Plasma Fueling Research Articles

Impact of the JET ITER-like wall on H-mode plasma fueling

JET ITER-like wall (ILW) experiments show that the edge density evolution is strongly linked with the poloidal distribution of the ionization source. The fueling profile in the JET-ILW is more delocalized as compared to JET-C (JET with carbon-based plasma-facing components PFCs). Compared to JET-C the H-mode pedestal fueling cycle is dynamically influenced by a combination of plasma–wall interaction features, in particular: (1) edge-localized modes (ELMs) induced energetic particles are kinetically reflected on W divertor PFCs leading to distributed refueling away from the divertor depending on the divertor plasma configuration, (2) delayed molecular re-emission and outgassing of particles being trapped in W PFCs (bulk-W at the high field side and W-coated CFCs at the low field side) with different fuel content and (3) outgassing from Be co-deposits located on top of the high-field side baffle region shortly after the ELM. In view of the results of a set of well diagnosed series of JET-ILW type-I ELMy H-mode discharges with good statistics, the aforementioned effects are discussed in view of H-mode pedestal fueling capacity. The ongoing modelling activities with the focus on coupled core-edge plasma simulations and plasma–wall interaction are described and discussed also in view of possible code improvements required.

Pellet injectors for EAST and KSTAR tokamaks

The paper presents high frequency pellet injectors developed for edge localized mode mitigation and plasma fuelling of the EAST and KSTAR tokamaks. Each pellet injector is able to inject solid deuterium or hydrogen pellets at steady state mode. Injectors consist of modules including screw extruder cooled by liquid helium and pneumatic punchers for pellet fabrication and acceleration which are optimized to reach maximal injection frequency for specified pellet parameters.The EAST pellet injector is capable of injecting 1.5mm diameter and 1.2–1.8mm length pellets at frequency up to 50Hz with velocities 200–250m/s using two injection modules working in an alternating mode at 25Hz each. Injection reliability over 90% has been confirmed during several cycles of continuous D2 pellet injection at 50Hz. The KSTAR pellet injector has been designed to inject 2mm size pellets at frequency up to 20Hz and velocity 200m/s. Due to improved design of pellet fabrication and acceleration system the world record in continuous injection of 20,164 deuterium pellets (Ø2.0×1.5mm) for 1039s at 20Hz with reliability 97% has been achieved.

Plasma fuelling with cryogenic pellets in the stellarator TJ-II

Cryogenic pellet injection is a widely used technique for delivering fuel to the core of magnetically confined plasmas. Indeed, such systems are currently functioning on many tokamak, reversed field pinch and stellarator devices. A pipe-gun-type pellet injector is now operated on the TJ-II, a low-magnetic shear stellarator of the heliac type. Cryogenic hydrogen pellets, containing between 3 × 1018 and 4 × 1019 atoms, are injected at velocities between 800 and 1200 m s−1 from its low-field side into plasmas created and/or maintained in this device by electron cyclotron resonance and/or neutral beam injection heating. In this paper, the first systematic study of pellet ablation, particle deposition and fuelling efficiency is presented for TJ-II. From this, light-emission profiles from ablating pellets are found to be in reasonable agreement with simulated pellet ablation profiles (created using a neutral gas shielding-based code) for both heating scenarios. In addition, radial offsets between recorded light-emission profiles and particle deposition profiles provide evidence for rapid outward drifting of ablated material that leads to pellet particle loss from the plasma. Finally, fuelling efficiencies are documented for a range of target plasma densities (~4 × 1018– ~2 × 1019 m−3). These range from ~20%– ~85% and are determined to be sensitive to pellet penetration depth. Additional observations, such as enhanced core ablation, are discussed and planned future work is outlined.

