Showing papers in "Plasma Physics and Controlled Fusion in 1992"
TL;DR: In this article, the authors summarize the present status of experimental tokamak confinement studies and compare the results of stellarators and tokammaks with those from W7-AS.
Abstract: In this paper we summarize the present status of experimental tokamak confinement studies. Under quiescent conditions, ion heat and impurity transport can be close to the neoclassical level. Generally, however, radial transport is enhanced by instabilities. There is evidence that anomalous ion heat and momentum transport may be caused by turbulence driven by the ion temperature gradient. The same level of understanding is not reached in electron heat and particle transport. Electron heat transport is characterized by a highly nonlinear relation between heat flux and temperature gradient. Particle transport is strongly governed by off-diagonal contributions. Where possible, the tokamak results are compared with those from stellarators, in particular with those from W7-AS. Such a comparison is meaningful because stellarators and tokamaks share many transport aspects. In both cases transport is generally anomalous, degrades with heating power and increases toward the edge. Bifurcations such as the tokamak H-mode transition are also observed in the W7-AS stellarator. Transport in stellarators, although anomalous, seems to be less confused by additional complexities such as large off-diagonal contributions, profile resilience, disparities between steady-state and perturbatively determined transport coefficients and an isotopic mass dependence in energy and particle transport. Differences in magnetic field and minor radius scaling seem to be introduced by operational restrictions: rotational transform iota is constant in stellarator field or size scans whereas qa generally varies with current, field, or size in tokamaks.
243 citations
TL;DR: In this article, the basic paradigm of electric field shear stabilization has been discussed in the light of the most recent data, and the experimental results with various theories have been compared.
Abstract: Combined theoretical and experimental work has resulted in the creation of a paradigm which has allowed semi-quantitative understanding of the edge confinement improvement that occurs in the H-mode. Shear in the E*B flow of the fluctuations in the plasma edge can lead to decorrelation of the fluctuations, decreased radial correlation lengths and reduced turbulent transport. Changes in the radial electric field, the density fluctuations and the edge transport consistent with shear stabilization of turbulence have been seen in several tokamaks. The purpose of this paper is to discuss the most recent data in the light of the basic paradigm of electric field shear stabilization and to critically compare the experimental results with various theories.
199 citations
TL;DR: In this article, a signal processing method based on the singular value decomposition (SVD) of a rectangular matrix has been proposed to disentangle the poloidal structure without a priori knowledge.
Abstract: The spatial structure and temporal evolution of coherent MHD modes in fusion plasma devices have been so far inferred from the experimental signals by using spectral techniques. Considering the data as a collection of n-dimensional discrete time series xi(t) permits one to introduce in this context a signal processing method based on the singular value decomposition (SVD) of a rectangular matrix. This paper shows that, whereas the SVD is equivalent to the discrete Fourier transform in the case of travelling sinusoidal waves, in more realistic cases it is a clear improvement on the spectral methods, since it disentangles the poloidal structure without a priori knowledge. Various applications of SVD, first on artificial signals and then on magnetic and SXR signals detected in the JET Tokamak in several plasma regimes, are shown with the purpose of illustrating the power and limits of the method.
126 citations
TL;DR: In this paper, the authors considered the linear MHD stability problem for 3D plasmas with noninteracting hot electron layers and nested magnetic flux surfaces in the absence of dissipation mechanisms.
Abstract: The linear MHD stability problem for 3D plasmas with noninteracting hot electron layers and nested magnetic flux surfaces in the absence of dissipation mechanisms is formulated in variational form. Boozer magnetic coordinates are extended to anisotropic pressure plasmas with nested magnetic flux surfaces for this purpose. A modified energy principle in which the hot electron species current is imperturbable is considered. Plasma incompressibility is imposed and thus a simplified kinetic energy is employed to determine the conditions of marginal stability. The vacuum region surrounding the plasma is treated as a shearless, pressureless and massless pseudoplasma. Fourier decomposition of the perturbations in the periodic angular variables of the magnetic coordinates is applied and a finite element discretization scheme reduces the stability problem to a special block pentadiagonal matrix eigenvalue equation which is amenable to solution with an inverse vector iteration technique. The formulation described is useful to evaluate the linear MHD stability properties of 3D plasma configurations with rigid hot electrons to global internal and external modes. The conditions for linear MHD stability to local modes are determined with the application of the ballooning mode representation to the internal plasma potential energy to derive the corresponding ballooning mode equation. The asymptotic analysis of this equation yields the Mercier criterion.
