Real-time plasma boundary reconstruction in the KSTAR tokamak using finite element method

A study is carried out on the real-time plasma shape identification in the KSTAR device. An improved form of the finite current element (FCE) method is utilized in this study. Results are shown that the plasma boundary can be reproduced in 7 mm accuracy for any plasma configuration in ideal cases without invoking measurement errors. A design guideline for magnetic diagnostics (MD) is established when the measurement signals are subject to Gaussian noise. It is found that the measurement errors in poloidal field (PF) coil currents have substantial influence on the determination of the plasma shape.     

Progress and improvement of KSTAR plasma control using model-based control simulators

Superconducting tokamaks like KSTAR, EAST and ITER need elaborate magnetic controls mainly due to either the demanding experiment schedule or tighter hardware limitations caused by the superconducting coils. In order to reduce the operation runtime requirements, two types of plasma simulators for the KSTAR plasma control system (PCS) have been developed for improving axisymmetric magnetic controls. The first one is an open-loop type, which can reproduce the control done in an old shot by loading the corresponding diagnostics data and PCS setup. The other one, a closed-loop simulator based on a linear nonrigid plasma model, is designed to simulate dynamic responses of the plasma equilibrium and plasma current (I_p) due to changes of the axisymmetric poloidal field (PF) coil currents, poloidal beta, and internal inductance. The closed-loop simulator is the one that actually can test and enable alteration of the feedback control setup for the next shot. The simulators have been used routinely in 2012 plasma campaign, and the experimental performances of the axisymmetric shape control algorithm are enhanced. Quality of the real-time EFIT has been enhanced by utilizations of the open-loop type. Using the closed-loop type, the decoupling scheme of the plasma current control and axisymmetric shape controls are verified through both the simulations and experiments. By combining with the relay feedback tuning algorithm, the improved controls helped to maintain the shape suitable for longer H-mode (10–16 s) with the number of required commissioning shots largely reduced.     

MHD stability analysis for advanced tokamak modes in the KSTAR device

The Korea Superconducting Tokamak Advanced Research (KSTAR) device aims to demonstrating the steady-state operation of high-performance advanced tokamak (AT) modes. In order to meet this research goal it is critical to have a good magnetohydrodynamic (MHD) stability, so that KSTAR adopted a strong plasma shape and a conducting wall close to plasma for such stability. An early calculation during the KSTAR design phase had shown that a target AT mode stable up to β_N above 5 can be then obtained. A recent work by Katsuro-Hopkins et al. [O. Katsuro-Hopkins, S.A. Sabbagh, J.M. Bialek, H.K. Park, J.G. Bak, J. Chung, et al., Equilibrium and global MHD stability study of KSTAR high beta plasmas under passive and active mode control, Nucl. Fusion 50(2010) 025019] showed, however, that the maximum β_N value can be substantially lower than 5, unlike the earlier result. In this work, we present a more detailed study on the MHD stability limit of the KSTAR target AT mode and try to clarify the discrepancy observed in the previous two works. It is shown that in the reverse-shear plasma the target mode with β_N above 5 can be obtained if the pressure profile is relatively peaked, but the maximum β_N value is substantially reduced below 5 if the pressure profile becomes broader. This result suggests the importance of a proper control of the pressure profile to get the high-beta AT mode in KSTAR.     

A repetitive pellet injection system for steady state fuelling in EAST superconducting tokamak

A new hydrogen/deuterium pellet injector has been developed for Experimental Advanced Superconducting Tokamak (EAST). The pellet injector based on a screw extruder is able to fire pellets (∅2 mm × 2 mm; frequency 1–10 Hz and velocity 150–300 m/s) in steady state mode with reliability greater than 95%. An injection line was designed for pumping propellant gas and for diagnostic purpose also. A guide tube for magnetic high-field side (HFS) injection was developed and theoretical calculation has been done. After successful engineering commissioning, the injection system served at EAST 2012 campaign and first experimental results were obtained.     

