Showing papers in "Plasma Physics and Controlled Fusion in 1993"
TL;DR: In this article, a status report on developments in the gyrofluid approach to simulating tokamak turbulence is given, with a detailed description of toroidal ITG-driven toroidal turbulence simulations.
Abstract: A status report is given on developments in the gyrofluid approach to simulating tokamak turbulence. 'Gyrofluid' (r 'gyro-Landau fluid') equations attempt to extend the range of validity of fluid equations to a more collisionless regime typical of tokamaks, by developing fluid models of important kinetic effects such as Landau-damping and gyro-orbit averaging. The fluid moments approach should converge if enough moments are kept, though this may require a large number of moments for some processes. Toroidal gyrofluid equations have been extended from 4 to 6 moments, and to include the mu Del B magnetic mirroring force. An efficient field-line coordinate system for toroidal turbulence simulations (useful for both particle and fluid simulations) is presented. Nonlinear 3-D simulations of toroidal ITG-driven turbulence indicate that turbulence-generated sheared flows play an important role in the development and saturation of the turbulence. There is a strong enhancement of the flows when the electrons are assumed adiabatic on each flux surface, which is partially offset by toroidal drift effects which reduce the flows.
289 citations
TL;DR: In this article, the evolution of the width and frequency of the rotating magnetic islands in a single-fluid MHD was studied and the dynamics of unstable m=2 and m=1 magnetic islands based on this equation was described.
Abstract: The nonlinear behaviour of tearing modes in a plasma with density and temperature gradients is reviewed. The effects of inhomogeneities can essentially modify the evolution of small scale islands from that predicted by Rutherford theory. Plasma gradient effects provide the mechanism for island excitation even in situations when the linear tearing mode stability parameter Delta is negative. The magnetic islands are sustained by the differential response of electron and ion components of a plasma in a fluctuating electric field. Such nonlinear magnetic islands are not related to the linear instability of drift-tearing modes. The nonlinear equations describing the evolution of the width and frequency of the rotating islands are derived. In the framework of one-fluid MHD, the general equation for a neighbouring equilibrium in a finite pressure plasma is considered. The dynamics of unstable m=2 and m=1 magnetic islands based on this equation is described. The quasilinear saturation of island growth in a finite pressure plasma leads to the bifurcation of the island type equilibrium into states without islands. A new evolution equation of m=1 islands is derived. For monotonic safety factor and temperature profiles this equation predicts saturation of the m=1 island growth.
171 citations
TL;DR: In this paper, the implementation and application of beam emission spectroscopy as a quantitative diagnostic tool on the Joint European Torus (JET) experiment is reviewed, where the beam attenuation, beam geometry, beam-divergence and species mix are investigated.
Abstract: A beam of injected fast atomic hydrogen presents a superb probe for hot fusion plasmas. The neutral particles experience excitation and ionization by collisions with electrons and ions as they penetrate into a plasma. The emitted characteristic line radiation is Doppler shifted and the spectral lines are split due to motional Stark fields. Measurements of wavelength, intensity and polarization of the Balmer- alpha emission reveal information about the neutral beam, such as beam attenuation, beam-geometry, beam-divergence and species mix. Local pitch angles and toroidal fields can be derived from the simultaneous measurement of the polarization pattern and the wavelength separation of the Stark multiplet. The implementation and application of beam emission spectroscopy as a quantitative diagnostic tool on the Joint European Torus (JET) experiment is reviewed.
117 citations
TL;DR: In this article, the feasibility of the cold radiative plasma mantle (CRM) concept for the energy exhaust problem in a fusion reactor has been investigated and compared with the high density divertor.
Abstract: Experimental results from TEXTOR are presented to provide strong evidence for the feasibility of the 'cold radiative plasma mantle', a concept which might be a possible solution for the energy exhaust problem in a fusion reactor. The concept is compared with the high density divertor. The compatibility to other constraints, limitations and open problems are discussed, in particular the issues of stationarity (feed-back control, thermal instabilities, q=2), energy confinement, He-exhaust and fuel dilution.