Benchmark studies of BOUT++ code and TPSMBI code on neutral transport during SMBI

SMBI (supersonic molecule beam injection) plays an important role in tokamak plasma fuelling, density control and ELM mitigation in magnetic confinement plasma physics, which has been widely used in many tokamaks. The trans_neut module of BOUT++ code is the only large-scale parallel 3D fluid code used to simulate the SMBI fueling process, while the TPSMBI (transport of supersonic molecule beam injection) code is a recent developed 1D fluid code of SMBI. In order to find a method to increase SMBI fueling efficiency in H-mode plasma, especially for ITER, it is significant to first verify the codes. The benchmark study between the trans_neut module of BOUT++ code and the TPSMBI code on radial transport dynamics of neutral during SMBI has been first successfully achieved in both slab and cylindrical coordinates. The simulation results from the trans_neut module of BOUT++ code and TPSMBI code are consistent very well with each other. Different upwind schemes have been compared to deal with the sharp gradient front region during the inward propagation of SMBI for the code stability. The influence of the WENO3 (weighted essentially non-oscillatory) and the third order upwind schemes on the benchmark results has also been discussed.

Fast piezoelectric valve offering controlled gas injection in magnetically confined fusion plasmas for diagnostic and fuelling purposes

In magnetically confined fusion plasmas controlled gas injection is crucial for plasma fuelling as well as for various diagnostic applications such as active spectroscopy. We present a new, versatile system for the injection of collimated thermal gas beams into a vacuum chamber. This system consists of a gas pressure chamber, sealed by a custom made piezo valve towards a small capillary for gas injection. The setup can directly be placed inside of the vacuum chamber of fusion devices as it is small and immune against high magnetic fields. This enables gas injection close to the plasma periphery with high duty cycles and fast switch on/off times ≲ 0.5 ms. In this work, we present the design details of this new injection system and a systematic characterization of the beam properties as well as the gas flowrates which can be accomplished. The thin and relatively short capillary yields a small divergence of the injected beam with a half opening angle of 20°. The gas box is designed for pre-fill pressures of 10 mbar up to 100 bars and makes a flowrate accessible from 1018 part/s up to 1023 part/s. It hence is a versatile system for both diagnostic as well as fuelling applications. The implementation of this system in ASDEX Upgrade will be described and its application for line ratio spectroscopy on helium will be demonstrated on a selected example.

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 ϕ 2 mm × 2 mm, a frequency of 1 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.

Impact of fuelling and impurity on pedestal dynamics and instabilities in the HL-2A tokamak

In recent experiments of the HL-2A tokamak, the effect on the pedestal dynamics by the plasma fuelling with supersonic molecular beam injection (SMBI) has been intensively investigated. Experimental results in several tokamaks suggested that SMBI is a promising technique for ELM mitigation. In addition to the fuelling, the impact of impurities on the pedestal dynamics and instabilities has been investigated in HL-2A. Experimental results have shown that during the H-mode phase, a broadband electromagnetic (EM) turbulence was driven by peaked impurity density profile at the edge plasma region, and governed by double critical gradients of the impurity density. The absolute value of the threshold in positive gradient region is much lower than that in the negative region. This strong asymmetry in the critical gradients has been predicted by theoretical simulation. The results reveal that pedestal dynamics and heat loads can be actively controlled by exciting or changing pedestal instabilities.

Guest Editorial Special Issue on Symposium on Fusion Engineering

This special issue is the first one published in the IEEE Transactions of Plasma Science covering the 26th Symposium on Fusion Engineering (SOFE 2015) chaired by Jean Paul Allain and Mark Tillack. SOFE 2015 was colocated with the 20th Pulsed Power Conference in Austin, TX, USA, from May 31 to June 4, 2015. The Symposium included an exciting collection of papers on the technological, engineering, and scientific advances in the field. Notable new technical areas, including fusion materials, concepts for a Fusion Nuclear Science Facility, and design and analysis tools for stellerator DEMO devices (a timely topic with W7-X at Max Planck Insitut in Greifswald coming online in 2015, and innovative divertor concepts) were emphasized, unlike previous SOFE meetings. Other topics in the Symposium covered engineering and scientific advances in both inertial confinement and magnetic confinement fusion, with attendees from major fusion energy research centers worldwide. Fusion engineering continues to be an important area to the fusion community, given the continued engineering and technology advances and challenges in magnetic and inertial thermonuclear fusion. The objective of this special issue is to bring the most up-to-date developments reported at the SOFE 2015 to the readership of the IEEE Transactions of Plasma Science. In addition to the above-listed new topics, SOFE 2015 highlighted continued interest in the areas of fusion engineering, including ITER and experimental fusion devices including alternate fusion confinement devices, new device design and reactor studies, divertors and plasma-material interactions, chambers, vacuum vessels related to fusion device technology, plasma diagnostics, data acquisition and plasma control systems, safety and environmental engineering, plasma fueling, pumping and tritium handling systems, IFE drivers, targets, and related technologies; fusion device power systems; magnet engineering and electromagnetics; and electromechanics. SOFE 2015 has an unprecedented participation from our Asian colleagues, as nuclear fusion programs are broadening, particularly the technological and engineering efforts across many major devices and facilities. This is one of the reasons for our decision to proudly hold the next SOFE 2017 in Shanghai, China, led by General Chair Dr. Hutch Neilson and General Co-Chair Dr. Jiangang Li.

