Magnetic Fusion Plasmas Research Articles

Electron tail suppression and effective collisionality due to synchrotron emission and absorption in mildly relativistic plasmas

Synchrotron radiation losses are a significant cause of concern for high-temperature aneutronic fusion reactions such as proton–Boron 11. The fact that radiation losses occur primarily in the high-energy tail, where the radiation itself has a substantial impact on the electron distribution, necessitates a self-consistent approach to modeling the diffusion and drag induced by synchrotron absorption and emission. Furthermore, an accurate model must account for the fact that the radiation emission spectrum is momentum-dependent, and the plasma opacity is frequency-dependent. Here, we present a simple Fokker–Planck operator, built on a newly solved-for blackbody synchrotron diffusion operator, which captures all relevant features of the synchrotron radiation. Focusing on magnetic mirror fusion plasmas, we show that significant suppression of the electron distribution occurs for relativistic values of the perpendicular electron momentum, which therefore emit much less radiation than predicted under the assumption of a Maxwell–Jüttner distribution.

Measurement of the Gamma-Ray-to-Neutron Branching Ratio for the Deuterium-Tritium Reaction in Magnetic Confinement Fusion Plasmas.

At present, magnetic confinement fusion devices rely solely on absolute neutron counting as a direct way of measuring fusion power. Absolute counting of deuterium-tritium gamma rays could provide the secondary neutron-independent technique required for the validation of scientific results and as a licensing tool for future power plants. However, this approach necessitates an accurate determination of the gamma-ray-to-neutron branching ratio. The gamma-ray-to-neutron branching ratio for the deuterium-tritium reaction ^{3}H(^{2}H,γ)^{5}He/^{3}H(^{2}H,n)^{4}He was determined in magnetic confinement fusion plasmas at the Joint European Torus in predominantly deuterium beam heated plasmas. The branching ratio was found to be equal to (2.4±0.5)×10^{-5} over the deuterium energy range of (80±20) keV. This accurate determination of the deuterium-tritium branching ratio paves the way for a direct and neutron-independent measurement of fusion power in magnetic confinement fusion reactors, based on the absolute counting of deuterium-tritium gamma rays.

Development of a new magnetic mirror device at the Korea Advanced Institute of Science and Technology

A new magnetic mirror machine named KAIMIR (KAIST mirror) has been designed and constructed at the Korea Advanced Institute of Science and Technology (KAIST) to study mirror plasma physics and simulate the boundary regions of magnetic fusion plasmas such as in a tokamak. The purpose of this paper is to introduce the characteristics and initial experimental results of KAIMIR. The cylindrical vacuum chamber has a length of 2.48 m and a diameter of 0.5 m and consists of three sub-chambers, namely the source, centre and expander chambers. A magnetic mirror configuration is achieved by electromagnetic coils with a maximum magnetic field strength of 0.4 T at the mirror nozzles and 0.1 T at the centre. The source plasma is generated by a plasma washer gun installed in the source chamber with a pulse forming network system. The typical discharge time is ~12 ms with a ~6 ms (1–7 ms) steady period. Initial results show that the on-axis electron density at the centre is 1019–20 m−3 and the electron temperature is 4–7 eV. Two parameters were varied in this initial phase, the source power and the mirror ratio, which is the ratio of highest to lowest magnetic field strength in the mirror-confined region. We observed that the increase of the electron density was mitigated for a source power above 0.2 MW. It was also found that the electron density increases almost linearly with the mirror ratio. Accordingly, the stored electron energy was also linearly proportional to the mirror ratio, similar to the scaling of the gas dynamic trap.

