Results of ICRF Heating Experiments from the EAST 2010 Campaign

Radio frequency(RF) plasma heating in ion cyclotron range of frequencies(ICRF)was successfully performed on the Experimental Advanced Superconducting Tokamak(EAST).This is mainly because lithium wall conditioning was routinely used to reduce both impurity and hydrogen(H) recycling and to improve the ICRF power absorption.Mainly ICRF heating of the H minority regime at 27 MHz has been applied in deuterium plasmas.The ion cyclotron resonance heating(ICRH) is found to depend strongly on plasma preheating.The ICRH efficiency can be much improved in conjunction with the lower hybrid wave(LHW).Effective ion and electron heating was observed with the H minority heating mode.The increase of the stored energy reached30 kJ in L-mode plasma by using the ICRF power of 1.0 MW alone when the H cyclotron resonance layer was at plasma center.     

Optimization Study of ICRF Hydrogen Minority Heating in a Deuterium Plasma of EAST

The full wave TORIC code and the Kinetic Fokker-Planck SSFPQL code are combined to perform self-consistent simulations of the ICRF heating in the EAST 2D magnetic configuration.The combined package is applied to the ICRF hydrogen minority heating in a deuterium plasma with the hydrogen concentration up to 10%.The fast wave propagation and absorption properties,power partitions among the plasma species and the RF driven energetic tails have been analyzed.Meanwhile,in order to optimize the ICRF heating,changing the resonance locations has also been considered in EAST plasmas.     

Observation of Central Toroidal Rotation Induced by ICRF on EAST

Core plasma rotation of both L-mode and H-mode discharges with ion cyclotron range of frequency(ICRF) minority heating(MH) scheme was measured with a tangential X-ray imaging crystal spectrometer on EAST(Experimental Advanced Superconducting Tokamak).Cocurrent central impurity toroidal rotation change was observed in ICRF-heated L-and H-mode plasmas.Rotation increment as high as 30 km/s was generated at ~1.7 MW ICRF power.Scaling results showed similar trend as the Rice scaling but with significant scattering,especially in L-mode plasmas.We varied the plasma current,toroidal field and magnetic configuration individually to study their effect on L-mode plasma rotation,while keeping the other major plasma parameters and heating unchanged during the scanning.It was found that larger plasma current could induce plasma rotation more efficiently.A scan of the toroidal magnetic field indicated that the largest rotation was obtained for on-axis ICRF heating.A comparison between lower-single-null(LSN)and double-null(DN) configurations showed that LSN discharges rendered a larger rotation change for the same power input and plasma parameters.     

EAST装置高功率宽带射频发射机系统研制

离子回旋波加热系统是EAST装置最重要的辅助加热工具,作为系统最核心的分系统之一,高功率射频发射机为加热等离子体提供射频波能量,对提高等离子体运行参数起着极为重要的作用。基于电路分析、传输线和波导谐振腔等相关工程理论,本文系统地总结了射频发射机系统高功率放大器输入输出回路、放大器级间匹配、寄生振荡抑制、腔体冷却等部分的设计原理和实现方法。在假负载上进行了系统测试,在设计频段内获得了1.5 MW的射频输出功率,测试结果表明系统达到了设计的技术指标。通过两轮EAST射频加热实验验证,发射机系统运行稳定可靠,满足射频加热等相关物理实验要求。     

Enhanced Plasma Performance by ICRF Boronization

Boronization with carborane(C2B10H12)by ICRF been applied routinely to the walls of HT-7 super-conducting tokamak for the reduction of impurity influx.especially carbon and oxygen.Significant suppression of metallic impurties and radiating power fraction are achieved.The imporved confinement for both particle and energy is observed in full range of operation parameters.Energy balance analysis shows that electron heat diffusion coefficient is strongly reduced.Measurements by Langmuir probes at the edge plasma show that poloidal velocity shear after boronization is changed to a profile to profile favoring to good confinement.The main emphasis of this paper is to describe effects of boronization on aspects of the enhanced plasma performance.     

