Magnetic Fusion Energy Research Research Articles

Patterned Adhesion Layer Enables Rugged Pd-MIS Hydrogen Sensors.

Rugged Pd-metal-insulator-semiconductor (Pd-MIS) hydrogen sensors for detecting charge-exchange particles in fusion reactors have been constructed by utilizing a novel patterned adhesion layer. Poor adhesion at the interface between Pd and SiO2 is a common failure mode for Pd-MIS devices, severely limiting the Pd thickness and their usefulness as hydrogen sensors. The mechanical integrity of the Pd coatings is of particular importance in magnetic fusion energy research where the Pd-MIS diodes are used to measure hydrogen charge-exchange neutral fluence at the wall in tokamaks. In this application, particularly thick Pd contacts are desirable to prevent damage caused by high-energy particles; however, such thick Pd coatings are prone to mechanical failure due to blistering and wire bond detachment during construction or operation. A continuous Ti or Cr adhesion layer is not possible for this application since it would interfere with H uptake at the SiO2 interface, which is essential for the device to generate a response. In this work, we demonstrate that a patterned Cr interlayer substantially improves adhesion while still providing access for hydrogen to reach the SiO2-Pd interface.

Data-driven model for divertor plasma detachment prediction

We present a fast and accurate data-driven surrogate model for divertor plasma detachment prediction leveraging the latent feature space concept in machine learning research. Our approach involves constructing and training two neural networks: an autoencoder that finds a proper latent space representation (LSR) of plasma state by compressing the multi-modal diagnostic measurements and a forward model using multi-layer perception (MLP) that projects a set of plasma control parameters to its corresponding LSR. By combining the forward model and the decoder network from autoencoder, this new data-driven surrogate model is able to predict a consistent set of diagnostic measurements based on a few plasma control parameters. In order to ensure that the crucial detachment physics is correctly captured, highly efficient 1D UEDGE model is used to generate training and validation data in this study. The benchmark between the data-driven surrogate model and UEDGE simulations shows that our surrogate model is capable of providing accurate detachment prediction (usually within a few per cent relative error margin) but with at least four orders of magnitude speed-up, indicating that performance-wise, it has the potential to facilitate integrated tokamak design and plasma control. Comparing with the widely used two-point model and/or two-point model formatting, the new data-driven model features additional detachment front prediction and can be easily extended to incorporate richer physics. This study demonstrates that the complicated divertor and scrape-off-layer plasma state has a low-dimensional representation in latent space. Understanding plasma dynamics in latent space and utilising this knowledge could open a new path for plasma control in magnetic fusion energy research.

Modification of short-range repulsive interactions in ReaxFF reactive force field for Fe–Ni–Al alloy* *Project supported by the National Magnetic Confinement Fusion Energy Research Project (Grant Nos. 2019YFE03120003, 2018YFE0307100, and 2017YFE0302500) and the National Natural Science Foundation of China (Grant Nos. 11975034, 11921006, 12004010, and U20B2025).

The short-range repulsive interactions of any force field must be modified to be applicable for high energy atomic collisions because of extremely far from equilibrium state when used in molecular dynamics (MD) simulations. In this work, the short-range repulsive interaction of a reactive force field (ReaxFF), describing Fe–Ni–Al alloy system, is well modified by adding a tabulated function form based on Ziegler–Biersack–Littmark (ZBL) potential. The modified interaction covers three ranges, including short range, smooth range, and primordial range. The short range is totally predominated by ZBL potential. The primordial range means the interactions in this range is the as-is ReaxFF with no changes. The smooth range links the short-range ZBL and primordial-range ReaxFF potentials with a taper function. Both energies and forces are guaranteed to be continuous, and qualified to the consistent requirement in LAMMPS. This modified force field is applicable for simulations of energetic particle bombardments and reproducing point defects' booming and recombination effectively.

Microcalorimeter measurement of x-ray spectra from a high-temperature magnetically confined plasma.

