Table of contents

Review and topical invited lectures of the 23rd European Physical Society Conference on Controlled Fusion and Plasma Physics

The 23rd EPS Conference on Controlled Fusion and Plasma Physics was organized on behalf of the European Physical Society by the National Academy of Sciences of Ukraine, State Committee on Science, Technologies, and Industrial Policy of Ukraine, and the Bogolyubov Institute for Theoretical Physics (BITP) in Kiev, Ukraine, June 24 - 28 1996. The 1996 conference included the topics: tokamaks; stellarators; alternative magnetic confinement; plasma edge physics; plasma heating and current drive; diagnostics; basic collisionless plasma physics; high-intensity laser-produced plasmas and inertial confinement; astrophysical and geophysical plasmas; low-temperature plasmas.

The conference was attended by 345 delegates from 22 countries. The program included five invited review lectures, 21 invited topical lectures, and 371 contributed papers (24 of which were selected for oral presentation).

The conference program has been the responsibility of the international program committee:

D Gresillon (Chairman) (Ecole Polytechnique, CNRS, Palaiseau, France) G Fussmann (MPI fur Plasmaphysik, Bereich Berlin, Germany) X Garbet (CEA, Cadarache, France) C Hidalgo (CIEMAT Fusion, Madrid, Spain) L Kovrizhnykh (General Physics Institute, Moscow, Russia) S Prager (University of Wisconsin, Madison, USA) A Sitenko (Chairman of the local organizing committee) (BITP, Kiev, Ukraine) F Sluijter (Chairman of the EPS Plasma Physics Division) (Eindhoven University, The Netherlands) M Tendler (Royal Institute of Technology, Stockholm, Sweden) P Thomas (JET Joint Undertaking, Abingdon, UK) H Weisen (CRPP-EPFL, Lausanne, Switzerland) J Winter (KFA, Juelich, Germany)

The four-page texts of the contributed papers will be published as three volumes of Europhysics Conference Abstract series (volume 20C) and will be distributed to all registered participants by the local organizing committee. The review and topical lectures (except one review lecture that will be published in a regular issue of PPCF later and one topical lecture that has been withdrawn by the author) are published in this special issue which will also be sent to the conference delegates.

The papers were refereed and edited by the members of the program committee, and are published from final versions presented by the authors. The Guest Editors are grateful to the Publishers for their cooperation.

D Gresillon, A G Sitenko and A Zagorodny Guest Editors

The fast magnetosonic wave, launched in the ion cyclotron range of frequency (ICRF) is one of the main heating schemes in tokamak plasmas. Great flexibility of the heating and current drive scenarios is possible, playing both on the various wave - particle interaction mechanisms and on the mode conversions that the fast wave can experience. When properly designed, an ICRF system can be employed for ion or electron heating and current drive, with the asset of localized deposition profiles. Almost all the existing tokamaks are now using ICRF heating and current drive, and an ICRF system is being studied for ITER.

Parametric phenomena in plasma which occur due to varying electric fields with the ion cyclotron frequency are reviewed. Beam-like lower hybrid instability emerges in strong pumping fields provided that the transverse relative velocity of particles is larger than the ion thermal speed . The resulting turbulence and the following numerous manifestations observed experimentally are addressed. The turbulence may prove important for experiments aimed at plasma production or radio frequency (RF) cleaning of metallic surfaces of vacuum chambers in stellarators, tokamaks and helicon devices. In contrast, for a weak field the kinetic parametric instabilities of ion cyclotron oscillations arise due to electrons. The issues of the turbulence, mathematical modelling, its role in turbulent heating observed on the torsatron Uragan-3M, decay instabilities associated with ion cyclotron oscillations and the triggering of ion quasimodes are considered.

A survey of theoretical and experimental results on plasmas with strong Coulomb interactions is given. First the basic theoretical concepts describing nonideality, degeneracy and screening are introduced. The general structure of the thermodynamic functions and the phase diagram in the density - temperature plane is explained. Relevant experiments including shock waves and pulse-produced plasmas are briefly reviewed. Results on the critical data, thermodynamic and electric properties around the critical point and in supercritical states are discussed. The existence of high-pressure, high-temperature plasma phase transitions is investigated. After a brief discussion of transport properties the kinetics of ionization and reaction processes is studied. The influence of nonideality on ionization and nuclear fusion rates is investigated.