First application of data assimilation-based control to fusion plasma

Magnetic fusion plasmas, which are complex systems comprising numerous interacting elements, have large uncertainties. Therefore, future fusion reactors require prediction-based advanced control systems with an adaptive system model and control estimation robust to uncertainties in the model and observations. To address this challenge, we introduced a control approach based on data assimilation (DA), which describes the system model adaptation and control estimation based on the state probability distribution. The first implementation of a DA-based control system was achieved at the Large Helical Device to control the high temperature plasma. The experimental results indicate that the control system enhanced the predictive capability using real-time observations and adjusted the electron cyclotron heating power for a target temperature. The DA-based control system provides a flexible platform for advanced control in future fusion reactors.

The radial localization of the transition from low to high confinement mode in the ASDEX Upgrade tokamak

A novel experimental method is applied to localize the initial suppression of turbulence, in the form of density fluctuations, at the transition from the low (L-) to the high (H-) confinement mode in toroidal magnetic fusion plasmas. The high radial and temporal resolution, combined with the unprecedented statistical significance, provided the awaited information on a possible dominant E×B shear layer in L-H transition physics. We show, for the first time, that the H-mode turbulence suppression is initiated at the inner E×B shear layer in the ASDEX Upgrade tokamak possibly shedding light on the causality behind the L-H transition process.

Enhancement of fusion reactivity under non-Maxwellian distributions: effects of drift-ring-beam, slowing-down, and kappa super-thermal distributions

Non-Maxwellian distributions of particles are commonly observed in fusion studies, especially for magnetic confinement fusion plasmas. The particle distribution has a direct effect on fusion reactivity, which is the focus of this study. We investigate the effects of three types of non-Maxwellian distributions, namely drift-ring-beam, slowing-down, and kappa super-thermal distributions, on the fusion reactivities of D-T (Deuterium-Trillium) and p-B11 (proton-Boron) using a newly developed program, where the enhancement of fusion reactivity relative to the Maxwellian distribution is computed while keeping the total kinetic energy constant. The calculation results show that for the temperature ranges of interest to us, namely 5–50 keV for D-T and 100–500 keV for p-B11, these non-Maxwellian distributions can enhance the fusion reactivities. In the case of the drift-ring-beam distribution, the perpendicular ring beam velocity leads to decreased enhancement in low temperature range and increased enhancement in high temperature range. This effect is favorable for p-B11 fusion reaction and unfavorable for D-T fusion reaction. This is because the important temperature range for fusion reaction significantly overlaps with the high temperature enhancement range, while the important temperature range for fusion significantly overlaps with the low temperature reduction range. In the slowing-down distribution, the birth speed plays a crucial role in both reactions, and increasing birth speed leads to a shift in the enhancement ranges towards lower temperatures, which is beneficial for both reactions. Finally, the kappa super-thermal distribution results in a relatively large enhancement in the low temperature range with a small high energy power-law index . Overall, this study provides insight into the effects of non-Maxwellian distributions on fusion reactivity and highlights potential opportunities for enhancing fusion efficiency.

Interplay among turbulence, flow and impurities for sustaining magnetic island

As ubiquitous structures in magnetized fusion plasmas, magnetic islands (MIs) would short-circuit adjacent magnetic flux surfaces and result in a reduced pressure gradient and fluctuations inside the island; it is widely accepted that due to the stabilizing of drift wave instability, the turbulence intensity inside MIs is much lower for larger islands. Here, we provide the first observations that strong turbulence could be generated inside a large radiation MI, which is probably driven by the electron temperature dip due to strongly localized impurity radiation. Moreover, the flow velocity inside the MI is strongly correlated with the turbulence intensity, and the impurity concentration rate suddenly increases as the flow velocity reaches a threshold value, strongly suggesting that turbulence and flow inside the island play important roles in trapping heavy impurities and sustaining radiative MIs.

Impact of avalanche type of transport on internal transport barrier formation in tokamak plasmas.