A NASA-built x-ray microcalorimeter spectrometer has been installed on the MST facility at the Wisconsin Plasma Physics Laboratory and has recorded x-ray photons emitted by impurity ions of aluminum in a majority deuterium plasma. Much of the x-ray microcalorimeter development has been driven by the needs of astrophysics missions, where imaging arrays with few-eV spectral resolution are required. The goal of our project is to adapt these single-photon-counting microcalorimeters for magnetic fusion energy research and demonstrate the value of such measurements for fusion science. Microcalorimeter spectrometers combine the best characteristics of the x-ray instrumentation currently available on fusion devices: high spectral resolution similar to an x-ray crystal spectrometer and the broadband coverage of an x-ray pulse height analysis system. Fusion experiments are increasingly employing high-Z plasma-facing components and require measurement of the concentration of all impurity ion species in the plasma. This diagnostic has the capability to satisfy this need for multi-species impurity ion data and will also contribute to measurements of impurity ion temperature and flow velocity, Zeff, and electron density. Here, we introduce x-ray microcalorimeter detectors and discuss the diagnostic capability for magnetic fusion energy experiments. We describe our experimental setup and spectrometer operation approach at MST, and we present the results from an initial measurement campaign.

Observation of trapped and passing runaway electrons by infrared camera in the EAST tokamak* *Project supported by the National Natural Science Foundation of China (Grant No. 11775263) and the National Magnetic Confinement Fusion Energy Research Project of China (Grant No. 2015GB111003).

In EAST, synchrotron radiation is emitted by runaway electrons in the infrared band, which can be observed by infrared cameras. This synchrotron radiation is mainly emitted by passing runaway electrons with tens of MeV energy. A common feature of radiation dominated by passing runaway electrons is that it is strongest on the high field side. However, the deeply trapped runaway electrons cannot reach the high field side in principle. Therefore, in this case, the high field side radiation is expected to be weak. This paper reports for the first time that the synchrotron radiation from trapped runaway electrons dominates that from passing runaway electrons and is identifiable in an image. Although the synchrotron radiation dominated by trapped runaway electrons can be observed in experiment, the proportion of trapped runaway electrons is very low.

Determination of activation energy of ion-implanted deuterium release from W-Y2O3**Project supported by the National Magnetic Confinement Fusion Energy Research Project, Ministry of Science and Technology of China (Grant No. 2015GB109002), the Innovation Fund of Postgraduate, Xihua University, China (Grant No. ycjj2018017), and the National Natural Science Foundation of China (Grant No. 21401173).

The retention and release of deuterium in W–2%Y2O3 composite materials and commercially pure tungsten after they have been implanted by deuterium plasma (flux ∼ 3.71 × 1021 D/m2⋅s, energy ∼ 25 eV, and fluence up to 1.3 × 1026 D/m2) are studied. The results show that the total amount of deuterium released from W–2%Y2O3 is 5.23 × 1020 D/m2(2.5 K/min), about 2.5 times higher than that from the pure tungsten. Thermal desorption spectra (TDS) at different heating rates (2.5 K/min–20 K/min) reveal that both W and W–2%Y2O3 have two main deuterium trapped sites. For the low temperature trap, the deuterium desorption activation energy is 0.85 eV (grain boundary) in W, while for high temperature trap, the desorption activation energy is 1.57 eV (vacancy) in W and 1.73 eV (vacancy) in W–2%Y2O3.

Investigation on the drives of the poloidal flow in the ohmic and biased electrode experiments**Project supported by the National Natural Science Foundation of China (Grant No. 11875124), the National Magnetic Confinement Fusion Energy Research Project, China (Grant No. 2015GB120002), and the National Key Research and Development Program of China (Grant No. 2017YFE0300405).

The experimental investigation on the drives of the poloidal flow in KT-5D tokamak are presented. It is found that the poloidal flow is the main contributor to the radial electric field, and the Reynolds stress can drive significant poloidal flows in ohmic discharges. The investigation on the relationship between the radial gradient of Reynolds stress and the poloidal flow in biasing discharges indicates that not only Reynolds stress but also the Lorentz’s force can drive the poloidal flow.

Analysis of the Zeeman effect on Dα spectra on the EAST tokamak**Project supported by the National Natural Science Foundation of China (Grant Nos. 11275231 and 11575249) and the National Magnetic Confinement Fusion Energy Research Program of China (Grant No. 2015GB110005).

