Showing papers in "Plasma Physics and Controlled Fusion in 1994"

Electron cyclotron resonance heating and current drive in toroidal fusion plasmas

Abstract: A review of experiments and theory of electron cyclotron resonance heating (ECRH) and current drive (ECCD) is presented. An outline of the basic linear theory of wave propagation and absorption in the electron cyclotron range of frequencies and their harmonics is given and compared with experimental results from many devices. The experimental data base on quasilinear and nonlinear physics as well as on parametric wave decay is reviewed and compared to theory. Experiments and theory on doppler shifted absorption either by bulk or tail electrons (which can be created by other means) are discussed. ECRH provides means for controlled plasma breakdown and current ramp up in tokamaks and plays a key role in net current-free stellarator research. Start-up was investigated in many tokamaks and stellarators and the results are discussed in the light of the present day theoretical understanding. The role of ECRH to improve the understanding of both particle and energy confinement is described and special heating correlated features, such as 'density pump out' during ECRH are discussed. The application of modulated ECRH for perturbative heat wave studies and the comparison with both sawtooth heat pulse propagation and the steady state power balance analysis is presented. Electron cyclotron current drive is a possible method for current profile and MHD control in tokamaks and provides means for bootstrap current compensation in stellarators. The basic theory of electron cyclotron current drive is presented and compared to experiments in both tokamaks and stellarators. Experiments on sawtooth stabilization and MHD control by ECRH or ECCD are discussed and compared to theory. An increasing number of fusion devices is equipped with ECRH for bulk heating and sophisticated plasma physics investigations. A remarkable extension of the accessible plasma parameter range became possible by the recent development of sources with high power (1 MW) and frequency (110-160 GHz). Particular emphasis is given to new experiments and the refinement of theory incorporating plasma phenomena and the mutual impact on the wave physics.

An overview of charge-exchange spectroscopy as a plasma diagnostic

Abstract: Charge-exchange spectroscopy in fusion plasmas entails the use of optical transitions that follow electron transfer from a neutral atom into an excited state of an impurity ion. In most applications, the sources of neutral particles are high-energy beams employed either for heating or for the specific purpose of active plasma diagnosis. The transitions following charge exchange are particularly useful for determining the densities of fully stripped low-Z ions and for measuring ion temperatures and plasma rotation, although they have also been exploited for other purposes. In this review, the atomic physics considerations for interpreting the data, including the influence of the plasma environment, are reviewed, and examples of recent applications to fusion studies are presented.

Review of spheromak research

Abstract: Spheromak research from 1979 to the present is reviewed including over I60 references. Emphasis is on understanding and interpretation of results. In addition to summarizing results some new interpretations are presented. An introduction and brief history is followed by a discussion of generalized helicity and its time derivative. Formation and sustainment are discussed including five different methods, flux core, theta -pinch z-pinch, coaxial source, conical theta -pinch, and kinked z-pinch. All methods use helicity injections. Steady-state methods and rules for designing spheromak experiments are covered, followed by equilibrium and stability. Methods of stabilizing the tilt and shift modes are discussed as well as their impact on the reactor designs. Current-driven and pressure-driven instabilities as well as relaxation in general are covered. Energy confinement is discussed in terms of helicity decay time and and beta s limits. The confinement in high and low open-flux geometries are compared and the reactor implications discussed.

Survey of theories of anomalous transport

Abstract: Energy and particle confinement in tokamaks is usually anomalous, greatly exceeding neoclassical predictions. It is desirable to develop an understanding of the underlying processes to increase the confidence in extrapolation of tokamak behaviour towards reactor regimes. The literature abounds with theoretical expressions for anomalous transport coefficients based on turbulent diffusion due to various micro-instabilities. These often purport to provide explanations of tokamak confinement at the level of global scaling laws. However, comparison with experimental data from local transport analyses offers a far more stringent test of these theories. This review presents the available theories for turbulent transport coefficients, particularly ion and electron thermal diffusivities, in a way that will facilitate a programme of testing models against data. It provides a brief description of the basis for each theory to place it in context and then presents the resulting turbulent diffusivity. Particular emphasis is placed on the validity conditions under which the expressions may be used; this is important when subjecting them to meaningful tests against data. The present review emphasizes the more recent developments, building on earlier ones by Liewer and Ross et al. The results of this work have already been of value in carrying out a programme of testing theories against high quality JET data (Conner et al. (1993) and Tibone et al. (1994)).

