Showing papers in "Plasma Physics and Controlled Fusion in 1986"

Theory of Resistive Modes in the Ballooning Representation

Abstract: The linear theory of current-driven resistive modes is presented in terms of the ballooning formalism, which employs the extended poloidal angle as the independent variable. This variable is proportional to the radial wave number. In the vicinity of the singular boundary layer, this formalism is applied to modes with high as well as with low poloidal mode numbers. The MHD solutions in the external region impose singular boundary conditions on the inner solutions, which must be satisfied near the origin of the extended variable. This is in contrast with coordinate space where they are expressed in integral form. These conditions are characterized by a single parameter which is assumed to be known. The collisional tearing and Alfven modes and the m=1 internal kink mode are obtained from a common dispersion relation. This is also the case for the semi-collisional tearing and Alfven modes. The structure of the modes in kr-space as well as in coordinate space is discussed.

Plasma flux motion in a toroidal plasma guide

Abstract: The motion of a plasma flux is studied in the toroidal system with crossed radial electric and longitudinal magnetic fields. It is shown that the particle motion occurs within the dynamic boundary which is narrower than the region of electrostatic and magnetic confinement. In this case it is the electric field that plays a predominant role in the ion transport. The effect of input conditions on the flux passage through the system is analysed. It is found that the change of the radius of curvature of the magnetic field lines in the input section causes the appearance of the electric field component directed against the flux motion and essentially influences the effectiveness of particle transport. Good agreement between theory and experiment is obtained.

On the generation mechanism and the instability properties of anode double layers

Abstract: Experimental results which prove a connection between the generation mechanism of anode double layers (anode glows or spots) and the variation of the excitation cross section of the gas with the electron velocity and also between the ionization phenomena and the anode double layer instability appearance are presented.

Power limits for dynamical pinch discharges

Abstract: Discharge initiation and sheath formation are serious problems of high current (several MA) dynamical pinches because of the enormous power input. The fast 300 kV plasma focus SPEED 2 allowed studying the discharge behaviour in a wide range of power input (10-100 GW) previously not accessible. The main results are: (i) high power input causes fast sheath formation ( 100 keV) photons with which the insulator surface is irradiated during discharge initiation and sheath formation or using insulators with coppered surface.

Collisional drift instability in a variable radial electric field

Abstract: The collisional drift instability, which is observed in a weakly ionized argon plasma, is described by a cylindrical two-fluid model which allows for arbitrary density profiles and a sheared E*B rotation of the plasma. The waves are destabilized by a current drawn from a DC biased grid (in the path of the plasma) to ground. Variation of the grid-to-ground current is shown to alter the radial potential profile and thus the E*B rotation of the plasma. A threshold current destabilizes the plasma. Above this threshold, mode frequency is found to increase with current. Predictions of the model demonstrate both rigid body and sheared E*B rotation are destabilizing and that both provide the well known Doppler shift to the real frequency. When three versions of the model are compared, the best agreement with observed real frequency occurs when the radial electric field is included rigorously in the theory. The documented increase in the radial potential gradient with grid-to-ground current is shown to be responsible for the corresponding increase frequency. Through a transition from a stable to an unstable regime of grid operation the observed increase in radial potential gradient is strongly suggested as the destabilizing mechanism at work. The radial wave shape is predicted closely by two versions of the model in which the radial electric field is or is not included, and both provide excellent agreement with experiment.

Confinement studies of neutral beam heated discharges in TFTR

Abstract: The TFTR tokamak has reached its original machine design specifications (Ip=2.5 MA and BT=5.2 T). Recently, the D degrees neutral beam heating power has been increased to 6.3 MW. By operating at low plasma current (Ip approximately=0.8 MA) and low density (ne approximately=1*1019 m-3), high ion temperatures (9+or-2 keV) and rotation speeds (7*105 m/s) have been achieved during injection. At the opposite extreme, pellet injection into high current plasmas has been used to increase the line-average density to 8*1019 m-3 and the central density to 1.6*1020 m-3. This wide range of operating conditions has enabled the authors to conduct scaling studies of the global energy confinement time in both ohmically and beam heated discharges as well as more detailed transport studies of the profile dependence.