Based on the passive spectroscopy, the atomic emission spectra in the boundary region of the plasma have been measured by a high resolution optical spectroscopic multichannel analysis (OSMA) system in EAST tokamak. The Zeeman splitting of the spectral lines has been observed. A fitting procedure by using a nonlinear least squares method was applied to fit and analyze all polarization π and components of the atomic spectra to acquire the information of the local plasma. The spectral line shape was investigated according to emission spectra from different regions (e.g., low-field side and high-field side) along the viewing chords. Each polarization component was fitted and classified into three energy categories (the cold, warm, and hot components) based on different atomic production processes, in consistent with the transition energy distribution by calculating the gradient of the spectral profile. The emission position, magnetic field intensity, and flow velocity of a deuterium atom were also discussed in the context.

Irradiation-induced void evolution in iron: A phase-field approach with atomistic derived parameters**Project supported by the National Magnetic Confinement Fusion Energy Research Project of China (Grant No. 2015GB118001), the Fundamental Research Funds for the Central Universities, China (Grant No. DUT16RC(3)052), the National Basic Research Program of China (Grant No. 2012CB619402), and the NETL Project (Grant No. DE-FE0027776).

A series of material parameters are derived from atomistic simulations and implemented into a phase field (PF) model to simulate void evolution in body-centered cubic (bcc) iron subjected to different irradiation doses at different temperatures. The simulation results show good agreement with experimental observations — the porosity as a function of temperature varies in a bell-shaped manner and the void density monotonically decreases with increasing temperatures; both porosity and void density increase with increasing irradiation dose at the same temperature. Analysis reveals that the evolution of void number and size is determined by the interplay among the production, diffusion and recombination of vacancy and interstitial.

Irradiation Effects on the Retention of Hydrogen in Al2O3**Supported by the National Natural Science Foundation of China under Grant Nos 91426304 and 91226202, and the National Magnetic Confinement Fusion Energy Research Project under Grant No 2015GB113000.

Substantial defects are produced in Al2O3 by 4 MeV Au ion irradiation with a fluence of 4.4 × 1015 cm−2. Rutherford backscattering spectrometry/channeling and cross-sectional transmission electron microscopy methods are used to investigate the irradiation damage. The 190 keV H ions with a fluence of 1 × 1017 cm−2 are used for implanting pristine and Au ion irradiated Al2O3 to explore the irradiation damage effects on the hydrogen retention in Al2O3. The time-of-flight secondary ion mass spectrometry method is used to obtain the single hydrogen depth profile and ions mass spectra (IMS), in which we find that implanted hydrogens interacted with defects produced by Au ion irradiation. In IMS, we also obtain the hydrogen retention at a certain depth. Comparing the hydrogen retention in different Al2O3 samples, it is concluded that the irradiation damage improves the tritium permeation resistance property of Al2O3 under given conditions. This result means that Al2O3 may strengthen its property of reducing tritium permeation under the harsh irradiation environment in fusion reactors.

Method for measuring prompt γ-rays generated by D-T neutrons bombarding a depleted uranium spherical shell**Supported by National Special Magnetic Confinement Fusion Energy Research, China (2015GB108001) and National Natural Science Foundation of China (91226104)

The prompt γ-ray spectrum from depleted uranium (DU) spherical shells induced by 14 MeV D-T neutrons is measured. Monte Carlo (MC) simulation gives the largest prompt γ flux with the optimal thickness of the DU spherical shells 3–5 cm and the optimal frequency of neutron pulse 1 MHz. The method of time of flight and pulse shape coincidence with energy (DC-TOF) is proposed, and the subtraction of the background γ-rays discussed in detail. The electron recoil spectrum and time spectrum of the prompt γ-rays are obtained based on a 2″×2″ BC501A liquid scintillator detector. The energy spectrum and time spectrum of prompt γ-rays are obtained based on an iterative unfolding method that can remove the influence of γ-rays response matrix and pulsed neutron shape. The measured time spectrum and the calculated results are roughly consistent with each other. Experimental prompt γ-ray spectrum in the 0.4–3 MeV energy region agrees well with MC simulation based on the ENDF/BVI.5 library, and the discrepancies for the integral quantities of γ-rays of energy 0.4–1 MeV and 1–3 MeV are 9.2% and 1.1%, respectively.

