Showing papers in "Plasma Physics and Controlled Fusion in 2002"

The ITER design

Abstract: In 1998, after six years of joint work originally foreseen under the ITER engineering design activities (EDA) agreement, a design for ITER had been developed fulfilling all objectives and the cost target adopted by the ITER parties in 1992 at the start of the EDA. While accepting this design, the ITER parties recognized the possibility that they might be unable, for financial reasons, to proceed to the construction of the then foreseen device. The focus of effort in the ITER EDA since 1998 has been the development of a new design to meet revised technical objectives and a cost reduction target of about 50% of the previously accepted cost estimate. The rationale for the choice of parameters of the design has been based largely on system analysis drawing on the design solutions already developed and using the latest physics results and outputs from technology R&D projects. In so doing the joint central team and home teams converge towards a new design which will allow the exploration of a range of burning plasma conditions. The new ITER design, whilst having reduced technical objectives from its predecessor, will nonetheless meet the programmatic objective of providing an integrated demonstration of the scientific and technological feasibility of fusion energy. Background, design features, performance, safety features, and R&D and future perspectives of the ITER design are discussed.

Density limits in toroidal plasmas

Abstract: In addition to the operational limits imposed by MHD stability on plasma current and pressure, an independent limit on plasma density is observed in confined toroidal plasmas. This review attempts to summarize recent work on the phenomenology and physics of the density limit. Perhaps the most surprising result is that all of the toroidal confinement devices considered operate in similar ranges of (suitably normalized) densities. The empirical scalings derived independently for tokamaks and reversed-field pinches are essentially identical, while stellarators appear to operate at somewhat higher densities with a different scaling. Dedicated density limit experiments have not been carried out for spheromaks and field-reversed configurations, however, `optimized' discharges in these devices are also well characterized by the same empirical law. In tokamaks, where the most extensive studies have been conducted, there is strong evidence linking the limit to physics near the plasma boundary: thus, it is possible to extend the operational range for line-averaged density by operating with peaked density profiles. Additional particles in the plasma core apparently have no effect on density limit physics. While there is no widely accepted, first principles model for the density limit, research in this area has focussed on mechanisms which lead to strong edge cooling. Theoretical work has concentrated on the consequences of increased impurity radiation which may dominate power balance at high densities and low temperatures. These theories are not entirely satisfactory as they require assumptions about edge transport and make predictions for power and impurity scaling that may not be consistent with experimental results. A separate thread of research looks for the cause in collisionality enhanced turbulent transport. While there is experimental and theoretical support for this approach, understanding of the underlying mechanisms is only at a rudimentary stage and no predictive capability is yet available.

Quiescent H-mode plasmas in the DIII-D tokamak

Abstract: H-mode operation is the choice for next-step tokamak devices based either on conventional or advanced tokamak physics. This choice, however, comes at a significant cost for both the conventional and advanced tokamaks because of the effects of edge-localized modes (ELMs). ELMs can produce significant erosion in the divertor and can affect the β limit and reduced core transport regions needed for advanced tokamak operation. Recent experimental results from DIII-D have demonstrated a new operating regime, the quiescent H-mode regime, which solves these problems. We have achieved quiescent H-mode operation which is ELM-free and yet has good density control. In addition, we have demonstrated that an internal transport barrier can be produced and maintained inside the H-mode edge barrier for long periods of time (>3.5 s or >25 energy confinement times τE). By forming the core barrier and then stepping up the input power, we have achieved βNH89 = 7 for up to 10 times the τE of 160 ms. The βNH89 values of 7 substantially exceed the value of 4 routinely achieved in standard ELMing \mbox{H-mode.} The key factors in creating the quiescent H-mode operation are neutral beam injection in the direction opposite to the plasma current (counter injection) plus cryopumping to reduce the density. Density control in the quiescent H-mode is possible because of the presence of an edge MHD oscillation, the edge harmonic oscillation, which enhances the edge particle transport while leaving the energy transport unaffected.