Magnetic topology, disruptions and electron heat transport

Abstract: JET results are presented which show a 'profile consistency' of the electron temperature under a variety of plasma conditions. This experimental fact is interpreted as due to a topology of the magnetic field lines where ordered structures such as laminar surfaces and magnetic islands coexist with ergodic domains. The development of this concept identifies different plasma regions according to the value of the safety factor and relates the observed degradation in confinement time, when additional heating is applied, to the plasma entering a multiphase state where the electron temperature gradient is limited by a critical value defined by the global plasma parameters. A preliminary scaling law for the electron temperature is proposed which indicates a strong favourable dependence on major radius and plasma current. The near future JET experimental programme is briefly described and possible ways of improving the plasma confinement are presented.

Heat flux effects in Ohm's law

Abstract: The velocity dependence of the collision frequency nu in a plasma causes the appearance of the thermoelectric field and heat flux effects in Ohm's law. An artificial dependence on electron velocity nu of v varies as nu -2 allows an exact representation of the two terms valid for any isotropic lowest order distribution function. The thermoelectric coefficient and the electric conductivity are isotropic (i.e. independent of the Hall parameter) in this model, and there is a magnetomotive force, additional to the usual v*B/c term, equal to 2(qT*B)/5(pec) where qT is the total electron flux (measured in the ion rest frame) and pe is the electron pressure. Similarly the inertial contribution to Ohm's law depends on delta qT/ delta t. Comparison is made with the full Braginskii transport (as modified by Epperlein and Haines 1986), including electron-electron collisions, for a lowest order Maxwellian distribution. If Ohm's law is written in a more physical form to include a term in qe*B (where qe is the electron heat flux measured in the electron fluid rest frame), it leads to a good approximation also to an isotropic thermoelectric coefficient and electric conductivity. Some applications of this new form of Ohm's law are outlined, particularly the role of the qe*B term.

Investigation of plasma confinement on Soviet tokamaks

Abstract: The results of plasma confinement investigations in ohmically and auxiliary heated discharges are presented. The confinement is shown to depend dramatically on the power input profile. The local transport coefficients depend not only on the local plasma parameters but on the profiles of the parameters. The phenomena observed so far can be explained by an effect of at least two instabilities driven by the current and pressure profiles.

Electron Cyclotron Resonance Heating in the Wendelstein VII-A Stellarator

Abstract: Plasma build-up and heating of net-current-free plasmas in W VII-A was investigated by ECRH Experiments were performed at two ECR-frequencies (28 and 70 GHz) and different heating scenarios were investigated such as first harmonic ordinary mode heating and second harmonic extraordinary mode heating The basic effects predicted by theory, ie localized wave absorption and optical thickness of the plasma were verified The electron heat conduction was found to be governed by neoclassical losses in the plasma core for high enough temperatures, whereas enhanced losses have to be assumed in the outer plasma regions Generation of a target plasma with sufficient parameters to allow further heating by NBI was successfully demonstrated Configuration studies showed a beneficial influence of small shear on the confinement, where internal currents have to be taken into account

ICRF studies on JET

Abstract: Two antennae have been installed in JET and operated to the maximum design capability of the generators. 4.5 MW, 10 MJ have been coupled to the plasma which heated up to a maximum stored energy of 3 MJ with central temperatures of Te0=5 keV and Ti0=4 keV without increase of the relative impurity concentration. Degradation of energy confinement is observed according to an L mode scaling. The effect of k/sub //// shaping is discussed using a quadrupole antenna. Hydrogen and helium 3 minority heating regimes give similar results.

Confinement of stellarator plasmas with neutral beam and RF heating in W VII-A

Abstract: WENDELSTEIN VII-A has been operated for ten years. It is a low-shear, high-aspect-ratio device. The confinement properties have been thoroughly studied for both ohmically heated and net-current free plasmas. For the latter case, NBI- and ECF-maintained plasmas were of particular importance. It was found that under optimized conditions the core of high-pressure, net-current free plasmas is mainly governed by collisional effects.