78 citations
TL;DR: In this paper, a numerical method for quantitative reconstruction of the plasma density exclusively from relative Li I 670.8 nm emission profiles as measured along the diagnostic beam has been developed, involving all relevant collisional interactions of the Li atoms with plasma constituents.
Abstract: The injection of 10-100 keV Li0 diagnostic beams into magnetically confined fusion plasmas causes collisionally induced Li I emission at 670.8 nm, in close relation to the edge plasma electron density. A numerical method for quantitative reconstruction of the plasma density exclusively from relative Li I 670.8 nm emission profiles as measured along the diagnostic beam has been developed, involving all relevant collisional interactions of the Li atoms with plasma constituents. The applicability of the described algorithm is illustrated by experimental results obtained for the TEXTOR Tokamak edge plasma at KFA Julich.
76 citations
TL;DR: In this paper, a general analysis of the sensitivity of reflectometry to perturbations of the plasma profile, using a full-wave description in one-dimensional space, is presented.
Abstract: A general analysis is presented of the sensitivity of reflectometry to perturbations of the plasma profile, using a full-wave description in one-dimension. The square of the wavenumber is allowed to have an imaginary part, to account for absorption or divergence of the wave. Correlation reflectometry is investigated. It is found that the phase correlation is substantial, regardless of the correlation length of the fluctuations, unless either the wave attenuation is substantial or else the fluctuation correlation function is non-monotonic, corresponding to narrow-band turbulence. Curves are presented that provide fairly general information for the purpose of experimental interpretation.
74 citations
TL;DR: In this paper, two classes of stellarators to which this general picture does not apply are described: quasi-helically symmetric stellarators and a class of stellarator with vanishing bootstrap current in which the collisionless alpha-particle confinement sufficiently improves at finite beta.
Abstract: In many stellarators-envisaged as fusion devices-any alpha -particle which ever gets reflected ( nu /sub ///=0) is collisionlessly lost in a time which is orders of magnitude smaller than the typical slowing-down time of approximately=10-1 s. Two classes of stellarators to which this general picture does not apply are described: quasi-helically symmetric stellarators and a class of stellarators with vanishing bootstrap current in which the collisionless alpha -particle confinement sufficiently improves at finite beta . The influence of the modular ripple in optimized coil systems realizing these configurations, the angular distribution of the fast alpha -particle losses, and the application of the results to alpha -particle confinement simulation experiments in next-generation stellarators are also discussed.
73 citations
TL;DR: In this article, the experimental techniques for measuring the current density profile or the associated magnetic fields within the plasma are reviewed and an assessment is made of the currently available techniques with respect to the state of development and the obtained results.
Abstract: Tokamak plasmas are, at present, the most studied objects in controlled thermonuclear fusion research. The basic concept of these devices relies on externally induced currents which serve the twofold purpose of heating the plasma and creating an appropriate magnetic field structure for confinement. A detailed knowledge of the current density distributions and the dependencies on the operational conditions are important for both the theoretical understanding and the practical improvement of Tokamaks. The experimental techniques for measuring the current density profile or the associated magnetic fields within the plasma are reviewed. After a brief description of the equilibrium magnetic field configuration in toroidal geometry and a short account of indirect and passive diagnostic methods, an assessment is made of the currently available techniques with respect to the state of development and the obtained results.
66 citations
TL;DR: In this article, the MHD spectrum of circular cross-section tokamak plasmas with small aspect ratio was studied for low mode numbers and it was shown that for asymptotically small resistivity the damping is finite and independent of eta.
Abstract: The MHD spectrum of circular cross-section tokamak plasmas with small aspect ratio is studied for low mode numbers. Particular attention is given to the continuous part of the ideal MHD spectrum of such plasmas. Poloidal mode coupling in finite aspect ratio tokamaks yields gaps in the Alfven continuum. Global Alfven modes are found with a frequency inside these gaps. By interaction with the continuum branches the global Alfven modes experience damping via phase-mixing. This damping is computed in resistive MHD. It is shown that for asymptotically small resistivity the damping is finite and independent of eta .