New results on collisionless beam - plasma dynamics in open systems with a non-equilibrium electron velocity distribution are reviewed and demonstrated via computer simulations based on the Vlasov - Poisson equations. It is shown that the known instability of the quasilinear theory (QLT) for sufficiently high wave amplitudes (in respect of the enhancement of wave - particle correlation and formation of space - time coherent structures) makes it invalid for describing plasma dynamics and turbulent transport in the majority of open plasma configurations. A typical example is the classical problem concerning a warm electron beam steadily injected into Maxwellian plasma half-space. Here, the real self-consistent relaxation picture is dramatically different from that predicted by QLT. The discovered fundamental deviations from QLT and their macroscopic consequences may be very important for a deep understanding of plasma transport processes, and are worthy of special laboratory testing. A short review of the present state of the problem and its background is given.

The theoretical basis of the dimensionless parameter scaling technique is derived and the limitations in its application are discussed. The use of the technique is illustrated by the production on JET of a steady-state ITER similarity pulse having the same and collisionality as the ignited ITER. The key issue of the scaling of the transport with the main dimensionless parameter is discussed in detail. Finally, possible shortcomings of the technique are examined.

This paper describes the major objectives of the LHD divertor experiment which is proposed to produce currentless-steady-state plasmas with high performance and without any current disruption. Since further improvement in confinement is a common and general requirement for fusion research including the LHD project, it is also necessary to develop the edge plasma control techniques and to understand the physical behaviour in the LHD divertor, i.e. the newly developed continuous helical divertor and a local island divertor (LID) concepts. In order to achieve these objectives, there were several key issues in physics and technology, which had to be resolved through careful investigation before the LHD experiment could start. In this paper, we summarize the recent progress of the physics understanding of divertor functions, divertor plasma operation scenarios, and properties of the LHD magnetic field structure in addition to the experimental planning. We also describe the recent result of an LID experiment in the CHS device, which demonstrated the possibility of edge particle and heat control by the LID.

A description of the GOL-3-II facility and the first experimental results obtained on this new device are presented. This facility was constructed as part of the second stage of the GOL-3 programme for studying the interaction of microsecond electron beams with a plasma and its fast heating.

The plasma column of the GOL-3-II facility is 12 m in length instead of 7 m for the first stage of GOL-3. The energy content of the capacitive storage for magnetic field supply has been increased from 10 MJ to 15 MJ. This allows us to have a magnetic field of up to 5 T in the homogeneous part of the solenoid and up to 10 T in its end mirrors. The expected key feature of the upgrading is an essential increase in the energy content of the electron beam injected into the plasma. The high-power electron beam generator U-2 is now used for the new beam - plasma interaction experiments with possible beam energy content of up to 0.3 MJ. It is shown experimentally that transportation of a 0.2 MJ electron beam through the 12 m plasma column is possible under conditions of almost full current neutralization. The collective beam - plasma interaction results in a plasma heating of up to 1 - 2 keV electron temperature at density.

Statistical and relaxation properties of dusty plasmas are studied numerically and analytically. The fluctuation electrodynamics of dusty plasmas is determined taking into account the dynamics of charging processes associated with plasma currents. The influence of grain charging on the effective interaction between dust particles is discussed. Grain attraction and dusty crystal formation are shown to be possible. The results of numerical simulation of dusty plasma relaxation caused by grain charging and plasma-layer expansion are presented in detail. Monte Carlo simulations are used to prove the existence of dust particle effective attraction within a model with constant grain charge.

Experimental data, analysis and simulations are used to describe the physics of divertor detachment and He exhaust under detached conditions in TdeV. With increasing density, the plasma is found to detach progressively from the outboard divertor plates with a marked reduction of the ion flux to the plates, the generation of a pressure gradient between an ionization front and the target plate, and strong cross-field transport in the divertor. Local interactions between the divertor plasma and the plates are described, with evidence for carbon sputtering and molecular processes near the divertor plates. Divertor exhaust and retention continue to increase through detachment and He exhaust is not affected although the divertor He enrichment remains low but constant at about 0.2. A moderate density of seems to be sufficient both for efficient peak power load reduction at the divertor plate and good He exhaust through the divertor. Simulations of the edge and divertor plasmas using the B2/EIRENE and DIVIMP codes give reasonable agreement with the measurements and indicate possible divertor geometry improvements

An effective electron and ion heating by LH waves was obtained in the FT-2 tokamak . It was observed in a narrow density range close to the current drive cut-off density. A transition from the parametric to the classical mechanism may take place during a single pulse. The electron temperature appears to be the main `hidden' parameter determining the heating.