Characteristics and scaling of energy and particle losses during Type I ELMs in JET H-modes

Abstract: Recent experiments on the Type I ELMy H-mode regime performed at JET with improved diagnostics have expanded the range of parameters for the study of Type I ELM energy and particle losses. Deviations from the standard behaviour of such losses in some areas of the Type I ELMy H-mode operating space have revealed that the ELM losses are correlated with the parameters (density and temperature) of the pedestal plasma before the ELM crash, while other global ELM characteristics (such as ELM frequency) are a consequence of the ELM-driven energy and particle flux and of the in-between ELM energy and particle confinement. The relative Type I ELM plasma energy loss (to the pedestal energy) is found to correlate well with the collisionality of the pedestal plasma, showing a weak dependence on the method used to achieve those pedestal plasma parameters: plasma shaping, heating, pellet injection and impurity seeding. Effects of edge plasma collisionality and transport along the magnetic field on the Type I ELM particle and energy fluxes onto the divertor target have also been observed. Two possible physical mechanisms that may give rise to the observed collisionality dependence of ELM energy losses are proposed and their consistency with the experimental measurements investigated: collisionality dependence of the edge bootstrap current with its associated influence on the ELM MHD origin and the limitation of the ELM energy loss by the impedance of the divertor target sheath to energy flow during the ELM event.

Fluctuation-driven transport in the DIII-D boundary

Abstract: Cross-field fluctuation-driven transport is studied in edge and scrape-off layer (SOL) plasmas in the DIII-D tokamak using a fast reciprocating Langmuir probe array allowing local measurements of the fluctuation-driven particle and heat fluxes. Two different non-diffusive mechanisms that can contribute strongly to the cross-field transport in the SOL of high-density discharges are identified and compared. The first of these involves intermittent transport events that are observed at the plasma separatrix and in the SOL. Intermittence has qualitatively similar character in L-mode and ELM-free H-mode. Low-amplitude ELMs observed in high-density H-mode produce in the SOL periods with cross-field transport enhanced to L-mode levels and featuring intermittent events similar to those in L-mode. The intermittent transport events are compatible with the concept of plasma filaments propagating across the SOL due to E×B drifts. The intermittent character of the transport in the SOL is also in agreement with predictions of the non-linear numerical simulations performed with an imposed driving flux. Another type of non-diffusive transport is often seen in high-density H-modes with prolonged ELM-free periods, where the transport near the separatrix is dominated by quasi-coherent modes driving particle and/or heat fluxes exceeding L-mode levels. These modes may play an important role by providing particle and/or heat exhaust between ELMs.

Lithium divertor concept and results of supporting experiments

Abstract: The ITER project development has shown that considerable difficulties are encountered when already known engineering solutions and materials are used for divertor and divertor plates for tokamaks of such a scale. We offer to use a Li capillary-pore system (CPS) as a plasma facing material for tokamak divertor. Evaporated Li serves as a gas target and redistributes thermal load. The heat flux from the plasma is transported to the first wall by Li radiation in the plasma periphery. This allows the divertor plate to reduce the heat flux. A solid CPS filled with liquid lithium has a high resistance to surface damage in the stationary mode and during plasma transitions (disruptions, ELMs, VDEs, runaways) to assure normal operation of the divertor target plates. These materials are not the sources of impurities giving rise to Zeff and they will not be collected as dust in the divertor area and in ducts. Experiments with lithium CPS in a steady-state mode (up to 25 MW m-2) and in plasma disruption simulation conditions (~5 MJ m-2, ~0.5 ms) have been performed. High stability of these systems have been shown. Li limiter tests on T-11M tokamak have revealed the lithium CPS compatibility with the edge plasma for energy loads of up to 10 MW m-2. In a stable discharge mode at lithium limiter temperature of 20-600°C, no Li abnormal erosion and injection to plasma have been detected. A high sorption of D+ and H+ ions on the vessel walls was the main substantial result of the replacement of a graphite limiter by lithium one. He and D sorption was terminated by wall heating up to 50-100°C and above 350°C, respectively. T-11 tests on helium discharge allowed to reduce limiter heat load by a factor of two due to lithium radiation. All the experimental results have shown considerable progress in the development of lithium divertor.

Disruption heat loads on the JET MkIIGB divertor

Abstract: Combined analysis of divertor thermocouple and IR camera measurements during JET disruptions can provide valuable information on the distribution of the energy loads, even if the stored energy of the JET plasmas is small compared to that foreseen for the next-generation tokamaks. Typically the energy collected at the divertor represents a small fraction of the pre-disruption plasma energy; this is consistent with the high level of radiation observed and with part of the magnetic energy being transferred to the plasma-coupled conductors. The data for this paper are taken from the whole set of disruptive plasmas of JET operation in the years 2000 and 2001. In most of the MkIIGB disruptions, the plasma displaces upwards (away from the divertor); therefore, only a small number of downward events are available for analysis. However, divertor heat loads seem to be more strongly correlated to the delay of the loss of the X-point with respect to the thermal quench than the direction of the plasma displacement. When the plasma thermal energy is lost with the plasma still in X-point configuration, the septum and the tiles wetted by the strike-points, often more than one tile per strike-point, experience a sharp increase in temperature, equivalent to up to 1 MJ m-2. When the thermal quench occurs at the same time as, or after, the loss of plasma vertical control, no significant divertor tile temperature obreak increase can be observed for both upwards and downwards events. Most of the disruptions purposely made to produce runaway electrons went towards the divertor and, although not systematically, lead to local (mostly at the septum) temperature increase equivalent to a load up to 2 MJ m-2, often toroidally asymmetric.