Imaging Methods for the Observation of Plasma-Density Fluctuations

Abstract: Imaging techniques offer attractive alternatives to small-angle Thomson scattering for scale lengths of density fluctuations causing diffraction in the Raman-Nath regime. This is the case for fluctuations with wavelengths above about 3 mm for Tokamak sized plasmas, when a CO2 laser probe beam is used. Four methods ('phase scintillation', phase contrast, wavefront shearing and interferometry) are compared, on the basis of their transfer properties. They offer new means of studying density fluctuations of magnitude up to the dimensions of the plasma, such as those associated with turbulence, magnetic islands, convective cells or driven waves. The long wavelength limitations are discussed in detail and are related to those encountered for far-field techniques. An experimental comparison of 'phase scintillation' and phase contrast is given, and the criteria relative to the depth of field of imaging instruments in this context are considered.

Review of lower hybrid wave heating and current drive

Abstract: The author reviews the interaction of lower hybrid waves and plasmas which is a very versatile method. The method has proven to be effective in a large range of applications which the author discusses: (1) bulk ion heating; (2) bulk electron heating; and (3) noninductive current drive.

Enhancement of confinement in tokamaks

Abstract: A plausible interpretation of the experimental evidence is that energy confinement in tokamaks is governed by two separate considerations: (1) the need for resistive MHD kink-stability, which limits the permissible range of current profiles-and therefore normally also the range of temperature profiles; and (2) the presence of strongly anomalous microscopic energy transport near the plasma edge, which calibrates the amplitude of the global temperature profile, thus determining the energy confinement time tau E. The present analysis begins by identifying a hypothetical model of tokamak confinement that is designed to take into account the conflict between Te(r)-profile shapes arising from microscopic transport and J(r)-profile shapes required for gross stability. On the basis of this model, a number of hypothetical lines of advance are developed. Some TFTR experiments that may point the way to a particularly attractive type of tokamak reactor regime are discussed.

Ion cyclotron resonance heating on TEXTOR

Abstract: Ion cyclotron heating on TEXTOR has now reached the Megajoule level. The heating scenario is normally mode conversion but occasionally minority heating in a D-(H) plasma. With appropriate wall conditioning by carbonization more than 1 MW of RF power has been injected for long pulse durations ( approximately 1 s). The ICRF heated plasma is characterized by a quasi-stationarity of all plasma parameters, little if no impurity increase and a loop voltage reduction resulting in the total power coupled to the plasma reaching six times the remaining ohmic power input. Evidence of the coupling of the RF power to the plasma is obtained from the increase of the thermal load on the limiters and central energy deposition is supported from analysis of the sawtooth heating rate.

Latest results from JET

Abstract: JET tokamak performance has been progressively raised, culminating in operation at the full design level of 5 MA plasma current in a toroidal field at 3.4 T. the plasma current, position, shape and line-average electron density are controlled by feed-back systems. By glow discharge cleaning in H2/methane mixtures, the interior of the vessel has been coated with carbon. This reduces the fraction of power radiated

Confinement in ASDEX with neutral beam and RF heating

Abstract: Neutral beam (NI), ion cyclotron resonance (ICRH) and lower hybrid resonance (LHRH) heating on ASDEX are discussed with regard to their effect on plasma confinement. Comparison of NI and ICRH shows that the L and H-regimes are universal confinement modes of auxiliary-heated tokamak plasmas (i.e. independent of the heating method), and that the edge electron temperature (or a related parameter) dictates which mode prevails. In this connection it is noted that carbonization of the vessel walls impedes transition to the H-mode in the case of NI heating. Studies of energy confinement in the intermediate regime from ohmic to NI heating reveal a gradual transition from ohmic ( approximately ne) to neutral injection L-mode ( approximately I) scaling. At the same time a remarkable invariance of electron temperature profile shape with increasing heating power is observed. Changing the NI power deposition profiles from central to off-axis leaves gross energy confinement times unchanged while central confinement is substantially improved.