113 citations
TL;DR: The interaction between the ion cyclotron resonance heating and the plasma edge is a vast and interlinked domain this paper, and the identification of the sheath rectification mechanism as the main cause for the influence of the RF on the edge has permitted a better understanding of this domain and significant progress in the avoidance of the effects.
Abstract: The interaction between the ion cyclotron resonance heating and the plasma edge is a vast and interlinked domain: the edge affects the radio frequency heating and the radio frequency heating affects the edge. This interaction between the radio frequency heating and the edge is-in view of the relation between the edge and overall plasma properties and performance-of utmost importance. The paper gives an overview of the topic, emphasizing basic concepts and recent results. The identification of the sheath rectification mechanism as the main cause for the influence of the RF on the edge, has permitted a better understanding of this domain and significant progress in the avoidance of the effects.
102 citations
TL;DR: In this paper, a review examines results from all non-circular tokamaks with a distinct emphasis on investigations in ASDEX-Upgrade, where a major fraction of the experimental time has been dedicated to studying vertical displacement events of single null plasmas over a large range of q-values in an attempt to obtain the scaling of both the displacement dynamics and the splitting of forces between those associated with poloidal and toroidal plasma currents as a function of q and Bt.
Abstract: This review examines results from all non-circular tokamaks with a distinct emphasis on investigations in ASDEX-Upgrade. There a major fraction of the experimental time has been dedicated studying vertical displacement events of single null plasmas over a large range of q-values in an attempt to obtain the scaling of both the displacement dynamics and the splitting of forces between those associated with poloidal and toroidal plasma currents as a function of q and Bt. These studies on different tokamaks are accompanied by simulations with-among other codes-the tokamak simulation code TSC, in a version where halo currents flowing in the plasma scrape-off layer (SOL) evolve self-consistently. The technical consequences of VDEs for the machine design, measures taken and first predictions are discussed. Safety setups that have been developed and possible avoidance strategies are briefly described.
98 citations
TL;DR: An overview of recent results on the creation of electric fields in the edge of limiter or divertor tokamaks is presented in this article, where the practical implementation and theoretical basis of several schemes, generating radial and/or poloidal fields, are outlined.
Abstract: An overview is presented of recent results on the creation of electric fields in the edge of limiter or divertor tokamaks. The practical implementation and theoretical basis of several schemes, generating radial and/or poloidal fields, are outlined. The manipulation of edge and scrape-off-layer profiles and the control of particle exhaust is discussed. Contributions of biasing experiments to H-mode physics are highlighted. Some prospects for biasing in next generation tokamaks are finally given.
89 citations
TL;DR: In this paper, a collective infra-red laser scattering diagnostic has been installed on the TORE SUPRA tokamak for the measurement of plasma density fluctuations, and the results of cross-correlation performed on the signals obtained by two simultaneous probing beams have been obtained.
Abstract: A collective infra-red laser scattering diagnostic has been installed on the TORE SUPRA tokamak for the measurement of plasma density fluctuations. For the range of wavenumbers explored (3-15 cm-1), the scattering angles are very weak ( approximately 1 mrad). Consequently, the scattering signals are averaged along the whole observation chord, resulting in poor longitudinal spatial localization. By virtue of the pitch angle variation of the magnetic field lines in the tokamak, and of the perpendicularity of the turbulence wavevector to these field lines, it has been possible to obtain partial spatial resolution along the direction of the beam. Good agreement between the experimental and theoretical angular resolution of the diagnostic as well as the results of cross-correlation performed on the signals obtained by two simultaneous probing beams also justify this novel concept.
85 citations
TL;DR: In this paper, the local electron and ion heat transport as well as the particle and impurity transport properties in stellarators are reviewed, and the comparison of the experimental results of the quite different confinement concepts is made.
Abstract: The local electron and ion heat transport as well as the particle and impurity transport properties in stellarators are reviewed. In this context, neoclassical theory is used as a guideline for the comparison of the experimental results of the quite different confinement concepts. At sufficiently high temperatures depending on the specific magnetic configuration, neoclassical predictions are confirmed by experimental findings. The confinement properties in the LMFP collisionality regime are discussed with respect to the next stellarator generation, for which at higher temperatures the neoclassical transport is expected to become more important.