Optimization of an in-vessel visible inspection system for a long-pulse operation in KSTAR

To monitor the global formation of shaped plasmas, motion, and damage to the internal structures of the vacuum vessel, an in-vessel visible inspection system has been developed and operated on the Korean superconducting tokamak advanced research (KSTAR) device. The system contributed much to research progress on KSTAR such as the plasma start-up, plasma wall interactions, edge-localized modes, and disruptions. Moreover the need to perform inspections became important with high plasma power operation because of the increased frequency of first wall damage following off-normal events. Therefore the system is being improved from its original concept, and its final goal is operation during steady-state operation of the tokamak. The system consists of three fast visible cameras and two light-emitting diode illuminators. They are designed to be controlled fully from the control room to provide inspection capability at any time during hostile operating conditions. In this paper, we describe the upgrade of the system and recent results of the visible inspection system with the images of the KSTAR discharges for the last four years. Finally, we discuss the technical issues for a long pulse steady-state operation.     

Development of the pellet injection system on the J-TEXT tokamak

Pellet injection is an attractive technology for core-fueling and magnetohydrodynamic study in magnetic-confinement fusion devices like tokamaks and stellarators. It can inject solid hydrogen/deuterium pellets into the plasma with deeper density deposition compared with other fueling methods, such as gas puffing. A three-barrel H₂ pellet injection system was installed on the J-TEXT tokamak and experiments were carried out. The pellets are formed in three barrels cooled by a cryocooler and compressor system at around 9 K, and are 0.8 mm/1 mm diameter and 0.8 mm length. The pellet is launched by helium propellant gas and injected from the low-field side of the plasma. The normal range of pellet speed is 210–310 m s⁻¹ for different propellant gas pressures. Due to the three-barrel structure, the number of injected pellets can be adjusted between one and three. Pellets can be launched sequentially with arbitrary time intervals, which enables flexible applications. The results of the experiments show that pellet fueling efficiency can reach 50%. The energy confinement time increased by about 7.5‒10 ms after pellet injection.     

Achieving temporary divertor plasma detachment with MARFE events by pellet injection in the EAST superconducting tokamak

A new pellet injection system has been equipped on the experimental advanced superconducting tokamak(EAST) in the 2012 campaign,with a pellet size of Ф 2 mm?×?2 mm,a frequency of1 Hz–10 Hz and velocity of 150 m s~(-1)–300 m s~(-1).The deuterium pellet is well-known for plasma fuelling as well as for? triggering the edge localized mode(ELM).In the 2012 campaign,pellet injection experiments were successfully carried out on EAST.Temporary plasma detachment achieved by deuterium pellets has been observed in a double null(DN) divertor configuration,with multi-pellet injections at a repetition frequency of 2 Hz.The partial detachment of the outer divertors and complete detachment of the inner divertors was achieved after 35 ms of each pellet injection,which have a duration of 30–60 ms with the maximum degree of detachment(DOD) reaching 3.5 and 37,respectively.Meanwhile,the multifaceted asymmetric radiation from the edge(MARFE) phenomena was also observed at the high field side(HFS) near both the lower and upper X-points with radiation loss suddenly increased to about 15%–70%,which may be the main cause of divertor plasma detachment.The temporary detachment induced by pellet injection may act as a new way to study divertor detachment behaviors.     

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.     

Upgrade of plasma density feedback control systemin HT-7 tokamak

The HT-7 is a superconducting tokamak in China used to make researches on the controlled nuclear fusion as a national project for the fusion research. The plasma density feedback control subsystem is the one of the subsystems of the distributed control system in HT-7 tokamak (HT7DCS). The main function of the subsystem is to control the plasma density on real-time. For this reason, the real-time capability and good stability are the most significant factors, which will influence the control results. Since the former plasma density feedback control system (FPDFCS) based on Windows operation system could not fulfill such requirements well, a new subsystem has to be developed. The paper describes the upgrade of the plasma density feedback control system (UPDFCS), based on the dual operation system (Windows and Linux), in detail.     

