Table of contents

The first measurement of the energy distribution of the neutron emission from the nuclear burn-up of tritons produced at 1 MeV in reactions is reported. The results refer to deuterium plasmas with a strongly pulsed neutron production attained with neutral beam heating in the JET tokamak. The experimental results are compared with simple plasma model calculations in order to study the triton burn-up for strongly transient plasmas and to illustrate the diagnostic information that neutron spectroscopy can provide in addition to the previously conducted neutron intensity measurements.

In recent years, the method of incoherent Thomson scattering has been applied to make measurements of electron properties of glow discharges used for industrial applications. These plasmas have electron density of the order of and electron temperature of a few eV. This paper reviews the recent progress in this research area. Details of the experimental systems are given and examples of measurements made in several different types of glow discharges are presented. A method in which Thomson scattering is combined with Rayleigh scattering to provide direct measurements of the neutral density is also described and an example of measurements is given. The electron velocity and energy distribution functions are also an important property of these plasmas, and the extent to which Thomson scattering can measure the distribution function is discussed. Future trends for Thomson scattering in glow discharges are also discussed.

The structure of fluctuations and turbulent transport have been investigated in the plasma boundary region of the L2-M stellarator. Normalized fluctuation levels are in the range (3-20)% and fluctuations are dominated by frequencies below 300 kHz. In the edge plasma region located inside the last closed magnetic flux surface the radial coherence of fluctuations is due to high-frequency fluctuations (> 100 kHz). The poloidal coherence is dominated by low frequencies. Linear coupling of resistive interchange modes is considered a candidate to explain the existence of highly radially correlated fluctuations in the high-frequency range.

Plasmas with the highest deuterium fusion neutron rates yet achieved in JET have been produced by combining a hollow or flat central current profile with a high-confinement (H-mode) edge. In these discharges, lower hybrid current drive (LHCD) and ion cyclotron resonance heating (ICRH) preheating, applied early in the current ramp-up phase, `freezes in' a hollow or flat current density profile. When the combined neutral beam injection (NBI) and ICRH heating power is increased, a region of reduced transport and highly peaked profiles forms during the L-mode phase and persists into the later H-mode phase when the fusion reactivity reaches a maximum. Transport analysis shows the formation of a central region of good confinement (the internal transport barrier or ITB) which expands with radial velocity . The clearest signatures of this region are large gradients in the ion temperature and toroidal rotation profiles. Ion thermal diffusivities in the central region are of the order of the neoclassical value. The position and rate of expansion of the ITB radius correlates well with the calculated rational q = 2 surface. The confinement improvement can also be seen in electron density, and, to a lesser extent, in electron temperature. The ITB can persist in combination with the edge transport barrier characteristics of the H-mode.

We have investigated the topological modification of an equilibrium with nested magnetic flux surfaces computed with the free boundary variational moments equilibrium code (VMEC) subject to an applied magnetic field perturbation. Magnetic field lines are traced in the domain interior to the plasma-vacuum interface either in the coordinates of the VMEC or using canonical coordinates that allow a Hamiltonian description of the magnetic field lines. The perturbation models the effect of the magnetic flux leakage from the saturated iron core of the JET device. Island formation around resonant surfaces near the boundary is observed, but these islands do not overlap. The Chirikov parameters and the Liapunov exponents are specifically calculated to verify this.

The effect of a small fraction of hot ions with anisotropic temperature on spontaneous reconnection of the neutral current sheet magnetic field is studied using a hydrodynamical approach. The linear dispersion equations are obtained for symmetric and antisymmetric current sheet perturbations. Solutions of these equations near the instability boundaries are found in analytical form. The numerical solutions of full dispersion equations are also represented. In the model under discussion, the magnetic reconnection is fast. The typical reconnection time for the conditions in the solar outer chromosphere and corona is of the order of several Alfvén times, which is considerably less than the values given by the current fast reconnection models.

Ion acoustic type shock-like structures based on the extended steady-state theory in multi-species plasma with different ion masses are studied. Using theoretical treatments, three simple models using continuous electron distribution functions at the separatrix , have been considered. From numerical studies on the existence of shock-like structures, we find the lowest limits of propagation velocity and amplitude of shock-like structures as a function of mass ratio (or density). In particle simulations, it is established for the case of a current driven system. Simulations are performed with an axially bounded electrostatic particle-in-cell (PIC) code XPDP1, which is a workstation version of the one-dimensional electrostatic bounded plasma code PDW1 [35-37]. The effect of the massive ion is to decrease the propagating velocity of the shock-like structures and decrease its maximum (or net) amplitude. Also, by increasing the density of the massive ion component, both the velocity of propagation and the maximum (or net) amplitude of the shock-like structure decreases.