74 citations
TL;DR: In this paper, a 2D full-wave simulation of ordinary mode propagation has been developed in an effort to model effects seen in reflectometry experiments but not properly explained by 1D analysis.
Abstract: A 2D full-wave simulation of ordinary mode propagation has been developed in an effort to model effects seen in reflectometry experiments but not properly explained by 1D analysis. The geometric fall-off of the fields, together with the effects of both refraction and diffraction, considerably modify the results obtained. The now commonly seen experimental observations of large amplitude and phase variations of the echo signal and occasional ramping of the phase can be explained by these 2D effects in the presence of fluctuations.
73 citations
TL;DR: In this article, a new kind of spectroscopy called MHD Spectroscopy has been proposed, which is based on edge localised modes and external excitation of toroidal Alfven eigenmodes.
Abstract: Limitations of present knowledge of plasma equilibrium profiles hamper a proper identification of MHD modes. However, through continued improvement of both numerical calculations and experimental observations the authors may witness the birth of a new kind of spectroscopy, properly called MHD spectroscopy, in the coming decade. This is illustrated by studies of edge localised modes and external excitation of toroidal Alfven eigenmodes.
TL;DR: A review of fast electron dynamic studies in the tokamaks TS, JET, and ASDEX during lower hybrid current drive is presented in this article, where the fast electron diffusion coefficient is found between 0.1-0.3m2s-1, close to the value determined on ASDEX in similar plasma conditions.
Abstract: A review of fast electron dynamic studies in the tokamaks TS, JET, and ASDEX during lower hybrid current drive is presented. In almost all experiments, the collisional slowing down is predominant, so a straightforward assessment of the fast electron radial transport is difficult, especially for large size tokamaks. From recent LH power modulation experiments performed on TS at low density, the fast electron diffusion coefficient, Dst, is found between 0.1-0.3m2s-1, close to the value determined on ASDEX in similar plasma conditions. An estimate of Dst is also obtained in TS and JET by modeling the plasma dynamics, using ray-tracing+Fokker-Planck codes. For JET, the fast electron transport is fully taken into account in the Fokker-Planck equation. Despite some quantitative discrepancies between Dst values determined from different methods, the diffusion rate of the fast electrons is weak enough for current profile control scenarii to be relevant on large tokamaks. A determination of the type of the turbulence - electrostatic or magnetic - that could drive the radial transport of fast electrons requires finer LH experiments.
TL;DR: The first experimental results of anisotropic slowing-down features observed in JET helium beam fuelling experiments are reported in this paper, where two independent observation ports, one with a view perpendicular to the magnetic field in the centre of the plasma and a second multichord viewing arrangement, approximately tangential to the toroidal field, provide radially and temporally resolved information on the velocity distribution function comprising the populations of both fast and thermalized alpha particles.
Abstract: The first experimental results are reported of anisotropic slowing-down features observed in JET helium beam fuelling experiments. Two independent observation ports, one with a view perpendicular to the magnetic field in the centre of the plasma and a second multichord viewing arrangement, approximately tangential to the toroidal field, provide radially and temporally resolved information on the velocity distribution function comprising the populations of both fast and thermalized alpha particles. The fuelling process is characterized by a change-over from a distinctly non-Maxwellian distribution function to a dominantly Maxwellian distribution and also by a broadening of the deduced fast ion density radial profile. The fast particle component in the observed composite charge exchange spectrum is found to be in excellent agreement with predictions are based on anisotropic velocity distribution functions obtained from the analytical solution of the neutral injection Fokker-Planck equation. Signal-to-noise levels in the measurement of fast alpha particle in the JET helium fuelling campaign are extrapolated to thermonuclear-fusion alpha particle density levels expected for the D-T phase of JET. It is shown that beam penetration and not competing continuum radiation is a major constraint, and that acceptable (hydrogen or helium) neutral beam power and energy requirements promise a feasible CX alpha particle diagnosis in the core of next-step devices such as ITER.
TL;DR: In this article, the Next-Step Tokamak should be optimized for continuous operation as well as high current pulsed operation with a high bootstrap fraction, and it is shown that high magnetic fields (Bt=8-11T) and high Troyon factors ( beta N>3.5) are required to attain high beta p operation and a reasonably high fusion power density.