Dynamical processes in complex plasmas

Abstract: In this review, a systematic overview of dynamical processes in complex (dusty) plasmas is given. Complex plasmas consist of electrons, ions, neutrals, and microparticles of nanometre to micrometre size, which are responsible for the unusual properties of this kind of plasmas, such as the formation of liquid or solid phases at strong electrostatic coupling. The examples represent the progress in this field within the last five years and mostly such cases were chosen, in which experimental results could be compared with theory or where the phenomenon has a diagnostic application. The presentation begins with single particle effects, where levitation, confinement in plasma traps, charging, and oscillations are involved. It is shown that vertical oscillations can be used for particle charge measurements. Nonlinear and parametric effects as well as self-excited oscillations are discussed. Then the interaction force between the particulates is explored in few-particle systems. Scattering experiments show that the interaction in the levitation plane can be described by shielded Yukawatype potentials, while the vertical interaction is governed by additional attractive wake fields. The latter are shown to be asymmetric and lead to the formation of vertical strings. The normal modes of one-dimensional or two-dimensional particle arrangements are useful tools for diagnostics. Many-particle effects are discussed in terms of low-frequency electrostatic waves. The dust-acoustic wave, the dust ion-acoustic wave, and two types of dust lattice waves (shear and compressional) are discussed with respect to diagnostic applications, among them Mach cones in plasma crystals. Laser methods are now established for the excitation of such modes. Two novel types of instabilities, the wakefield instability and the void-forming instability, as well as first results on dust ion-acoustic shocks are presented.

Improved Performance of ELMy H-Modes at High Density by Plasma Shaping in JET

Abstract: We present the results of experiments in JET to study the effect of plasma shape on high density ELMy H-modes, with geometry of the magnetic boundary similar to that envisaged for the standard Q = 10 operation in ITER. The experiments described are single lower null plasmas, with standard q profile, neutral beam heating and gas fuelling, with average plasma triangularity ? calculated at the separatrix ~0.45-0.5 and elongation ?~1.75. In agreement with the previous results obtained in JET and other divertor Tokamaks, the thermal energy confinement time and the maximum density achievable in steady state for a given confinement enhancement factor increase with ?. The new experiments have confirmed and extended the earlier results, achieving a maximum line average density ne~1.1nGR for H98~0.96. In this plasma configuration, at 2.5?MA/2.7?T (q95~2.8), a line average density ~95%?nGR with H98 = 1 and ?N~2 are obtained, with plasma thermal stored energy content Wth being approximately constant with increasing density, as long as the discharge maintains Type I ELMs, up to nped~nGR (and ne~1.1nGR). A change in the Type I ELMs behaviour is observed for pedestal densities nped70%?nGR, with their frequency decreasing with density (at constant Psep), enhanced divertor D? emission and increased inter-ELM losses. We show that this change in the ELM character at high pedestal density is due to a change in transport and/or stability in the pedestal region, with the ELMs changing from Type I to mixed Type I and Type II. The similarity of these observations with those in the Type II ELM regime in ASDEX?Upgrade and with other small ELM regimes in DIII-D, JT-60U and Alcator C-MOD is discussed. Finally, we present the first results of experiments by studying in more detail the effects of the plasma boundary geometry, in particular by investigating separately the effect of the upper and lower triangularity, at high average ?. We show that the changes to the lower ? (or of the radial position of the x-point) affect the pedestal parameters, the size of ELM energy losses as well as the global energy confinement of the plasma.