In magnetic fusion plasmas, a transport barrier is essential to improve the plasma confinement. The key physics behind the formation of a transport barrier is the suppression of the micro-scale turbulent transport. On the other hand, long-range transport events, such as avalanches, has been recognized to play significant roles for global profile formations. In this study, we observed the impact of the avalanche-type of transport on the formation of a transport barrier for the first time. The avalanches are found to inhibit the formation of the internal transport barrier (ITB) observed in JT-60U tokamak. We found that (1) ITBs do not form in the presence of avalanches but form under the disappearance of avalanches, (2) thesurface integral of avalanche-driven heat fluxe is comparable to the time rate change of stored energy retained at the ITB onset, (3) the mean E × B flow shear is accelerated via the ion temperature gradient that is not sustained under the existence of avalanches, and (4) after the ITB formation,avalanches are damped inside the ITB, while they remain outside the ITB.

Development of a High Sampling Rate Data Acquisition System Working in a High Pulse Count Rate Region for Radiation Diagnostics in Nuclear Fusion Plasma Research

In this study, a high sampling rate data acquisition system with the ability to provide timestamp, pulse shape information, and waveform simultaneously under a sub megahertz pulse counting rate was developed for radiation diagnostics for magnetic confinement nuclear fusion plasma research. The testing of the data acquisition system under the high pulse counting rate condition using real signals was performed in an accelerator-based deuterium-deuterium fusion neutron source (Fast Neutron Source) at the Japan Atomic Energy Agency. We found that the pulse counts acquired by the system linearly increased up to 6 × 105 cps, and the count loss at 106 cps was estimated to be ~10%. The data acquisition system was applied to deuterium-deuterium neutron profile diagnostics in the deuterium gas operation of a helical-type magnetic confinement plasma device, called the Large Helical Device, to observe the radial profile of neutron emissivity for the first time in a three-dimensional magnetic confinement fusion device. Time-resolved measurements of the deuterium-deuterium fusion emission profile were performed. The experimentally observed radial neutron emission profile was consistent with numerical predictions based on the orbit-following models using experimental data. The data acquisition system was shown to have the desired performance.

Lagrangian Chaotic Mixing in a Nonlinear Model of Resistive Drift-wave Turbulence in Tokamak Plasmas

In fusion plasma, numerical simulations are commonly employed to investigate the confinement properties of plasma in the bulk region of tokamaks. The modified Hasegawa-Wakatani (MHW) equations are used to model the behavior of the plasma, which enables us to understand the radial transport in two-dimensional numerical simulations of electrostatic resistive drift-wave turbulence. By utilizing the MHW equations, we have gained insights into the low-to-high confinement (L-H) transitions that occur spontaneously in the plasma when it moves from a low confinement stage, characterized by turbulent flow, to a turbulence-suppressed regime known as zonal flow. To investigate these transitions, we vary the value of a control parameter \(\alpha\), which is related to adiabaticity, in numerical simulations, and observe the transition between the two regimes. This simplified model of L-H transitions can provide valuable information for tokamaks. To identify the Lagrangian coherent structures (LCS) and to better characterize the chaotic mixing during the L-H transition, we computed the finite-time Lyapunov exponent (FTLE) of the calculated velocity field derived from the electrostatic potential. We further compared the statistics of the chaotic mixing of the two regimes. The results of our study offer insights into the turbulent transport processes in magnetic confinement fusion plasmas.

Surface instability of static liquid metal in magnetized fusion plasma

Understanding surface instability in magnetized fusion plasma supports the appropriate implementation and handling of liquid metal as plasma facing components (PFCs) in future fusion reactors. A Lagrange equation describing a viscous liquid surface deformation in a magnetized plasma is derived using Rayleigh’s method. Its solution justifies the general instability criterion and helps in characterizing the key interactions driving such instability under fusion conditions. Surface tension and gravity, especially with the poloidal angles of the lower part of a plasma chamber, mainly stabilize the liquid surface at small and large disturbance wavelengths, respectively. The sheath electric field and the external tangential magnetic field cause the liquid surface to disintegrate at an intermediate wavelength. Practically, a magnetic confinement fusion (MCF) device requires a strong magnetic field for confinement. The study suggests that such a strong field dominates the rest and governs instability. In addition, this implies that the configuration of a static planar free liquid surface is difficult to adopt as a candidate for handling the liquid metal as PFCs in next step MCF devices.

