射频传输系统功率补偿研究

平行分布的天线阵电流带之间的磁耦合将会引起电流带单元之间的功率串扰,造成系统功率传输不平衡,这种功率串扰将会对射频系统的控制与稳定运行产生严重影响;为了抑制电流带之间的功率串扰,一种去耦网络被用来补偿由互耦产生的功率流,实现了功率馈入端口的有效隔离,功率平衡问题得到解决。本文基于微波网络工程理论,运用导纳矩阵对功率补偿的原理进行了深入分析,并对去耦网络的调试和测试方法进行了详细介绍。     

Water-cooling effects on the high-voltage and long-pulse capabilities of the KSTAR ICRF antenna

A new ICRF antenna originating from the prototype antenna was constructed for the KSTAR tokamak in 2002. The performance of the antenna was experimentally estimated at the RF test stand without a plasma. Recently three series of RF tests were performed at a frequency of 30 MHz; without any cooling, with a water-cooling for only the antenna, and with a water-cooling of the antenna and the transmission line connected to the antenna. In the tests, a half of the current strap was connected to a RF source via a matching circuit with the other half one connected to an open terminated coaxial line, and the other three straps were shorted at the input ports. During the RF pulse, the temperatures at several positions of the antenna cavity wall were measured by embedded thermocouples and the temperature profile of the front face of the antenna was measured by an IR camera. The line voltage, forward and reflected powers, and the RFTC pressure were also measured. The water-cooled antenna showed several enhanced performances in a comparison with the non-cooled case, and the standoff voltage was significantly increased. By utilizing a water-cooling of the antenna and the transmission line, we achieved a standoff voltage of 41.3 kVp for a pulse length of 300 s, and we could extend the pulse length up to 600 s at a maximum voltage of 35.0 kVp without encountering any problems, which considerably exceeds the design requirements.     

Design of improved compact decoupler based on adjustable capacitor for EAST-ICRF antenna

The Ion Cyclotron Radio Frequency (ICRF) heating antenna on EAST adopts a decoupling device to constrain power coupling among the radiation straps, which was discovered shortcomings such as long size, poor contact, and etc. In order to improve these weak points, a new type decoupler with terminal-loaded tunable capacitor is designed to replace the previous design. Besides the capability of the tunable admittance parameters of decoupler, the withstand voltage of the capacitor is the most significant consideration for working under high power. Therefore, the theoretical analysis carefully elaborates the capacitor withstand voltage, and the detailed analytical equations and criteria for design are given. After the comparative analysis of theoretical calculation and 3D simulation results, the decoupler design scheme is finalized. The capacitor-loaded decoupler has been successfully adopted for ICRF antenna at port N on EAST, and achieved the optimization of adjacent port isolation from −22 to −58 dB at 37 MHz without plasma to restrict mutual coupling. The new design of the decoupler has greatly improved its compactness and automatic adjustment performance, and could be good solution for the decoupling network of ICRF antennas.     

Design, fabrication and testing of UHV compatible high power RF devices for lower hybrid current drive system on SST-1 tokamak

Maintaining plasma current under steady state conditions is one of the most important pre-requisites for a tokamak-based reactor. Lower hybrid current drive (LHCD) system aims to drive tokamak plasma current by means of RF power. The LHCD system on SST-1 tokamak is based on two 500 kW, CW klystrons operating at 3.7 GHz. A waveguide transmission line transmits power from source to the antenna. A phased array waveguide antenna is used to couple power to the plasma. The antenna side of the transmission line is placed inside the tokamak vacuum vessel. The design and fabrication of this In-Vessel system has to satisfy the demands of high power RF as well as ultra high vacuum (UHV) compatibility. This paper describes some of the critical UHV compatible In-Vessel RF devices, their design, fabrication, and test results.     