Principles of Plasma Diagnostics: Second Edition

Abstract: Although diagnostics are the essential tools of the experimental plasma physicist, there are surprisingly few up-to-date books in this field. Several excellent volumes appeared in the early days of fusion research but these, though containing much material that is still relevant, are now seriously out of date with respect to the substantial advances that have taken place in this field over the past decades. There is of course a vast amount of information in journal articles, conference papers and reviews, but these generally are aimed at the experienced specialist and consequently usually omit, or at most give a quick overview of, the basic principles. Moreover most research papers are concerned either with recent developments or with specific applications of existing techniques making it difficult for a newcomer to the field to find a good introduction to the basic principles. So when the first edition of Ian Hutchinson's book entitled Principles of Plasma Diagnostics appeared some 15 years ago it filled an important niche and quickly established a high reputation. This book deals with the fundamental physics of plasma diagnostics and develops an understanding of the methods from first principles. Instrumentation and experimental techniques are not discussed in great detail-as the author explains, to do so would have resulted in an unwieldy volume and moreover some of the technology is developing so rapidly that it would soon become outdated-but there is sufficient discussion of the applications to guide the reader towards more specialist literature. So this is not a book that will give a step-by-step account of how to build or operate a diagnostic-but it is the place to turn to in order to learn about the physics underlying the measurement method. After a brief introduction, the book deals comprehensively with the broad range of diagnostic methods, categorising them according to the physical process or property of the plasma that is used in the measurement. Successive chapters deal with magnetic measurements; plasma particle flux measurements-essentially probe techniques; refractive index methods, including interferometry, polarimetry and reflectometry; emission from free electrons-cyclotron emission, bremsstrahlung etc.; emission from bound electrons-the classic emission spectroscopy; scattering of electromagnetic radiation, incoherent and coherent Thomson scattering; neutral atom diagnostics; and finally a chapter on fast ions and fusion products. There are appendices dealing with various mathematical tools and some special issues, a glossary of symbols and a list of references. And as well as these specific references, the author gives suggestions for further reading and exercises for the student at the end of each chapter. With over 25 years experience in experimental plasma physics, Ian Hutchinson has made significant contributions to the development of the physics of many of the measurement methods that are described here, so this is book of considerable authority and insight. It is well written and will provide much interesting reading for the experienced physicist as well as the student. The material has been thoroughly revised and updated for this second edition, with new sections and chapters covering recent developments in the field. A small amount of outdated material has been removed, but some additional hundred pages compared to the first edition have extended the work quite considerably. Although the book is based quite firmly on diagnostics for magnetic confinement fusion plasmas, the author's own field of research, the basic principles of these methods are sufficiently general and they are so well explained here that this book will serve as a valuable asset also for physicists working in other areas of plasma research-in particular astrophysical and processing plasmas. The treatment is generally at graduate student level; knowledge of basic plasma physics is useful but not essential to work through the book. But this is much more than a book for the student-it is a valuable work of reference that merits a place on every diagnostician's desk. Peter Stott

Overview on stationary and transient divertor heat loads

Abstract: The divertor concept and the divertor materials envisaged for ITER FEAT restrict the maximum values of the stationary and transient heat fluxes. The maximum stationary heat flux is limited by the active cooling structure. The limit for transient events is given by the maximum tolerable surface temperature. This paper will review different options for divertor heat flux reduction. Proper geometric orientation of the divertor targets reduces the heat flux to the target plates at most by a factor of 60 relative to the parallel heat flux determined by upstream transport. The radiation capability of closed divertor configurations is doubled compared to open configurations. A further reduction of maximum heat load can be achieved by increasing the wetted area in double-null plasma configurations. The energy transported by transient events (type I ELMs) is up to 30% of the total energy deposited in the divertor. The heat flux profile during ELMs is not significantly broadened and is comparable to the profile between ELMs. The duration of the energy deposition varies with edge plasma parameters between 0.1 and 1 ms. The expected transient heat flux for ITER FEAT conditions is above the tolerable value. This requires the further development of operation scenarios with tolerable ELMs (e.g. type II) preserving the high energy confinement of type I ELM scenarios.

Runaway acceleration during magnetic reconnection in tokamaks

Abstract: In this paper, the basic theory of runaway electron production is reviewed and recent progress is discussed. The mechanisms of primary and secondary generation of runaway electrons are described and their dynamics during a tokamak disruption is analysed, both in a simple analytical model and through numerical Monte Carlo simulation. A simple criterion for when these mechanisms generate a significant runaway current is derived, and the first self-consistent simulations of the electron kinetics in a tokamak disruption are presented. Radial cross-field diffusion is shown to inhibit runaway avalanches, as indicated in recent experiments on JET and JT-60U. Finally, the physics of relativistic post-disruption runaway electrons is discussed, in particular their slowing down due to emission of synchrotron radiation, and their ability to produce electron–positron pairs in collisions with bulk plasma ions and electrons.