Abstract: Based on recent progress in world tokamak research, the prospects of continuous operation of a tokamak fusion power reactor are discussed. Efficient steady state operation (Q>30) becomes feasible when a large fraction of the plasma current (>70%) is driven by the bootstrap current. Moderate current (Ip approximately 10-20 MA) and an enhanced confinement mode of operation are the key for efficient steady state operation. High magnetic fields (Bt=8-11T) and high Troyon factors ( beta N>3.5) are required to attain high beta p operation and a reasonably high fusion power density. Control of the current and edge pressure profiles is necessary for high beta N operation with a high bootstrap fraction. It is proposed that the Next Step Tokamak should be optimized for continuous operation as well as high current pulsed operation.
TL;DR: In this paper, a multichannel microwave reflectometer system is used to measure the electron density profile at JET and a new measurement technique is described which gives continuous information on the electron profile in time.
Abstract: A multichannel microwave reflectometer system is used to measure the electron density profile at JET. A new measurement technique is described which gives continuous information on the electron profile in time. Profile measurements using swept-frequency measurements are combined with accurate measurements of the evolution of the density using fixed-frequency operation. The analysis of the data can be carried out routinely for each discharge at JET. The method includes estimates for the uncertainties on the measurements, corrections for finite sweep on the data and relativistic effects. The results compare well with other measurements of the electron density in JET.
TL;DR: In this article, an intensity interferometer was built and installed on the Wendelstein 7-AS stellarator, which consists of two identical but independent multichannel heterodyne radiometers viewing the same emitting plasma volume along crossed lines of sight.
Abstract: Transport-relevant fluctuations of the electron temperature can in principle be measured by radiometry of the electron cyclotron emission (ECE), but they might be buried completely in natural fluctuations of the ECE due to the thermal nature of this radiation. The spatial coherence properties of thermal radiation can be utilized in correlation experiments to overcome the principal difficulties and to make temperature fluctuations accessible to measurement. On the basis of these considerations an intensity interferometer was built and installed on the Wendelstein 7-AS stellarator. The interferometer consists of two identical but independent multichannel heterodyne radiometers viewing the same emitting plasma volume along crossed lines of sight. The angle between the sightlines is chosen sufficiently large to decorrelate the natural fluctuations of the radiation field. A quasithermal microwave radiation source is used to test the intensity interferometer and to simulate the correlation experiment. Measurements of the spatial coherence and the statistical properties are in agreement with theoretical predictions.
TL;DR: An overview of reflectometry measurements of density profiles and density fluctuations in fusion devices, is presented in this paper, where the results are analyzed in the light of the expectations from recent theoretical and numerical studies.
Abstract: An overview of reflectometry measurements of density profiles and density fluctuations in fusion devices, is presented. The results are analysed in the light of the expectations from recent theoretical and numerical studies. The spatial resolution of the diagnostic and the wave number sensitivity are discussed. The diagnostic capabilities are considered in view of its application in next generation devices.
TL;DR: The VH-mode regime is characterized by thermal confinement twice that seen in H-mode, with the edge transport barrier penetrating deeper into the plasma as mentioned in this paper, and two mechanisms have been identified as important in achieving this high level of confinement.
Abstract: The VH-mode regime of high confinement has been observed in both DIII-D and JET. VH-mode is characterized by thermal confinement twice that seen in H-mode, with the edge transport barrier penetrating deeper into the plasma. Two mechanisms have been identified as important in achieving this high level of confinement. Expansion of the E*B velocity shear turbulence suppression zone is important in allowing reductions in local transport, while access to the second ballooning stability regime in the edge allows avoidance or elimination of ELMs which impede the confinement improvement. The high performance phase of these discharges is usually terminated by an MHD event which removes energy from a large portion of the plasma cross-section, and is followed by an H-mode phase.
TL;DR: In this article, a theory of the L-mode confinement in tokamaks is developed based on the microscopic ballooning instability which is destabilized by the plasma transport below the critical pressure gradient against the ideal MHD instability.
