Magnetic Confinement Fusion Plasmas Research Articles

Radio-frequency current drive and flow drive in magnetic confinement fusion plasma

Radio-frequency (rf) waves can penetrate fusion plasmas and deposit energy and momentum through collisionless mechanisms, resulting in plasma heating and, in some cases, current drive and flow drive. The advantage of this noninductive current drive is that a tokamak-type fusion reactor can be operated at a steady state. Meanwhile, an appropriate profile of plasma current and a significant plasma flow are important in stabilizing magnetohydrodynamics instabilities and realizing and sustaining high-performance confinement regimes. Therefore, rf current drive and flow drive are important in research on the physics of magnetic confinement fusion plasma. This paper presents the elementary physics of wave-particle interactions in plasmas, reviews the present status and challenges of research on rf current drive and flow drive, and suggests possible research strategies. Several problems are highlighted. These include the intrinsic relation of resonant absorption to the reduction of rf current drive efficiency in high-density plasmas; the existence/feasibility of a nonresonant drive scheme; the possibility of increasing the drive efficiency within the resonant mechanism; the direct and indirect effects of rf waves in flow drive, especially the generalized ponderomotive force generated by rf waves; and nonlinear processes during the coupling and propagation of waves, such as parametric decay instabilities, and their possible effects on current drive and flow drive.

Progress towards numerical and experimental simulations of fusion relevant beam instabilities

In certain plasmas, non-thermal electron distributions can produce instabilities. These instabilities may be useful or potentially disruptive. Therefore the study of these instabilities is of importance in a variety of fields including fusion science and astrophysics. Following on from previous work conducted at the University of Strathclyde on the cyclotron resonance maser instability that was relevant to astrophysical radiowave generation, further instabilities are being investigated. Particular instabilities of interest are the anomalous Doppler instability which can occur in magnetic confinement fusion plasmas and the two-stream instability that is of importance in fast-ignition inertial confinement fusion. To this end, computational simulations have been undertaken to investigate the behaviour of both the anomalous Doppler and two-stream instabilities with the goal of designing an experiment to observe these behaviours in a laboratory.

Recent Advances in the Worldwide Fusion Gyrotron Development

Progress in the worldwide development of high-power gyrotrons for magnetic confinement fusion plasma applications is presented. Gyrotron oscillators are used for electron cyclotron heating, electron cyclotron current drive, stability control, and plasma diagnostics. After technology breakthroughs in the research on gyrotron components in the 1990s, significant progress has been achieved in the 2000s, in the field of long-pulse and continuous wave (CW) operation for a wide range of frequencies. Currently, the development of 1-MW-class CW gyrotrons for the tokamak ITER (170 GHz), the stellarator Wendelstein 7-X (140 GHz), and the tokamaks DIII-D and JT-60SA (110 GHz) has been very successful in EU, Japan, Russia, and USA. The Japan 170-GHz ITER gyrotron holds the energy world record of 2.88 GJ (0.8 MW at 1-h pulse duration). For this progress in the field of high-power long-pulse gyrotrons, innovations such as the realization of high-efficiency stable oscillation in very high-order cavity modes with low ohmic losses, the use of single-stage depressed collectors for energy recovery (efficiency enhancement and simpler power supplies), highly efficient internal quasi-optical (q.o.) mode converters (low level of internal stray radiation), and synthetic diamond windows have essentially contributed. The total tube efficiencies are around 50% and the purity of the linearly polarized fundamental Gaussian output mode is 97% and higher. Power modulation technologies for stabilization of neoclassical tearing modes have proceeded. Future prospects of advanced high-power fusion gyrotrons are in the areas of two- and three-frequency gyrotrons, fast step-wise frequency tuneability, higher unit power (coaxial cavities), and higher frequencies for more efficient plasma stabilization and noninductive current drive as well as reliability, availability, maintainability and inspectability for next step fusion power stations. The GYCOM step-tuneable 1-MW gyrotron for ASDEX Upgrade employing a broadband travelling-wave-resonator window (with two diamond disks) operates at 105, 117, 127 and 140 GHz. The EU 170 GHz coaxial-cavity gyrotron prototype achieved in millisecond pulses the power of 2.1 MW at 46% efficiency and 96% Gaussian mode purity. A new power record of second harmonic oscillation has been achieved in the subterahertz band (83 kW/389 GHz/3 ms) in Japan for application to collective Thomson scattering diagnostics. A fundamental frequency 670-GHz gyrotron with pulsed magnet generated 210 kW in 20-30- μs pulses at 20% efficiency (collaboration of institutions in Russia and USA).

