Fusion Plasma Devices Research Articles

Defining and identifying three-dimensional magnetic reconnection in resistive magnetohydrodynamic simulations of Earth’s magnetosphere

Magnetic reconnection is thought to be the primary mode by which the solar wind couples to the terrestrial magnetosphere, driving phenomena such as magnetic storms and aurorae. While the theory of two-dimensional reconnection is well developed and has been applied with great success to axisymmetric and toroidal systems such as laboratory plasma experiments and fusion devices, it is difficult to justify the application of two-dimensional theory to nontoroidal plasma systems such as Earth’s magnetosphere. Unfortunately, the theory of three-dimensional magnetic reconnection is much less well developed, and even defining magnetic reconnection has turned out to be controversial. In this paper, recent progress in the use of magnetohydrodynamics (MHD) to address the physics of three-dimensional reconnection in Earth’s magnetosphere is reviewed. The paper consists of two parts. In the first part, various definitions of three-dimensional reconnection are reviewed, with the goal of mapping these definitions to sets of physical phenomena that have been identified as “reconnection” in various contexts. In the second part of the paper, MHD simulation results for the magnetosphere are presented, and two qualitatively distinct types of reconnection phenomena are identified: (1) Steady separator reconnection under generic northward interplanetary magnetic field (IMF) conditions, involving plasma flow across magnetic separatrices, and (2) time-dependent reconnection under generic southward IMF conditions, involving a locally detectable change in the magnetic field topology. It is concluded that magnetic reconnection phenomena at Earth’s dayside magnetopause are adequately captured by two distinct definitions: The Vasyliunas definition [V. M. Vasyliunas, Rev. Geophys 13, 303 (1975)], which identifies magnetic reconnection with plasma flow across magnetic separatrices, and the Greene definition [J. Greene, Phys. Fluids B 5, 2355 (1993)], which identifies magnetic reconnection with a violation of magnetic flux conservation. Further generalizations of the definition of magnetic reconnection—e.g., the Schindler–Hesse [K. Schindler and M. Hesse, J. Geophys. Res. 93, 5547 (1988)] definition, which identifies reconnection with spatially localized violations of ideal MHD—are, while potentially useful in characterizing reconnection phenomena in the absence of magnetic nulls, separators, or separatrices, unnecessary in the magnetospheric context.

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.

Toward coupling flow driven and magnetically driven dynamos

Most large-scale dynamo research for astrophysical rotators focuses on interior flow driven helical dynamos (FDHDs), but larger scale coronal fields most directly influence observations. It is thus important to understand the relationship between coronal and interior fields. Coronal field relaxation is actually a type of magnetically dominated helical dynamo (MDHD). MDHDs also occur in fusion plasma devices where they drive a system toward its relaxed state in response to magnetic helicity injection that otherwise drives the system away from this state. Global scale fields of astrophysical rotators and jets are thus plausibly produced by a direct coupling between an interior FDHD and a coronal MDHD, interfaced by magnetic helicity transport through their mutual boundary. Tracking the magnetic helicity also elucidates how both FDHD and MDHDs evolve and saturate. The utility of magnetic helicity is unhampered by its non-gauge invariance since physical fields can always be recovered.

Controlling the cross-field flux of cold α-particles with resonant magnetic perturbations in a helical fusion plasma device

A new scenario to control transport of ‘cold’ α-particles in a helical fusion device by changing poloidal field coil current during plasma discharge is proposed here. A way to enhance radial transport of α-particles in the intermediate energy range is considered which relies on specific features of helical magnetic field. To remove helically trapped, cold α-particles, a β-induced change in the B/B0 modulation along the particle trajectory is employed. To remove passing particles resonant effects connected with the poloidal field coil currents are used.

Fractal Reactor: Re-Creating the Sun

The author recounts his quest to design an alternative plasma fusion device that could generate limitless energy through nuclear fusion. The proposed Fractal Reactor is based on fractal geometry rather than the Euclidean geometry used in the designs for the containment systems of plasma fusion devices. Fusion energy systems might become more effective if they more closely embody the geometry and physics of stars, nature's “fractal reactors.” The author aims to work with nature and not against it in controlling the forces that govern burning plasmas. Instead of jamming the square peg of Euclidean geometry into the round hole of fractal geometry, the author considers exerting intense forces on plasmas that approximate the gravitational forces in a star.