61 citations
TL;DR: In this paper, the authors present a simple physical description of the basic MHD instabilities and describe the main difference between laboratory and coronal plasmas due to the presence of an extremely dense plasma at the footpoints of the coronal magnetic fieldlines.
Abstract: The review begins with an introduction to some of the various solar phenomena that may be investigated using stability theory. The geometry of the coronal magnetic field for many models is either a loop or an arcade and it is the stability properties of these structures that are investigated. The author presents a simple physical description of the basic MHD instabilities and describes the main difference between laboratory and coronal plasmas due to the presence of an extremely dense plasma at the footpoints of the coronal magnetic fieldlines. The implications of this density interface are discussed. Linear stability theory is applied to some solar situations and some non-linear simulations are discussed.
55 citations
TL;DR: In this paper, the authors compared and contrasted the properties of electrostatic and magnetic turbulence in the edge of tokamaks, stellarators, and reversed field pinches and discussed the scalings of turbulent fluctuation levels and wave numbers.
Abstract: Characteristics of electrostatic and magnetic turbulence in the edge of tokamaks, stellarators and reversed field pinches are compared and contrasted. Scalings of turbulent fluctuation levels and wave numbers are discussed. Electrostatic turbulence is shown to explain edge density scale lengths in all three device types. Electrostatic fluctuations also determine edge thermal transport in tokamaks and stellarators, while magnetic fluctuations explain edge thermal transport in reversed field pinches and runaway electron transport in tokamaks and stellarators.
TL;DR: In this article, it has been shown that an excited Faraday-shielded fast-wave antenna draws a large DC electron current from the plasma due to the sheath rectification effect.
Abstract: In the authors' previous study (1989) on TEXTOR it has been shown that an excited Faraday-shielded fast-wave antenna draws a large DC electron current from the plasma due to the sheath rectification effect. They show for the first time that this DC current (up to 400 A) extends toroidally all around the circumference of the machine and returns to the wall via conducting objects which are electrically connected to the wall. Preliminary measurements of r.f. currents in the scrape-off layer were performed using a specially designed Rogowski coil.
TL;DR: In this article, the authors used the laser blow-off technique to study impurity transport in JET using the Ohmic, L-mode and H-mode plasmas.
Abstract: Impurity transport has been studied in JET using the laser blow-off technique. Results are presented for Ohmic, L-mode and H-mode plasmas. In all cases impurity transport was found to be small inside r/a approximately 0.3 with values for D approximately 0.03-0.3 m2 s-1 close to neoclassical predictions. Outside r/a approximately 0.4, the transport was faster than neoclassical with values for D approximately 0.3-0.6, 3-5 and 0.8-1.2 m2 s-1 in H-mode, L-mode and Ohmic plasmas, respectively. These values apply only between sawtooth crashes, during the sawtooth phase itself ( approximately 100 mu s) the transport is greatly perturbed over the central region allowing the impurities to quickly leave or enter this region. In an Ohmic plasma the impurity confinement time tau imp was between 250 and 350 ms. This value was reduced to approximately 150-200 ms in the L-mode plasma due to an increase of D in the anomalous transport region. During H-mode plasmas, on the other hand, very long impurity confinement times, of the order of several seconds, were observed. This was explained by a sharp increase of the ratio V/D in a thin region near the plasma edge and a general reduction of D in the outer half of the plasma radius.
TL;DR: In this article, techniques for controlling the instabilities that lead to major disruptions are reviewed and their applicability to different types of disruptions (density limit beta -limit, low-q, and disruptions driven by error fields) in investigated.
Abstract: Techniques for controlling the instabilities that lead to major disruptions are reviewed and their applicability to different types of disruptions (density limit beta -limit, low-q, and disruptions driven by error fields) in investigated. Particular emphasis is given to schemes for controlling the m=2, n=1 resistive tearing mode, whether arising from error fields or from natural plasma instability.
TL;DR: A decrease in the edge magnetic and density fluctuations is always seen at the L-H transition in DIII-D as mentioned in this paper, which indicates the formation of shear in the radial electric field.
