Table of contents

Results are presented of transport studies conducted on trace impurities injected with the laser blow-off technique in a variety of JET L mode pulses. In the core of the discharge, the transport is much slower, the impurity diffusion coefficient D is more than an order of magnitude below the values it assumes further out, but still above the neoclassical predictions. The extent of the slow transport core region varies with the magnetic field and with the total plasma current and is broadly correlated with the edge value of the safety factor. Closer analysis reveals that the current profile is essential in determining the radial dependence of D. This parameter appears to undergo a rapid transition to highly anomalous levels in the vicinity of the radial position where the dimensionless shear parameter is equal to 0.5. Within that region D stays moderate even when the electron temperature gradient is high. A marked increase of D in the outer region of the discharge is observed when the power per particle is raised or, alternatively, when the temperature and its gradient grow in that region, but no clear dependence of D on plasma density is found when the electron temperature profile is kept constant. Transport modelling based on the critical Del T_c assumption leads to D profiles that are similar, although not in detailed quantitative agreement, to the experimental ones when the temperature profiles are flat in the centre; when the temperature profiles are peaked in the centre, even the radial dependence of the predicted diffusion profiles is very different from the one observed. Recent theoretical attempts to analyse the radial structure of the microturbulent fluctuations predict a strong positive dependence of anomalous diffusion on the magnetic shear as observed in our experiments

Divertor impurity radiation is considered as one possibility of reducing the target heat load in ITER-grade devices. The radiative power fraction in the divertor is, however, limited in principle and, under standard high recycling conditions, it is relatively small. In this paper we discuss the possible increase of the maximum radiative fraction by a 'neutral cushion' that provides a pressure drop in the divertor due to ion-neutral interaction. For ITER EDA parameters and the present ITER divertor design a pressure drop by a factor of about ten would allow the radiative fraction to increase sufficiently to satisfy constraints on ITER target loads ( approximately=5 MW/m²). Experimental and theoretical evidence has been given in the literature for gas targets with the required properties. Supersonic flow conditions at the target are also included in the discussion, but are found to be insufficient, by themselves, to provide the required effect

Without a knowledge of the normalized viscosity, a Mach probe may be just a directional probe or a flow meter with large uncertainties. To solve this problem, a Visco-Mach probe has been developed to measure the normalized viscosity ( alpha ) and flow Mach number (M_infinity ) simultaneously. By generating a free pre-sheath and a bounded pre-sheath by a small Mach probe (SMP) within a free pre-sheath generated by a large Mach probe (LMP), one can measure two ratios of current densities: one R_s by the SMP, and the other R_l by the LMP. Then simultaneous deduction of alpha and M_infinity is made by an iteration scheme from R_s and R_l. The applicability of this new probe to the reciprocating probe of JET is discussed

The bootstrap current in a tokamak is examined by implementing the Hirshman-Sigmar model and comparing the predicted current profiles with those from two popular approximations. The dependences of the bootstrap current profile on the plasma properties are illustrated. The implications for steady state tokamaks are presented through two constraints; the pressure profile must be peaked and β_p must be kept below a critical value

Theoretical investigations on the use of an advanced launcher to couple lower hybrid waves to a plasma, for current drive in tokamaks, have been developed. A study of the coupling has been carried out in a rigorous way, through the solution of the scattering by an array of cylinders with parallel axes in the presence of a plane of discontinuity for the electromagnetic constants. The general features of the proposed method are presented together with results on the launched spectra and coupled power. The numerical results of this work are relevant to the optimization of the single layer quasi-optical grill launching RF power towards a constant density plasma: it is shown that the power coupled in the plasma can reach values of about 25-30% in the -1 diffracted order and about 40% in all other orders. As a consequence it is expected that the coupling values for a suitable double layer configuration should approach those for total transmission

A simulation of an anticipated TFTR deuterium-tritium supershot is described. The simulation is based on a reproducible, high performance, long duration D-only supershot. The TRANSP plasma analysis code is used to model fast ion (D, T and alpha) parameters, including their distributions in energy and pitch angle

Plasma heating and potential formation by ion cyclotron range of frequencies (ICRF) waves are investigated in the HIEI tandem mirror. It is demonstrated that ions and electrons are selectively heated by the appropriate waves that have been mode converted from the fast magnetosonic wave in a two-ion-species plasma. In the central cell, the minority ions can be heated up to 0.3 keV with the presence of the mode-converted slow ion cyclotron wave. Plug cell electrons are heated by the mode-converted electrostatic slow wave resulting in the formation of an ion confining potential. The dependence of the ion confining potential on the plug electron temperature is in qualitative agreement with values predicted by the improved modified Boltzmann relation which includes velocity space diffusion of electrons due to the ICRF wave. The axial confinement of the core plasma is improved by the ICRF produced potential

The role of the neutral-beam fuelling profile in determining the stored energy and neutron emission in neutral-beam-heated discharges on TFTR is investigated. It is found that the neutral-beam fuelling profile is sensitive to both the magnitude and the shape of the electron density. The ion stored energy correlates well with the peaking of the neutral-beam fuelling profile, whereas the electron stored energy is insensitive to this parameter. The DD fusion neutron emission is also well correlated with the peaking of the neutral-beam fuelling profile. The derived empirical scalings of the ion stored energy and neutron yield include discharges from the L mode to the supershot regime

A new method is described for inducing energy-selective transport by 'sweeping' the frequency of applied low n magnetic perturbations. The mechanism, formally analogous to the 'rising buckets' concept in accelerator physics, can move particles with a selected velocity in a non-diffusive fashion from one specified radius to another. The technique is considered principally as a means for removal of helium ash. Other likely applications are as a method for burn control, profile control, as a diagnostic and perhaps as a nonstochastic means of effecting the direct coupling of alpha power recently discussed by Fisch and Rax (1992)

Report on the IAEA Technical Committee Meeting held at Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey, United States of America, 25-27 January 1994.