Numerical simulation of plasma response to externally applied resonant magnetic perturbation on the J-TEXT tokamak

Nonlinear magnetohydrodynamic (MHD) simulations of an equilibrium on the J-TEXT tokamak with applied resonant magnetic perturbations (RMPs) are performed with NIMROD (non-ideal MHD with rotation, open discussion). Numerical simulation of plasma response to RMPs has been developed to investigate magnetic topology, plasma density and rotation profile. The results indicate that the pure applied RMPs can stimulate 2/1 mode as well as 3/1 mode by the toroidal mode coupling, and finally change density profile by particle transport. At the same time, plasma rotation plays an important role during the entire evolution process.     

托卡马克电场漂移电子注入的物理分析研究

对电场漂移电子注入中的电子束物理,电子注入情况下手 马克等离子的平衡,电子注入器中的物理问题等进行了分析研究。结果表明,漂移电子束的稳定条件是外加电场必须大于电子束本身所产生的电场;为了保持等离子体环的平衡,外加电压愈低愈好;     

A novel technique for the measurement of plasma displacement in IR-T1 tokamak

In this paper we present a novel technique based on poloidal magnetic flux for determination of plasma displacement in IR-T1 tokamak. This instrument consists of a two semicircle wires which installed toroidally on inner and outer sides of tokamak chamber and connected with each other. In order to receive the poloidal flux on Last Closed Flux Surface (LCFS); this instrument installed on polar coordinate so as projection of it on midplane lie on LCFS. Really, this instrument receives the difference between poloidal flux on inner and outer sides of LCFS, which we needed in calculating of the Shafranov shift. Main benefits of our proposed instrument are that it is a simple, solid, and also its output is directly related to the Shafranov shift. Based on this technique we determined the plasma position and to compare the result obtained using this method, multipole moments method is also experimented on IR-T1. Results of the two techniques are in good agreement with each other.     

Development of real-time plasma analysis and control algorithms for the TCV tokamak using Simulink

One of the key features of the new digital plasma control system installed on the TCV tokamak is the possibility to rapidly design, test and deploy real-time algorithms. With this flexibility the new control system has been used for a large number of new experiments which exploit TCV's powerful actuators consisting of 16 individually controllable poloidal field coils and 7 real-time steerable electron cyclotron (EC) launchers. The system has been used for various applications, ranging from event-based real-time MHD control to real-time current diffusion simulations. These advances have propelled real-time control to one of the cornerstones of the TCV experimental program. Use of the Simulink graphical programming language to directly program the control system has greatly facilitated algorithm development and allowed a multitude of different algorithms to be deployed in a short time. This paper will give an overview of the developed algorithms and their application in physics experiments.     

Comparative study of plasma position measurements using multipole moments and discrete magnetic probes methods on IR-T1 tokamak

In this paper, we present a comparative study for estimating plasma column displacement using multipole moments and discrete magnetic probes methods. It is shown that multipole moments method gives a better performance in the real time determination of the plasma column displacement in IR-T1 tokamak.     

Scaling of the Density Peak with Pellet Injection in ITER

Scalings of the density peak and pellet penetration length in ITER are developed based on simulations using 1.5D BALDUR integrated predictive modeling code. In these simula- tions, the pellet ablation is described by the Neutral Gas Shielding (NGS) model with grad-B drift effect taken into account. The NGS pellet model is coupled with a plasma core transport model, which is a combination of an MMM95 anomalous transport model and an NCLASS neoclassical transport model. The BALDUR code with a combination of MMM95 and NCLASS models, together with the NGS model, is used to simulate the time evolution of plasma current, ion and electron temperatures, and density profiles for ITER standard type I ELMy H-mode discharges during the pellet injection. As a result, the scaling of the density peak and pellet penetration length at peak density can be established using this set of predictive simulations that covers a wide range of ITER plasma conditions and pellet parameters. The multiple regression technique is utilized in the development of the scalings. It is found that the scaling for density at center is sensitive to both the plasma and pellet parameters; whereas the scalings for density and location of the additional peak are sensitive to the pellet parameters only.     