Creation and control of variably shaped plasmas in TCV

Abstract: During the first year of operation, the TCV tokamak has produced a large variety of plasma shapes and magnetic configurations, with 1.0

Neutron measurement techniques for tokamak plasmas

Abstract: Reviews the neutron measurement techniques that are applied to the study of tokamak plasmas. The range of neutron energies of primary interest is limited to narrow bands around 2.5 and 14 MeV, and the variety of measurements that can be made for plasma diagnostic purposes is also restricted. To characterize the plasma as a neutron source, it is necessary only to measure the total neutron emission, the relative neutron emissivity as a function of position throughout the plasma, and the energy spectra of the emitted neutrons. In principle, such measurements might be expected to be relatively easy. That this is not the case is, in part, attributable to practical problems of accessibility to a harsh environment but is mostly a consequence of the time scale on which the measurements have to be made and of the wide range of neutron emission intensities that have to be covered; for tokamak studies, the time-scale is of the order of 1 to 100 ms and the neutron intensity ranges from 1012 to 1019 s-1.

Tokamak plasma diagnosis by electrical probes

Abstract: This paper reviews progress in the understanding and application of electrical probes for the diagnosis of tokamak edge plasmas. Langmuir probes are still one of the most commonly applied diagnostics in tokamaks but our understanding of how to interpret them in strong magnetic fields is still rather limited. Recent results from flush-mounted Langmuir probes in divertor plates are used to highlight the problem areas, in particular non-saturation of the ion current and low electron to ion saturation current ratios. The importance of measuring the parallel flow velocity in the plasma edge is highlighted and recent developments in theories for interpretation of Mach probes are discussed. Finally, the importance of diagnostics for ion temperature and impurity content is emphasized, and progress in the application of advanced electrical probes such as retarding field analysers and mass spectrometers to the tokamak boundary is reviewed in this context.

Resonance wave discharge and collisional energy absorption in helicon plasma source

Abstract: The concept of the 'resonance' wave discharge is introduced to explain the high absorption efficiency of inductively coupled helicon plasma source. This discharge is supported by long-scale weakly damped eigenmodes excited in plasma resonator. The RF power absorbed in it is inversely proportional to electron collision frequency. The fields excited by simple single-loop inner antenna and its impedance are calculated in a cylindrical metal resonator. At low collision frequencies the field amplitudes and antenna resistance are shown to peak sharply near the helicon dispersion branches.

Modelling of detachment in a limiter tokamak as a nonlinear phenomenon caused by impurity radiation

Abstract: A numerical one-dimensional radial time-dependent model of particle and energy transport in tokamak plasmas taking the interaction of background and impurity particles through the radiation losses of energy and ion dilution is developed. Detachment in ohmic discharges caused by an increase in the electron density is modelled and the effect of particle and energy transport on detachment conditions is studied. Investigation of dynamic changes in plasma parameters during detachment provoked by an increase in impurity influx is performed. The effect of the nature of heat source on the stabilization of the radiating layer is elucidated. The influence of neo-classical transport and charge-exchange with hydrogen neutrals on the impurity behaviour in 'attached' and 'detached' plasmas is discussed.