Ion collection by a sphere in a flowing plasma: I. Quasineutral

Abstract: The spatial distribution of collisionless ions collected by a spherical object of radius much larger than the Debye length, in a flowing plasma, is calculated using a particle-in-cell code. The results provide the first rigorous theoretical calibration of a `Mach probe' in a plasma with negligible magnetic field. They are also applicable, for example, to spacecraft-plasma interactions. Ion to electron temperature ratios 0.1

Impurity behaviour in the ASDEX Upgrade divertor tokamak with large area tungsten walls

Abstract: At the central column of ASDEX Upgrade, an area of 5.5 m2 of graphite tiles was replaced by tungsten-coated tiles representing about two-thirds of the total area of the central column. No negative influence on the plasma performance was found, except for internal transport barrier limiter discharges. The tungsten influx ΓW stayed below the detection limit only during direct plasma wall contact or for reduced clearance in divertor discharges spectroscopic evidence for ΓW could be found. From these observations a penetration factor of the order of 1% and effective sputtering yields of about 10-3 could be derived, pointing to a strong contribution by light intrinsic impurities to the total \mbox{W-sputtering}. The tungsten concentrations ranged from below 10-6 up to a few times 10-5. Generally, in discharges with increased density peaking, a tendency for increased central tungsten concentrations or even accumulation was observed. Central heating (mostly) by ECRH led to a strong reduction of the central impurity content, accompanied by a very benign reduction of the energy confinement. The observations suggest that the W-source strength plays only an inferior role for the central W-content compared to the transport, since in the discharges with increased W-concentration neither an increase in the W-influx nor a change in the edge parameters was observed. In contrast, there is strong experimental evidence, that the central impurity concentration can be controlled externally by central heating.

Formation and detection of internal transport barriers in low-current tokamaks

Abstract: In low-current tokamaks, in the absence of radial electric fields (Er), the widths of the drift orbits are large and the direct orbit losses can extend deep into the plasma. Furthermore, in such a plasma even a modest Er can produce rotation with a poloidal Mach number (Mp) that exceeds unity. Using the Monte Carlo code ASCOT, which follows charged particle orbits in the five-dimensional phase space, the formation of an internal transport barrier (ITB) in such a tokamak is investigated. Carrying out the simulations for the geometry corresponding to the FT-2 tokamak, it is shown that if, under these conditions, a steep density gradient is created in the plasma, the plasma responds by generating a strong (much stronger than needed to compensate the diamagnetic drift) Er in the region of the strong gradient. The generation appears to be a pure neoclassical effect, but a global solution over the entire plasma cross section is required to fully identify it. As a result, an ITB-like situation with a strongly sheared E×B flow is obtained inside the plasma. In these circumstances Mp>1, and thus the orbits of the majority of ions become strongly squeezed. The neutral fluxes observed by neutral particle analysers are also simulated to find out if the neutral spectra can be utilized to estimate the Er values across the plasma cross section in the FT-2 tokamak.

Divertor modelling and extrapolation to reactor conditions

Abstract: Recent results of divertor modelling are summarized and some modelling results incorporated in the ITER-FEAT divertor design, such as the beneficial effect of V-shaped targets, the importance of high gas conductivity between the divertors, and the role of deep core fuelling in maintaining the plasma density, are discussed. Differences between carbon and seeded impurities are pointed out, and the beneficial effect of helium elastic scattering is demonstrated. A number of considerations for modelling reactor-like operation and additional requirements imposed on the code under these conditions are discussed.

Steady state advanced scenarios at ASDEX Upgrade

Abstract: Recent experiments at ASDEX Upgrade have achieved advanced scenarios with high β N (>3) and confinement enhancement over ITER98(y, 2) scaling, H H98y2 = 1.1-1.5, in steady state. These discharges have been obtained in a modified divertor configuration for ASDEX Upgrade, allowing operation at higher triangularity, and with a changed neutral beam injection (NBI) system, for a more tangential, off-axis beam deposition. The figure of merit, β N H ITER89-P , reaches up to 7.5 for several seconds in plasmas approaching stationary conditions. These advanced tokamak discharges have low magnetic shear in the centre, with q on-axis near 1, and edge safety factor, q 95 in the range 3.3-4.5. This q-profile is sustained by the bootstrap current, NBI-driven current and fishbone activity in the core. The off-axis heating leads to a strong peaking of the density profile and impurity accumulation in the core. This can be avoided by adding some central heating from ion cyclotron resonance heating or electron cyclotron resonance heating, since the temperature profiles are stiff in this advanced scenario (no internal transport barrier). Using a combination of NBI and gas fuelling line, average densities up to 80-90% of the Greenwald density are achieved, maintaining good confinement. The best integrated results in terms of confinement, stability and ability to operate at high density are obtained in highly shaped configurations, near double null, with δ = 0.43. At the highest densities, a strong reduction of the edge localized mode activity similar to type II activity is observed, providing a steady power load on the divertor, in the range of 6 MW m -2 , despite the high input power used (> 10 MW).