Recent results on turbulence and MHD activity achieved by reflectometry

Over the last years, owing to hardware progress and the development of new methods, reflectometry has become a common diagnostic on plasma fusion devices. This paper presents some results obtained with reflectometry on transport, turbulence and magnetohydrodynamic (MHD). The emphasis is put on some new results from Tore-Supra. Combining the density profile and fluctuation measurement, it was shown on Tore-Supra that the particle pinch inside the q = 1 surface is close to the neoclassical value in ohmic plasma, while the observed small diffusion is in agreement with a very low level of density fluctuations inside the q = 1 surface. In β scaling experiments, no change in the fluctuation levels was found on Tore-Supra, in agreement with the observation of weak confinement degradation with increasing β. Zonal flows have been detected by Doppler reflectometry in ASDEX-U and with correlation reflectometry in T-10. On Tore-Supra, a fast decrease in the density fluctuation level at high poloidal wavenumbers was measured with Doppler reflectometry, suggesting a minor role of electron temperature gradient driven modes. Various forms of Alfvén eigenmodes (toroidal Alfvén eigenmodes, Alfvén cascades and possibly beta Alfvén eigenmodes) have been detected with reflectometry in TFTR, JET and Tore-Supra. The density fluctuations induced by the mode were found to be higher on the high-field side.

Imaging system for hypervelocity dust injection diagnostic on NSTX

The novel hypervelocity dust injection diagnostic will facilitate our understanding of basic aspects of dust-plasma interaction and magnetic field topology in fusion plasma devices, by observing “comet tails” associated with the injected micron-size dust particles. A single projection of the tail onto an image plane will not provide sufficient information; therefore, we plan to use two views, with intensified DiCam-Pro cameras on two NSTX ports. Each camera can furnish up to five overlaying sequential images with gate times greater than 3ns and 1280×1024pixel resolution. A coherent fiber bundle with 1500×1200 fibers will relay the image from an imaging lens installed directly on the port to the camera optics. The lens receives light from the outer portion of the NSTX cross section and focuses a 1cm tail onto at least 60 fibers for adequate resolution. The estimated number of photons received by the camera indicates signal-to-noise ratios of 102–104, with the use of a 10nm bandwidth filter. The imaging system with one camera was successfully tested on NSTX in 2005. Photographing lithium pellets yielded bright and distinctive pictures of the tails nearly aligned with B lines. We also observed that the bright “filaments”—plasma cords with high density and temperature—are present in both top and bottom portions of the machine.

Edge stability and transport control with resonant magnetic perturbations in collisionless tokamak plasmas

A critical issue for fusion-plasma research is the erosion of the first wall of the experimental device due to impulsive heating from repetitive edge magneto-hydrodynamic instabilities known as 'edge-localized modes' (ELMs). Here, we show that the addition of small resonant magnetic field perturbations completely eliminates ELMs while maintaining a steady-state high-confinement (H-mode) plasma. These perturbations induce a chaotic behaviour in the magnetic field lines, which reduces the edge pressure gradient below the ELM instability threshold. The pressure gradient reduction results from a reduction in the particle content of the plasma, rather than an increase in the electron thermal transport. This is inconsistent with the predictions of stochastic electron heat transport theory. These results provide a first experimental test of stochastic transport theory in a highly rotating, hot, collisionless plasma and demonstrate a promising solution to the critical issue of controlling edge instabilities in fusion-plasma devices.

Recombination processes involving H and D atoms interacting with a graphite surface: collisional data relevant to fusion plasma devices

The recombination of atomic hydrogen on graphite surface via the Eley–Rideal mechanism has been studied in the framework of the semiclassical time-dependent method and a potential energy surface assumed in our previous work. The results show that at the surface temperature, TS=500 K, the probability for H2 formation is large in a rather narrow range of collisional energies, from 0.01 up to 0.2 eV. At Ekin > 0.3 eV the recombination reaction is suppressed. Three other different mass combinations have been considered in the simulation: D+Dad, D+Had and H+Dad. A large isotope effect has been pointed out on the energy thresholds, the energy structure of the recombination probability as well as for the energy partitioning in the final states of the molecular species scattered in the gas phase.