Use of Polycarbonate Vacuum Vessels in High-Temperature Fusion-Plasma Research

Magnetic fusion energy (MFE) research requires ultrahigh-vacuum conditions, primarily to reduce plasma contamination by impurities. For radiofrequency (RF)-heated plasmas, a great benefit may accrue from a nonconducting vacuum vessel, allowing external RF antennas to avoid the complications and cost of internal antennas and high-voltage high-current feedthroughs. In this paper we describe these and other criteria, e.g., safety, design flexibility, structural integrity, access, outgassing, transparency, and fabrication techniques that led to the selection and use of 25.4-cm OD, 1.6-cm wall polycarbonate pipe as the main vacuum vessel for an MFE research device whose plasmas are expected to reach keV energies for durations exceeding 0.1 s.

Tritium well depth, tritium well time and sponge mechanism for reducing tritium retention

New simulation results are predicted in a fusion reactor operation process. They are somewhat similar to, but quite different from, the xenon poisoning effects resulting from fission-produced iodine during the restart-up process of a fission reactor. We obtained completely new results of tritium well depth and tritium well time in magnetic confinement fusion energy research area. This study is carried out to investigate the following: what will be the least amount of tritium storage required to start up a fusion reactor and how long the fusion reactor needs to be operated for achieving the tritium break-even during the initial start-up phase due to the finite tritium-breeding time, which is dependent on the tritium breeder, specific structure of the breeding zone, layout of the coolant flow pipes, tritium recovery scheme and applied extraction process, the tritium retention of plasma facing component (PFC) and other reactor components, unrecoverable tritium fraction in the breeder, leakage to the inertial gas container and the natural radioactive decay time constant. We describe these new issues and answer these problems by setting up and solving a set of equations, which are described by a dynamic subsystem model of tritium inventory evolution in a fusion experimental breeder (FEB). Reasonable results are obtained using our simulation model. It is found that the tritium well depth is about 0.319 kg and the tritium well time is approximately 235 full power operation days for the reference case of the designed FEB configuration, and it is also found that after one-year operation the tritium storage reaches 0.767 kg, which is more than the least amount of tritium storage required to start up another FEB-like fusion reactor. The results show that the tritium retention in the PFC is equivalent to 11.9% of tritium well depth that is fairly consistent with the result of 10–20% deduced from the integrated particle balance of European tokamaks. Based on our experimental and theoretical studies, some new mechanisms are proposed for reducing the tritium retention in PFC and structure materials of tritium-breeding blanket. In this paper, a qualitative analysis of the ‘sponge effect’ is carried out. The ‘sponge effect’ may help us to reduce tritium retention by ∼20% in the PFC.

Progress in characterization of the H-mode pedestal

The H-mode pedestal remains a critical subject for magnetic fusion energy research. Recent experimental efforts to characterize the edge pedestal, for tests of pedestal models now under development, are summarized in this report. The pedestal transport is characterized by the profiles and energy and particle fluxes through both the electron and ion channels. The flux measurements are made difficult by the short scale lengths of the pedestal and the difficulty of measuring the neutral ionization source within the pedestal. Recent results on pedestal scaling with a number of parameters are also summarized. Measurements of the pedestal scaling with the ion gyroradius, global beta, input power, shape, critical gradients, neutral density, aspect ratio, rotation, toroidal ripple and the toroidal field direction are presented. These measurements provide a strong test for emerging pedestal models as well as improve confidence of extrapolating existing experiments towards future devices. Recent work on the scaling of the H-mode power threshold is also presented.