Parametric study of one triplet of the ITER ICRH antenna by numerical modeling

Each of the two ITER ICRF antennas consists of a close-packed array of 24 straps arranged in a 6 poloidal by 4 toroidal array. Three poloidally adjacent straps (a “triplet” of straps) are fed together through a 4-port junction from one 20 Ohm feeding line. The complete array has to radiate 20 MW of RF power over a frequency range of 40 MHz to 55 MHz and for different toroidal phasings. The RF optimization of the antenna has been performed numerically on one triplet of straps (1/8th of the antenna) [1], [2]. In parallel a number of reduced-scale mock-ups of one triplet of the ITER ICRH antenna were constructed in order to validate the results of the numerical optimization [1], [3].The aim of this work is primarily to benchmark the CST MWS^® [4] numerical modeling against numerous measurements done on the mock-up of the 2007 design. Moreover MWS calculates the 3D distribution of the currents and of the fields of the triplet. Hence it gives the possibility to check the fields and current distributions resulting from the optimisation study of the ITER ICRH antenna triplet done by changing geometrical parameters of the straps and antenna box of the mock-up of 2007 design [1], [2], [3]. The considered parameters are: strap width, antenna box depth and vertical septum recess with respect to the front of the current strap. The impact of the presence of the Faraday screen is also evaluated.Excellent agreement between modeled and measured S parameters is obtained. Analysis of the fields and currents distributions on the straps is reported. Excellent current balance is confirmed.     

Mechanical design of the second ICRF antenna for EAST

In order to satisfy the requirements of heating plasma on EAST project, 3 MW ion cyclotron range of frequency (ICRF) heating system will be available at the second stage. Based on this requirement, the second ICRF antenna, has been designed for EAST. The antenna which is planned to operate with a frequency ranging from 30 MHz to 110 MHz, comprises four poloidal current straps. The antenna has many cooling channels inside the current straps, faraday shield and baffle to remove the dissipated RF loss power and incoming plasma heat loads. The antenna is supported via a cantilever support box to the external support structure. Its assembly is plugged in the port and fixed on the support box. External slideway and bellows allow the antenna to be able to move in the radial direction. The key components of the second ICRF antenna has been designed together with structural and thermal analysis presented.     

Commissioning the ICRF system and an ICRF assisted discharge cleaning at the KSTAR

Korean superconducting tokamak advanced research (KSTAR) is a national superconducting tokamak with the aim of a high beta operation based on advanced tokamak (AT) scenarios, and an ion cyclotron ranges of frequency (ICRF) heating is one of the essential tools to achieve this goal. The fabrication and high voltage (HV) test of the antenna and the matching system were finished in 2006 and the installation of the antenna, matching system and the transmitter at the KSTAR site was completed in 2007. Antenna conditioning was carried out to improve the HV holding condition of the antenna installed on the KSTAR and to check on the electro-magnetic (EM) interference with other equipments such as the superconducting magnet monitoring system and other machine and/or plasma diagnostic systems. The first KSTAR tokamak experimental campaign started by a vacuum pumping, a cryostat cooling and an ICRF system contributed to the successful tokamak shots through an ICRF assisted discharge cleaning of the vacuum vessel. In this paper, the installation processes of the ICRF system (with an emphasis on the quality assurance procedures of KSTAR), as well as the results from the first RF discharge experiment for the discharge cleaning and FWEH (fast wave electron heating) experiment for the KSTAR 1st experimental campaign are outlined.     

EASTICRF天线电流带电磁分析

为了实现EAST托卡马克1000s以上的稳态先进模式运行的最终物理目标,两电流带双环共振(RDL)离子回旋共振（ICRF）天线被选择用来加热,电流带是ICRF天线关键部件,它通过近场区的耦合把能量传输到等离子体中。本文通过有限元方法对电流带在等离子体破裂和等离子体垂直位移事件两种工况下进行了电磁计算,给出了电流带感应电流密度大小分布情况、磁感应强度大小分布情况以及电流带所受的电磁力。利用电流带所受的电磁力作为载荷对电流带进行了结构分析,分析结果为验证电流带结构的可行性提供理论依据,分析方法对未来更高功率的ICRF天线电流带进行电磁分析具有一定的借鉴价值。     

Influence of the plasma profile and the antenna geometry on the matching and current distribution control of the ITER ICRF antenna array. Optimization of the decoupling-matching system