Properties, control and ICR-heating of the plasma in TEXTOR

Abstract: The TEXTOR research programme focuses on the systematic analysis of plasma wall interaction, the development of a suitable wall system and the production of quasi-stationary long-pulse high-temperature plasmas with tolerable impurity concentrations, with well-defined boundary layer and with relevant particle and power fluxes through the boundary. The authors describe the method of wall carbonization developed in Julich and some characteristic features of the TEXTOR plasma obtained with this method for both metal limiters and graphite limiters. Moreover, the effect of ICRH heating (2 MW, 1 sec) on plasma parameters and on the boundary layer is discussed, together with the application of the single head pump limiter ALT-I and of the localized set of magnetic perturbation coils for boundary 'ergodization'.

Theory of MHD waves

Abstract: Reviews recent developments in the theory of MHD waves in connection with radio-frequency heating in the Alfven wave and ion cyclotron ranges of frequencies (AWRF, ICRF). The account focuses on the discussion of full wave solutions and the oscillation spectra in bounded, generally inhomogeneous, plasmas. Original results are presented concerning forced ICRF oscillations in a current-carrying torus. The effects on the wave structures of the equilibrium current, the size of the device, the minority concentration and the phenomenological damping are investigated. The resonant surfaces coincide with the magnetic surfaces as in the AWRF. The poloidal extension of the resonance is small in cases where a WKB approach is permitted so that there is no conflict between full wave solutions and the WKB method.

The influence of self-generated magnetic fields on the Rayleigh-Taylor instability

Abstract: In the absence of magnetic diffusion the self-generated magnetic field in a plasma is proportional to the fluid vorticity. The ratio of magnetic to fluid energy then shows that self-generated magnetic fields can only affect the Rayleigh-Taylor growth rate for large k (wavelengths less than a few microns). Reduction of the plasma thermal conductivity on the other hand is more important for small k, i.e. long wavelength modes.

Review on pellet fuelling

Abstract: For several years now a growing number of experiments with pellet injection have been conducted on hot plasmas. So far different influences on the plasma performance have been reported. Strong peaking of the density profile is usually also seen in cases where the pellet does not penetrate to the centre. In ohmic discharges with sufficient edge recycling it was observed that the averaged density increases and in parallel the particle and energy confinement are substantially improved. With growing neutral injection power, this improvement degrades, while the central particle and energy confinement remain enhanced. The author presents a review of pellet injection work which only briefly summarizes the technical development and status of injectors. Ablation theory and comparison with experiments are also reported in condensed form. The author concentrates more on contributions to plasma confinement and reviews the most essential highlights of plasma experiments.

Low voltage start-up in the cleo tokamak using ecrh

Abstract: Studies of ECRH-assisted start-up in the CLEO Tokamak have resulted in the production of well-controlled discharges with Ip approximately 13 kA, ne approximately 5*1018 m-3 in the presence of a loop voltage Vl<2 V throughout. The average loop voltage during the start-up phase is approximately 1.1 V, corresponding to an electric field E approximately 0.19 V m-1, and results in an average rate of current rise of dIp/dt approximately 0.44 MA s-1. Calculated over the whole of the current rise there is a approximately 50% reduction in volt-second consumption when compared with the best case in the absence of RF power. It is estimated that during the plasma current rise approximately 30-50% of the electromagnetic energy input from the poloidal field system is converted to stored magnetic energy.

A review of ECRH experiments

Abstract: The absorption of electron cyclotron waves in the electron component of the plasma in both tokamaks and stellarators has led to most interesting results on energy confinement and instability control. The ability to heat the electrons in a relatively localised region has enabled the electron temperature profile and thereby the current profile to be modified allowing the stabilisation of the m=1 and m=2 modes in the tokamak. Thermal transport has also been studied by this means with results now available on changes in tau E and on chi e using the power modulation technique. At low densities the formation of a high energy tail accompanies an increase in tau E. At high densities tau E decreases with power but for heating on axis the value of tau E lies above that of L mode scaling. In the stellarator diffusion due to collisions is adequate to explain thermal transport in the inner regions of low density ECRH discharges.