Transport into and across the scrape-off layer in the ASDEX Upgrade divertor tokamak

Abstract: The elements of transport into and across the scrape-off layer in the poloidal divertor tokamak ASDEX Upgrade are analysed for different operational regimes with emphasis on enhanced confinement regimes with an edge barrier. Utilizing the existing set of edge diagnostics, especially the high-resolution multi-pulse edge Thomson scattering system, in combination with long discharge plateaus, radial sweeps and advanced averaging techniques, detailed radial mid-plane profiles of diverted plasmas are obtained. Profiles are smooth across the separatrix, indicating strong radial correlation, and there is no remarkable variation across the second separatrix either. Together with measured input, recycling, pumping and bypass fluxes, a corrected separatrix position is determined and transport characteristics are derived in the different radial zones generally identified in the profile structure. Transport in the steep gradient region inside and across the separatrix shows typical ballooning-type critical electron pressure gradient scaling and, in parallel, even a clear correlation between radial electron density and temperature decay lengths (e.g. η e = d(ln T)/d(ln n) ∼ 2 for type-I ELMy H-modes). These findings indicate the importance of stiff profiles in this region, while diffusion coefficients are secondary parameters, determined essentially by the source distribution. The outer scrape-off layer wing exhibits a more filamentary structure with preferential outward drift especially in high-performance discharges, with formal diffusion coefficients far above the Bohm value in agreement with results on the old ASDEX experiment. A basic mechanism involved there seems to be partial loss of equilibrium and fast curvature-driven outward acceleration, in principle well known from theory, investigated decades ago in pinch experiments and utilized recently in high-field-side pellet fuelling.

Recent advances in collisionless magnetic reconnection

Abstract: One of the recurring problems in magnetic reconnection is the identification of the appropriate generalized Ohm's law. In weakly collisional plasmas with a strong magnetic guide field component, a fluid model may be adopted, where electron inertia and the electron pressure gradient play important roles. In the absence of collisions, electron inertia provides the mechanism for magnetic field-line breaking. Electron compressibility alters significantly the structure of the reconnection region and allows for faster reconnection rates, which are consistent with the fast relaxation times of sawtooth oscillations in tokamak plasmas. The Hall term may also become important when the guide field is weak. The very possibility of nonlinear, irreversible magnetic reconnection in the absence of dissipation is addressed. We show that in a collisionless plasma, magnetic islands can grow and reach a saturated state in a coarse-grained sense. Magnetic energy is transferred to kinetic energy in smaller and smaller spatial scale lengths through a phase mixing process. The same model is then applied to the interpretation of driven reconnection events in the vicinity of a magnetic X-line observed in the VTF experiment at MIT. The reconnection is driven by externally induced plasma flows in a background magnetic configuration that has a hyperbolic null in the reconnection plane and a magnetic guide field component perpendicular to that plane. In the limit where the guide field is strong, assuming the external drive to be sufficiently weak for a linear approximation to hold, a dynamic evolution of the system is obtained which does not reach a stationary state. The reconnection process develops in two phases: an initial phase, whose characteristic rate is a fraction of the Alfven frequency, and a later one, whose rate is determined by the electron collision frequency.

Physics behind transport barrier theory and simulations

Abstract: The common physics elements behind various transport barrier theories and simulations, and the related experimental observations mainly from tokamaks, are discussed. These include: a hierarchy of the E×B shear effects on turbulence, the role of q profiles, the electron thermal transport mechanism, basic transport barrier dynamics, and the causality of transitions and the possible role of turbulence generated zonal flows.

Measurements with an emissive probe in the CASTOR tokamak

Abstract: An emissive probe has been used in the edge region of the CASTOR tokamak in order to test the possibility of direct measurements of the plasma potential. The difference between the floating potential of a cold probe and that of an emissive probe has been found to be approximately 1.3 times the electron temperature, which is less than predicted by the probe theory. Several possible reasons to explain this discrepancy are offered, such as secondary electron emission, uncertainties in the ion temperature, different collecting areas for electrons and ions, etc. The possible impact of a space charge formed by the emitted electrons is also discussed.