The eight triplets of straps of the ITER ICRF antenna array are fed through 8 matching circuits and 4 hybrids to ensure load resilience. Decouplers are used to mitigate the effects of triplet mutual coupling. They also control the array phasing. The electrical constraints on the decouplers for different layouts with heating (H) or current drive (CD) phasing are compared starting from the TOPICA matrix computed for the last antenna plug design and the reference (most pessimistic) plasma profile “2010low” provided by IO. It is shown that this last profile provides a significant decrease of plasma coupling and increase of mutual coupling with respect to the previous reference profile “Sc2short17”. This results in a larger range of decoupler reactance X_dec and voltage V_Xdec needed. This range can be reduced when using 10 decouplers instead of the 7 needed for the same forward power P_Gk+ of the 4 power sources. For H phasing only 4 decouplers could be used but with different P_Gk+ (P_Gk+ ratio up to 1.5–2.5). For CD phasing and same plasma profile the power capability P_tot is increased by 25% with a decoupler layout allowing much smaller poloidal phasing than the 90° provided by the hybrids. A decrease of the distance antenna-plasma profile reduces the normalized decoupler voltage V_Xdec/√P_tot with no significant change of the X_dec range. The recess of the vertical septa between the strap boxes increases the plasma coupling but has the drawback of also increasing the mutual coupling between triplets: the needed range of X_dec and of V_Xdec/√P_tot is increased in proportion.     

Status of the ITER Ion Cyclotron H&CD system

The ongoing design of the ITER Ion Cyclotron Heating and Current Drive system (20 MW, 40–55 MHz) is rendered challenging by the wide spectrum of requirements and interface constraints to which it is subject, several of which are conflicting and/or still in a high state of flux. These requirements include operation over a broad range of plasma scenarios and magnetic fields (which prompts usage of wide-band phased antenna arrays), high radio-frequency (RF) power density at the first wall (and associated operation close to voltage and current limits), resilience to ELM-induced load variations, intense thermal and mechanical loads, long pulse operation, high system availability, efficient nuclear shielding, high density of antenna services, remote-handling ability, tight installation tolerances, and nuclear safety function as tritium confinement barrier. R&D activities are ongoing or in preparation to validate critical antenna components (plasma-facing Faraday screen, RF sliding contacts, RF vacuum windows), as well as to qualify the RF power sources and the transmission and matching components. Intensive numerical modeling and experimental studies on antenna mock-ups have been conducted to validate and optimize the RF design. The paper highlights progress and outstanding issues for the various system components.     

Development status of KSTAR 5 GHz LHCD system

A steady-state lower hybrid current drive (LHCD) system is under development for advanced tokamak experiments of the Korea Superconducting Tokamak Advanced Research (KSTAR) device. The KSTAR 5 GHz steady-state LHCD system is being designed to couple an input power of 2 MW for 300 s generated by four 5 GHz klystrons. For the development of this system, there are two critical issues. One is the development of a 5 GHz CW klystron for the RF source of the system. The other is the design of a steady-state LH launcher with active water cooling. In this paper, the current status of the development and design for the KSTAR steady-state LHCD system is described. For the LHCD system, aiming at a basic experimental study of 5 GHz LH wave propagation and operational experience with an LHCD system, the installation of an initial LHCD system with a capacity of 0.5 MW for 2 s is scheduled in 2010 using a 5 GHz prototype klystron and an un-cooled 1 MW launcher. The design and progress for the initial LHCD system are also presented.     

Recent ICRF coupling experiments on EAST

Recent ion cyclotron resonance frequency(ICRF) coupling experiments for optimizing ICRF heating in high power discharge were performed on EAST. The coupling experiments were focus on antenna phasing and gas puffing, which were performed separately on two ports of the ion cyclotron resonance heating(ICRH) system of EAST. The antenna phasing was performed on the I-port antenna, which consists of four toroidally spaced radiating straps operating in multiple phasing cases; the coupling performance was better under low wave number ∣k_‖∣(ranging from 4.5 to 6.5). By fuelling the plasma from gas injectors, placed as uniformly spaced array from top to bottom at each side limiter of the B-port antenna, which works in dipole phasing, the coupling resistance of the B-port antenna increased obviously.Furthermore, the coupling resistance of the I-port antenna was insensitive to a smaller rate of gas puffing but when the gas injection rate was more than a certain value(1021 s~(-1)), a sharp increase in the coupling resistance of the I-port antenna occurred, which was mainly caused by the toroidal asymmetric boundary density arising from gas puffing. A more specific analysis is given in the paper.     