The supersonic molecular beam injector as a reliable tool for plasma fueling and physics experiment on HL-2A.

On HL-2A tokamak, supersonic molecular beam injection (SMBI) has been developed as a routine refueling method. The key components of the system are an electromagnetic valve and a conic nozzle. The valve and conic nozzle are assembled to compose the simplified Laval nozzle for generating the pulsed beam. The appurtenance of the system includes the cooling system serving the cooled SMBI generation and the in situ calibration component for quantitative injection. Compared with the conventional gas puffing, the SMBI features prompt response and larger fueling flux. These merits devote the SMBI a good fueling method, an excellent plasma density feedback control tool, and an edge localized mode mitigation resource.

Radial transport dynamics studies of SMBI with a newly developed TPSMBI code**Project supported by the National Natural Science Foundation of China (Grant Nos. 11575055, 11375053, and 11475219) and the National Magnetic Confinement Fusion Science Program of China (Grant Nos. 2013GB111005, 2014GB108004, and 2015GB110001).

In tokamak plasma fueling, supersonic molecule beam injection (SMBI) with a higher fueling efficiency and a deeper penetration depth than the traditional gas puffing method has been developed and widely applied to many tokamak devices. It is crucial to study the transport dynamics of SMBI to improve its fueling efficiency, especially in the high confinement regime. A new one-dimensional (1D) code of TPSMBI has also been developed recently based on a six-field SMBI model in cylindrical coordinate. It couples plasma density and heat radial transport equations together with neutral density transport equations for both molecules and atoms and momentum radial transport equations for molecules. The dominant particle collisional interactions between plasmas and neutrals, such as molecule dissociation, atom ionization and charge-exchange effects, are included in the model. The code is verified to be correct with analytical solutions and also benchmarked well with the trans-neut module of BOUT++ code. Time-dependent radial transport dynamics and mean profile evolution are studied during SMBI with the TPSMBI code in both slab and cylindrical coordinates. Along the SMBI path, plasma density increases due to particle fuelling, while plasma temperature decreases due to heat cooling. Being different from slab coordinate, the curvature effect leads to larger front densities of molecule and atom during SMBI in cylindrical coordinate simulation.

A study of hydrogen isotopes fuel control by wall effect in magnetic fusion devices

Determination of plasma density behavior in magnetic confinement system needs to study the plasma materials interaction in the facing components such as first wall, limiter and divertor. Recycling of hydrogen isotope is an effective parameter in plasma density rate and plasma fueling. Recycling coefficient over the long pulse operation, gets to the unity, so it has a significant effect on steady state in magnetic fusion devices. Typically, sputtered carbon atoms from the plasma facing components form hydrocarbons and they redeposit on the wall. In this case little rate of hydrogen loss occurs. In present work a zero dimensional particle equilibrium model has been represented to determine particles density rate in main plasma and wall inventory under recycling effect and codeposition of hydrogen in case of continues and discontinues fueling methods and effective parameters on the main plasma decay has been studied.