Conceptual design of a GEM (gas electron multiplier) based gas Cherenkov detector for measurement of 17MeV gamma rays from T(D, γ)5He in magnetic confinement fusion plasmas.

The only method for assessing the fusion power throughput of a deuterium-tritium (DT) reactor presently relies on determining the absolute number of 14MeV neutrons produced in the DT plasma. An independent method, developed and investigated during the recent DT campaign at the Joint European Torus, is based on the absolute counting of 17MeV gamma rays produced by the competing T(D, γ)5He reaction that features a very weak branching ratio (about 3-6 × 10-6) when compared to the main T(D,n)4He reaction. The state-of-the-art spectrometer used for gamma-ray measurements in magnetic confinement fusion plasmas is LaBr3(Ce) scintillator detectors, although they require significant neutron shielding to extract a relatively weak gamma-ray signal from a much more abundant neutron field. A better approach relies on a gamma-ray detector that is intrinsically insensitive to neutrons. We have advanced the design of a gamma-ray counter based on the Cherenkov effect for gamma-rays whose energy exceeds 11MeV, optimized to work in the neutron-rich environment of a steady-state, magnetically confined fusion plasma device. The gamma-rays interact with an aluminum window and extract electrons that move into the radiator emitting photons via the Cherenkov effect. Since the Cherenkov light consists of few photons (25 on average) in the far UV band (100-200nm), a pre-amplifier is required to transport the photons to the neutron-shielded location, which may be a few meters away, where the readout elements of the detector, either a silicon or standard photomultiplier tube, are placed. The present work focuses on the development of a scintillating GEM (Gas Electron Multiplier) based pre-amplifier that acts as a Cherenkov photon pre-amplifier and wavelength shifter. This paper presents the result of a set of Garfield++ simulations developed to find the optimal GEM working parameters. A photon gain of 100 is obtained by biasing a single GEM foil to 1kV.

Investigation of ICRF-NBI synergetic heating induced fast ion distribution and transport in EAST tokamak

In magnetic confinement fusion plasmas, radio-frequency wave heating in the ion cyclotron range of frequencies (ICRF) and neutral beam injection (NBI) are two main heating methods. Their synergetic heating has long been a key topic in fusion research. In this work, we clarify the basic principles of ICRF high harmonic heating and the synergetic heating between ICRF and NBI. Then, we perform a series of experiments on EAST tokamak and carry out the corresponding TRANSP simulations. The results indicate that the ICRF-NBI synergetic heating not only significantly increases the plasma parameters (including poloidal beta, plasma stored energy, ion temperature and neutron yield), but also generates a large number of energetic particles and develops an energetic particle tail in its distribution function. For instance, the ICRF third harmonic heating with 1 MW of power can increase the energy of NBI fast ions from 60 to 600 keV. By changing the hydrogen minority concentration, improving the ICRF and NBI heating power, using the on-axis ICRF heating or optimizing the NBI injection angle, the ICRF-NBI synergetic heating effect can be further enhanced, accompanied with an increase of fast ion energy. Moreover, by using the fast ion distribution as input in the orbit tracing code, the transport and loss of energetic particles are calculated. The results show that the initial positions of the lost energetic particles are on the low field side, and their orbits are mainly trapped orbits. The loss of energetic particles is mainly located in the middle and upper plane of the main limiter, ICRF and LH antenna limiters. The lost of these energetic particles are considered as one of the main reasons why hot spots occur on the limiters.

