Table of contents

Using the spatial coherency present in a reflected microwave signal (Conway et al 1994 Rev. Sci. Instrum. 65 2920) it is possible to measure a coherent, , and an incoherent, , reflection coefficient (proportional to the radar cross section) from a turbulent plasma cutoff layer. Results acquired with a 17 GHz reflectometer from the STOR-M tokamak edge region give significant and , which suggests two-dimensional structure in the reflection layer. Using a `distorted-mirror' model for the plasma fluctuations, estimates of an effective radial width, , and poloidal correlation length, , can be derived from the reflection coefficients. STOR-M results typically give a of a few millimetres and an of a couple of centimetres.

An experimental investigation into rotating MHD modes has been performed in the TEXTOR tokamak. The effects on the stability of the MHD tearing modes of coupling between m/n = 2/1 and 1/1 modes and of the slowing down of the mode rotation by wall friction have been studied. Tangential neutral beam injection (NBI) has been used to change the toroidal rotation to observe the influence of plasma rotation on the development of MHD modes.

Two phases in the rotation frequency behaviour of the modes can be distinguished: a slow reversible decrease and a fast exponential decay, which starts with a short transient increase in frequency. The last phase finally results in mode locking, which mostly leads to a disruption.

By injecting low power NBI in the last phase of the slowing-down process, mode locking could be prevented. The mode amplitude and rotation frequency have been stabilized, without major changes to the pressure profile. By decreasing the central plasma velocity, using NBI, a coupling of the two modes has been forced. The coupling of the modes had a destabilizing effect on the MHD activity.

A new maximum entropy approach has been applied to analyse three types of transient transport experiments. For sawtooth propagation experiments in the ASDEX Upgrade and ECRH power modulation and power-switching experiments in the Wendelstein 7-AS Stellarator, either the time evolution of the temperature perturbation or the phase and amplitude of the modulated temperature perturbation are used as non-linear constraints to the profile to be fitted. Simultaneously, the constraints given by the equilibrium temperature profile for steady-state power balance are fitted. In the maximum entropy formulation, the flattest profile consistent with the constraints is found. It was found that determined from sawtooth propagation was greater than the power balance value by a factor of five in the ASDEX Upgrade. From power modulation experiments, employing the measurements of four modulation frequencies simultaneously, the power deposition profile as well as the profile could be determined. A comparison of the predictions of a time-independent model and a power-dependent model is made. The power-switching experiments show that the profile must change within a millisecond to a new value consistent with the power balance value at the new input power. Neither power deposition broadening due to suprathermal electrons nor temperature or temperature gradient dependences of can explain this observation.

The application of tomographic techniques for the study of vector fields has been demonstrated only in the last few years. We show its potential for remote sensing of plasma vector fields such as the fluid velocity v and the magnetic field B using Doppler and Zeeman spectroscopy. Simulations suggest the possibility of time-resolved reconstruction of ion velocity flow fields and poloidal magnetic fields from direct measurements of the low-order spectral moments of line-integrated spectroscopic measurements. Other possible applications of vector tomography are examined.

We report observations of a relativistic plasma wave excited by two copropagating laser beams with frequency difference close to the plasma frequency in a fully ionized hydrogen plasma. The laser beams were at (Nd:YAG) and (Nd:YLF) of 200 ps duration and focused to a diffraction limit spot with peak irradiance of . The plasma wave was detected by monitoring the forward-scattered spectrum of a third copropagating laser beam at . A novel use of Moire deflectrometry ensured the pump beams and probe beam were copropagating at focus. The inferred plasma wave density modulation is about 1 - 2% and corresponds to a longitudinal electric field of about . The plasma wave was observed to exist for a time ps. These results are not in agreement with theoretical predictions for relativistic detuning which give , and suggest that there is some mechanism limiting the growth of the plasma wave. Simulations indicate that this is due to either ponderomotive blowoff taking the plasma off resonant density or to the modulational instability.

A theoretical model of giant ELMs (type-I ELMs) is developed. The theory of the self-sustained turbulence of the current-diffusive ballooning mode (CDBM) is developed in the presence of a steep pressure gradient and a radial electric field shear. Multifold states for the L-mode, H-mode and the third state with magnetic braiding are obtained. Transition to the state with magnetic braiding is found to occur if the pressure gradient becomes high enough. At this critical point, nonlinear excitation of the magnetic perturbation takes place, the growth time of which is of the order of the poloidal Alfven time. This event can cause catastrophic enhancement of the transport coefficient by a factor of almost 10. The avalanche of the transport catastrophe is also analysed, showing a very rapid radial propagation velocity. The magnetic braiding terminates if the pressure gradient becomes small, leading to a back transition to the H- (L-) mode. Under a constant power supply these processes can repeat themselves, causing periodic bursts. The period becomes shorter as the average power flux increases.