An interpretation of the collisionality dependence of , which has been observed in many tokamaks, is presented in terms of neoclassical tearing mode theory. Magnetic islands are predicted to be driven to large saturated widths by the perturbed bootstrap current, provided their width exceeds a certain threshold value. This width is of the order of the ion banana width for low collisionality plasmas, but is substantially larger in collisional plasmas (though still within the banana regime); below this threshold value the stabilizing ion polarization current causes an island to decay. Assuming that `background' MHD events such as ELMs or sawteeth can provide an initial `seed' magnetic island of width (through toroidal coupling, for example) one can develop a model equation which retains the essential properties of neoclassical tearing mode theory and is able to reproduce many of the experimental trends associated with the reduced -limit.

Tungsten-coated tiles, manufactured by plasma spray on graphite, were mounted in the divertor of the ASDEX Upgrade tokamak and cover almost 90% of the surface facing the plasma in the strike zone. Over 600 plasma discharges have been performed to date, around 300 of which were auxiliary heated with heating powers up to 10 MW. The production of tungsten in the divertor was monitored by a W I line at 400.8 nm. In the plasma centre an array of spectral lines at 5 nm emitted by ionization states around W XXX was measured. From the intensity of these lines the W content was derived. Under normal discharge conditions W-concentrations around or even lower were found. The influence on the main plasma parameters was found to be negligible. The maximum concentrations observed decrease with increasing heating power. In several low power discharges accumulation of tungsten occurred and the temperature profile was flattened. The concentrations of the intrinsic impurities carbon and oxygen were comparable to the discharges with the graphite divertor. Furthermore, the density and the limits remained unchanged and no negative influence on the energy confinement or on the H-mode threshold was found. Discharges with neon radiative cooling showed the same behaviour as in the graphite divertor case.

New improved confinement regimes with reversed shear and high triangularity configurations have been exploited in JT-60U by optimizing the current profile and using shape control.

The reversed shear regime was established with the formation of an internal transport barrier (ITB) and enhanced confinement for both ions and electrons. The stored energy was 8.2 MJ at where an NB power of 15 MW was applied. The energy confinement time and were 0.71 s and 0.41, respectively. The radial position of the ITB was as large as . The ITB manifested in both temperature and density profiles was sustained in the radiative divertor condition. In addition, the reversed magnetic shear configuration has been sustained with LHCD for 7.5 s, which is encouraging for future steady-state operation.

The effect of increased triangularity has also been intensively investigated, particularly regarding the improvement of confinement properties for the H-mode plasmas. The edge density and temperature at the onset of ELMs were improved by factor of 1.3 - 1.4 for and 1.5 - 2.0 for . In a high triangularity discharge, a fully non-inductive current drive at 1 MA has been demonstrated for about 2 s in combination with a high bootstrap current and beam current drive fraction with , and H = 2 - 2.5.

A negative-ion-based neutral beam was successfully injected into plasmas for the first time in March 1996. The initial injection power was approximately 100 kW with an energy of 180 keV and beam current of 3.2 A.

Wendelstein 7-AS is the first large-scale modular stellarator, partially optimized with respect to reduced equilibrium currents . It therefore plays an important role for the optimized next step stellarator W7-X as a pilot experiment for manufacturing techniques of coils and vessel and for equilibrium and neo-classical transport properties of the Wendelstein stellarator line of IPP. Experimental results from W7-AS of importance for the continuation and optimization of the advanced stellarators are summarized: with respect to magnetic optimization we address improved equilibrium, stability and neo-classical transport and the feasibility of the modular coil system. Regarding operational enhancements we focus on demonstrated high-density operation and the soft edge density limit, improved confinement, verification of the 3D plasma edge topology, stability of island structures for exhaust properties and demonstrated stable divertor high recycling in the island divertor configuration.