Existing and new applications of micropellet injection (MPI) in magnetic fusion

The intense heat and energetic particle fluxes expected in ITER and future magnetic fusion reactors pose prohibitive problems to the design, selection and maintenance of the first wall and divertor. Micropellet injection (MPI) technologies can offer some innovative solutions to the material and extreme heat challenges. Basic physics of micropellet motion, ablation and interactions with high-temperature plasmas and energetic particles are presented first. We then discuss MPI technology options and applications. In addition to plasma diagnostic applications, controlled injection of micropellets of different sizes, velocities and injection frequencies will offer several possibilities: (1) better assessment of the core plasma cooling due to dust produced in situ; (2) better understanding of the plasma–material interaction physics near the wall; (3) new methods for plasma fuelling and impurity control; and (4) techniques for edge cooling with minimal impact on the plasma core. Dedicated small-scale laboratory experiments will complement major fusion experiments in development and applications of MPI.

Test bench operation of a plasma fuelling device at IPR

A single barrel pellet injection system has been developed at Institute for plasma research (IPR). It is an in-situ pipe gun type injector, in which, a solid hydrogen pellet formed at freezing zone is accelerated to high speed by using high pressure propellant gas. The pellet injection system is equipped with a GM-cycle cryogenic refrigerator for pellet formation, a fast opening valve for pellet acceleration and diagnostic system to measure the ejected pellet parameter. A pellet is formed in-situ in the gun barrel connected to cryogenic refrigerator maintained at a temperature < 10 K. With proper thermal heat load minimization on cryo region, solid hydrogen pellets are obtained. In test bench operation various sized cylindrical pellets of dimension (length x diameter) 1.6 mm × 1.8 mm and 4 mm × 4.25 mm are produced. The ejected pellet speed is varied over a range from 700 to 1000 m/s by regulating propellant pressure. Whereas, the speed of a pellet is measured by time of flight based light gate diagnostic system, pellet quality and its size are estimated from fast camera imaging system. The reliability of pellet formation and injection in present experimental system is greater than 95%.

The ITER project construction status

The pace of the ITER project in St Paul-lez-Durance, France is accelerating rapidly into its peak construction phase. With the completion of the B2 slab in August 2014, which will support about 400 000 metric tons of the tokamak complex structures and components, the construction is advancing on a daily basis. Magnet, vacuum vessel, cryostat, thermal shield, first wall and divertor structures are under construction or in prototype phase in the ITER member states of China, Europe, India, Japan, Korea, Russia, and the United States. Each of these member states has its own domestic agency (DA) to manage their procurements of components for ITER. Plant systems engineering is being transformed to fully integrate the tokamak and its auxiliary systems in preparation for the assembly and operations phase. CODAC, diagnostics, and the three main heating and current drive systems are also progressing, including the construction of the neutral beam test facility building in Padua, Italy. The conceptual design of the Chinese test blanket module system for ITER has been completed and those of the EU are well under way. Significant progress has been made addressing several outstanding physics issues including disruption load characterization, prediction, avoidance, and mitigation, first wall and divertor shaping, edge pedestal and SOL plasma stability, fuelling and plasma behaviour during confinement transients and W impurity transport. Further development of the ITER Research Plan has included a definition of the required plant configuration for 1st plasma and subsequent phases of ITER operation as well as the major plasma commissioning activities and the needs of the accompanying R&D program to ITER construction by the ITER parties.

Fuelling and density control for DEMO

Plasma fuelling and density control are an open issue regarding EU DEMO studies and solutions may be different from present day experiments. The present paper addresses through JINTRAC core transport code simulations the feasibility of different fuelling methods such as gas puff and pellet injection and the influence of neoclassical and anomalous inward pinch in the edge transport barrier in order to achieve and control the target DEMO density. Given the expected high fusion power production, He accumulation in the plasma core is a critical issue, and an estimation of the influence of impurities (He, Ar, and W) on core fuelling and plasma dilution is given together with a discussion on D–T core balance. The DEMO reference scenario investigated in this work is characterized by a peaked density profile, which requires a careful core fuelling. Due to the large pedestal temperature gradient, gas puff may not be a feasible option for core density control, unless assuming a large anomalous inward pinch in the edge transport barrier of more than ~2 m s−1. Pellet injection from the high field side of the torus, on the contrary, may represent a viable solution for core fuelling and D–T ratio control. The effect of pellet mass, speed, and injection geometry is also discussed in the present paper. Regardless, core fuelling efficiency with pellet injection is almost entirely determined by the presence of E × B drift.