Abstract: A decrease in the edge magnetic and density fluctuations is always seen at the L-H transition in DIII-D. Also, a sudden change of the perpendicular rotation vperpendicular to in the edge has been observed at the transition, which indicates the formation of shear in the radial electric field. This region coincides with the region where the density fluctuations are suppressed. These experimental observations are consistent with earlier theoretical models. Comparisons between the theories and the experimental observations will be discussed.
TL;DR: In this article, the second root of the dispersion relation is also seen at low B fields for a 1 cm radius helicon plasma source, and the wave acceleration of primary electrons is shown to be very strong at these small diameters.
Abstract: Evidence is given for wave acceleration of primary electrons in a 1 cm radius helicon plasma source. The second root of the dispersion relation is also seen at low B fields. Electrostatic confinement of primaries appears to be important at these small diameters.
TL;DR: In this paper, the anomalous energy transport can be described by Del.Qj*, where Qj* = 3/5Qj, with all remaining source terms such as (pj Del.Vj) cancelling.
Abstract: Quasilinear expressions for anomalous particle and energy fluxes arising from electrostatic plasma turbulence in a Tokamak are reviewed. Further clarifications are made, and the position taken in a previous report (Ross, 1989, Comments Plasma Phys. Contr. Fusion 12, 155) is modified. There, the total energy flux, Qj, and the conductive heat flux, qj, were correctly defined, and the anomalous Qj was correctly calculated. It was shown that the anomalous energy transport can be described by Del .Qj*, where Qj*=3/5Qj, with all remaining source terms such as (pj Del .Vj) cancelling. A revised discussion is given of the identification of the anomalous conductive flux, qj, in which the distinction between Qj and Qj* is reconsidered. It is shown that there is more than one consistent way to define qj. Transport calculations involving only theoretical electrostatic turbulent fluxes are unaffected by these distinctions since Qj or Qj*, rather than Qj, is the quantity naturally calculated in theory. However, an ambiguity remains in experimental transport analysis if the measured particle flux Gamma j=njVj is to be used in the energy equation to identify the convective terms. This is because the author cannot be sure in general how properly to treat the source terms pj Del .Vj or (pj Del .Vj).
TL;DR: In this paper, the authors view the transport of Tokyoamak plasma as a self-organisation process, and propose nonlinear relations between fluxes and gradients provide a variety of confinement modes and profile resilience.
Abstract: Tokamak plasma transport is viewed as a phenomena of self-organisation. Nonlinear relations between fluxes and gradients provide a variety of confinement modes and profile resilience.
TL;DR: In this paper, the radial and poloidal correlation lengths from beam emission spectroscopy and correlation reflectometry density fluctuation diagnostics on TFTR have been compared and the results show that the radial correlation lengths are roughly 2-3 cm near the plasma edge and gradually drop to 1-2 cm in the region of r/a=0.5-0.7.
Abstract: New beam emission spectroscopy and correlation reflectometry density fluctuation diagnostics on TFTR allow examination of the crucial long wavelength (kperpendicular to rho i<0.1) region, where microwave scattering has indicated where the bulk of the power density lies. The turbulent spectrum for low k is broadband and dominated by low frequencies (<200 KHz), and the exact nature of the spectrum is dominated by Doppler effects. These core diagnostics show n/n rising as tau E decreases. Radial and poloidal correlation lengths from BES are comparable and are roughly 2-3 cm near the plasma edge and gradually drop to 1-2 cm in the region of r/a=0.5-0.7. Values <1.5 cm are indicated deeper in the core region, but precise values await further analysis which takes into account the effects of finite spatial resolution of the BES measurements. A strong anisotropy in the radial and poloidal wavenumber spectra is indicated from the shape of the correlation functions. The poloidal wavenumber spectrum does not increase indefinitely as k goes to 0 cm-1, but has a peak in the range of 1-2 cm-1.
TL;DR: In this paper, the refractive indices and cutoff conditions for electromagnetic waves in plasmas are investigated for cold, hot and relativistic plasma models, and significant relatvistic modifications of refractive index and locations of cutoffs are found in regimes relevant for reflectometry in large Tokamaks.
Abstract: The refractive indices and cutoff conditions for electromagnetic waves in plasmas are investigated for cold, hot and relativistic plasma models. Significant relativistic modifications of refractive indices and locations of cutoffs are found in regimes relevant for reflectometry in large Tokamaks. For X-mode it is demonstrated that these effects may shift the location of the reflecting layer by a significant fraction of the minor radius and that the cold model may lead to considerable underestimations of the density profile. Relativistic effects predicted for O-mode reflectometry are smaller than for X-mode, but not negligible. An algorithm for reconstruction of density profiles which allows a relativistic plasma model to be used is presented.