High-power gyrotron development at Forschungszentrum Karlsruhe for fusion applications

In the first part of this paper, the status of the 140-GHz continuously operated gyrotrons with an output power of 1 MW for the stellarator Wendelstein 7-X will be described. With the first series tube, an output power of 1000 kW has been achieved in short pulse operation (milliseconds) with an electron beam current of 40 A, and of 1150 kW at 50 A. With a pulse length of 3 min limited by the available high-voltage (HV) power supply, an output power of 920 kW at an electron beam current of about 40 A with an efficiency of 45% and a mode purity of 97.5% has been obtained. At a reduced beam current of 29 A, an output power of 570 kW was measured with a pulse length of 1893 s without significant increase in tube pressure. The energy content of this pulse is almost 1.1 GJ. For the next fusion plasma device, International Thermonuclear Experimental Reactor (ITER), gyrotrons with a higher output power of about 2 MW are desirable. In short-pulse experiments, the feasibility of the fabrication of coaxial cavity gyrotrons with an output power up to 2-MW, continuous wave (CW), has been demonstrated, and the information necessary for a technical design has been obtained. The development of a long-pulse 2-MW coaxial cavity gyrotron started within a European cooperation. In parallel to the design and fabrication of an industrial prototype gyrotron, a short-pulse preprototype gyrotron has been operated to verify the design of critical components. An output power of 1.2 MW with an efficiency of 20% has been achieved. The development of frequency tunable gyrotrons operating in the range from 105 to 140 GHz for stabilization of current driven plasma instabilities in fusion plasma devices (neoclassical tearing modes) is another task in the development of gyrotrons at the Forschungszentrum Karlsruhe.

Multifractal analysis of plasma turbulence in biasing experiments on Castor tokamak

Experiments as well as some stochastic models have shown that behaviour of the transport in the edge plasma in fusion devices should be considered as a multi-scale phenomenon. The multifractal statistics describes processes as a multiplicative cascade with a complex self-similarity. The multifractal analysis, which enables to clearly demonstrate the turbulent properties of plasma, was applied to fluctuating signals in the biasing experiments. The influence of the biasing parameters is shown. A comparison with other types of the analysis is illustrated, too.

Introduction to plasma physics: with space and laboratory applications

The emphasis of this text is on basic plasma theory, with applications to both space and laboratory plasmas. All mathematical concepts beyond those normally covered in an advanced calculus course are fully explained. Topics covered include single-particle motions, kinetic theory, magnetohydrodynamics, small amplitude waves in both cold and hot plasmas, nonlinear phenomena and collisional effects. Applications include planetary magnetospheres and radiation belts, the confinement and stability of plasmas in fusion devices, the propagation of discontinuities and shock waves in the solar wind, and the analysis of various types of plasma waves and instabilities that can occur in planetary magnetospheres and laboratory plasma devices. This book is structured as a text for a one- or two-semester introductory course in plasma physics at the advanced undergraduate or first-year graduate level. It can also serve as a resource book on the basic principles of plasma physics.

A Spectral Global Gyrokinetic Approach to Plasma Linear Microinstability Analysis

With the term microinstabilities, we indicate a class of plasma drift waves such as the ion temperature gradient (ITG) mode, the trapped electron mode (TEM), and the electromagnetic Alfvénic ion temperature gradient (AITG) mode. They are usually driven by spatial gradients in the plasma and they are held responsible for anomalous transport and enhanced heat flux in tokamaks. Their understanding is therefore crucial for the development of magnetically confined plasma fusion devices. Our approach is based on the gyrokinetic model (Hahm 1988) for particle dynamics. The gyrokinetic nonlinear Vlasov equation, in the presence of an electromagnetic perturbation (Brizard 1989, 1995), is derived using Lie perturbation theory (Cary and Littlejohn 1983) to average out the fast particle motion around the gyrocenter. We have linearized the gyrokinetic Vlasov equation and added the quasi‐neutrality equation and Ampère's law, providing closure to the system. A temporal spectral formulation is used, ensuring the absence of numerical instabilities due to high frequency modes. The equations are simplified with a large aspect ratio approximation for an axisymmetric toroidal configuration. The fields are expanded in a Fourier series in the three spatial directions (radial, poloidal, and toroidal) reducing the problem in a matrix form. The code EM‐GLOGYSTO (Brunner et al. 1998; Falchetto et al. 2003; Joint Varenna‐Lausanne International Workshop 2002) has been developed to look at its solutions. Recently, we have included in the model an electrostatic equilibrium potential in order to provide E×B sheared flow. The code has been optimized for running in a parallel environment using MPI routines, and it allows us to study frequencies, growth rates, and mode structures of electromagnetic microinstabilities. Results on an AITG mode are shown to illustrate these features.