A crucial problem in the development of appropriate electrodynamic systems of feedback control of low-order nonlinear rotating resistive modes in tokamaks consists of isolating, in the measurement of the Mirnov magnetic signals, the contribution due to the actual plasma perturbation from that due to the reflection of the controlling field. The final engineering objective is heavily dependent on the physical insight into the process of plasma response to the time-dependent controlling fields with a helical pitch resonant to that of the closed lines of force of a magnetic surface with rational safety factor q. Accordingly, we develop from first principles the theory of the time-harmonic response of a plasma to single helicity perturbations in the frame of the reduced magnetohydrodynamic (MHD) resistive model. The external field perturbation is split into an incident and a reflected component and an appropriate reflection frequency response function is evaluated. The results for modes of different helicities are presented which can guide the construction of the compensation network of feeedback stabilization systems of the type in use on the JET tokamak.

A visible endoscope system and an infrared camera system have been used to study the flux of carbon from an inertially-cooled graphite limiter in Tore Supra. From the variation in the carbon flux with plasma parameters, new data have been obtained, describing the dependence of radiation enhanced sublimation (RES) and chemical sputtering on incident ion flux. Other characteristics of RES under plasma operation conditions have also been studied. The dependence of RES on incident deuterium particle flux density is found to be in reasonable agreement with the expected particle flux scaling over a range of particle fluxes varying by a factor of , extending the present scaling to higher flux density values. Chemical sputtering has been observed, but only in regions of the limiter with low incident deuterium fluxes. Values inferred for the chemical sputtering yield are similar to those measured with a temperature-controlled test limiter in Textor.

Ignition characteristics are studied using zero-dimensional equations and are compared for both the ITER89P and ITER93HP scalings with the H-mode power threshold in the International Thermonuclear Experimental Reactor-intermediate design (ITER-ID). With the help of the operation path method on the -T plane, the ignition behaviour for both scalings is compared. The comparison study illustrates that the confinement factor of 2.6-2.9 over the ITER89P scaling is necessary for obtaining the equivalent performance to the ITER93HP scaling with 85% degradation due to an ELMy discharge. If the confinement degradation effect of due to the radiative divertor operation is taken into account, the confinement factor should be increased by up to . This result implies that the `advanced' tokamak operation regime may be required even for MA nominal operation from the outset. Modifications of the plasma shape, the central solenoid and divertor would be necessary to increase the confinement time for reducing the ignition risk. The ignition behaviour for both scalings are entirely different, where the ITER93HP scaling is more favourable for the safer ignited operation. Thus, in this sense, ITER89P scaling is highly recommended for designing the ITER.

The propagation and absorption of ordinary waves in fully relativistic plasma are investigated. Several interesting effects introduced by the relativistic mass increase associated with an increase in the electron temperature are examined. In particular, it is found that the cutoff density of ordinary waves increases significantly as the electron temperature increases. The effects of anomalous wave dispersion, which appear near the fundamental electron cyclotron frequency and its second and third harmonic, are greatest for low temperatures and subsist up to intermediate temperatures ( keV). An efficient damping of ordinary waves is obtained in a wide temperature range. The harmonic structure of the absorption profiles and the effects of anomalous dispersion disappear almost simultaneously at high electron temperatures. The results obtained may have important implications for the wave absorption and in particular, for emission in relativistic plasmas.

The aim of this paper is to calculate, in a new manner, a general expression for the anomalous particle flux due to electromagnetic turbulence, including the parallel components of the magnetic field fluctuations. The calculus of the flux uses the anomalous distribution function defined as the difference between the total distribution function and the neoclassical one. Besides the well known low-beta result, we also obtain the transport flux induced by parallel (to the equilibrium magnetic fields) fluctuations. Comparing the different components of the anomalous radial particle flux we specify the conditions when the effect of parallel fluctuations can be important.

Measurement of electron density fluctuations in magnetically confined fusion plasmas by the beam emission spectroscopy (BES) method is discussed and an algorithm is given for the correct reconstruction of correlation functions of the electron density fluctuation from measured correlation functions of beam light fluctuations. The technique is generally applicable to all kinds of neutral beams, provided an appropriate model for beam light emission is available and the beam penetration time through the observation volume is shorter than the characteristic times of the plasma turbulence. Light from a 48 keV Lithium beam on W7-AS plasmas is analysed in detail, whereby electron density fluctuations in the scrape-off layer and in the edge plasma are analysed for a low-density electron cyclotron resonance heated (ECRH) heated discharge. Characteristic changes in the autocorrelation function are found at the last closed flux surface. Limitations of the technique are discussed.

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.nsk.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