Exact equilibria for nonlinear force-free magnetic fields with its applications to astrophysics and fusion plasmas

AbstractKnowledge of the structure of coronal magnetic field originating from the photosphere is relevant to the understanding of many solar activity phenomena, e.g. flares, solar prominences, coronal loops, and coronal heating. In most of the existing literature, these loop-like magnetic structures are modeled as force-free magnetic fields (FFMF) without any plasma flow. In this paper, we present several exact solution classes for nonlinear FFMF, in both translational and axisymmetric geometries. The solutions are considered for their possible relevance to astrophysics and solar physics problems. These are used to illustrate arcade-type magnetic field structures of the photosphere and twisted magnetic flux ropes through the coronal mass ejections (CMEs), as well as magnetic confinement fusion plasmas.

Relevance of Uncorrelated Lorentzian Pulses for the Interpretation of Turbulence in the Edge of Magnetically Confined Toroidal Plasmas

Recently, it has been proposed that the turbulent fluctuations measured in a linear plasma device could be described as a superposition of uncorrelated Lorentzian pulses with a narrow distribution of durations, which would provide an explanation for the reported quasiexponential power spectra. Here, we study the applicability of this proposal to edge fluctuations in toroidal magnetic confinement fusion plasmas. For the purpose of this analysis, we introduce a novel wavelet-based pulse-detection technique that offers important advantages over existing techniques. This technique allows for extracting the properties of individual pulses from the experimental time series, and for quantifying the distribution of pulse duration and energy as well as temporal correlations. We apply the wavelet technique to edge turbulent fluctuation data from the W7-AS stellarator and the JET tokamak, and find that the pulses detected in the data do not have a narrow distribution of durations and are not uncorrelated. Instead, the distributions are of the power-law type, exhibiting temporal correlations over scales much longer than the typical pulse duration. These results suggest that turbulence in open and closed field line systems may be distinct, and cast doubts on the proposed ubiquity of exponential power spectra in this context.

Application of spatially resolved high resolution crystal spectrometry to inertial confinement fusion plasmas

High resolution (λ∕Δλ ∼ 10 000) 1D imaging x-ray spectroscopy using a spherically bent crystal and a 2D hybrid pixel array detector is used world wide for Doppler measurements of ion-temperature and plasma flow-velocity profiles in magnetic confinement fusion plasmas. Meter sized plasmas are diagnosed with cm spatial resolution and 10 ms time resolution. This concept can also be used as a diagnostic of small sources, such as inertial confinement fusion plasmas and targets on x-ray light source beam lines, with spatial resolution of micrometers, as demonstrated by laboratory experiments using a 250-μm (55)Fe source, and by ray-tracing calculations. Throughput calculations agree with measurements, and predict detector counts in the range 10(-8)-10(-6) times source x-rays, depending on crystal reflectivity and spectrometer geometry. Results of the lab demonstrations, application of the technique to the National Ignition Facility (NIF), and predictions of performance on NIF will be presented.

CENTORI: A global toroidal electromagnetic two-fluid plasma turbulence code

A new global two-fluid electromagnetic turbulence code, CENTORI, has been developed for the purpose of studying magnetically-confined fusion plasmas on energy confinement timescales. This code is used to evolve the combined system of electron and ion fluid equations and Maxwell equations in toroidal configurations with axisymmetric equilibria. Uniquely, the equilibrium is co-evolved with the turbulence, and is thus modified by it. CENTORI is applicable to tokamaks of arbitrary aspect ratio and high plasma beta. A predictor–corrector, semi-implicit finite difference scheme is used to compute the time evolution of fluid quantities and fields. Vector operations and the evaluation of flux surface averages are speeded up by choosing the Jacobian of the transformation from laboratory to plasma coordinates to be a function of the equilibrium poloidal magnetic flux. A subroutine, GRASS, is used to co-evolve the plasma equilibrium by computing the steady-state solutions of a diffusion equation with a pseudo-time derivative. The code is written in Fortran 95 and is efficiently parallelised using Message Passing Interface (MPI). Illustrative examples of output from simulations of a tearing mode in a large aspect ratio tokamak plasma and of turbulence in an elongated conventional aspect ratio tokamak plasma are provided.