The effect of diamagnetic drift on the boundary conditions in tokamak scrape-off layers and the distribution of plasma fluxes near the target

Abstract: The standard model for the two-dimensional simulation of the scrape-off layer (SOL) in the divertor is based on the equations for anomalous cross-field particle and energy transport and classical parallel transport. Recently, it has been complemented by the fluid equations of Braginskii to include all classical fluxes in a self-consistent way. The main physical effect of introducing classical fluxes is the appearance of two new terms-an electrical E*B drift and a diamagnetic B* Del P drift, as well as B* Del T contributions to the energy transport. Although the new models provide an adequate consideration of all the classical fluxes inside the plasma, a mistake has typically been made in the literature in formulating the new boundary conditions. The poloidal component of the B* Del Pi drift, existing inside the plasma, is assumed to continue through to the target surface, thus altering the Bohm criterion. Likewise, the poloidal current due to the B* Del (Pi+Pe) term is assumed to reach the surface, changing the surface electric potential drop. In this paper we demonstrate, however, that the diamagnetic fluxes in the whole plasma, including the SOL region, in their major part can be represented by the curl of a vector and, therefore, are self-terminating. We have made a detailed analysis of the distribution of plasma pressure and fluxes at the boundary between the plasma and the surface, encompassing the magnetic presheath and Debye sheath layers. After entering the presheath entrance, the poloidal component of the diamagnetic flux is diverted in the direction along the material surface due to the sharp pressure gradient in the presheath region. This creates boundary drift flows along the surface. Because of that conversion of the poloidal diamagnetic fluxes into boundary fluxes they do not deliver particles and current to the surface and therefore must not be included in the modified boundary conditions. Kinetic analysis of the distribution of the fluxes near the target also reveals an additional ExB drift in the radial direction, which mainly affects the ion motion in the magnetic presheath and Debye sheath layers. This drift is expected to shift the density profile of the plasma near the target in the radial direction a distance of the order of the ion poloidal Larmor radius.

Summary of experimental progress and suggestions for future work (H mode confinement)

Abstract: Since the last H-mode workshop in 1991, there has been significant progress in a number of areas. In addition to H-modes in tokamaks, H-mode has been achieved in a current-free stellarator, a heliotron/torsatron with some net toroidal current and in a linear, tandem mirror. Because H-mode has been seen in a variety of magnetic confinement devices and has been produced by a variety of methods, a universal explanation is needed for the H-mode confinement improvement. The hypothesis of turbulence stabilization by sheared E*B flow has this universality. New diagnostics have confirmed the structure of Er at the plasma edge in tokamaks and have lead to direct determination of the reduction in turbulence-driven transport in the H-mode. Improved measurements of the edge rotation and pressure gradients in tokamaks have lead to better understanding of the physics of the Er formation and to tighter tests of L to H transition theories. Significant improvements in core confinement have also been seen in several tokamaks; the data here indicate that sheared E*B flow may be playing a role in the core confinement improvement. Power balance studies of the change in core thermal diffusivity after the L to H transition have raised fundamental questions about the relationship between heat flux and the temperature gradient. Finally, helium transport studies have shown that helium transport is quite similar to deuterium transport indicating that helium ash removal in a reactor operating in ELMing H-mode should be feasible.

Confinement and turbulent transport in a plasma torus with no rotational transform

Abstract: In the BLAAMANN device a weakly ionized hydrogen plasma is produced by electrons accelerated from a hot, negatively biased tungsten filament and confined in a toroidal magnetic field of strength up to 0.4 T. The plasma is turbulent, with relative fluctuation levels in ne, phi and Te of 10% or more. The time-averaged state exhibits nested toroidal surfaces of constant potential and pressure, which requires an anomalous cross-field current to remove the space-charge injected by the cathode and the charge accumulated due to the Del B- and curvature drifts. Typical plasma parameters are ne approximately 1016 m-3, Te approximately 1-20 eV, Ti approximately 1 eV. The cross-field diffusion coefficient is typically Dperpendicular to approximately 30 m2 s$ -1 approximately 104*Dperpendicular to classical approximately 101*Dperpendicular to Bohm. Evidence is presented in support of the hypothesis that the plasma goes turbulent because it needs to develop an anomalous current channel, and this turbulence in turn determines the plasma transport and the time-averaged state.