Two-dimensional density and density fluctuation diagnostic for the edge plasma in fusion devices

A technique is described for the two-dimensional measurement of electron density profile and fluctuations in edge regions of magnetically confined fusion plasmas. The method is based on existing lithium beam beam emission spectroscopy technique, two-dimensional resolution is achieved by electrostatically scanning the beam. If scanning is performed faster than the lifetime of the turbulent structures in the plasma, the diagnostic is capable of measuring the structure of electron density fluctuations as well. The beam strength of currently available beams makes the detection of single fluctuation events impossible, but the full two-dimensional spatial structure of correlations can still be determined. The article describes the technique and fast beam deflection tests up to 250kHz. The capabilities of such a diagnostic for fluctuation measurement are explored by simulating measurement signals. Measurement of both the two-dimensional density profile, fluctuation correlation function and poloidal flow velocity are demonstrated at the Wendelstein 7-AS stellarator. The shape of the density profile, the radial and poloidal correlation lengths and the flow velocity are in agreement with expectations and previous Langmuir probe measurement.

Development of Multimegawatt Gyrotrons for Fusion Plasma Heating and Current Drive

High-frequency gyrotrons with high output power are mainly used for microwave heating and current drive in plasmas for thermonuclear fusion. The development of high-power gyrotrons in continuous wave (CW) operation has been in progress for several years in a joint collaboration between different European research centers and an industrial partner. The status of the development of the 140-GHz continuously operating gyrotrons with an output power of 1 MW for the stellarator Wendelstein 7-X will be described. An output power of 890 kW has been achieved with a pulse length of 3 min. limited by the available high-voltage power supply at an electron beam current of 40 A. At a reduced beam current of 27 A , an output power of 540 kW was measured with a pulse length of 939 s. For the next fusion plasma device international thermonuclear experimental reactor, gyrotrons with a higher output power of about 2 MW are desirable. In short-pulse experiments, the feasibility of fabrication of coaxial cavity gyrotrons with an output power up to 2 MW, CW, has been demonstrated, and the information necessary for a technical design has been obtained. An output power of 2.2 MW has been reached in stable operation (without mode competition). At the nominal output power of 1.5 MW an efficiency of 48% could be obtained with single-stage depressed collector. The development of frequency tunable gyrotrons operating in the range from 105 to 140 GHz for stabilization of current driven plasma instabilities in fusion plasma devices (neoclassical tearing modes) is another task in the development of gyrotrons at the Forschungszentrum Karlsruhe.

Understanding of edge plasmas in magnetic fusion energy devices

A limited overview is given of the theoretical understanding of edge plasmas in fusion devices. This plasma occupies the thin region between the hot core plasma and material walls in magnetic confinement devices. The region is typically formed by a change in magnetic topology from close magnetic field lines (i.e. the core region) and open field lines that contact material surfaces (i.e. the scrape-off layer (SOL)), with the most common example being magnetically diverted tokamaks. The physics of this region is determined by the complex interaction of plasma with neutral species in the presence of plasma turbulence, and impurity radiation is often an important component. Recent advances in modelling strong, intermittent micro-turbulent edge-plasma transport are given, as is the closely coupled self-consistent evolution of the edge-plasma profiles in tokamaks. In addition, selected new results are given for the characterization of edge-plasmas behaviour in the areas of edge-pedestal relaxation and SOL transport via edge-localized modes, impurity formation including dust and magnetic field-line stochasticity at the edge of tokamaks.