TL;DR: In this article, the authors measured the Doppler shifted X-ray lines emitted from He-like nickel present in the centre of the plasma and concluded the rotation is connected with the creation of fast ions during ion cyclotron resonance heating.
Abstract: Changes of the toroidal rotation velocity of up to 3*104 m s-1 have been observed during ICRH in JET. The plasma increases its toroidal rotation in the direction of the plasma current. The change in velocity is obtained from the measurement of the Doppler shifted X-ray lines emitted from He-like nickel present in the centre of the plasma. By analyzing various heating scenarios and also measuring the fast anisotropic ion energy content of the plasma the authors conclude the rotation is connected with the creation of fast ions during ion cyclotron resonance heating. Toroidal acceleration of the plasma in connection with MHD instabilities leading to similar magnitude of rotational velocity is also seen.
TL;DR: In this article, collisional drift wave turbulence is studied numerically in a sheared slab configuration, where nonlinear mode structure changes allow self-sustaining of the turbulence, even though no unstable linear modes exist.
Abstract: As a paradigm for electron-based anomalous transport involving several fluctuating quantities, collisional drift wave turbulence is studied numerically in a sheared slab configuration. Nonlinear mode structure changes allow self-sustaining of the turbulence, even though no unstable linear modes exist. The responsible physics violates all the assumptions behind mixing-length, or turbulence-as-diffusion models, and so the turbulence is necessarily not of that type. The key ingredients are shared by all important transport candidates. Further, this single model displays several important properties known from tokamak transport-isotope mass trends, up-gradient transport ('inward pinch'), and ExB shear effects-which have lacking or dubious explanations within the existing framework. It can therefore serve as a useful basis for building new, and more realistic, scenarios for inhomogeneous turbulent transport.
TL;DR: In this article, the so-called coherent scattering of electromagnetic waves from nonuniform, irregularly moving plasmas is investigated in the case where the scattering wavelength is large compared to the Debye length, but of the order of the irregularities correlation length.
Abstract: Magnetized plasmas occurring in nature as well as in fusion laboratories are often irregularly shaken by magnetic field fluctuations. The so-called 'coherent scattering' of electromagnetic waves from nonuniform, irregularly moving plasmas is investigated in the case where the scattering wavelength is large compared to the Debye length, but of the order of the irregularities correlation length. The scattered signal frequency spectrum is shown to be a transform of the plasma motion statistical 'characteristics'. When the scattering wavelength is larger than the plasma motion correlation length, the frequency spectrum is shown to be of a Lorentzian shape, with a frequency width that provides a direct measurement of the cross-B particle diffusion coefficient. This is illustrated by two series of experimental results: radar coherent backscattering observations of the auroral plasma, and far infrared scattering from tokamak fusion plasma. Radar coherent backscattering shows the transition from Gauss to Lorentz scattered frequency spectra. In infrared laser coherent scattering experiments from the Tore-Supra tokamak, a particular frequency line is observed to present a Lorentzian shape, that directly provides an electron cross-field diffusion coefficient. This diffusion coefficient agrees with the electron heat conductivity coefficient that is obtained from the observation of temperature profiles and energy balance.
TL;DR: In this article, an ion-temperature gradient driven instability was investigated in the Columbia Linear Machine (CLM), where a transit-time RF technique was used to heat the core of the plasma column.
Abstract: An ion-temperature-gradient driven instability is investigated in the Columbia Linear Machine (CLM). A transit-time RF technique is used to heat the core of the plasma column. This produces the peaked ion temperature profile with the parameter eta i= delta ln Ti/ delta ln n exceeding the critical value ( approximately 1.4) required for onset of the instability. Along with the peaked temperature profile, a new feature in the density fluctuation spectrum is observed. This mode has an azimuthal mode number m=2 and propagates in the ion diamagnetic drift direction. It is identified as an eta i mode. The mode is similar to the one studied previously in the CLM using DC biased meshes for heating the ions.
