Table of contents

The q-profile has been determined inside the q = 1 surface of ion cyclotron resonance frequency (ICRF) heated plasmas from the observed toroidicity induced Alfvén eigenmodes (TAE) that appeared before giant sawtooth crashes. The q-profile deduced from the TAE spectrum is in good agreement with the one obtained from motional Stark effect measurements.

It is found that the Mercier criterion can be violated in a reversed shear plasma of a tokamak. A reversed shear plasma has negative magnetic shear and negative pressure gradient in the inner region of a plasma. In the negative shear region, stabilizing terms due to the parallel current and the magnetic well produced by the poloidal current change to destabilizing ones. As increases, the destabilizing effects increase and the Mercier criterion can be violated. Here, and are the safety factor on the plasma axis and the minimum value along the minor radius, respectively. In JT-60U reversed shear plasmas, the value of becomes large. The violation of the Mercier criterion seems to be consistent with the observation of magnetohydrodynamic activity localized near the internal transport barrier.

A review is made of the crash phenomena in toroidal plasmas. The emphasis is placed on the physics that causes the crash of global plasma parameters. Recent progress in the measurement has provided a unified view of various crash phenomena, i.e. the sudden occurrence of the crash, the sensitivity (or probabilistic dependence) of the occurrence of the crash on the global parameters, and the abrupt excitation of a symmetry-breaking perturbation (magnetic trigger). Essential observations that describe the physics of collapse are surveyed. The theoretical study of the nonlinear plasma dynamics is overviewed. Theories of the onset and explosive growth, which are based on magnetic braiding, are discussed. As an example, a picture based on a turbulence-turbulence transition is explained. A picture based on hysteresis and bifurcation, not on the linear instability criterion, emerging from advanced measurements and recent progress, describes the basic physics of the collapse.

The properties of classically confined high-energy alpha particles near the outer midplane of a tokamak torus are considered in order to provide the basic information required for theoretical studies of the superthermal ion cyclotron emission which has been observed in tokamak experiments. An analytical expression for the velocity distribution function of fusion reaction products has been derived, which generalizes previous results. The alpha-particle distribution function for test case data corresponding to typical parameters of the JET preliminary tritium experiment has been calculated using a three-dimensional Fokker-Planck code and it has been shown that this function can be well approximated by the derived analytical expression. The number of alpha particles produced in a pure deuterium plasma due to secondary fusion reactions has been evaluated. It is shown that, due to orbital effects, the local alpha-particle density in a deuterium plasma can be comparable to the density of protons which are produced directly by the DD reactions.

It is believed that radial anomalous transport in tokamaks is caused by plasma turbulence. Using infra-red laser scattering techniques on the Tore Supra tokamak, statistical properties of the density fluctuations are studied as a function of the scales in ohmic as well as additional heating regimes using the lower hybrid or the ion cyclotron frequencies. The probability distributions are compared to a Gaussian in order to estimate the role of intermittency which is found to be negligible. The temporal behaviour of the three-dimensional spectrum is thoroughly discussed; its multifractal character is reflected in the singularity spectrum. The auto-correlation coefficient is shown to reflect a short-time coherence of the dissipative structures as well as their long-time incoherence and statistical independence. We also put forward the existence of fluctuations transfer between two distinct but close wavenumbers. A rather clearer image is thus obtained about the way energy is transferred through the turbulent scales.

At the advanced stellarator W7-AS, active charge exchange recombination spectroscopy (CXRS) is used to measure the toroidal and poloidal impurity rotation velocity. The radial electric field is determined experimentally from the radial force balance equation. In parallel, is calculated by means of the neoclassical drift kinetic equation solver (DKES) transport code. Neoclassical calculation and measurement are, in general, in good agreement. A selection of discharges is presented in this paper which shows typical properties of at W7-AS, and which highlights the mutual relationship between and transport.

We propose a new kind of transit-time interaction in which the reversal (or any localized transition) in the phase velocity of a wave within a wavelength of an antenna results in a high rate of work done by the wave near the antenna for conditions where the wave phase velocity is greater than a few times the thermal speed. This enhanced rate of work near the antenna can significantly exceed the far-field value due to Landau damping. For the conditions of typical low-field (< 0.01 T) and low-density helicon wave-driven plasma sources, where the phase velocity parallel to the magnetic field can be a few times the thermal speed of electrons, it has been demonstrated that this spatial transient overshoot in the rate of work done by the wave is the dominant kinetic energy coupling process to electrons). In this paper it is demonstrated that, within a half wavelength of the antenna, it is the high-energy electrons that gain energy from both the wave and low velocity electrons as a result of this process. An important practical consequence is that the ionization rate of neutral gas can be significantly enhanced above the Maxwellian rate. The phenomenon is not restricted to helicon sources. This process may also explain the production of high-energy electrons in the near fields of antennas used in fusion plasma heating by radiofrequency waves.