Innovation on high-power long-pulse gyrotrons

Progress in the worldwide development of high-power gyrotrons for magnetic confinement fusion plasma applications is described. After technology breakthroughs in research on gyrotron components in the 1990s, significant progress has been achieved in the last decade, in particular, in the field of long-pulse and continuous wave (CW) gyrotrons for a wide range of frequencies. At present, the development of 1 MW-class CW gyrotrons has been very successful; these are applicable for self-ignition experiments on fusion plasmas and their confinement in the tokamak ITER, for long-pulse confinement experiments in the stellarator Wendelstein 7-X (W7-X) and for EC H&CD in the future tokamak JT-60SA. For this progress in the field of high-power long-pulse gyrotrons, innovations such as the realization of high-efficiency stable oscillation in very high order cavity modes, the use of single-stage depressed collectors for energy recovery, highly efficient internal quasi-optical mode converters and synthetic diamond windows have essentially contributed. The total tube efficiencies are around 50% and the purity of the fundamental Gaussian output mode is 97% and higher. In addition, activities for advanced gyrotrons, e.g. a 2 MW gyrotron using a coaxial cavity, multi-frequency 1 MW gyrotrons and power modulation technology, have made progress.

Scientific and Computational Challenges of the Fusion Simulation Program (FSP)

This paper highlights the scientific and computational challenges facing the Fusion Simulation Program (FSP) a major national initiative in the United States with the primary objective being to enable scientific discovery of important new plasma phenomena with associated understanding that emerges only upon integration. This requires developing a predictive integrated simulation capability for magnetically-confined fusion plasmas that are properly validated against experiments in regimes relevant for producing practical fusion energy. It is expected to provide a suite of advanced modeling tools for reliably predicting fusion device behavior with comprehensive and targeted science-based simulations of nonlinearly-coupled phenomena in the core plasma, edge plasma, and wall region on time and space scales required for fusion energy production. As such, it will strive to embody the most current theoretical and experimental understanding of magnetic fusion plasmas and to provide a living framework for the simulation of such plasmas as the associated physics understanding continues to advance over the next several decades. Substantive progress on answering the outstanding scientific questions in the field will drive the FSP toward its ultimate goal of developing the ability to predict the behavior of plasma discharges in toroidal magnetic fusion devices with high physics fidelity on all relevant time and space scales. From a computational perspective, this will demand computing resources in the petascale range and beyond together with the associated multi-core algorithmic formulation needed to address burning plasma issues relevant to ITER - a multibillion dollar collaborative experiment involving seven international partners representing over half the world's population. Even more powerful exascale platforms will be needed to meet the future challenges of designing a demonstration fusion reactor (DEMO). Analogous to other major applied physics modeling projects (e.g., Climate Modeling), the FSP will need to develop software in close collaboration with computers scientists and applied mathematicians and validated against experimental data from tokamaks around the world. Specific examples of expected advances needed to enable such a comprehensive integrated modeling capability and possible co-design approaches will be discussed. __________________________________________________

Simulation science for fusion plasmas

The world fusion effort has embarked into a new age with the construction of ITER in Cadarache, France, which will be the first magnetic confinement fusion plasma experiment dominated by the self-heating of fusion reactions. In order to operate and control burning plasmas and next generation demo fusion reactors, an advanced capability for comprehensive integrated computer simulations that are fully verified and validated against experimental data will be necessary. The ultimate goal is to predict reliably the behaviour of plasmas in toroidal magnetic confinement devices on all relevant scales, both in time and space. In addition to developing a sophisticated integrated simulation codes, directed advanced research in fusion physics, applied mathematics, computer science and software is envisaged. In this paper we review the basic strategy and main research efforts at the Department of Simulation Science of the National Institute for Fusion Science (NIFS)- which is the Inter University Institute and the coordinating Center of Excellence for academic fusion research in Japan. We overview a simulation research at NIFS, in particular relation to experiments in the Large Helical Device (LHD), the world's largest superconducting heliotron device, as a National Users' facility (see Motojima et al. [1]). Our main goal is understanding and systemizing the rich hierarchy of physical mechanisms in fusion plasmas, supported by exploring a basic science of complexity of plasma as a highly nonlinear, non-equilibrium, open system. The aim is to establish a simulation science as a new interdisciplinary field by fostering collaborative research in utilizing the large-scale supercomputer simulators. A concept of the hierarchy-renormalized simulation modelling will be invoked en route toward the LHD numerical test reactor.