TL;DR: In this article, the effect of TF ripple on the confinement of fast particles and, more generally, on the plasma behaviour, was performed using only 16 TF coils and the toroidal field was limited to 1.4 T, with plasma currents in the range between 2 and 3 MA.
Abstract: The JET machine is equipped with 32 toroidal field coils. In order to study the effect of TF ripple on the confinement of fast particles and, more generally, on the plasma behaviour, a series of experiments was performed using only 16 TF coils. At the position of the outer limiter, this led to an increase of the ripple, delta =(Bmax-Bmin)/(Bmax+Bmin), from 1% to 12.5%. The toroidal field was limited to 1.4 T, with plasma currents in the range between 2 and 3 MA. Additional heating power-levels and energy-input were kept low in order to avoid possible damage to some first wall components made out of Inconel. Experiments were carried out using 140 keV NBI injected deuterons, ICRF accelerated protons and deuterons ( approximately 0.5 to approximately 2 MeV) and 1 MeV tritons from DD reactions.
TL;DR: Spectral observations of ions of beryllium in the JET plasma are described and are interpreted in terms of the interplay of atomic reactions and local plasma conditions as mentioned in this paper, where emphasis is placed on the role of metastable ion states.
Abstract: Spectral observations of ions of beryllium in the JET plasma are described and are interpreted in terms of the interplay of atomic reactions and local plasma conditions The methods of generalized-collisional-radiative theory are extended and used to provide a quantitative model Emphasis is placed on the role of metastable ion states The diagnostic scope of beryllium observations is explored for the influx ions Be0+ and Be1+ in the visible spectral range, for Be2+ in the XUV, and for Be3+ by charge exchange spectroscopy Useful results for general diagnostic interpretation are presented in tables and graphs The calculation procedures and data form part of the 'Atomic Data and Analysis Structure' in use at JET
TL;DR: In this article, the development of H- and D- sources for neutral beam heating on the next generation Tokamaks such as NET or ITER is discussed, where either the ion is formed in the plasma by dissociative attachment or on the surface of a negatively biased electron of low work function.
Abstract: A description is presented of the development of H- and D- sources for neutral beam heating on the next generation Tokamaks such as NET or ITER. The two basic types of such ion sources are discussed, where either the ion is formed in the plasma by dissociative attachment or on the surface of a negatively biased electron of low work function. Also described are the acceleration techniques to form high energy and power beams needed for neutral beam injectors. Finally, a short discussion is presented of the future development of these sources.
TL;DR: In this paper, the basic theoretical results regarding stochastic edge layers are reviewed and compared to the experimental data, and a lowering and flattening of the edge temperature profile is found.
Abstract: The basic theoretical results regarding stochastic edge layers are reviewed and compared to the experimental data. A lowering and flattening of the edge temperature profile is found. The increase of the electron transport leads to both a change in the electric field and to a shrinking of the current channel and hence to a stabilizing effect on the (2,1) MHD mode. The screening of the particles influx leads to a decontamination of the plasma core allowing high purity plasmas, nD/ne>0.99.
TL;DR: In this article, a review of the experimental observation of plasma net currents in several stellarator devices and the comparison with theoretical predictions on the neoclassical bootstrap current is given.
Abstract: A review of the experimental observation of plasma net currents in several stellarator devices and the comparison with theoretical predictions on the neoclassical bootstrap current is given. Good agreement of the experimental findings with neoclassical predictions is found with respect to the parameter dependence, the local distribution of the currents and the magnitude. The compensation of the bootstrap current by electron cyclotron current drive is experimentally demonstrated. Experiments on the ECCD-efficiency are compared with simple theoretical models.
TL;DR: In this article, the authors describe measurements of nonlinear interactions between drift waves and low-frequency flute-like modes in the UMIST quadrupole, and propose a model of energy flow in the drift-wave spectrum.
Abstract: The authors describe measurements of nonlinear interactions between drift waves and low-frequency flute-like modes in the UMIST quadrupole. They present experimental measurements of the bispectrum, and also results from a technique of 'amplitude correlation' which they believe provides a more direct way of determining the direction of the energy flow. On the basis of the results they propose a model of energy flow in the drift-wave spectrum: the observed spectrum contains regions of growth at high frequency and damping at low frequency, with flow down the spectrum via a cascade mediated by the flute-like modes.