M.Z. Tokar

Identification and analysis of transport domains in the stochastic boundary of TEXTOR-DED for different mode spectra

At the TEXTOR tokamak an external resonant magnetic perturbation is applied with the dynamic ergodic divertor to control the edge transport properties. The approaches to analyse the impact of such a kind of edge stochastization on transport apply mostly a shell-like picture which includes a dependence of transport from magnetic field topology in the radial direction only. In this paper multiple experimental evidence is presented in contrast to these approaches the perturbation applied forms a poloidally heterogenous edge layer in which the transport characteristics are determined by the poloidally alternating field line behaviour. A thorough analysis of density and temperature profiles and their gradients for base mode spectra with poloidal/toroidal mode numbers of m/n = 12/4 and m/n = 6/2 is worked out in comparison with the modelled magnetic field topology and results from three dimensional transport modelling with EMC3/EIRENE. Hereby two poloidally adjacent transport domains are identified for the first time in such detail. A domain representing a helical scrape off layer is formed by field lines with short connection and therefore prevailing parallel transport to the wall elements. Here, the field lines are clustered into extended flux tubes embedded into a long connection length ergodic domain with diffusive transport characteristics and enhanced radial transport.

Plasma edge physics with siliconization in TEXTOR

Abstract By using silicon as a material for wall coatings significant improvements in the tokamak performance have been obtained. These include the lowest oxygen level ever achieved in TEXTOR, an improved density limit and an enhanced energy confinement also at high central electron densities. The present paper gives a survey on the plasma properties under siliconized wall conditions with special emphasis on plasma edge physics. The impurity fluxes released from the limiter have been determined as a function various parameters. An outstanding property of silicon as an impurity in the plasma is its strong influence on plasma edge parameters caused by line radiation. A high radiation level of up to 70% of the input power (1.5 MW) has been achieved, limited by the mechanisms which are responsible for the release of silicon. To describe and understand these processes at the plasma edge Od- and 1d-models are used to simulate the highly nonlinear system. The properties are compared to plasmas in a boronized machine. Silicon and neon are compared with respect to radiation characteristics and transport to the centre. Remarkable differences in the penetration depths and central densities are reported.

Evidence of hot spot formation on carbon limiters due to thermal electron emission

Carbon test limiters have been exposed in TEXTOR to high heat loads up to about 30 MW/m2. The evolutions of the surface temperature distribution and of the carbon release have been observed by means of local diagnostics. A sudden acceleration of the rise of the surface temperature has been found at a critical temperature of approximately= 2400 degrees C. The increase of the rate of the temperature rise is consistent with an enhancement of the power loading by a factor of 2.5-3. Following the temperature jump (hot spot), a quasi-equilibrium temperature establishes at approximately= 2700 degrees C. The development of the hot spot is explained by an increase of the local power loading due to the breakdown of the sheath potential by thermal emission of electrons from the carbon surface. Simultaneously with the appearance of the hot spot, the carbon release from the surface increases sharply. This increase can be explained by normal thermal sublimation. Sublimation cooling contributes to the establishment of the quasi-equilibrium temperature at about 2700 degrees C

Overview of radiative improved mode results on TEXTOR-94

The radiative improved (RI) mode is a tokamak regime offering many attractive reactor features. In the article, the RI mode of TEXTOR-94 is shown to follow the same scaling as the linear ohmic confinement regime and is thus identified as one of the most fundamental tokamak operational regimes. The current understanding derived from experiments and modelling of the conditions necessary for sustaining the mode is reviewed, as are the mechanisms leading to L-RI mode transition. The article discusses the compatibility of high impurity seeding with the low central power density of a burning reactor, as well as RI mode properties at and beyond the Greenwald density.

Localized recycling as a trigger of MARFE

An analytical model taking into account the plasma cooling due to localized hydrogen recycling is proposed to interpret the conditions of the MARFE onset above a critical plasma density in TEXTOR-94. Results of numerical modeling confirm that under conditions of a good plasma contact with the inner wall this mechanism of the MARFE triggering is more important than the usually considered cooling instability on impurity radiation.