ICRH ITER-like antenna tested on TS commissioning,electrical modeling and load resilience studies

A prototype of ICRF antenna based on the load-resilient electrical layout anticipated for ITER has been built at CEA-Cadarache. It consists of two toroidally adjacent resonant double loops (RDLs) based on the conjugate-T concept proposed for the ITER ICRF array. This prototype has been recently validated in Tore Supra plasmas exhibiting fast density perturbations in front of the antenna. This paper reports on the load resilience properties of the antenna prototype, as well as the RF modeling and the commissioning. Significant effort on modeling, coupled to an extensive low power campaign, has allowed characterization of the antenna in both vacuum and plasma loading conditions. Plasma load modelings computed with the code TOPICA – very helpful to set up the matching points on plasmas – are found to be in good agreement with the experimental results. The main studies focus on load resilience properties have been carried out in L-mode plasmas. Supersonic molecular beam injection (SMBI), able to launch a series of very short/dense gas jets at Mach number up to 5, was used to mimic the sudden increases of the antenna coupling provoked by ELMs. The results are found to be in good agreement with RF circuit calculations that include 3D modeling.     

Current Status of ICRF Heating Experiments on EAST

Radio frequency (RF) heating in the ion cyclotron range of frequencies (ICRF) is one of the primary auxiliary heating methods for EAST. The ICRF system provides 6 MW power in primary phase and will be capable of 10 MW later. Three 1.5 MW ICRF systems in a frequency range of 25 to 70 MHz have already been in operation. The ICRF heating launchers are designed to have two current straps with each driven by a RF power source of 1.5 MW. In this paper a brief introduction of the ICRF heating system capability in EAST and the preliminary results in EAST are presented.     

Experimental study of density pump-out on EAST

In the experimental advanced superconducting tokamak,density pump-out phenomena were observed by using a multi-channel polarimeter-interferometer system under different heating schemes of ion cyclotron resonant heating,electron cyclotron resonance heating,and neutral beam injection.The density pump-out was also induced with application of resonant magnetic perturbation,accompanied with a degradation of particle confinement.For the comparison analysis in all heating schemes,the typical plasma parameters are plasma current 400 k A,toroidal field 2 T,and line average density 2?×?10~(19)m~(-3).The experimental results show that the degree of pump-out is concerned with electron density and heating power.Low density deuterium low confinement(L-mode) plasmas(3.5?×?10~(19)m~(-3)) show strong pump-out effects.The density pump-out correlated with a significant drop of particle confinement.     

EAST ion cyclotron resonance heating system for long pulse operation

Radio frequency (RF) power in the ion cyclotron range of frequencies (ICRF) is one of the primary auxiliary heating techniques for Experimental Advanced Superconducting Tokamak (EAST). The ICRF system for EAST has been developed to support long-pulse high-β advanced tokamak fusion physics experiments. The ICRF system is capable of delivering 12 MW 1000-s RF power to the plasma through two antennas. The phasing between current straps of the antennas can be adjusted to optimize the RF power spectrum. The main technical features of the ICRF system are described. Each of the 8 ICRF transmitters has been successfully tested to 1.5 MW for a wide range of frequency (25–70 MHz) on a dummy load. Part of the ICRF system was in operation during the EAST 2012 spring experimental campaign and a maximum power of 800 kW (at 27 MHz) lasting for 30 s has been coupled for long pulse H mode operation.     

Theory on dynamic matching with adjustable capacitors for the ICRF system of ASDEX Upgrade

The coupling of electromagnetic waves in the Ion Cyclotron Range of Frequencies (ICRF) is an important method to heat magnetically confined plasmas. Changing plasma conditions, which originate from processes like L-mode to H-mode transition or gas puffing, vary the load impedance of the ICRF antennas. To optimize the power transfer from the radio frequency (RF) generators to the antennas and consequently to the plasma, as well as to protect the RF sources against too high reflected power, a system that matches (i.e. transforms) the antenna input impedance to the impedance required by the generator is necessary. At ASDEX Upgrade this matching system consists of two stub tuners for each antenna, which match the antenna impedance for a value preset before the discharge. The length of the stubs cannot be changed fast enough to compensate plasma variations even on the moderate timescale of the confinement time in ASDEX Upgrade. The use of 3 dB-couplers allows operation even with varying load, at the cost of a reduced power to the plasma.When adjustable capacitors are applied in parallel to the stubs, dynamic matching becomes possible on the tens of ms timescale. The paper describes first the calculation of the required capacitance using transmission line theory. In a second model a minimum search algorithm finds, for a given antenna impedance, the length of the stubs needed for matching, now including the initial values of the capacitors. For the chosen pre-match point in the Smith chart, the range of impedances around this point is calculated for which the voltage standing wave ratio (VSWR) can be lowered below a minimum value by readjusting the capacitors within their maximum and minimum values. The matching range is thereby significantly larger than without the application of adjustable capacitors, at least with a frequency of 30 MHz and 36.5 MHz.     