The ignition access and ignited operation regime are studied using a POPCON analysis with the H-mode power threshold scalings for a D - T experimental tokamak reactor, such as the International Thermonuclear Experimental Reactor (ITER). The H-mode power threshold scaling provides two H-mode regimes in the low-density and high-density regions. The L-mode regime exists between them. As the two H-mode regimes are connected at a plasma temperature around 30 keV, the operation path must make a detour around this connecting area to reach ignition within a reasonable heating power. Although the heating power to reach ignition depends on the coefficient in the H-mode power threshold scaling and plasma parameters, the heating power of 50 MW, as originally proposed by Rebut et al, is marginal and not sufficient to reach ignition for the ITER tokamak even if the L - H transition can be first achieved in the initially low-density regime of less than . Auxiliary heating power of 100 MW may satisfy the various requirements for D - T ignition experiments in the ITER tokamak. The thermal equilibrium operating point with fusion power of 1.5 GW in the ignition regime can exist only when the H-mode power threshold has a hysteresis for the ASDEX Upgrade threshold scaling.

The Rayleigh - Taylor instability is proposed as a mechanism to explain the occurrence of `oscillating striations', the regular fluctuations in the emitted light intensity from a pellet ablation cloud. The instability is driven by the rotation of the ablation column about its symmetry axis parallel to the B field. The radial E field comes from a radially varying floating potential, arising as a result of attenuation of the field-aligned hot plasma electrons streaming through the inhomogeneous ablation cloud density. Based on a model ellipsoidal density profile, the angular rotation speed was found to be nearly uniform. A radial eigenmode, eigenvalue equation was solved for the m = 1 shift mode, which predicts frequencies and threshold instability conditions consistent with experimental observations. Implications for fast confined alpha diagnostics using pellet injection are discussed briefly.

A theoretical investigation of infinitely periodic quasi-optical grill structures for excitation of lower-hybrid waves in toroidal plasmas has been undertaken for cylinder grill elements with an arbitrary cross section. The basis for investigation is the expansion of the scattered RF field into the infinite series of spatial harmonics which makes it possible to obtain the integral equation for the distribution of magnetic current over the surface of grill elements and to calculate the efficiency of LH wave excitation depending upon the LH wave slowing-down and parameters of the grill structure and of the plasma layer. The numerical simulation performed for the single-layer grill, with elements of elliptical cross section irradiated from vacuum by infinite plane waves, demonstrates the possibility of achieving a high efficiency of up to 25 - 30% of the unidirectional LH wave excitation with the slowing-down , which is relevant to large fusion installations. The calculations have also been performed for the grill structure irradiated by quasi-optical RF beams with finite cross section; it is shown that the coupling efficiency degrades insignificantly if the RF beam aperture (which may be of the order of the size of the RF port in the direction of the toroidal magnetic field) essentially exceeds the vacuum wavelength of radiation. The results obtained seem to be encouraging for construction of a double-layer grill with high efficiency close to unity and low Q-value of the quasi-optical cavity, resulting in a comparatively low value of RF field intensity near the grill elements and in a weak dependence of grill efficiency upon plasma parameters.

The turbulent heat transport in high-temperature fusion plasmas is not understood. It increases with heating power. The physical mechanism of this process is investigated in the W7-AS stellarator. The fundamental question addressed here is whether transport is governed by local plasma parameters or by a global quantity. Three different types of experiments with critical sensitivity are carried out. They can consistently be described on the basis of a non-local dependence of the transport coefficient on the global heating power.

The sequence of equilibria of two-dimensional reduced magnetohydrodynamics has been studied as a function of the tearing mode stability parameter . After a symmetry-breaking bifurcation occurring at , which originates a state with a small magnetic island, the system undergoes a second bifurcation, of tangent type, at . Above this value, no stationary solutions with small islands exist. The system rapidly develops an island of macroscopic size. This general property is proposed as a basic ingredient of the intermittent events observed in magnetically confined plasmas.

The paper discusses the difference between kinetic and hydrodynamic descriptions of nonlinear wave scattering and transformation in plasmas with external magnetic fields. In particular, it gives a detailed study of electromagnetic wave scattering by fluctuations in a magnetoactive plasma. For the limiting case of cold plasmas, the results of the kinetic description are shown to reproduce the expressions obtained in the fluid approach. The applicability conditions of the fluid description are analysed. The method of moments is applied to solve the kinetic equation and is thus shown to be efficient in the study of nonlinearity and thermal dispersion of fluctuations responsible for the wave scattering. The quantitative description of thermal effects can be provided only in terms of the kinetic approach.

The cold fluid (CF) and the more general low-temperature kinetic (LTK) models of bilinear wave interaction and collective Thomson scattering have been the subject of exhaustive investigations. These have revealed that the CF model is inappropriate for its intended use: the modelling of scattering from thermal fluctuations in the limit where the plasma may be treated as cold for the incident and scattered waves, but not cold for the fluctuations. Assuming the plasma to be cold for the fluctuations would be inconsistent with their thermal nature. In response it has been argued that the CF model is in accord with the LTK model when scattering is from cold collective oscillations. This is true, but irrelevant.