Magnetic field line flow as a Hamiltonian problem

Abstract: The stochastic stability of a plasma torus with nested magnetic surfaces of arbitrary cross-section is investigated. The equations for field lines are put into Hamiltonian form, exhibiting both gauge and canonical symmetry. A careful application of Chirikov's (1979) overlap criterion leads to a formula for critical resonant fluctuations which should be valid for the whole cross-section and for arbitrary mode numbers m, n.

Initial experiments in JT-60

Abstract: Initial ohmic heating experiments in JT-60 were performed for a three month period of April-June 1985. A maximum plasma current of 1.6 MA was obtained for both divertor and limiter discharges. Low-q discharges of qeff=2.5 and high density discharges of 4.8*1019 m-3 in line-averaged density were obtained in the divertor configuration. In divertor discharges radiated loss from the main plasma can be kept at 20-30% of the ohmic input.

Surface waves in a two-ion-species magnetized plasma

Abstract: The spectrum of waves in a two-ion-species plasma at frequencies up to the ion-ion hybrid resonance frequency is investigated. A cylindrical plasma column of uniform density and magnetic field, and surrounded by a vacuum, is firstly analysed, and the results are checked when plasma and magnetic field nonuniformities are allowed for. A new m=-1 mode is found to exist at frequencies just above the ion-ion hybrid resonance frequency. This mode has much in common with a previously reported m=-1 mode above the Alfven resonance frequency in that it can have surface wave properties and its dispersion curve merges into the ion-ion hybrid resonance at a critical value of the axial wavenumber. The mode survives as an ion-ion resonance damped wave in a nonuniform density plasma or magnitude field. The m=+1 wave dispersion relation is found to pass through the ion-ion hybrid resonance via a mode crossing, with connection to a hybrid global eigenmode below the hybrid resonance, and connection to a surface wave above the hybrid resonance.

A Review of Alfven-Wave Heating

Abstract: Reviews the experimental status and recent advances in Alfven wave heating (AWH). The authors discuss the underlying physics of Alfven wave heating, as seen by the experiments. The use of Alfven waves for plasma diagnosis is discussed. The results of plasma heating on TCA are summarized. Apparent changes in the current profile are inferred, caused by changes in the evolving Alfven wave spectrum excited. The implications for AWH on larger devices are mentioned.

Some effects of MHD activity on impurity transport in the PBX Tokamak

Abstract: The effects of MHD activity on intrinsic impurity transport are studied in ohmic discharges of the Princeton Beta Experiment (PBX) by measuring the Zeff profile from visible bremsstrahlung radiation and spectral line intensities from ultraviolet spectroscopy. A diffusive/convective transport model, including an internal disruption model, is used to simulate the data. The Zeff profile with no MHD activity is fitted with a strong inward convection, characterized by a peaking parameter cv(=-a2 nu /2rD)=11 (-3.5, +4.5). At the onset of MHD activity (a large m=1 n=1 oscillation is followed by sawteeth), this strongly peaked profile is flattened and subsequently reaches a new quasi-equilibrium shape. This new profile is characterized by reduced convection (cv=3.6 (-1.1, +1.6), D=1.4 (-0.7, +5.6)*104 cm2 s-1) in addition to the particle redistribution which accompanies the sawtooth internal disruptions, indicating that the transport processes themselves change with the onset of sawtooth activity.

High power ICRF heating on the divertor tokamak ASDEX

Abstract: First ICRF experiments on ASDEX have been performed at 67 MHz, corresponding to 2 Omega CH-heating of a hydrogen plasma at B0=2.2 T. Despite divertor operation ICRH is accompanied by a significant increase of impurity production which can drastically be reduced by means of wall carbonisation. RF power up to 2.3 MW is routinely coupled to the plasma for pulse lengths of up to 1 sec. The RF heating is found to depend strongly on plasma preheating. In combination with neutral beam injection the ICRF heating efficiency is even higher than the one of NI. Confinement degrades with ICRH to values in between NI-L-type and OH confinement.