Heliotron E Results and Large Helical Project

Heliotron E(H-E) experiment was started in 1980. Until 1987 high power heating experiments for improving plasma parameters have almost finished. H-E firstly demonstrated that ECR heated plasmas are usable for target plasmas of NBI or ICRF heating to obtain high density and high temperature currentless plasmas. The highest electron temperature is 1.5keV and ion temperature is 1.6keV and both are realized in the low density regime of <n> (average density) ≤10¹³cm^?3.

H-E also showed that the currentless plasmas have no major disruption and quasi-steady plasmas are confined with controlling impurity ions by titanium gettering and carbon coating.

H-E also obtained <β> (average β) –2%, which is the highest value realized in helical systems, with <n–8×l0¹³cm^?3 and Te(0)–T_i(0)–350 eV at B₀ (magnetic field at the magnetic axis) =0.94 T. In the high β experiments pressure-driven instabilities were observed for peaked pressure profiles and sometimes relaxation oscillations similar to the tokamak internal disruptions were observed.

In the ECRH plasmas neoclassical transport is dominant in the region inside the half radius. However, global confinement time τ_E follows the scaling law τ_E ∝<n>^0.66P_heat ^?0.53 which is different from the neoclassical scaling law. Here P_heat denotes the net heating power.

Based on the H-E results, a new large helical system design study has started in 1986. The plasma parameters entering the regime of <n>τ_E<T> (2–3)× 10¹⁹m^?3?S?keV is investigated, which is about one tenth of fusion plasma condition. From the transport code studies and empirical scaling law based on the H-E results, R=(4×5)m, ā=(50–60)cm and B_o=4T are required to satisfy the above condition with P_heat=20MW. The design study to fix the magnetic field configuration is progressing. Expected one is l=2 and m=10 with additional poloidal coils, where m is a toroidal period number. The magnetic field is produced by superconducting coil and long pulse operation will be tested, if continuous heating is available.     

JET and the Physics Basis of ITER

JET has made unique contributions to the physics basis of ITER by virtue of its ITER-like geometry,large plasma size and D-T capability.The paper discusses recent JET results and their implications for ITER in the areas of standard ELMy H-mode,D-T operation and advanced tokamak modes.In ELMy H-mode the separation of plasma energy into core and pedestal contributions shows that core confinement scales like gyroBohm transport.High triangularity has a beneficial effect on confinement and leads to an integrated plasma performance exceeding the ITER Q=10 reference case.A revised type I ELM scaling predicts acceptable ELM energy losses for ITER,while progress in physics understanding of NTMs shows how to control them in ITER.The D-T experiments of 1997 have validated ICRF scenarios for heating ITER/a reactor and identified ion minority schemes (e.g.(^3He)DT) with strong ion heating.They also show that the slowing down of alpha particles is classical so that the self-heating by fusion alphas should cause no unexpected problems.With the Pellet Enhanced Performance mode of 1988,JET has produced the first advanced tokamak mode,with peaked pressure profiles sustained by reversed magnetic shear and strongly reduced transport.More recently,LHCD has provided easy tuning of reversed shear and reliable access to ITBs.Improved physics understanding shows that rational q-surfaces play a key role in the formation and development of ITBs.The demonstration of real time feedback control of plasma current and pressure profiles opens the path towards fully controlled steady-state tokamak plasmas.     

Electron Cyclotron Heating (ECH) of Tokamak Plasmas

Electron cyclotron heating (ECH) is one of the intense methods of plasma heating, and which utilizes the collisionless electron-cyclotron-resonance-interaction between the launched electromagnetic waves (called electron cyclotron waves) and electrons which are one of the constituents of the high temperature plasmas. Another constituent, namely the ions which are subject to nuclear fusion, are heated indirectly but strongly and instantly (in about 0.1 s) by the collisions with the ECH-heated electrons in the fusion plasmas. The recent progress on the development of high-power and high-frequency millimeter-wave-source enabled the ECH experiments in the middle size tokamaks such as JFT-2M (Japan), Doublet III (USA), T-10 (USSR) etc., and ECH has been demonstrated to be the sure and intense plasma heating method. The ECH attracts much attention for its remarkable capabilities; to produce plasmas (pre-ionization), to heat plasmas, to drive plasma current for the plasma confinement, and recently especially by the localization and the spatial controllability of its heating zone, which is beneficial for the fine controls of the profiles of plasma parameters (temperature, current density etc.), for the control of the magnetohydrodynamic instabilities, or for the optimization/improvement of the plasma confinement characteristics. Here, the present status of the ECH studies on tokamak plasmas are reviewed.     