Abstract: A theory of the L-mode confinement in tokamaks is developed based on the microscopic ballooning instability which is destabilized by the plasma transport below the critical pressure gradient against the ideal MHD instability. Destabilization by the current diffusivity and stabilization by the thermal transport and ion viscosity are analysed. The least stable mode determines the anomalous transport coefficients. The formula of the thermal transport coefficient is derived, which explains major experimental observations on L-mode confinement.
TL;DR: The kinetic theory of radiofrequency current drive in tokamak plasmas is investigated in detail in this article, where the impact of anomalous transport on the driven current profile and efficiency is considered in detail.
Abstract: The kinetic theory of radiofrequency current drive in tokamak plasmas is investigated. The problem of the impact of anomalous transport on the driven current profile and efficiency is considered in detail. Among the possible candidates for explaining anomalous transport in tokamaks, magnetic turbulence is known to have a strong influence on the dynamics of superthermal electrons and is assumed to be the basic mechanism responsible for radial diffusion of the RF-driven current. The 3-dimensional kinetic equation in the presence of RF heating and magnetic turbulence is studied. Its properties are first investigated by a nonlocal response function technique. Both the adjoint formalism and the Langevin equations method are extended to the case of radially diffusing electrons. The full kinetic equation is numerically solved by means of a 3-D Fokker-Planck code. Applications to lower-hybrid current drive are presented and several kinetic effects are discussed.
AeA1
TL;DR: In this article, the scale invariance technique is employed to discuss pressure gradient driven turbulent transport when an Ohm's law with electron inertia, rather than resistivity, is relevant.
Abstract: The scale invariance technique is employed to discuss pressure gradient driven turbulent transport when an Ohm's law with electron inertia, rather than resistivity, is relevant. An expression for thermal diffusivity obtained by Itoh et al. (1992), which has many features appropriate to L-mode transport in tokamaks, is seen to have greater generality than indicated by their particular calculation. The results of applying the technique to a more appropriate collisionless Ohm's law are discussed.
TL;DR: In this article, the authors considered the inhomogeneities in Tokamak peripheral plasmas due to localized recycling of neutrals (plasma fuelling) and found that a dense cloud of cold charged particles arises in the vicinity of such a localized particle source, for example a gas-puffing valve, as the plasma density exceeds a certain critical level.
Abstract: Inhomogeneities in Tokamak peripheral plasmas due to localized recycling of neutrals (plasma fuelling) are considered. A dense cloud of cold charged particles arises in the vicinity of such a localized particle source, for example a gas-puffing valve, as the plasma density exceeds a certain critical level. A dependence of the critical density on such parameters as dimensions of the device, heat flux from the discharge core and diffusivity of charged particles at the edge is found. The results of calculation are compared with experimental data.
TL;DR: A survey of the literature on theoretical models for the anomalous ion thermal diffusivity is reported in this article, emphasising in particular the experimental conditions for which each is valid.
Abstract: A survey of the literature on theoretical models for the anomalous ion thermal diffusivity is reported, emphasising in particular the experimental conditions for which each is valid. The results of a systematic comparison of these models against JET observations on anomalous ion transport, taking full account of the validity conditions, are given. This provides a basis for assessing the value of these various models.
TL;DR: A discussion of the initial phase in plasma focus devices is given, in the light of the available experimental and theoretical knowledge on this subject as mentioned in this paper, and the influence of this phase on the focus phase is also discussed.
Abstract: A discussion of the initial phase in plasma focus devices is given, in the light of the available experimental and theoretical knowledge on this subject. The influence of this phase on the focus phase is also discussed.
TL;DR: In this paper, the distribution of near 7 MeV alpha-particles formed in close collisions between 3.5 MeV and RF-heated minority ions in a fusion plasma was determined.
Abstract: As fusion-generated alpha -particles of RF-heated minority ions in a fusion plasma collide with impurity ions, the latter can be accelerated to energies of several MeV, a fact which may be useful for diagnostic purposes. The source strength and the distribution function of the hot ion population thus created is calculated. Account is taken of the effects of nuclear elastic scattering and velocity-space anisotropy. In addition, the distribution of near 7 MeV alpha -particles formed in close collisions between 3.5 MeV alpha -particles is determined.