NSTX Plasma Start-Up Using Transient Coaxial Helicity Injection

Future toroidal magnetic confinement fusion plasma devices such as the Component Test Facility (CTF) require non-inductive toroidal current drive. A new method of non-inductive startup, referred to as transient coaxial helicity injection (Transient CHI), has been developed on the Helicity Injected Torus (HIT-II) experiment and the National Spherical Torus Experiment NSTX). In this method, plasma current is produced by discharging a capacitor bank between coaxial electrodes in the presence of toroidal and poloidal magnetic fields chosen such that the plasma rapidly expands into the chamber. When the injected current is rapidly decreased, magnetic reconnection occurs near the injection electrodes with the toroidal plasma current forming closed flux surfaces. In NSTX, transient CHI has demonstrated closed-flux current generation of up to 160 kA, without the use of a central solenoid. Detailed experimental measurements made on NSTX include fast time-scale visible imaging of the entire process.

A ‘multi-colour’ SXR diagnostic for time and space-resolved measurements of electron temperature, MHD activity and particle transport in MCF plasmas

A fast (⩽0.1 ms) and compact ‘multi-colour’ soft x-ray array has been developed for time and space-resolved electron temperature (Te) measurements in magnetically confined fusion (MCF) plasmas. The electron temperature is obtained by modelling the slope of the continuum radiation from ratios of the available 1D-Abel inverted radial emissivity profiles over different energy ranges, with no a priori assumptions of plasma profiles, magnetic field reconstruction constraints or need of shot-to-shot reproducibility. This technique has been used to perform fast Te measurements in the National Spherical Torus Experiment (NSTX), avoiding the limitations imposed by the well-known multi-point Thompson scattering, electron cyclotron emission and electron Bernstein wave mode conversion diagnostics. The applicability of this ‘multi-colour’ technique for magnetohydrodynamic (MHD) mode recognition and a variety of perturbative electron and impurity transport studies in MCF plasmas is also discussed. Reconstructed ‘multi-colour’ emissivity profiles for a variety of NSTX scenarios are presented here for the first time.

Measurement of the deuterium/tritium fuel mixture in magnetic confinement fusion plasmas by Doppler-free two-photon spectroscopy

Doppler-free two-photon excitation of fluorescence may be used to measure the deuterium/tritium fuel mix in magnetic confinement fusion plasmas. Two atomic transition schemes of the hydrogen isotopes come into question for this purpose: (I) excitation of the first excited energy level (n=2) and observation of Lyman-α, (II) excitation of the second excited energy level (n=3) and observation of Balmer-α. Comparison of the two schemes indicates that both may have their merits in particular situations. Due to lack of information about the future laser characteristics achievable for scheme II, the detailed performance of this scheme cannot yet be definitely assessed.

Isoelectronic behavior of resonant and intercombination lines in MgI-like ions

The observed intensity ratio of the 3s 2 1S 0 - 3s3p 1P 1 to the 3s 2 1S 0 - 3s3p 3P 1 transitions in MgI-like ion has always presented those who model plasma spectra with a challenge; the obseved intensity of the intercombination line is always several times greater than what simple models predict. We offer a model that takes into account the contribution to the MgI-like emission features from autoionizing levels of the adjacent All-like charge state. Models in the present work give good agreement with the observed resonant/intercombination (R/I) emission ratio only when a departure from ionization equilibrium is assumed. We also identify, for the first time, intercombination lines of the form 3s3p 1P 1 · 3p 2 3P 2 in several elements relevant to both astrophysical and magnetically-confined fusion plasmas.