Power Compensation for ICRF Heating in EAST

The source system covering a working frequency range of 24 MHz to 70 MHz with a total maximum output power of 12 MW has already been fabricated for Ion Cyclotron Range of Frequency(ICRF) heating in EAST from 2012. There are two continuous wave(CW) antennas consisting of four launching elements each fed by a separate 1.5 MW transmitter. Due to the strong mutual coupling among the launching elements, the injection power for launching elements should be imbalance to keep the k||(parallel wave number) spectrum of the launcher symmetric for ICRF heating. Cross power induced by the mutual coupling will also induce many significant issues,such as an uncontrollable phase of currents in launching elements, high voltage standing wave ratio(VSWR), and impedance mismatching. It is necessary to develop a power compensation system for antennas to keep the power balance between the feed points. The power balance system consists of two significant parts: a decoupler and phase control. The decoupler helps to achieve ports isolation to make the differential phase controllable and compensate partly cross power. After that, the differential phase of 0 or π will keep the power balance of two feed points completely. The first power compensation system consisting of four decouplers was assembled and tested for the port B antenna at the working frequency of 35 MHz. With the application of the power compensation system, the power balance, phase feedback control, and voltage standing wave ratio(VSWR) had obviously been improved in the 2015 EAST campaign.     

Time-dependent simulations of feedback stabilization of neoclassical tearing modes in KSTAR plasmas

A simulation is performed for feedback stabilization of neoclassical tearing mode (NTM) by electron cyclotron current drive (ECCD) for KSTAR in preparation for experiments. An integrated numerical system is constructed by coupling plasma transport, NTM stability, and heating and current drive modules and applied to a KSTAR plasma by assuming similar experimental conditions as ASDEX Upgrade to predict NTM behaviors in KSTAR. System identification is made with database produced by predictive simulations with this integrated numerical system so that three plasma response models are extracted which describe the relation between the EC poloidal launcher angle and the island width in KSTAR. Among them, the P1DI model exhibiting the highest fit accuracy is selected for designing a feedback controller based on the classical Proportional–Integral–Derivative (PID) concept. The controller is coupled with the integrated numerical system and applied to a simulation of NTM stabilization. It is observed that the controller can search and fully stabilize the mode even though the poloidal launch angle is misaligned with the island initially.     

Design and implementation of power and phase feedback control system for ICRH on EAST

Ion cyclotron wave resonance heating(ICRH) is one of the most important auxiliary methods to heat plasma in the Experimental Advanced Superconducting Tokamak(EAST). Several megawatts of power is transmitted through separate coaxial lines and coupled with the plasma through arrays of loop antennas. The parameters of the ICRH system, including the injected power and phasing between antenna straps, are critical to the coupling efficiency of the power as well as the resulting impact on the heating efficiency. In this paper, we present a system for feedback control of the phase between the current straps and the ICRH power on EAST. The feedback control system was tested using both a matched dummy load and a plasma load, and it successfully maintained stable operation in the 2016 EAST campaign. Good control of the injected power and wave phases was achieved during edgelocalized mode operation.     

Summary and results of the study of the hybrid matching option implementation of the ITER ICRH system

The hybrid option has been chosen for the load-resilient matching of the ITER ICRH system. The corresponding 3 dB hybrid matching circuit has been developed and fully implemented on a low-powered scaled mock-up at the LPP-ERM/KMS. The circuit features 23 simultaneously active feedback loops to minimize reflections on the 8 triplet lines, reduce the mutual coupling between the triplets and control the antenna array strap current distribution. The article discusses the employed methods for the implementation on a test-bench of the matching system including the derivation of closed-form analytical matching solutions for the tuners, the development of real-time control algorithms and the feedback systems. Furthermore, the achieved results are compared to numerical simulations of the ideal transmission line model of the matching system and the significance of these results for the extrapolation to the ITER case is discussed.     

Analysis and design of the Alfven wave antenna system for the SUNIST spherical tokamak

Analysis and design of the Alfven wave antenna system for the SUNIST spherical tokamak are presented. Two candidate antenna concepts, folded and unfolded, are analyzed and compared with each other. In the frequency range of Alfven resonance the impedance spectrums of both two concept antennas for major modes are numerically calculated in a 1-D MHD framework. The folded concept is chosen for engineering design. The antenna system is designed to be simple and requires least modification to the vacuum vessel. The definition of the antenna shape is guided by the analyses with constraints of existing hardware layouts. Each antenna unit consists of two stainless steel straps with a thickness of 1 mm. A number of boron nitride tiles are assembled together as the side limiters for plasma shielding. Estimation shows that the structure is robust enough to withstand the electromagnetic force and the heat load for typical discharge duty cycles.     