TL;DR: In this paper, the authors presented the results from JT-60U on high performance experiments and studies on steady state plasma and showed that up to 10 seconds of quasi steady state high confinement discharges with a substantial bootstrap current fraction have been experimentally demonstrated.
Abstract: Recent results from JT-60U on high performance experiments and studies on steady state plasma are presented. High fusion performance of nD(0) tau ETi(0)=1.1*1021m-3s keV, Sn=5.6*1016s-1, QDT=0.6, Ti(0)=40 keV has been obtained in the regime names 'high- beta p H-mode'. By intensive survey on pressure/current profile control, beta limit has been substantially expanded: beta p
TL;DR: In this article, the theory of three-wave mixing and Thomson scattering in plasmas is re-examined in the low-temperature limit with a kinetic model, giving a more complete description of Thomson scattering.
Abstract: The theory of three-wave mixing and Thomson scattering in plasmas is re-examined in the low-temperature limit with a kinetic model, giving a more complete description of Thomson scattering in this limit. Errors in the traditional fluid approach to three-wave mixing and scattering are identified and a new corrected fluid approach is given. These corrections to the theory may have important consequences for the analysis of collective Thomson scattering experiments, such as fast ion diagnostics and for the assessment of the feasibility of certain measurements.
TL;DR: In this article, the authors discuss the existing evidence and arguments, both direct and circumstantial, that might help to resolve this question and place predictions for the confinement properties of tokamaks on a sound basis if the level of fluctuations and the resulting transport could be calculated.
Abstract: Anomalous transport in tokamaks is usually explained in terms of turbulent fluctuations. Predictions for the confinement properties of tokamaks would be placed on a more sound basis if the level of fluctuations and the resulting transport could be calculated. At present there is no clear agreement on whether the responsible fluctuations are electrostatic or magnetic. This paper discusses the existing evidence and arguments, both direct and circumstantial, that might help to resolve this question.
TL;DR: In this paper, the locations of the R-cutoff, the L-and the O-mode cutoff for electromagnetic waves in relativistic plasmas are investigated. But the ion motion is ignored, and it is assumed that the plasma is collisionless and in thermodynamic equilibrium.
Abstract: Expressions suitable for numerical evaluation, are given for the parameters characterizing the locations of the R-cutoff (upper X-mode cutoff), the L-cutoff (lower X-mode cutoff) and the O-mode cutoff for electromagnetic waves in relativistic plasmas. The ion motion is ignored, and it is assumed that the plasma is collisionless and in thermodynamic equilibrium. A new derivation of the relativistic cutoff expressions, starting from first principles, is given, together with a heuristic explanation for the relativistic shift of the O-mode cutoff.
TL;DR: In this article, a real-time electronic neural network was used for feedback control of the equilibrium in a cyclotron current drive in Compass-D tokamak, where the fundamental resonance was used and the waves were launched from the high field side of the torus through four mirror-antennae.
Abstract: Electron cyclotron current drive experiments have been carried out in Compass-D using a power of approximately 500 kW at a frequency of 60 GHz. The fundamental resonance was used and the waves were launched from the high field side of the torus through four mirror-antennae. Significant asymmetry in the loop voltage behaviour is observed when comparing co and counter current drive cases, suggesting a driven current approximately 15 kA (in a plasma current of 130 kA). The BANDIT-3D Fokker Planck code has been used to model these discharges revealing a similar value for the predicted driven current at zero loop voltage but strong synergistic effects with the toroidal electric field at the approximately 0.4 Volts/turn typical of the experiments. This synergy appears to be sufficient to explain the inferred net 'co' current drive in most of the 'counter' current drive shots. In H-mode discharges featuring a strongly asymmetric single null separatrix configuration, discrete ELM's sometimes produce sufficient relaxation of the current profile to cause loss of vertical control, leading to vertical displacement events. Application of resonant magnetic perturbations (RMPs) of predominantly n=1 (with various low m values) is seen reliably to increase the frequency of the ELM's, although the exact mechanism underlying this effect has still to be elucidated. A real-time electronic neural network has been used for the first time for feedback control of the equilibrium in a tokamak. The neural network was used for feedback control of the plasma elongation throughout the discharge, while simultaneously monitoring the plasma vertical and horizontal position.