Finite element modelling of transport in a tokamak edge and divertor

Abstract: A finite element approach is described, for modelling transport in tokamak edge and divertor plasma. The method discretizes all transport equations on an unstructured triangular mesh. The advantages and difficulties of this approach are discussed. Results are presented and compared with experimental measurements made on TdeV. Example results are also given for JET, using a simplified physics model (single ion species, no neutrals and no flux limits), and using a more comprehensive model with helium impurity ions and neutrals.

Transport in island divertors: physics, 3D modelling and comparison to first experiments on W7-AS*

Abstract: Basic plasma transport properties in island divertors are compared to those of standard tokamak divertors. A realistic plasma transport modelling of high-density discharges in island divertors has become possible by implementing a self-consistent treatment of impurity transport in the EMC3-EIRENE code. In contrast to standard tokamak divertors, the code predicts no high recycling prior to detachment, with the downstream density never exceeding the upstream density. This is mainly due to momentum losses arising from the cross-field transport associated with the specific island divertor geometry. This momentum loss is effective already at low densities, high temperatures and is responsible for the high upstream densities needed to achieve detachment. Numerical scans of carbon concentration for high-density plasma typically show first a smooth, then a sharp increase of the carbon radiation, the latter being accompanied by a sharp drop of the downstream temperature and density indicating detachment transition. The jumps of the radiation and temperature are due to a thermal instability associated with the form of the impurity cooling rate function and can be reproduced by a simple 1D radial energy model based on cross-field transport and impurity losses. This model is used as a guideline to illustrate and discuss the detachment physics in details, including detachment condition and thermal instability. Major EMC3-EIRENE code predictions have been verified by the first W7-AS divertor experiments. A comparison of calculations and measurements is presented for high-density, high-power W7-AS divertor discharges and the physics related to rollover and detachment is discussed in detail. The code has been recently extended to general SOL configurations with open islands and arbitrary ergodicity by using a new highly accurate field-line mapping technique. The method correctly reproduces flux surfaces and islands over a high number of toroidal field periods, thus ensuring a clear distinction between parallel and radial transport. The technique has been tested successfully on W7-AS, W7-X, LHD and TEXTOR DED, and first applied to solve the coupled heat conduction equations for a typical ergodic W7-AS configuration.

ELM energy scaling in DIII-D

Abstract: The energy lost from the pedestal region due to an average ELM in DIII-D is determined from changes to the electron density and temperature profiles as measured by Thomson scattering. The ELM energy due to loss of temperature in the pedestal is associated with conduction and is found to decrease with increasing pedestal density. The ELM energy from lost pedestal density, or convective transport, remains constant as a function of density. The scaling of the two transport channels, conduction and convection, are examined in terms of parallel transport processes in the scrape-off-layer and divertor.

Confinement and stability on the TJ-II stellarator

Abstract: This paper describes a series of experiments performed in TJ-II stellarator with the aim of studying the influence of magnetic configuration on stability and transport properties of TJ-II plasmas. Plasma potential profiles have been measured in several configurations up to the plasma core with the heavy ion beam probe diagnostic. Low-order rational surfaces have been positioned at different plasma radii observing the effect on transport features. Plasma edge turbulent transport has been studied in configurations that are marginally stable due to decreased magnetic well. Results show a dynamical coupling between gradients and turbulent transport. Experiments on the influence of magnetic shear on confinement are reported. Global confinement issues as well as enhanced confinement regimes found in TJ-II are discussed as well.

Pedestal characteristics and extended high-βp ELMy H-mode regime in JT-60U

Abstract: With the high-field-side multiple pellet injection into high-triangularity high-βp type I ELMy H-mode discharges, JT-60U has extended the density range with a favourable integrated performance. The pellet injected discharge has a high pedestal pressure compared with the gas fuelled discharges, and the pedestal temperature does not decrease even at high pedestal density. At high triangularity, the pedestal βp and the edge α-parameter increase with increasing total βp. Possible linkage among pedestal and core parameters has been proposed based on a variety of JT-60U experiments. Grassy ELMy discharges have been achieved at the low safety factor regime q95<4 for the first time in JT-60U with high triangularity (~0.6). The full non-inductive operation has been demonstrated with grassy ELMs.