Commissioning of 3.7 GHz LHCD system on ADITYA tokamak and some initial results

To drive plasma current non-inductively, a lower hybrid current drive (LHCD) system has been designed, fabricated and successfully installed on ADITYA tokamak. The system is designed to launch 120 kW of RF power, at a frequency of 3.7 GHz. The system mainly consists of a high power CW klystron source, a long waveguide transmission line of about 100 m length, a UHV compatible modular waveguide line of about 2.65 m, and a conventional grill type antenna. Independent phase shifters, one each in the eight lines, are used to adjust the antenna phasing and also provides the flexibility to launch a composite spectrum. The antenna is designed to launch lower hybrid waves (LHW) with parallel refractive index (N_||), in the range, 1 < N_|| < 4.5, by appropriately phasing the antenna. Antenna is positioned in the shadow of the poloidal limiter and is provided with 100 mm radial movement to achieve optimum coupling conditions.The complete system development includes design, fabrication and testing of number of waveguide components, modular waveguide lines and their integration. Different cost effective fabrication techniques are adopted to achieve good RF performance. Special attention is paid on the flanged joint seals in the long transmission line to minimize the RF losses. The entire LHCD system is calibrated, especially, in terms of phase, insertion loss and return loss measurements.After the successful integration of the system on ADITYA tokamak, some initial experiments have been carried out to assess the system commissioning and its performance. The experiments were done with a plasma (hydrogen) density of 2-5 × 10¹² cm⁻³ at a toroidal magnetic field of 0.8 T with 10-25 kA of plasma current. Initial results indicate that, good coupling is achieved in the presence of proper edge density. Measurements obtained from second harmonic electron cyclotron emission (ECE) and hard X-ray diagnostics suggest generation of suprathermal electrons in the presence of LH pulse. Plasma current pulse elongation with LH power is observed but needs further investigation to derive conclusions.This paper presents the design, fabrication, testing and integration of the waveguide lines, waveguide components and UHV compatible modular transmission lines of the LHCD system on ADITYA tokamak and discusses some of the initial results.     

Electrical model parameters identification of radiofrequency discharge in argon through 1D3V/PIC-MC model

This work represents a contribution to the modeling of a radiofrequency (RF) discharge in argon at low pressure (from 25 to 200 mTorr). It is started by the validation of the collision cross sections used in the particle model through a comparison between the transport coefficients calculated by these data and the measurements of the transport coefficients already exist in the literature, the particle model is also validated by a comparison between the calculated plasma density and that measured in the literature. The electrical model proposed in this work consists of replacing the RF discharge by a passive circuit (resistance in series with a capacitor), where the resistance represents the plasma medium and the obstruction of the passage of the electronic current, and the capacitor represents the sheaths and the appearance of the displacement current in these regions. The parameters of the electrical model are obtained through particle modeling. The electrical model presented accurately reproduces the current of the discharge, but without considering the phenomenon of distortion. The total harmonic distortion rate follows the variation of the plasma density; its maximum value is 5.75％ at 100 mTorr.     

Design criteria and current status of LHCD system on SST1 machine

The lower hybrid current drive (LHCD) system for superconducting steady state tokamak-1 (SST-1) machine is in advanced stage of integration and commissioning. The system is designed to launch 1 MW of RF power at a frequency of 3.7 GHz to sustain 220 kA of plasma current non-inductively for 1000 s. The system employs a conventional grill antenna (having 2 × 32 waveguides), vacuum and air transmission lines and high power source system. A new design for vacuum transmission line, which enables better vacuum and RF compatibility has been successfully executed and tested. The transition from the narrow waveguide to WR284 waveguide system is achieved through a simple design, having stacks of copper plates with the waveguides milled in them, referred as transforming module and is successfully tested for mechanical, vacuum and RF performance. Many of the critical components have been successfully fabricated and tested as per the designs. The components and sub-systems are made ready and the integration is in progress. All the components are actively cooled and are compatible with 1000 s operation.The performance tests and current status of various sub assemblies and partially integrated LHCD system is discussed in detail in this paper.