20 - 24 May 1996 12th International Conference on Plasma Surface Interactions in Controlled Fusion Devices Saint Raphael, France Contact: PSI 96 c/o Ms V Poli, Association EURATOM-CEA sur la Fusion, DRFC/SPPF, Centre d'Etudes de Cadarache, F-13108 Saint-Paul-lez-Durance Cedex, France. Tel +33-42-25-62-60, Fax +33-42-25-62-55, E-mail psi96@drfc.cad.cea.fr.

3 - 5 June 1996 7th International Conference on Plasma Science Boston, MA, USA Contact: M Silevitch, ECE Department, Northeastern University, Boston, MA 02115, USA.

24 - 28 June 1996 23rd European Physical Society Conference on Controlled Fusion and Plasma Physics Kiev, Ukraine Contact: Dr Ihor Goutych, Scientific Secretary, 23rd EPS CFPP, Bogolyubov Institute for Theoretical Physics, 252143 Kiev, Ukraine. Tel +38-044-2665362, Fax +38-044-2665998, E-mail cfpp@gluk.apc.org.

8 - 19 July 1996 33rd Culham Plasma Physics Summer School (UKAEA/Euratom) Oxford, UK Contact: Mrs J Bright, UKAEA Fusion, Culham, Abingdon, Oxfordshire, OX14 3DB, UK. Tel +44-1235-463773, Fax +44-1235-463474, E-mail julie.bright@ukaea.org.uk.

8 - 18 August 1996 International Summer School on Plasma Physics and Technology La Jolla, CA, USA Contact: Mr V Stefan, Institute for Advanced Physics Studies, PO Box 2964, La Jolla, CA 92038, USA. Tel +1-619-456-5737.

26 - 30 August 1996 Joint Varenna - Lausanne International Workshop on Theory of Fusion Plasmas Villa Monastero, Varenna, Italy Contact: Centro di Cultura Villa Monastero, 1 Piazza Venini, 22050 Varenna (Lecco), Italy. Tel +39-341-831261, Fax +39-341-831281. Application and abstract deadline: 15 June 1996.

2 - 5 September 1996 EU - US Workshop on Transport in Fusion Plasmas Villa Monastero, Varenna, Italy Further information: G Gorini, ISPP, 16 Via Celoria, I-20133 Milano, Italy. Tel +39-2-2392637, Fax +39-2-2392205, E-mail ggorini@mi.infn.it. Administrative contact: Centro di Cultura Villa Monastero, 1 Piazza Venini, 22050 Varenna (Lecco), Italy. Tel +39-341-831261, Fax +39-341-831281. Application and abstract deadline: 15 June 1996.

9 - 13 September 1996 International Conference on Plasma Physics Nagoya, Japan Contact: Conference Secretariat, c/o Prof. Hiromu Momota, National Institute for Fusion Science, Nagoya 464-01, Japan. Tel +81-52-789-4260, Fax +81-52-789-1037, E-mail icpp96@nifs.ac.jp. Abstract deadline: 31 March 1996.

16 - 20 September 1996 19th Symposium on Fusion Technology Lisbon, Portugal Contact: Professor Carlos Varandas, Centro de Fusão Nuclear, 1096 Lisboa Codex, Portugal. Fax +351-1-8417819, E-mail cvarandas@cfn.ist.utl.pt. General information will be available via WWW with URL http://www.cfn.ist.utl.pt.

25 - 29 September 1996 Summer University of Plasma Physics Garching, Germany Contact: Ms Ch Stahlberg, Max-Planck-Institut für PlasmaPhysik, Boltzmannstr 2, D-85748 Garching, Germany. Tel +49-89-3299-2232, Fax +49-89-3299-1001.

2 - 4 October 1996 6th European Conference on Fusion Theory Utrecht, The Netherlands Contact: T J Schep, FOM - Institute for Plasma Physics, Postbus 1207, NL-3430 BE Nieuwegein, Netherlands. Tel +31-3402-312-24.

6 - 11 April 1997 10th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating Ameland, The Netherlands Contact: J Hamers-Smit, FOM - Instituut voor Plasmafysica `Rijnhuizen', Postbus 1207, 3430 BE Nieuwegein. Tel +31-30-6096999, Fax +31-30-6031204, E-mail ec10@rijnh.nl. Application and abstract deadline: 17 January 1997.

8 - 12 September 1997 12th International Conference on Gas Discharges and their Applications Greifswald, Germany Contact: Dr G Babucke, Inst. f. Niedertemperatur-Plasmaphysik, Robert-Blum-Str. 8 - 10, 17489 Greifswald, Germany. Tel +49-3834-554411, Fax +49-3834-554301, E-mail gd97@public.inp.uni-greifswald.de.