Triggering of internal transport barrier in JET

Abstract: Internal transport barriers (ITBs) can be produced in JET by the application of strong additional heating during the current rise phase of the plasma discharge. Using up to 3 MW of lower hybrid power to tailor the q-profile prior to the main heating phase, a large variety of q-profiles ranging from low positive to strong negative central shear have been obtained during the current rise (0.4 MA s−1). With negative central magnetic shear s = (r/q)(dr/dq), the analysis of ITB triggering reveals a correlation between the formation of the ITB and q min reaching an integer value (q = 2 or q = 3). This observation is confirmed by the analysis of the Alfven cascades. The minimum power required to access regimes with ITBs is probably related to the transport and magnetohydrodynamic properties of integer magnetic surfaces. Laser ablation and shallow pellet injection have also been attempted in recent JET ITB triggering experiments. This article was scheduled to appear in issue 7 of Plasma Phys. Control. Fusion. To access this special issue on advanced Tokamak research in EFDA-JET please follow this link: http://stacks.iop.org/0741-3335/44/7

Marginal β-limit for neoclassical tearing modes in JET H-mode discharges

Abstract: A neoclassical tearing mode (NTM) requires a finite size seed island to become unstable. Usually the local pressure gradient is relatively large at the beta-values needed for these seed islands to destabilize the NTMs. Therefore, the island has a large growth rate at mode onset and grows rapidly to its saturated island width. This width is proportional to as long as it is well above the marginal beta-limit below which the mode is stable. The marginal beta-limit is independent of the seed island trigger mechanism and provides detailed information on the stabilizing terms in the modified Rutherford equation, which are not unambiguously determined theoretically. It is shown that in JET the marginal normalized beta-limit for the 3/2 mode, beta(N,marg), is of the order of 0.5-1 for magnetic fields between 3.3 and 1T, with q(95) approximate to 3.3, and near the H-L transition. Therefore, all H-modes with typical q-profiles (q(95) approximate to 3.3) are metastable in JET to 3/2 NTMs. In addition, the marginal island width is of the order of 2-4 cm and the stabilizing terms are such that they influence the saturated island width when it is smaller than 4-6 cm in these H-mode discharges. It is also shown that detailed analyses of the time evolution of the island width with slow beta ramp-down suggest that the convective form of the stabilization term due to the 'chi(perpendicular to) model' is more appropriate and can explain the island decay between 4 and 6 cm to the marginal island width, while the polarization current model can explain the rapid stabilization when beta < beta(marg). The range of values of the different stabilizing terms are discussed in detail. In particular, it is shown that the mode is stabilized and has a large negative growth rate, when the effect of the stabilizing terms is such as to reduce the saturated width by a factor of 2.

Towards fully non-inductive current drive operation in JET

Abstract: Quasi-steady operation has been achieved at JET in the high-confinement regime with internal transport barriers (ITBs). The ITB has been maintained up to 11 s. This duration, much larger than the energy confinement time, is already approaching a current resistive time. The high-performance phase is limited only by plant constraints. The radial profiles of the thermal electron and ion pressures have steep gradients typically at mid-plasma radius. A large fraction of non-inductive current (above 80%) is sustained throughout the high-performance phase with a poloidal beta exceeding unity. The safety factor profile plays an important role in sustaining the ITB characteristics. In this regime where the self-generated bootstrap current (up to 1.0 MA) represents 50% of the total current, the resistive evolution of the non-monotonic q-profile is slowed down by using off-axis lower-hybrid current drive.

The divertor program in stellarators

Abstract: Two significant problems that need to be solved for any future fusion device are heat removal and particle control. A very promising method to attack these problems in tokamaks and helical devices is the use of a divertor, providing a controlled interaction zone between plasma and wall. By carefully designing a divertor, conditions can be created in front of the divertor targets, which lead to a sufficient reduction of the power load on the targets by strong radiation redistribution. Any solution of course needs to allow for an energy confinement which is at least sufficient for the realization of a fusion reactor. Since energy confinement has been found to be strongly related to edge anomalous transport and edge plasma profiles, the ultimate aim is to find an integral solution which is optimum with respect to exhaust, heat load and energy confinement. Two different types of divertors are presently being investigated in helical devices: the `helical divertor' and the `island divertor'. So far divertor concepts have been investigated only in a few helical devices. Theoretical and experimental efforts have mainly concentrated on the suitability of divertor magnetic field structures, while detailed studies of the divertor plasma properties for the two types of divertor configurations have only recently begun. In the course of this exploration, a promising new high-density H-mode (HDH) plasma operational regime has been discovered on the Wendelstein stellarator W7-AS. It benefits from high-energy (up to twice the value of the International Stellarator Scaling ISS95) and low impurity confinement times, complemented by edge radiated power fractions of up to 90% in detached regimes. This allowed quasi-steady-state operation for up to 50 energy confinement times and so far was only constrained by machine operability.