Experimental results concerning the plasma response to the biasing of the tokamak ISTTOK localized limiters, on a strong flat-top plasma current reference discharge, are reported. Modifications of central beta as well as of energy confinement time are determined through time-resolved measurements of the line-averaged plasma density, electron density profile, electron temperature and ohmic power. Gross particle confinement variations are confirmed by the associated changes of the ratio between the line-averaged electron density and the radiation level. Plasma stability modifications are analysed by measurements of the plasma column transverse displacement, plasma poloidal rotation frequency and sliding fast Fourier transform spectra of both the magnetic and the electron density fluctuations. The evolution of the amplitude as well as the frequency of the most important tearing modes is determined. Negative bias leads to better particle and energy confinement, and improved stability. Positive bias reduces both confinement and stability, causing a significant transitory vertical displacement of the plasma column as well as of its current axis.

In this paper, the toroidal and poloidal rotation velocities measured in the reversed field pinch device RFX from the Doppler shift of impurity ion lines are presented and their relation with the plasma velocity is discussed. It is found that the toroidal velocity at the edge has the opposite direction with respect to the core toroidal velocity, indicating a shear of the plasma rotation velocity of about .

Starting from the rotation measurements and the momentum equation, a one-dimensional impurity diffusion model for the core plasma and a Monte Carlo code for the edge region are used to estimate the radial electric field, that is found to be around outward directed in the plasma core, and about inward directed at the edge, consistent with that measured at the edge by Langmuir probes.

The electron heat diffusivity has been measured in the sawtoothing core of the TEXT-U tokamak plasma during electron cyclotron heating (ECH). ECH creates a non-steady-state electron temperature gradient on a time scale faster than a single sawtooth ramp, whose relaxation is measured with two ECE systems. The measured diffusivity is considerably smaller than that measured in the steady state during the ohmic phase of the discharge outside the sawtooth inversion radius. In these conditions, high-resolution ECE has also shown the presence of small scale, non-symmetric structures characterized by very large temperature gradients. These spatially localized, temporally cyclical structures, previously described as plasma filamentation, have been identified as plasma magnetic islands. If these small-scale gradients are taken into account then the reported measurements imply an electron heat diffusivity which approaches the neoclassical prediction.

Low-pressure silane plasmas (2-20 Pa) diluted with the noble gases helium and argon as well as hydrogen were generated by microwave excitation in order to determine plasma parameters and absolute particle number densities. Specific silane radicals (SiH, Si, , H) were measured by means of optical emission spectroscopy, whereas particle densities of silane, disilane and molecular hydrogen were measured with mass spectroscopy. Experimental results confirm model calculations, which were carried out to determine number densities of all silane radicals and of higher silanes as well as electron temperature. The electron temperature varies from 1.5 to 4 eV depending on pressure and gas mixture. The temperature of heavy particles is 450 K and the electron number density is . The rotational temperatures of SiH are between room temperature and 2000 K due to increasing dissociative excitation. In the plasma the number density of silane is reduced, whereas the number density of molecular hydrogen is close to the silane density, which is fed in. Particle densities of , disilane and atomic hydrogen are in the range of a few per cent of the silane number density. At low pressure the density is similar to and decreases with increasing pressure due to heavy particle collisions with silane producing higher silanes. Particle densities of SiH and Si are only in the range of some of the silane density decreasing with increasing collisions of heavy particles with silane and molecular hydrogen. In mixtures with argon Penning reactions increase the silane dissociation.

Cascades of high-n tearing modes with mode bursts in order of increasing n number have recently been observed in ASDEX Upgrade in the vicinity of the q = 1 surface. The relationship between the poloidal and toroidal mode numbers is m = n + 1. A cascade commences with a low n number, typically , and can reach values around 22. They are observed in discharges with impurity accumulation and with a transient evolution of the q profile, where the magnetic shear is low and the resistivity high. Calculations with the CASTOR code have shown that high-n tearing modes become unstable if the magnetic shear is low, resistivity is high and the pressure gradient high enough to make mode coupling important. All three conditions are needed to obtain a positive and large growth rate. These CASTOR code calculations are consistent with the experimental observations.