H-mode of W7-AS stellarator

Abstract: In W7-AS the H-mode has been observed for the first time in a currentless stellarator plasma. H-modes are achieved with 0.4 MW Electron Cyclotron Resonance Heating with 140 GHz at 2.5 T and high density, with 70 GHz at 1.25 T and lower density and with neutral beam injection. The H-phases display all characteristics known from tokamak H-modes including the development of an edge transport barrier, an increase of the poloidal impurity flow velocity at the edge, the reduction of edge turbulence and ELMs. The power threshold for the H-mode seems to be lower than that in tokamaks and is in agreement with an neBT scaling. Major differences to the divertor H-mode is the small increase in energy content of maximally 30%, the lack of a strong isotope effect both in threshold and in H-mode characteristics and a peculiarly narrow operational range in iota.

Theory of improved confinement in high- beta p tokamaks

Abstract: The dissipative self-organization of the plasma pressure and current in tokamaks is investigated through interactions between bootstrap current and the anomalous transport due to self-sustained turbulence. The strongly reduced magnetic shear due to the pressure-driven current and the magnetic axis shift cause a reduction in the anomalous transport, which enhances the plasma pressure and bootstrap current and leads to a transition to a high- beta p plasma. The threshold is given by beta p approximately=1. Simulation results are compared with experimental observations.

Optimized profiles for improved confinement and stability in the DIII-D tokamak

Abstract: Simultaneous achievement of high energy confinement, tau E, and high plasma beta, beta , leads to an economically attractive compact tokamak fusion reactor. High confinement enhancement, H= tau E/ tau E-ITER89P=4, and high normalized beta beta N beta /(I/aB)=6%-m-T/MA have been obtained in DIII-D experimental discharges. These improved confinement and/or improved stability limits are observed in several DIII-D high performance operational regimes: VH-mode, high li H-mode, second stable core, and high beta poloidal. We have identified several important features of the improved performance in these discharges: details of the plasma shape, toroidal rotation or E*B flow profile, q profile and current density profile, and pressure profile. From our improved physics understanding of these enhanced performance regimes, we have developed operational scenarios which maintain the essential features of the improved confinement and which increase the stability limits using localized current profile control. The stability limit is increased by modifying the interior safety factor profile to be nonmonotonic with high central q, while maintaining the edge current density consistent with the improved transport regimes and the high edge bootstrap current. We have calculated high beta equilibria with beta N=6.5, stable to ideal n=1 kinks and stable to ideal ballooning modes. The safety factor at the 95% flux surface is 6, the central q value is 3.9 and the minimum in q is 2.6.

Magnetic electrostatic plasma confinement

Abstract: Electrostatic plasma confinement and magnetic electrostatic plasma confinement (MEPC) have been studied for four decades. The multiple potential well hypothesis, postulated to explain high neutron yields from Hirsch's colliding beam experiment (1968), has been supported by several pieces of evidence, but results were inconclusive. Magnetic shielding of the grid was developed to reduce the required beam current and to prevent grid overheating. Electrostatic plugging of magnetic cusps evolved to a similar configuration. Due to low budgets, early MEPC experiments used spindle cusps, which are poor for plasma confinement. Later experiments used multipole cusps or a linear set of ring cusps, which have larger volumes of field-free plasma. To keep the self-shielding voltage drop Delta phi

Ripple transport in helical-axis advanced stellarators: a comparison with classical stellarator/torsatrons

Abstract: Calculations of the neoclassical transport rates due to particles trapped in the helical ripples of a stellarator's magnetic field are carried out, based on solutions of the bounce-averaged kinetic equation. These calculations employ a model for the magnetic field strength, B, which is an accurate approximation to the actual B for a wide variety of stellarator-type devices, among which are Helical-Axis Advanced Stellarators (Helias) as well as conventional stellarators and torsatrons. Comparisons are carried out in which it is shown that the Helias concept leads to significant reductions in neoclassical transport rates throughout the entire long-mean-free-path regime, with the reduction being particularly dramatic in the nu -1 regime. These findings are confirmed by numerical simulations. Further, it is shown that the behaviour of deeply trapped particles in Helias can be fundamentally different from that in classical stellarator/torsatrons; as a consequence, the beneficial effects of a radial electric field on the transport make themselves felt at a lower collision frequency than is usual.