Magnetic fusion energy studies in Japan

The primary facility for magnetic fusion energy (MFE) research in Japan is JT-60, in which DD plasma is heated up to an electron temperature of T e>10 keV with neutral beam injection (NBI) and radio frequency (RF) powers. It is noted that a normalized β N of 2.3 was maintained for 22 s. The JT-60 team has discovered internal transport barrier (ITB) in high-beta-poloidal discharges as an additional improved confinement mechanism like a previously known edge transport barrier (ETB). The operation forming the ITB has remarkably improved the plasma confinement. The second major MFE facility is a large helical device (LHD) at National Institute for Fusion Science (NIFS). The world's largest superconducting coil system of LHD generates helical magnetic field of 3 T where the stored magnetic energy reaches 1 GJ. A quasi-stationary plasma with an electron density of 4×10 18 m −3 at a temperature of about 1 keV was sustained for 54 min with the help of ion cyclotron range of frequency (ICRF) and electron cyclotron range of frequency (ECRF). The maximum electron density so far achieved is 5×10 20 m −3 with T e∼1 keV. We report MFE programs related to International Thermonuclear Experimental Reactor (ITER).

The collaborative tokamak control room

Magnetic fusion experiments keep growing in size and complexity resulting in a concurrent growth in collaborations between experimental sites and laboratories worldwide. In the US, the National Fusion Collaboratory Project is developing a persistent infrastructure to enable scientific collaboration for all aspects of magnetic fusion energy research by creating a robust, user-friendly collaborative environment and deploying this to the more than 1000 US fusion scientists in 40 institutions who perform magnetic fusion research. This paper reports on one aspect of the project which is the development of the collaborative tokamak control room to enhance both collocated and remote scientific participation in experimental operations. This work includes secured computational services that can be scheduled as required, the ability to rapidly compare experimental data with simulation results, a means to easily share individual results with the group by moving application windows to a shared display, and the ability for remote scientists to be fully engaged in experimental operations through shared audio, video, and applications. The project is funded by the USDOE Office of Science, Scientific Discovery through Advanced Computing (SciDAC) Program and unites fusion and computer science researchers to directly address these challenges.

Security on the US fusion grid

The National Fusion Collaboratory project is developing and deploying new distributed computing and remote collaboration technologies with the goal of advancing magnetic fusion energy research. This has led to the development of the U.S. fusion grid (FusionGrid), a computational grid composed of collaborative, compute, and data resources from the three large U.S. fusion research facilities and with users both in the U.S. and in Europe. Critical to the development of FusionGrid was the creation and deployment of technologies to ensure security in a heterogeneous environment. These solutions to the problems of authentication, authorization, data transfer, and secure data storage, as well as the lessons learned during the development of these solutions, may be applied outside of FusionGrid and scale to future computing infrastructures such as those for next-generation devices like ITER.

Advances in the real-time interpretation of fusion experiments

The National Fusion Collaboratory Project is developing a persistent infrastructure to enable scientific collaboration for all aspects of magnetic fusion energy research by creating a robust, user-friendly collaborative environment and deploying this to the more than one thousand fusion scientists in forty institutions who perform magnetic fusion research in the US. Work specifically focusing on advancing real-time interpretation of fusion experiments includes collocated collaboration in tokamak control rooms via shared display walls, remote collaboration using Internet based audio and video, and pseudo-real-time data analysis via the National Fusion Energy Grid (FusionGrid). The technologies being developed and deployed will also scale to the next generation experimental devices such as ITER.

Varian's involvement with magnetic fusion energy research

This article provides an overview of the history, accomplishments, and spinoffs of over two decades of interaction between the U.S. Fusion Energy Research Program`s plasma heating scientists and Varian`s Electron Device organization; an interaction which has been mutually benficial and, as with other U.S., Industry interaction with the U.S. Fusion Research Program, has produced valuable benfits to the country`s economy in the form of spinoffs which have had positive commercial and economic benfits. Varian has had two major RD and the other the development of super power tetrodes for Ion Cyclotron Power Sources.

Progress in magnetic fusion energy research

The remarkable scientific progress that has been made in the Magnetic Fusion Energy Program since its inception 40 years ago is reviewed. This formalized international collaborative effort of design and development for a 1000-MW experimental reactor (ITER) has been entered into by the United States, Russia, Japan, and the European Community. In the United States, a national project to build a superconducting steady-state advanced tokamak (SSAT) to improve the reactor prospects of fusion is underway. Beginning with a quick summary of fundamentals, progress in research is traced through major steps in the program. This study concludes with a view of the future and of the problems remaining to be overcome before a demonstration (demo) power plant can be built by the 2025 date given in the US National Energy Strategy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>