The edge pedestal widths in ion and electron temperature profiles for H-mode discharges have been measured during an ELM-free phase over a wide range of plasma current of 1-4.5 MA in JT-60U. The pedestal width of the ion temperature indicates a dependence on both inverse poloidal field and edge ion temperature. As a result, it is shown that the pedestal width linearly scales like , where is the inverse aspect ratio and is the poloidal gyroradius for thermal ions. The pedestal width is also compared with theoretical predictions.

A method has been developed to calculate the dielectric tensor in a toroidal plasma from a distribution function represented numerically by a discrete set of orbit invariants. Assuming a locally homogeneous plasma, parasitic -particle absorption of the fast magnetosonic wave is studied for different orbit widths in JET-like scenarios. In addition, the beneficial effect observed in JET of introducing a small concentration in second-harmonic tritium heating scenarios is studied.

The influence of laser light components specularly reflected by the inner wall of an x-ray confining cavity was experimentally investigated by means of time-resolved x-ray imaging and shock wave measurements. The experiments were made for two different cavity shapes (circular and decagonal cylinders) to investigate the geometrical effect of the reflected light. The results were compared with calculations involving a simple illumination code. Time-resolved x-ray images for a gold-coated surrogate sphere at the cavity centre were replicated by the calculations, showing that the reflected light shines strongly on the sphere at the beginning of the laser pulse. It was also found that, for the experimental conditions adopted in this study, about one third of the drive pressure of a shock wave propagating through a planar sample placed at the cavity centre is due to the reflected light. The estimated intensity of the reflected light is high enough to impose noticeable imprinting onto a fusion capsule set in the vacuum cavity conventionally used for indirect-drive implosions.

Edge transport and confinement changes induced by radial electric fields, externally imposed in TEXTOR-94 by means of electrode biasing, were investigated. The edge profiles of the electric field were measured continuously by a specially developed nine-tip Langmuir probe, thus allowing the study of the relative spatial and temporal dynamics of and its radial gradient and possible ensuing transport changes. A particle transport barrier is found to be built up as the electric field gradient increases, thus strengthening the conjecture that stabilization is a viable mechanism for improved confinement in tokamaks.

In order to understand the recycling and emission processes of deuterium atoms, spectral profiles of the line emitted from the divertor region of JT-60U have been observed with a high-resolution spectrometer and analysed by simulation with a three-dimensional neutral particle transport code. The profile has been explained as composed of narrow and broad components; the narrow component is attributed to dissociative excitation and electron collisional excitation of the atoms produced by dissociation, and the broad component is attributed to electron collisional excitation of the atoms produced by reflection and charge exchange. In low-density plasmas, the simulated line profile agrees reasonably well with that observed, although the component attributed to the atoms reflected at the divertor tiles is overestimated by a factor of about two. Dissociative excitation from deuterium molecules and molecular ions plays an important role for the line intensity. The ratio of the line intensity to the deuterium atom flux for high-energy deuterium atoms, which are produced by the reflection and charge exchange, is reduced, because the fast atoms readily escape from the divertor plasma. The width of the narrow component in a low-density case corresponds to a temperature of deuterium atoms of 1.3 eV, and that in a high-density case corresponds to a temperature of 2.2 eV.

A novel type of stellarator device, the helical post stellarator (HPS), is introduced. Calculations are given for a particular HPS configuration, the single-helix stellarator (SHS) featuring a single-helix centre post. This configuration is significantly different from all previously known stellarators due to the combination of its unique geometrical characteristics and physical properties. Among these are the facts that the helical magnetic field has only one toroidal period (N = 1), the plasma has an extremely low aspect ratio, , and the variation of the magnetic field, B, along field lines features a helical ripple entirely on the inside of the torus. Among the main advantages of an HPS for the fusion program are an extremely compact, modular, and simple design compatible with significant rotational transform, large plasma volume, and improved particle transport characteristics.

In this paper we report the effect of both edge localized modes (ELMs) and drifts on divertor plasma parameters in COMPASS-D. Profile changes during ELMs are measured with high spatial resolution, and power and particle losses per ELM are estimated. The time evolution of the divertor parameters is shown to be consistent with the ELM being associated with a fast collapse of the transport barrier.

Asymmetries between inboard and outboard divertor targets in L-mode plasmas are shown to depend on the plasma parameters, in particular the direction of the toroidal magnetic field, the average plasma density and the input power. In general, the outer divertor region receives more power than the inner one, the asymmetry increasing for ion drift away from the X-point, contrary to observation in other devices. The drift together with scrape-off layer (SOL) currents are suggested as possible mechanisms causing the asymmetry.