The local electron energy flux produced by magnetic fluctuations has been measured directly in the edge plasma of the Madison symmetric torus (MST) reversed field pinch (RFP), continuous current tokamak (CCT), and the scrape-off layer of the TEXT-U tokamak. The flux produced by electrons travelling parallel to a fluctuating magnetic field is obtained from correlation between the fluctuations in the parallel heat flux and the radial magnetic field. The fluctuations in the parallel heat flux were measured with a fast insertable pyrobolometer. The measurements reveal fundamental differences in the nature of electron energy transport in the RFP and the tokamak. In the RFP the fluctuation-induced energy flux is large (, comparable to the total ohmic heating power) inside the reversal surface where the magnetic field is expected to be stochastic, and small in the edge. The magnetic fluctuation induced radial energy flux Q and radial particle flux (measured independently) are related by a `convective' formula . The electron heat transport is significantly lower than the value predicted by the Rechester - Rosenbluth transport model. This feature of the electron energy transport can be explained using self-consistent calculations that account for clumping of electrons streaming along the magnetic field. In the tokamak the magnetic fluctuations do not contribute to the total energy transport except in the vicinity of the q = 2 magnetic surface, where the transport is associated with large amplitude Mirnov oscillations.

Electron temperature fluctuations at a relative level of the order of 0.1% in the presence of strong inherent intensity fluctuations of thermal radiation can be measured applying correlation radiometry of electron cyclotron emission. The diagnostic problem and the technique are briefly reviewed. Examples are given which characterize the temperature fluctuations as measured in the core region of purely ECRH heated plasmas with respect to spectrum, relative level, and coherence length. Parameter dependences are discussed.

A review of the procedures which have been developed to measure suprathermal electron distribution functions is given. These experiments have been conducted on three tokamaks; Versator, Tore Supra and TdeV. The absorption of an electron cyclotron wave is measured at frequencies below the fundamental cyclotron frequency. Theory relates this absorption to the amplitude of the distribution function. Experimentally, refractive losses must be separated from absorption losses and contamination of the data by reflections within the system must be avoided. Illustrative results from each tokamak are presented.

Recent stationary improved confinement experiments with current density profile modifications in non-inductive Tore Supra operation are reported. Significant progress has been obtained by achieving long duration discharges using lower hybrid (LH) waves: (i) 2 min in the 4 T improved confinement LHEP (LH enhanced performance) regime, at a current MA, loop voltage V, LH power MW (, are the electron energy content and Rebut - Lallia - Watkins L-mode prediction), (ii) 75 s long in a fully non-inductive LHEP regime (, V, MA, MW) using a new plasma control scheme, (iii) 30 s at 1.7 MA ( V, MW) in the L-mode regime. MHD stability in full current drive operation and the role of weak - or reversed - central magnetic shear in the central LHEP electron temperature transition (reproduced in full current drive in a systematic manner and measured by independent electron temperature diagnostics) are discussed.

Stationary high- experiments were performed with fast-wave direct electron heating (FWEH) in a large range of operation: 42 - 76 MHz and - 3.9 T. Maximum coupled power of 9.5 MW is obtained in the 48 MHz/2T configuration with good electron heating performance and improved confinement at high density (central density ). By increasing the magnetic shear in the gradient region, stationary improved confinement states (6 MW during 5 s, ) are reached in a reproducible manner with poloidal beta, , approaching 1 and 40% of bootstrap current.

The high-bootstrap FWEH experiments have been recently combined with LHCD at reduced loop-voltage ( V and up to 70% of non-inductive current). A total injected power up to 7.2 MW (LH: 3.6 MW, FW: 3.6 MW) has produced stationary improved confinement with a peaked electron temperature profile (central value up to 8.0 keV) at a central density of .

Operating the JET Mark I divertor at high density and with high volume losses is shown to have many beneficial effects on divertor performance. On the other hand, combining high-density operation with high-confinement (H-mode) operation is difficult. In particular, the density at which the plasma naturally operates in ELMy H-mode discharges is difficult to vary. A scaling for how this natural density depends on toroidal field, plasma current, and input power suggests that high density may be achievable in reactors but requires a dedicated multi-machine database, in particular to verify size scalings. Experiments with and without active pumping in the divertor show that the confinement varies with the neutral pressure in the main chamber rather than the fuelling rate per se, making operation with a closed divertor attractive. Such a divertor, Mark IIA, has recently been installed at JET and is now operational. First results are presented demonstrating that the operating regimes relevant to JET's programmes have been re-established.