The Upgrade of Fast Ferrite Tuning Matching System for ICRF in EAST

The fast ferrite tuning (FFT) real-time matching system has been designed and tested for the ion cyclotron range of frequency (ICRF) in EAST tokamak, which is necessary to transfer ICRF power to the plasma against variations in the antenna impedance. Through the test results, we proved this FFT system is feasible in EAST. Therefore this system have been upgraded recently to achieve real-time matching by the upgrading of the coil power supply and optimizing of the tuning structure. Finally the new FFT system achieved a response time of 10 ms and operated with a peak power of 1.5 MW, which satisfied the requirements of matching system in EAST.     

Performance of the Scattering Matrix Arc Detection System on the JET ITER-like ICRF antenna

Ion Cyclotron Radio Frequency (ICRF) antennas operating under high voltage to couple high power to fusion plasmas are at risk of electrical arcing. The standard Voltage Standing Wave Ratio (VSWR) system does not protect low impedance areas, which are used in antennas to achieve load tolerance to variations in plasma loading during Edge Localised Modes (ELMs). The Scattering Matrix Arc Detection System (SMAD) was designed to create additional protection for these areas, and 4 complete systems were implemented and tested on the ITER-like antenna (ILA) on JET. This paper describes the performance under relevant experimental conditions of load tolerance and/or high voltage and electrical arcing causing generator trip events.     

Experimental study of a single tube high RF power amplifier for ICRF heating system

A concept of a single tube high RF power amplifier was developed for ion cyclotron range of frequency (ICRF) plasma heating system. In the concept, a tetrode was used with a grounded cathode and input power to drive a control grid of the tetrode was provided by a switching circuit. As the new amplifier arrangement can eliminate a low power (10 kW level) and an intermediate power (100 kW level) tetrode amplifiers, their high voltage DC (HVDC) power supplies, and control and monitor system for these amplifiers and HVDC power supplies in a conventional high RF power source of the ICRF heating system, this new high RF power source is more flexible on frequency change and more mechanically reliable than the conventional one. A test amplifier composed of the tetrode and a field effect transistor (FET) switching circuit was constructed. The FET switching circuit was so compact that it could be mounted close to the tetrode socket. The maximum output RF power of 8.5 kW was obtained with a plate efficiency of 82% at 70 MHz. The feasibility of the single tube high RF power amplifier was experimentally proved. The plate efficiency of 82% could not be explained by the standard class-C amplification but by high efficiency amplification under assumptions of a flat-topped plate current pattern and double resonance of an output cavity at the fundamental frequency and the third higher harmonic frequency.     

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.     

Properties of vacuum-deposited Li films and impact on plasma performance in TJ-II

Drastic changes in the operation of the Spanish TJ-II stellarator have been produced upon lithiation of its first wall in 2007, performed by direct vacuum evaporation. From the operational point of view, much more stable, reproducible plasmas have been achieved, mostly due to the excellent density control associated with the low recycling wall characteristics. It has allowed exploring new, high densities, high power plasma conditions never attained under the previous, boron coating scenarios. In addition to this, new physical phenomena arose. Thus, for example, clear transitions to enhanced confinement modes (H mode) and strongly peaked plasma profiles were observed under the new wall conditions. In this paper, a summary of the main findings is described, and recent results on the interchange of plasma species (H vs. He) and sputtering in the Li wall are described and discussed.     

Observation of MHD Instabilities Driven by Energetic Electrons in the Large Helical Device

Coherent magnetic fluctuations in an acoustic range of frequency have been regularly observed in low-density(n_e0.2×10~(19)m~(-3))plasmas with strong second harmonic electron cyclotron resonance heating(ECRH)on the Large Helical Device.Hard X-ray measurements indicated that energetic electrons are generated in these ECRH discharges.The magnetic fluctuations are suppressed in higher density discharges where energetic electrons are not present.The ECRH power modulation experiment indicated that the observed magnetohydrodynamic(MHD)mode has an acoustic nature rather than an Alfvenic nature.     

Removal of Particles by ICRF Cleaning in HT-7 Superconducting Tokamak

1. IntroductionThe tokamak is a magnetically confined fusion device, Which demands ultra-vacuum and low impuritylevel for the plasma discharge. At the same time, therecycling of the working gas including its isotopesmust be decreased in favor of long pulse plasma discharge. For a larger superconducting tokamak of thefuture with a high power and a long pulse plasmadischarge, the recycling must be very low and theimpurity needs to be removed quickly.For decreasing the impurity content in the de…