Rotation and radial electric field in the plasma edge with resonant magnetic perturbation at TEXTOR

In this paper the results of a systematic experimental assessment of the plasma edge rotation and radial electric field with application of resonant magnetic perturbation (RMP) are presented. The results are based on the radially resolved measurement of the poloidal (vpol) and toroidal (vtor) rotation. It is shown that the radial electric field Er can be deduced from the radial force balance when small amplitude resonant magnetic perturbations are applied to the plasma boundary (Br/Btor ~ 10?4).Both vpol and vtor spin-up in the ion-diamagnetic-drift and co-current direction, respectively, with increasing external perturbation field (?vpol ~ 15?km?s?1, ?vtor ~ 2?5?km?s?1) yielding an increase in Er by ?Er,max = 9?kV?m?1. The toroidal rotation increases over the whole radius while the poloidal rotation shows distinct local features driving the evolution of the Er-profiles. Depending on the edge safety factor a local (at the q = 5/2 rational surface) increase in the shear rate ?E?B (??q=5/2 = 1.4 ? 105?s?1) or reduced shearing can occur. Increased shearing is correlated with an improved particle confinement with an increase in the particle confinement time by ??p = +40%. Increasing the local resonant amplitude by 30% induces a reduced density level, the so-called RMP induced pump-out. At this confinement stage the shear rate decreases by 15% correlated with a significant drop in particle confinement (??p = ?30%).Field line tracing in the vacuum approximation gives indications towards explaining the threshold behaviour connecting the shearing rate, confinement stages and magnetic topology to the amount of applied RMP. However, this basic approach does not account for plasma response and the results presented are linked in the discussion section to recent results on the link between rotation and plasma response as well as on the transport features of RMP.

Improved kinetic test particle model for impurity transport in tokamaks

The expression for the Coulomb collision term in Fokker-Planck form is revised and generalized to the case of interaction of trace impurity ions with a fluid-like background plasma. The velocity distribution of the background ions is assumed as a Maxwellian in the 13 moment expansion, with parameters determined from a fluid consideration. The impurities are described in the framework of a linear drift kinetic test particle model. The new Coulomb term is suitable for linear Monte Carlo procedures and provides both a correct treatment of thermalization of the test particles and an accurate kinetic description of velocity distribution effects on the thermal and frictional forces. These features are missed out in existing codes for kinetic modelling of impurity transport, based on a reduced Coulomb term with these forces treated in the fluid approximation. Numerical results presented for TEXTOR specific edge plasma conditions emphasize the importance of the proposed more accurate treatment, in particular by consideration of the influence of thermal forces on low ionized impurities.

The possible nature of the localized recycling effect on the plasma edge in Tokamaks

Inhomogeneities in Tokamak peripheral plasmas due to localized recycling of neutrals (plasma fuelling) are considered. A dense cloud of cold charged particles arises in the vicinity of such a localized particle source, for example a gas-puffing valve, as the plasma density exceeds a certain critical level. A dependence of the critical density on such parameters as dimensions of the device, heat flux from the discharge core and diffusivity of charged particles at the edge is found. The results of calculation are compared with experimental data.

Nature of high-Z impurity accumulation in tokamaks

The recent experiments on accumulative behaviour of heavy impurities in TEXTOR with test limiters of molybdenum and tungsten and puffing of xenon are briefly reviewed. The results of the reconstruction of the transport coefficients of high-2 ions in the accumulation stage are presented. They confirm the neoclassical nature of the convective particle transport, leading to peaking of the impurity density. A mechanism triggering accumulation, invoking the temperature dependence of neoclassical flows of impurities, is discussed and the threshold of accumulation obtained is compared with experimental data. Processes which can lead to a saturation of the accumulation, caused by a modification of the flux component proportional to the temperature gradient, are analysed. The results of numerical modelling for experiments in TEXTOR with different durations and intensities of xenon puffing are presented. The role of high-2 ions under reactor conditions is analysed, and it is shown that in a reactor, such as ITER, the discussed heavy impurity driven instabilities should be suppressed.

Transport studies of high-Z elements in neon edge radiation cooled discharges in TEXTOR-94

High-Z materials as tungsten are intended to be used in future fusion reactors due to their low sputtering rates and high melting points. In this context the important question is whether the use of high-Z materials is compatible with the concept of a cold radiative boundary. To investigate the local release and transport behaviour of the high-Z impurities, Mo and W test limiters were used in auxiliary heated discharges under different radiation scenarios with neon seeding. In addition, laser blow-off of tungsten as well as xenon gas puffing were performed. In some particular discharge conditions impurity accumulation was observed in the plasma core, which in the case of ohmic discharges led to minor disruptions. The connection between the accumulation and the evolution of the current density profiles is discussed. A transport analysis is made, in order to compare the observations with the neoclassical theory. It is shown that with neon the impurity - impurity driven fluxes enhance the high-Z concentration in the plasma core. However, if the source of the high-Z elements is significantly reduced, by increasing the plasma density, a development of an accumulation instability can be avoided.