Introducing Machine-Learning in Spectroscopy for Plasma Diagnostics and Predictions

Artificial Intelligence (AI) and data science techniques are increasingly introduced in physics including plasma physics where Machine Learning (ML) is applied to emission spectroscopy for plasma parameter determination. Recently, the open-access python-based Sickit-Learn ML platform was used to analyze line intensities in the order to infer the plasma electron densities and temperatures for conditions relevant to tokamak divertors. In this paper, we discuss the application of deep-learning (DL) to synthetic line spectra for conditions of magnetic fusion plasmas with hydrogen-deuterium (H-D) mixtures. The idea will be illustrated through application of Artificial Neural Networks (ANN) to spectra of the Balmer-α line emitted by H-D mixtures, the aim being to obtain the isotopic ratios. The objective of our approach is to provide a new method to infer the hydrogen isotopic ratio sufficiently fast that can be exploited for real-time applications. We will demonstrate the proof-of-principle of our method through the application of a TensorFlow DL regression algorithm to theoretical line spectra generated with predetermined parameters.

A Data-Assimilation Based Method for Equilibrium Reconstruction of Magnetic Fusion Plasma: Solution by Adjoint Method

Accurately determining the magnetohydrodynamic (MHD) equilibrium is fundamentally important in controlling and stabilizing fusion plasmas. Development of reliable methods for equilibrium reconstruction is therefore crucial to advancing the fusion plasma research. However, since the dynamics of plasma phenomena are highly nonlinear and highly complicated dynamics, it is difficult to develop their rigorous mathematical model. We already proposed a method of equilibrium reconstruction for fusion plasmas based on data assimilation. In the method the problem is formulated as a parameter optimization and a solution by the sensitivity equation method is proposed. In this paper, we propose a solution of equilibrium reconstruction for fusion plasmas by the adjoint method for the purpose of reducing computation time. In order to ensure the applicability of the adjoint method to a wide class of fusion plasmas, the problem formulation and derivation of the reconstruction algorithm based on the adjoint method are performed in generic form. Furthermore, we implement the proposed method as equilibrium reconstruction algorithms applicable to axisymmetric toroidal plasmas, typical examples of which are tokamaks and reversed field pinches (RFPs). Finally, the proposed method is applied to a real RFP experiment, and it is shown that the proposed method has the capability of reconstructing the equilibrium with sufficient accuracy and dramatic reduction of computation time compared with the existing methods.

Analysis method for calculating radial correlation length of electron temperature turbulence from correlation electron cyclotron emission radiometer.

The radial correlation length (Lr) is one of the essential quantities to measure in order to more fully characterize and understand turbulence and anomalous transport in magnetic fusion plasmas. The analysis method for calculating Lr of electron temperature (Te) turbulence from correlation electron cyclotron emission (correlation ECE or CECE) radiometer measurements has not been fully developed partly due to the fact that the turbulent electron temperature fluctuations are generally imbedded in much larger amplitude thermal noise, which leads to a greatly reduced cross correlation coefficient (ϱ) between two spatially separated ECE signals. This work finds that this ϱ reduction factor due to thermal noise is a function of the local relative temperature fluctuation power and CECE system bandwidths of intermediate and video frequencies, independent of radial separations. This indicates that under the approximation of constant relative temperature fluctuation power for a small radial range of local CECE measurements, the original shape of ϱ as a function of radial separation without thermal noise is preserved in the CECE data with thermal noise present. For Te turbulence with a Gaussian radial wavenumber spectrum, a fit function using the product of Gaussian and sinusoidal functions is derived for calculating Lr. This analysis method has been numerically tested using simulated ECE radiometer data over a range of parameters. Using this method, the experimental temperature turbulence correlation length Lr in a DIII-D L-mode plasma is found to be ∼10 times the local ion gyroradius.

Design of a multi-detector, single line-of-sight, time-of-flight system to measure time-resolved neutron energy spectra.