The response of several luminescent materials to keV and MeV ions

We have quantified the ionoluminescence of several materials when irradiated with protons and He ions accelerated to keV and MeV energies. In particular, we have determined the absolute luminosity in terms of the number of photons emitted per incident ion from the front and back faces of thin screens of Y3Al5O12:Ce (0.15% CeO2), Al2O3:Ti (0.2% Ti), fused silica, and chemical vapour deposited diamond. This work has been motivated principally by their application in diagnostics for measuring fast ion losses at the edges of hot plasmas in fusion devices, where their radiation hardness, compared to that of standard phosphors, make them attractive candidates. Here, after presenting brief descriptions of the materials and summarizing the experimental set-up and the analysis method used, the results obtained are presented, and the afterglow and damage are evaluated.

Gyrokinetic particle-in-cell simulations of plasma microturbulence on advanced computing platforms

Since its introduction in the early 1980s, the gyrokinetic particle-in-cell (PIC) method has been very successfully applied to the exploration of many important kinetic stability issues in magnetically confined plasmas. Its self-consistent treatment of charged particles and the associated electromagnetic fluctuations makes this method appropriate for studying enhanced transport driven by plasma turbulence. Advances in algorithms and computer hardware have led to the development of a parallel, global, gyrokinetic code in full toroidal geometry, the gyrokinetic toroidal code (GTC), developed at the Princeton Plasma Physics Laboratory. It has proven to be an invaluable tool to study key effects of low-frequency microturbulence in fusion plasmas. As a high-performance computing applications code, its flexible mixed-model parallel algorithm has allowed GTC to scale to over a thousand processors, which is routinely used for simulations. Improvements are continuously being made. As the US ramps up its support for the International Tokamak Experimental Reactor (ITER), the need for understanding the impact of turbulent transport in burning plasma fusion devices is of utmost importance. Accordingly, the GTC code is at the forefront of the set of numerical tools being used to assess and predict the performance of ITER on critical issues such as the efficiency of energy confinement in reactors.

Formation of graphite re-deposition layer by hydrogen RF plasma

It has been revealed that a graphite re-deposition layer is formed in a fusion plasma device and can trap fuel atoms when a carbon-based material is applied as a plasma facing component. However, there are not sufficient data for quantitative prediction of hydrogen and carbon behavior with the formation of the re-deposition layer under several plasma conditions. We produced a graphite re-deposition layer on quartz substrates by a sputtering method using hydrogen RF plasma. The re-deposition layer was approximately uniformly formed on the substrate located on the ground electrode, but in some experimental conditions no deposition was observed there. This is due to the effect of re-sputtering by the incident hydrogen ions. It was found that the layer formed on and around the ground electrode had a dense columnar structure and a packed fibrous structure, respectively. With varying RF power or hydrogen pressure, the H/C ratio changed between approximately 0.1 and 0.45, and is considerably larger than that for absorption into graphite bulk. This indicates that the formation of graphite re-deposition layer makes tritium inventory in a D-T fusion reactor to dramatically grow. The plasma parameters in each experimental condition were investigated by the Langmuir probe method.

Drift‐ordered fluid equations for modelling collisional edge plasma

AbstractThe fluid description of collisional magnetized plasma by Braginskii is based on an ordering assuming the species flow velocities to be on the order of the ion thermal speed. This large flow assumption is normally not valid for plasma fusion devices in general and tokamak edge plasmas in particular. To remove this shortcoming Mikhailovskii and Tsypin developed a plasma fluid description assuming the flow velocities to be on the order of the diamagnetic drift velocities, which are smaller than the ion thermal speed. However, when deriving their equations Mikhailovskii and Tsypin missed important terms in their expressions for the species collisional viscous stress tensors. Starting from corrected Mikhailovskii‐Tsypin fluid equations, we derive a system of non‐linear reduced equations describing field‐aligned fluctuations in low‐beta collisional magnetized edge plasma; namely, equations for plasma density, electron and ion energies (or, equivalently, temperatures), parallel ion flow velocity, parallel current, vorticity (or, equivalently, electrostatic potential), perturbed parallel electromagnetic potential, and perturbed magnetic field. Our equations locally conserve particle number and total energy, and insure that perturbed magnetic field and total plasma current are divergence‐free. In addition, while intended primarily for modelling plasma turbulence, they contain the neoclassical results for plasma current, parallel ion flow velocity, and parallel gradients of equilibrium electron and ion temperatures. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)