-power gyrotron applications. The efficiency of the sub-THz beam-shaping mirror system procedure has been successfully tested in a sub-THz beam-shaping 4 -mirror system design for a single-mode (TE22,6/110GHz), 1.25 MW CPI gyrotron to shape the input beam from a QO launcher into the target Fundamental Gaussian Beam (FGB) to be injected into the corrugated waveguide through the diamond window, with up to 99.85% coupling between the designed output beam and the target FGB.

Optimal synthesis of quasi-optical launchers for high-power gyrotrons

Quasi-optical launcher designs for high-power gyrotrons use perturbations of the waveguide surface following the cavity to convert the high order cavity mode to a Gaussian-like beam, which is then radiated from a spiral cut of the waveguide. The design technique of these launchers is based on an analytic theory that specifies the mode amplitudes to obtain a Gaussian-like distribution of the field in the azimuthal and longitudinal directions. This approach can generate a beam with high Gaussian content and low diffraction losses. However, by adjusting the mode amplitudes to obtain a Gaussian-like distribution of the field in the radial (instead of the azimuthal) direction, designs with significantly less diffraction loss, reduced beam divergence angles, and higher Gaussian beam content can be achieved

Characterization of Quasi-Optical Launcher for ECRH in SST-1

The design and characterization of a high-power microwave launcher have been discussed. The launchers have been designed for electron cyclotron resonance heating of the plasma in the Steady-State Superconducting Tokamak (SST-1). High-power microwave beam launchers mainly consist of a focusing mirror and a plane mirror combination to focus the beam at a specified location. Two microwave beam launchers are fabricated to launch the waves from the low magnetic field side (radial port) as well as from the high magnetic field side (top port). The frequency of operation is 82.6 GHz, and the power is 200 kW/continuous wave. A gyrotron capable of delivering 200 kW power at 82.6 GHz is under fabrication at M/s. Gycom. The mirrors of the launchers are based on quasi-optical Gaussian beam theory. The mirrors provide cooling for long-pulse (1000-s) operation. Low-power microwave characterization is done to check the performance of the launchers. A low-power microwave beam incident on the focusing mirror of the launcher and focusing action is analyzed at the output of the launcher.

Surface integral equation analysis of quasi-optical launchers

Modern high-power gyrotrons typically use a mode converter and launcher to convert the high-order cavity mode to a Gaussian like output mode. Efficiencies of the conversion are usually only 85%-90%. The analysis codes used to design these systems are based on approximate techniques which are not sufficiently accurate to allow design of higher efficiency converters. We have developed an analysis code based on the magnetic field integral equation. This approach will give engineers a tool for design of more efficient converter and launcher systems.

Gaussian beam excitation of quasi-optical launchers for lower hybrid waves

An analysis of a quasi-optical launcher of lower hybrid plasma waves powered by an incident Gaussian beam is performed. This study, based on the solution for the scattering by an array of conducting cylinders with parallel axes close and parallel to a plane interface, is presented together with numerical results. An experimental configuration that gives high values of the coupled power is proposed

Gaussian beam diffraction by a quasi-optical grating for coupling to lower-hybrid plasma waves

This paper is dedicated to the theoretical approach to the problem of modelling quasi-optical launchers of plasma waves in the lower-hybrid range of frequencies, when a gaussian beam is the incident field. Numerical results are presented and discussed comparing the cases of a gaussian beam and a plane wave as an incident field [1].

Quasi-optical launchers for lower hybrid waves: a full wave approach

Theoretical investigations on the use of an advanced launcher to couple lower hybrid waves to a plasma, for current drive in tokamaks, have been developed. A study of the coupling has been carried out in a rigorous way, through the solution of the scattering by an array of cylinders with parallel axes in the presence of a plane of discontinuity for the electromagnetic constants. The general features of the proposed method are presented together with results on the launched spectra and coupled power. The numerical results of this work are relevant to the optimization of the single layer quasi-optical grill launching RF power towards a constant density plasma: it is shown that the power coupled in the plasma can reach values of about 25-30% in the -1 diffracted order and about 40% in all other orders. As a consequence it is expected that the coupling values for a suitable double layer configuration should approach those for total transmission

Electron Cyclotron Resonance Heating Transmission Line and Launching System for the Wendelstein VII-AS Stellarator

A 70-GHz electron cyclotron resonance heating (ECRH) system (4 {times} 0.2 MW for 3 s and 0.2 MW for 0.1 s) has been installed on the Wendelstein VII-AS stellarator for plasma buildup, heating, and current drive. Five commercially available gyrotrons, each delivering 0.2-MW radio-frequency (rf) output power, serve as the ECRH sources. The microwaves are transmitted to the stellarator by conventional oversized waveguides, and they convert the gyrotron output mode to a linearly polarized Gaussian free-space mode, which is launched to the plasma at arbitrary poloidal and toroidal angles by a quasi-optical launcher. The transmission line and launching system as well as the related rf diagnostics are discussed in detail. Experimental results on heating and current drive obtained with various kinds of microwave launching are presented.