Imaging polychromator for density measurements of polystyrene pellet cloud on the Large Helical Device.

Experimental data on spatial distributions of a pellet cloud electron density are necessary for the development of many applications of pellet injection, namely, plasma fuelling, discharge control, and plasma diagnostics. An improved approach of electron density measurements inside the cloud of a polystyrene pellet ablating in hot plasma of the large helical device is described. Density values of (1-30) × 10(16) cm(-3) depending on the background plasma parameters and distance from the solid pellet were measured.

DEMO diagnostics and burn control

The development of the control system for a tokamak demonstration fusion reactor (DEMO) faces unprecedented challenges. First, the requirements for control reliability and accuracy are more stringent than on existing fusion devices: any loss of plasma control on DEMO may result in a disruption which could damage the inner wall of the machine, while operating the device with larger margins against the operational limits would lead to a reduction of the electrical output power. Second, the performance of DEMO control is limited by space restrictions for the implementation of components (optimization of the tritium breeding rate), by lifetime issues for the front-end parts (neutron and gamma radiation, erosion and deposition acting on all components) and by slow, weak and indirect action of the available actuators (plasma shaping, heating and fuelling). The European DEMO conceptual design studies include the development of a reliable control system, since the details of the achievable plasma scenario and the machine design may depend on the actual performance of the control system. In the first phase of development, an initial understanding of the prime choices of diagnostic methods applicable to DEMO, implementation and performance issues, the interrelation with the plasma scenario definition, and the planning of necessary future R&D have been obtained.

50 Hz deuterium pellet injector for EAST tokamak

A new high frequency pellet injector has been developed for edge localized mode mitigation and plasma fuelling of the EAST tokamak. The pellet injector consists of two independent modules operating at 1–25Hz. Both modules are equipped with screw extruders cooled by liquid helium for solid deuterium ice production at steady state mode. An innovative pellet fabrication and acceleration system has been designed and applied to reduce gas load on the extruders in 4–5 times and to increase the pellet injection reliability. Pellets of 1.5mm diameter and 1.5mm length were injected at 250±50m/s velocity at 25Hz with reliability over 93% using one module in preliminary tests. Two modules working in turn are under tests now. It is anticipated they will be able to inject pellets at 50Hz after further technical improvements in near future.

Impact of localized gas injection on ICRF coupling and SOL parameters in JET-ILW H-mode plasmas

Recent JET-ILW [1,2] experiments reiterated the importance of tuning the plasma fuelling in order to optimize ion cyclotron resonance frequency (ICRF) heating in high power H-mode discharges. By fuelling the plasma from gas injection modules (GIMs) located in the mid-plane and on the top of the machine instead of adopting the more standardly used divertor GIMs, a considerable increase of the ICRF antenna coupling resistances was achieved with moderate gas injection rates (<1.5×1022e/s). This effect is explained by an increase of the scrape-off-layer density in front of the antennas when mid-plane and top fuelling is used. By distributing the gas injection to optimize the coupling of all ICRF antenna arrays simultaneously, a substantial increase in the ICRF power capability and reliability was attained. Although similar core/pedestal plasma properties were observed for the different injection cases, the experiments indicate that the RF-induced impurity sources are reduced when switching from divertor to main chamber gas injection.

First Results of Pellet Injection Experiments on EAST

A new pellet injection system was installed on the EAST tokamak and preliminary experiments were performed during the 2012 run campaign. Typical phenomena associated with deuterium pellet injection into a plasma discharge have been observed including sudden increases of the electron density and Hα/Dα emission intensity as well as a significant decrease in plasma electron temperature. Profiles have been studied in order to understand the influence of pellet fuelling on EAST discharges. Even though the injector was specifically designed for plasma fuelling, ELM triggering using the pellet injection has also been tested. In order to find appropriate parameters for triggering ELMs in H-mode plasmas, scanning of the pellet injection speed was employed for pellets injected from both the high field side and low field side of the plasma column. It has been observed that low-speed pellets injected into H-mode plasma from the low-field side could trigger an ELM followed by a number of smaller induced ELMs at about 300 Hz.