Rapid change of ion energy distribution and floating potential at L/H transition in the JFT-2M tokamak

Abstract: Rapid changes of the main ion energy distribution at transitions from L-to-H, H-to-L and during ELMs are studied by time of flight neutral measurements in the JFT-2M tokamak. At the L to H transition, 200-400 mu s prior to the start of Halpha drop, an increase of the high energy neutral flux above an energy of 200 eV is observed. An energy of higher than 200 eV for hydrogen corresponds to the collisionless condition v*i

The time behaviour of the thermal conductivity during L to H and H to L transitions in JET

Abstract: The change in thermal transport across the L to H transition is studied in detail for those JET high performance H-modes which have a very fast transition. It is found that in these pulses the transport changes very rapidly (<4 msecs) over a very large radial region 0.5< rho <1, and a very large transport barrier is formed. The reasons for the formation of this barrier are discussed.

H-mode results in ASDEX Upgrade

Abstract: The H-mode obtained in ASDEX Upgrade by three heating methods (Ohmic, NBI, ICRF) is analysed. The power threshold is shown to be relatively low compared with other tokamaks and to be independent of the heating method. The operational window for the different types of H-phases, (dithering, ELMing with type-III and I ELMs, ELM-free) is given and discussed. The features of the H-mode discharges, in particular quasi-stationary ELMing phases with type I ELMs, are described and discussed. Finally, it is shown that the power flux through the plasma edge is a key parameter for H-mode operation and control.

Global and local energy confinement properties of simple transport coefficients of the Bohm type

Abstract: Global and local energy transport analysis of experimental results in TFTR and JET indicate that transport models of the Bohm type allow a better representation of experimental data than models of the gyro-Bohm type. Hence a Bohm-like model has been developed by allowing a dependence on a few dimensionless parameters so as to reproduce the main features of global L-mode thermal confinement scaling laws in tokamaks and to obtain bowl-shaped thermal diffusivity profiles. The electron temperature profiles predicted by this model are shown to be in agreement with measured ones in a variety of steady-state and time-dependent JET L-mode discharges.

Expansion of parameter space for toroidal Alfven eigenmode experiments in TFTR

Abstract: Several techniques were used to excite toroidal Alfven eigenmodes (TAE) in the Tokamak Fusion Test Reactor (TFTR) (Proc. 13th Int. Conf. on Plasma Physics and Controlled Nuclear Fusion Research Vol 1 (Vienna: IAEA, 1990) 9) at magnetic fields above 10 kG. These involved pellet injection to raise the plasma density, variation of the plasma current to change the energetic ion orbit and the q-profile, and ICRF heating to produce energetic hydrogen ions at velocities comparable with 3.5 MeV alpha particles created in deuterium-tritium (d-t) fusion reactions. The amplitude of the TAE modes observed in ICRF-heated plasmas behaves differently from those driven by neutral beams. This can be explained by the difference in the energetic ion orbits. These experimental results are presented and relevance to fusion reactors are discussed.

Progress on spherical tokamaks

Abstract: The history of low-aspect-ratio 'spherical' tokamak research is described and the advantages claimed for the concept are outlined and compared with experimental results from the START device and elsewhere. Further development of the spherical tokamak, and its potential for a materials test facility or ultimately a fusion plant, are discussed.

Confinement improvement in H-mode-like plasmas in helical systems

Abstract: The reduction of the anomalous transport due to the inhomogeneous radial electric field is theoretically studied for toroidal helical plasmas. The self-sustained interchange-mode turbulence is analysed for a system with magnetic shear and magnetic hill. The ballooning mode turbulence is studied for a system with magnetic well-like conventional stellarators. The influence of the radial electric field inhomogeneity on the transport coefficients and fluctuations is quantitatively shown. Unified theories of the transport coefficients in the L-mode- and H-mode-like plasmas are presented.