A multiple-beam, collective scattering technique for spatially resolved measurement of plasma turbulencea)

Collective Thomson scattering has long been utilized to study electrostatic turbulence in magnetic confinement fusion plasmas with a view to demonstrating a causal link to the observed anomolous transport of heat and particles. However, this goal has been severly hampered by the relatively poor spatial resolution available at the dominant turbulence wave numbers using standard experimental techniques. Good spatial resolution is necessary if adequate comparison of theory and experiment is to occur. The present paper describes the basic principles of a new technique utilizing simultaneous, multiple input beams, a detector array, and coherent tomographic inversion algorithm, which can substantially improve the available spatial resolution for these important measurements. The proposed multiple beam scattering diagnostic offers a spatial resolution of ∼10 cm at a fluctuation wave number of 5 cm−1 when the angular envelope of the beams is 0.1 rad. Optical designs suitable for implementing the multiple beam diagnostic on a large tokamak have been developed.

A multiple-beam, collective scattering technique for spatially resolved measurement of plasma turbulence (abstract)a)

Collective Thomson scattering has long been utilized to study electrostatic turbulence in magnetic confinement fusion plasmas with a view to demonstrating a causal link to the observed anomolous transport of heat and particles. However, this goal has been severly hampered by the relatively poor spatial resolution available at the dominant turbulence wave numbers using standard experimental techniques. Good spatial resolution is necessary if adequate comparison of theory and experiment is to occur. The present paper describes the basic principles of a new technique utilizing simultaneous, multiple input beams, a detector array, and coherent tomographic inversion algorithm, which can substantially improve the available spatial resolution for these important measurements. The proposed multiple beam scattering diagnostic offers a spatial resolution of ∼10 cm at a fluctuation wave number of 5 cm−1 when the angular envelope of the beams is 0.1 rad. Optical designs suitable for implementing the multiple beam diagnostic on a large tokamak have been developed.

Atomic modeling and spectroscopic diagnostics (invited)

Atomic reaction models provide the link by which quantitative diagnostic comments on plasma behavior and parameters may be made from spectral observations of emission by ions in the plasma. This paper reviews progress in this area with emphasis on impurity species in magnetic confinement fusion plasmas. A systematic approach based on generalized collisional-radiative theory is adopted and items discussed include spectroscopy of edge and divertor environments, beam emission, and charge exchange spectroscopy. Case studies are based on experience at the JET Joint European Torus Experiment.

Interferometry Techniques on Fusion Plasmas

AbstractOptical interferometry techniques allow phase changes in a medium to be accurately determined. Since plasmas introduce a phase modification proportional to electron density, interferometry has been used routinely to determine the time development of electron density profiles in magnetic confinement fusion plasmas. The present paper describes the basic principles of interferometry of plasmas and establishes criteria regarding wavelength selection, vibrational phase noise, phase averaging etc. In addition, recent advances at UCLA in interferometry of fusion plasmas will be described. For example, high spatial sampling has allowed the detailed study of MHD phenomena such as sawteeth and Mimov oscillations, and 2-D imaging interferometry has permitted tomographic reconstruction of electron density contours. Difficulties associated with interferometry on future fusion devices, such as CIT, will also be discussed together with possible solutions.

FPPAC: A two-dimensional multispecies nonlinear Fokker-Planck package

Energetic electrons are of interest in many types of plasmas, however previous modeling of their properties has been restricted to the use of linear Fokker–Planck collision operators or non-relativistic formulations. Here, we describe a fully non-linear kinetic-equation solver, capable of handling large electric-field strengths (compared to the Dreicer field) and relativistic temperatures. This tool allows modeling of the momentum-space dynamics of the electrons in cases where strong departures from Maxwellian distributions may arise. As an example, we consider electron runaway in magnetic-confinement fusion plasmas and describe a transition to electron slide-away at field strengths significantly lower than previously predicted.Program title: NORSEProgram Files doi: http://dx.doi.org/10.17632/86wmgj758w.1 Licensing provisions: GPLv3Programming language: MatlabNature of problem: Solves the Fokker–Planck equation for electrons in 2D momentum space in a homogeneous plasma (allowing for magnetization), using a relativistic non-linear electron–electron collision operator. Electric-field acceleration, synchrotron-radiation-reaction losses, as well as heat and particle sources are included. Scenarios with time-dependent plasma parameters can be studied.Solution method: The kinetic equation is represented on a non-uniform 2D finite-difference grid and is evolved using a linearly implicit time-advancement scheme. A mixed finite-difference-Legendre-mode representation is used to obtain the relativistic potentials (analogous to the non-relativistic Rosenbluth potentials) from the distribution.