In the dynamic environment of burning, thermonuclear deuterium-tritium plasmas, diagnosing the time-resolved neutron energy spectrum is of critical importance. Strategies exist for this diagnosis in magnetic confinement fusion plasmas, which presently have a lifetime of ∼1012 longer than inertial confinement fusion (ICF) plasmas. Here, we present a novel concept for a simple, precise, and scale-able diagnostic to measure time-resolved neutron spectra in ICF plasmas. The concept leverages general tomographic reconstruction techniques adapted to time-of-flight parameter space, and then employs an updated Monte Carlo algorithm and National Ignition Facility-relevant constraints to reconstruct the time-evolving neutron energy spectrum. Reconstructed spectra of the primary 14.028MeV nDT peak are in good agreement with the exact synthetic spectra. The technique is also used to reconstruct the time-evolving downscattered spectrum, although the present implementation shows significantly more error.

Machine learning application in complicated burning plasmas for future magnetic fusion exploration

On October 24, 2021, the Chinese government unveiled a guiding document about state-level work to achieve carbon peaking and carbon neutrality goals under a new development philosophy, laying out key targets and measures for upcoming decades. The documment, titled “Working Guidance For Carbon Dioxide Peaking and Carbon Neutrality In Fully And Faithful Implementation Of The New Development Philosophy.” stated that China's carbon dioxide emissions per unit GDP would have dropped by 18% from the level in 2020 by 2025 and by more than 68% from the level in 2005 by 2030. Fusion energy, as an environment-friendly and intrinsically safe energy source with an abundant fuel supply, is considered an effective low-carbon ultimate solution. Fusion research has been conducted for 60 years, and has been demonstrated that magnetic confinement controlled fusion is very effective magnetic confinement fusion research has progressed significantly over the years. Generally, it is believed in the magnetic confinement fusion plasma community that the next pivotal step in magnetic confinement fusion plasma research is to create burning plasma, where the dominant method of heating plasmas is alpha particle self-sustaining heating. The research aims to understand the underlying physics of the confinement, heating, and instability of burning plasma and to explore the technologies associated with the power-producing fusion reactor. In future burning plasmas, multiple mode-number instabilities are very important but complicated physics issues because of the employment of strong power auxiliary heating schemes. Compared with the time-consuming large-scale computer simulation, the machine learning can bring an advantage for the investigations concerning multiple mode-number instabilities, especially in predictions compared to the time-consuming large-scale computer simulation.

Multi-scale turbulence simulation suggesting improvement of electron heated plasma confinement

Turbulent transport is a key physics process for confining magnetic fusion plasma. Recent theoretical and experimental studies of existing fusion experimental devices revealed the existence of cross-scale interactions between small (electron)-scale and large (ion)-scale turbulence. Since conventional turbulent transport modelling lacks cross-scale interactions, it should be clarified whether cross-scale interactions are needed to be considered in future experiments on burning plasma, whose high electron temperature is sustained with fusion-born alpha particle heating. Here, we present supercomputer simulations showing that electron-scale turbulence in high electron temperature plasma can affect the turbulent transport of not only electrons but also fuels and ash. Electron-scale turbulence disturbs the trajectories of resonant electrons responsible for ion-scale micro-instability and suppresses large-scale turbulent fluctuations. Simultaneously, ion-scale turbulent eddies also suppress electron-scale turbulence. These results indicate a mutually exclusive nature of turbulence with disparate scales. We demonstrate the possibility of reduced heat flux via cross-scale interactions.

Design of Thomson scattering diagnostic system on linear magnetized plasma device

In addition to the magnetic confinement fusion plasma, Thomson scattering has been applied to measure electron density and temperature of low-temperature plasmas. Based on a linear magnetized plasma device, a set of Thomson scattering diagnostic system is designed to diagnose the plasma with and eV. Due to low plasma temperature and density, this diagnostic system needs high spectral resolution and collection efficiency to meet the requirements of electron velocity distribution function measurements. Through the bench test, it is confirmed that the spectral resolution reaches 0.01 nm, and theoretical collection efficiency is high enough to obtain a Thomson scattering spectrum by 1000 accumulations.