The effect of BT reversal on the asymmetries between the strike zones in single null divertor discharges: experiment and theories

Abstract: Asymmetries in power and particle fluxes to the strike zones in single null divertor configuration are strongly affected by the toroidal field direction. In the present paper we review experimental data and examine physical mechanisms which redistribute density between the inner and outer divertor branches and, consequently, affect temperature and power deposition asymmetries between the two strike zones. Amongst these is the radial E*B drift caused by the Te drop along the field lines towards the target and recently proposed mechanism of momentum transfer into the SOL due to edge toroidal rotation. These two mechanisms compress the plasma at the inner side in the normal BT configuration (ion Del B drift towards the target), and at the outer side-when the BT is reversed. Since they do not supply additional power to the target apart from convective energy fluxes, the increased recycling and radiation at the side where the plasma is compressed should lead to a decrease in Te. The combined effect of the above mechanisms allows one to explain qualitatively experimental observation that plasma parameters at the target become more symmetric when the BT is reversed.

Advanced tokamak physics-status and prospects

Abstract: Experimental and theoretical results from around the world point to the possibility of high confinement, high- beta , and high-bootstrap-fraction steady-state tokamak operating modes. These modes of operation, if fully developed and extended to steady-state, could lead to much less expensive tokamak demonstration power reactors and to a significantly reduced cost-of-electricity from fusion, as compared to projections based on low- beta N, pulsed operating modes. Present results have clear implications in the areas of particle control, plasma shaping, and current-profile control. Thus they have strongly influenced the design of the steady-state advanced tokamak TPX, which has the mission to combine the best results from present experiments and extend them to steady-state. These results also have important implications for follow-up tests in ITER, which have the goal of studying advanced-tokamak operation in an ignited plasma, as well as for the eventual configuration of an advanced-tokamak fusion reactor.

Experimental survey of the L-H transition conditions in the DIII-D tokamak

Abstract: We present the global analysis of a recent survey of the H-mode power threshold in DIII-D using D degrees to D+ NBI after boronization of the vacuum vessel. Single parameter scans of BT, Ip, density, and plasma shape have been carried out on the DIII-D tokamak for neutral beam heated single-null and double-null diverted plasmas. In single-null discharges, the power threshold is found to increase approximately linearly with BT and ne but remains independent of Ip. In double-null discharges, the power threshold is found to be approximately independent of both BT and ne. Various shape parameters such as plasma-wall gaps had only a weak effect on the power threshold. Imbalancing the double null configuration resulted in a large increase in the threshold power.

Theoretical progress on H-mode physics

Abstract: Theoretical research into H-mode physics is reviewed. The topics cover elementary processes such as bifurcation physics, collisional transport theory and anomalous transport theory. Associated with these investigations, a global picture of H-mode phenomena, such as the spatial structure and temporal evolution has been constructed from theory. The author tries to clarify the contributions of theoretical presentations in this H-mode workshop, illustrating the relation to the overall progress on the H-mode theory.

Doppler shift of the TAE mode frequency in DIII-D

Abstract: The Doppler shift caused by toroidal plasma rotation in DIII-D complicates a comparison of the frequency of toroidicity-induced Alfven eigenmodes (TAE) to theoretical predictions, but also separates toroidal modes in frequency space and provides information about the radial location of the modes. Two independent techniques for estimating the mode frequency and Doppler shift are presented: one based upon the spectrum of multiple toroidal modes and one that utilizes the measured toroidal rotation of the plasma. Both methods indicate the presence of multiple TAE modes located between the q=1 and y=3/2 surfaces. The frequencies determined by the two methods agree within 20%, consistent with their estimated uncertainties.

The resistive interchange convection in the edge of tokamak plasmas

Abstract: Theoretical results are presented addressing the behaviour of plasmas outside the separatrix i.e. in the region where the magnetic field lines do not form closed magnetic surfaces. The resistive interchange model is used to describe self-consistently the generation of poloidal (or toroidal) flow and the onset of ELMs, which appear as bubbles of increased density (or higher temperature). In addition, the decrease of the turbulent transport processes as the velocity of the flow increases and its connection with the LH transition are discussed. The linear theory of stability is applied and complemented by a full numerical simulation of the two-dimensional nonlinear equations. Analytic solutions for asymptotically small parameters in the nonlinear model are derived and used for the explanation of the numerical results. Semi-phenomenological turbulence balance equations for the resistive interchange convection are presented as well. Finally, these results are applied for the explanation of existing experiments.