Turbulent transport simulation results from a large nonlinear three-dimensional (3D) toroidal electrostatic gyrokinetic particle-in-cell simulation model, including global equilibrium effects, are presented. The parallel implementation of the particle simulation model on massively parallel computers has allowed us to perform large-scale simulations of ion temperature gradient-driven turbulence and to include low-n and high-n toroidal mode numbers in a single calculation. Simulation results indicate a strong interaction between these short and long wavelength scales and that this is the possible origin of a Bohm-like ion thermal transport scaling. The inclusion of trapped electron dynamics and self-generated shear flows on the fluctuation dynamics is shown to produce quantitative differences in the ion thermal transport.

In a plasma, some of the energy of a high-power laser beam can be transferred to a longitudinal plasma wave with a high phase velocity. This wave can in turn accelerate relativistic charged particles to very high energies. Several mechanisms have been proposed to generate these intense electric fields and some of them have already been tested experimentally. Using the beat wave method, electric fields of 1 - 10 have been produced and electrons have been accelerated with an energy gain from 1 MeV to more than 30 MeV. Some preliminary experiments have shown that electrons can be accelerated in plasma waves generated by the wakefield method. In the case of self-modulated wakefield, electric fields larger than 100 trap electrons and eject them from the plasma with an energy up to 100 MeV. The perspectives in the near future are the production of intense and short electron beams of a few MeV and the acceleration of electrons up to 1 GeV. To reach an energy of 1 TeV and get closer to the parameters required by the high-energy physicists, one will have to test some new methods to be able to guide the laser beam over large distances.

The sign of the toroidal magnetic field has a major effect on the divertor in Alcator C-Mod, determining whether the higher recycling is on the inboard or outboard side and leading to inboard/outboard temperature and density differences of up to a factor of ten. A recently published paper (Hutchinson et al 1995 Plasma Phys. Control. Fusion 37 1389) reported in detail on these observations, which are an indication of the importance of plasma particle drifts for understanding and modelling the divertor. The conference presentation covered these published results and also some additional results, documented here, in which the plasma flows associated with the field-direction-dependent asymmetries are measured using a Mach probe and a localized impurity puffing plume analysis technique.

Silane gas has been injected through a hole in the surface of test limiters during ohmically heated tokamak discharges in TEXTOR-94. The gas injection led to the deposition of a thin layer around the injection hole which was close to the area of the highest heat load. The layer was eroded in discharges without silane injection and spectroscopic measurements prove that the erosion of limiter material is reduced in areas where the layer is deposited. Surface analyses reveal that the layer was formed not only by the injected silicon but also by a large amount of carbon which must originate from other plasma-facing components. The preferential direction (symmetry line) of the deposition pattern is tilted by relative to the magnetic field lines. Calculations with the Monte Carlo code ERO-TEXTOR were used to interpret the data. The shape of the Si-deposition pattern can be recovered quantitatively by including an drift in a radial electric field originating from the gradient in the electron temperature profile above the limiter surface. The deposition pattern allows the `imaging' of the complex plasma flow close to the surface. To obtain agreement between calculated and experimentally observed deposition efficiencies a sticking coefficient much smaller than 1 and the influence of an electric field parallel to B have to be considered.

Ionospheric investigations in Ukraine are being carried out with uniquely developed incoherent scatter radars. Data on the variations in the electron density and the electron and ion temperatures during periods of low (1994) and high (1989 - 1990) solar activity are presented. The F-layer peak height ranges from 450 at night to 240 km in the daytime; the electron density maximum varies approximately from over a cycle of solar activity throughout the winter period. The electron temperature has a maximum afternoon value as the solar activity is high in summer. In the periods of magnetic disturbances, a drop in is most frequently observed. In winter, the influence of the sunrise and sunset at the magnetic conjugate point is also observed. An increase in electron temperature, a drop in electron density, and the displacement of the heated region along the magnetic field lines are clearly seen in the ionosphere irradiated by high-power, HF radiation. Longitudinal effects in the ionospheric behaviour were observed in simultaneous measurements in Kharkiv and Millstone Hill (USA).