Simulation and test results of low-order band harmonic filters to decrease resonance phenomenon in the KSTAR power system

The superconducting magnet power supply which supplies superconducting magnet coil (SC) with the power to generate plasma during a KSTAR experiment for nuclear fusion research is a nonlinear load. Characteristic harmonics are generally produced since it converts AC to DC using 6 or 12 pulsed operation. However, non-characteristic harmonics or inter-harmonics are generated according to output control characteristics. Also, 95% out of the power generated from superconducting magnet coil is reactive power. Therefore, harmonic and reactive power occurring during operation have some bad influences such as voltage drop, voltage distortion and decrease in power factor on the KSTAR power system, and reactive power compensator (RPC) & harmonic filter (HF) system which is able to compensate harmonic and reactive power at the same time was established and has been operated [1]. However, out of non-characteristic harmonics and inter-harmonics caused by output control characteristics of superconducting magnet power supply, the more compensation volume of the RPC & HF system increases, the more voltage distortion with harmonic expansion is caused by harmonics in a low-order band due to the parallel resonance in a low-order band between the KSTAR power system and the RPC & HF system. As a result, it has serious effect on the injection capacity restriction of the RPC & HF system, the unstable operation control of superconducting magnet coil, and the operation of main cooling facilities. This paper presents reasons of the resonance phenomenon of the KSTAR power system and suggests a design plan of additional facilities for stable operation of the KSTAR power system, and proves their effects through the simulation and test results.     

Preliminary 2D code simulation of the quasi-snowflake divertor configuration in the FAST tokamak

The behaviour of the SOL in FAST for the new quasi snowflake configuration of the divertor is compared with that of the conventional single null case for three main scenarios: reference, advanced and extreme H-mode. The flexible, quick and versatile 2D code TECXY is used. The main physics processes occurring at the edge are carefully taken into account but the neutral dynamics is simplified. Even though the status close to detachment conditions cannot be detailed, new phenomena generated by the configuration are clearly highlighted. The heat load is strongly mitigated to a level that is much easier to tackle with the present technology. Mitigation is always stronger than expected from the magnetic topology only, especially in the high density regimes. This is attributed to the much longer time that the SOL particles spend in the quite cold region of the X point where mostly the connection length of the magnetic lines is strongly increased and hence to the much more intense interaction of the plasma particle with the background neutrals.     

Effect of resonant magnetic perturbation on boundary plasma turbulence and transport on J-TEXT tokamak

The effect of resonant magnetic perturbation(RMP) on boundary turbulence and transport in J-TEXT plasma is experimentally investigated.Edge plasma fluctuations in discharges with and without the(m/n=3/1) RMP currents are diagnosed by using Langmuir probe arrays.It was found that fluctuations in the edge and scrape-off layer(SOL) regions decrease with the application of a 6 kA RMP.The broadband turbulence at the radial location of ρ～0.9 which has a characteristic frequency of 40-150 kHz was strongly suppressed when applying RMP,as was the radial turbulent particle flux and blob transport in the near-SOL region.These experimental findings make RMP a promising method of suppressing and controlling turbulence and particle transport in a plasma boundary.     

The use of an electrothermal plasma gun to simulate the extremely high heat flux conditions of a tokamak disruption

Disruption damage conditions for future large tokamaks like ITER are nearly impossible to simulate on current tokamaks. The electrothermal plasma source SIRENS has been designed, constructed, and operated to produce high density (> 10²⁵/m³), low temperature (1–3 eV) plasma formed by the ablation of the insulator with currents of up to 100 kA (100 s pulse length) and energies up to 15 kJ. The source heat fluence (variable from 0.2 to 7 MJ/m²) is adequate for simulation of the thermal quench phase of plasma disruption in future fusion tokamaks. Different materials have been exposed to the high heat flux in SIRENS, where comparative erosion behavior was obtained. Vapor shield phenomena has been characterized for different materials, and the energy transmission factor through the shielding layer is obtained. The device is also equipped with a magnet capable of producing a parallel magnetic field (up to 16 T) over a 8 msec pulse length. The magnetic field is produced to decrease the turbulent energy transport through the vapor shield, which provides further reduction of surface erosion (magnetic vapor shield effect).