Heating during the early part of the current rise phase gives a low or negative magnetic shear (= (dq/dr)) in the centre of JET plasmas. Under these conditions the confinement improves with high additional heating power heating during the current ramp-up phase of the discharge. The reduction in the transport manifests itself as a peaking of the profiles with a large gradient region near = 0.55. The best discharges have no transport barrier at the edge of the plasma (L-mode). This allows central power deposition by the neutral beams in JET. A control of the plasma pressure, using feedback of the additional heating power in real-time, minimizes the impact of magnetohydrodynamic instabilities. As a result, these discharges achieve the highest D-D neutron rates in JET; , with , and .

Non-thermal electron cyclotron emission (ECE) spectra are measured by comparison of low-field side and high-field side observed ECE. The asymmetry in the respective spectra can be ascribed to the existence of non-thermal populations. Optical-depth effects are directly determined from electron cyclotron absorption measurements. Measurements are compared with theoretical ECE spectra, obtained on the basis of calculations with a quasi-linear bounce-averaged Fokker-Planck simulation code, including a simple model for radial diffusion. This yields information on the non-thermal population responsible for the measured non-thermal spectra. The good agreement between experiment and simulation corroborates the quasi-linear bounce-averaged Fokker-Planck model.

Dust can play an important role in fusion devices. Various mechanisms for dust production are discussed including the possible growth of particulates during the fusion plasma discharge itself. Samples of dust from TEXTOR-94 are analysed by scanning electron microscopy. The particle size ranges from millimetres down to < 100 nm. The morphology of the smallest particles suggests their plasma-induced growth. Part of the dust is ferromagnetic. The impact of dust particles on safety, plasma operations and performance is addressed.

29 June - 3 July 1998 1998 International Congress on Plasma Physics (Joint Conference of the 12th Kiev International Conference on Plasma Theory and 12th International Congress on Waves and Instabilities in Plasmas) and 25th Conference on Controlled Fusion and Plasma Physics Prague, Czech Republic Contact: ICPP 98, Institute of Plasma Physics, Za Slovankou 3, PO Box 17, 182 00 Prague 8, Czech Republic. Tel +420-2-688-3028, Fax +420-2-858-6389, E-mail plasma98@ipp.cas.cz

30 June - 1 July International Conference on Fusion and Industry Oxfordshire, UK Contact: Ms Sue Frye, Thomas Telford Conferences, One Great George Street, London SW1P 3AA, UK. Tel +44-171-665-2315, Fax +44-171-233-1743.

6 - 17 July 35th Culham Plasma Physics Summer School Abingdon, UK Contact: Geoff Maddison, UKAEA Fusion, Culham Science Centre, Abingdon, Oxon OX14 3DB, UK. Tel +44-1235-463-672, Fax +44-1235-464-192, E-mail geoff.maddison@ukaea.org.uk, Website http://www.fusion.org.uk/info/sschool98.htm

27 - 31 July International Conference on Open Magnetics Systems for Plasma Confinement Novosibirsk, Russian Federation Contact: Dr Peter Bagryansky, OS98, Budker Institute of Nuclear Physics, Prospect Lavent'eva 11, 630090 Novosibirsk, Russian Federation. Tel +7-3832-359-772, Fax +7-3832-352-163, E-mail os98@inp.nsk.su, Website http://www.inp.snk.su/OS98/

28 - 31 July Chapman Conference on Magnetic Helicity in Space and Laboratory Plasmas Boulder, CO, USA Contact: AGU Meetings Department, 2000 Florida Avenue NW, Washington, DC 20009, USA. Tel +1-202-462-6900 x215, Fax +1-202-328-0566, E-mail meetinginfo@kosmos.agu.org

7 - 11 September 20th International Symposium on Fusion Technology (SOFT) Marseille, France Contact: Mme V Poli - 20th SOFT, DRFC, CEA/Cadarache, F13108 Saint-Paul-lez-Durance Cedex, France. Tel +33-4-42-25-62-60, Fax +33-4-42-25-62-55.

18 - 24 October 17th International Conference on Fusion Energy Yokohama, Japan Contact: Conference Service Section, IAEA, PO Box 100, A-1400 Vienna, Austria. Tel +43-1-2060-21310, Fax +43-1-20607, E-mail official.mail@iaea.org