Influence of the dynamic ergodic divertor on the density limit in TEXTOR

A significant influence of the dynamic ergodic divertor (DED) on the density limit in TEXTOR has been found. In Ohmic discharges, where without DED detachment normally arises at the density limit, a MARFE (multifaceted asymmetric radiation from the edge) develops when the DED is operated in a static regime. The threshold of the MARFE onset in the neutral beam heated plasmas is increased by applying 1 kHz ac DED at the high-field side. The theoretical predictions based on the parallel energy balance taking poloidal asymmetries into account agree well with the experimental observation.     

Limiter Biasing Experiments in TEXTOR-94

Particle collection by the ALT-II belt limiter in the TEXTOR-94 tokamak is exclusively limited to the parallel outflux because the scoop walls that are oriented parallel to the field lines obstruct the poloidal E×B mass flow. With normal B _tor direction (E×B towards the scoops), a threefold decrease of plenum pressure is measured during negative biasing, while with reversed B _tor (E×B away from the scoops), a 60% pressure increase is observed. This behaviour is exactly opposite to that observed in X-point divertors. A simple fluid model explains this apparent contradiction, and gives good quantitative agreement with measurements of the parallel Mach number in the SOL. The essential physics is governed by the Bohm-Chodura criterion.     

Change of the magnetic-field topology by an ergodic divertor and the effect on the plasma structure and transport

The magnetic-field perturbation produced by the dynamic ergodic divertor in TEXTOR changes the topology of the magnetic field in the plasma edge, creating an open chaotic system. The perturbation spectrum contains only a few dominant harmonics and therefore it can be described by an analytical model. The modeling is performed in the vacuum approximation without assuming a backreaction of the plasma and does not rely on any experimentally obtained parameters. It is shown that this vacuum approximation predicts in many details the experimentally observed plasma structure. Several experiments have been performed to prove that the plasma edge behavior is defined mostly by the magnetic topology of the perturbed volume. The change in the transport can be explained with the knowledge of only the magnetic structures; i.e., the ergodic pattern dominates the plasma properties.     

Forced magnetic reconnection and field penetration of an externally applied rotating helical magnetic field in the TEXTOR tokamak

The magnetic field penetration process into a magnetized plasma is of basic interest both for plasma physics and astrophysics. In this context special measurements on the field penetration and field amplification are performed by a Hall probe on the dynamic ergodic divertor (DED) on the TEXTOR tokamak and the data are interpreted by a two-fluid plasma model. It is observed that the growth of the forced magnetic reconnection by the rotating DED field is accompanied by a change of the plasma fluid rotation. The differential rotation frequency between the DED field and the plasma plays an important role in the process of the excitation of tearing modes. The momentum input from the rotating DED field to the plasma is interpreted by both a ponderomotive force at the rational surface and a radial electric field modified by an edge ergodization.     

Phase transitions in Sr 0.61 Ba 0.39 Nb 2 O 6 :Ce 3+ : II. Linear birefringence studies of spontaneous and precursor polarization

The linear birefringence (LB) of Sr _0.61-x Ba _0.39 Nb ₂ O ₆ :Ce ³⁺ x (SBN61:Ce) has been measured as a function of temperature within the range of . Large tails have been observed above the ferroelectric phase transition temperatures T _c = 350, 328, 320 and 291 K for the concentrations x = 0, 0.0066, 0.0113 and 0.0207, respectively. Within an Ornstein-Zernike analysis the critical exponents , and are determined. It suggests that pure SBN61 belongs to the 3D Ising universality class. Doping with Ce ³⁺ ions, which seem to act as random fields, enhances the relaxor properties. The critical exponents and of SBN61:Ce shift against those of the three-dimensional random-field Ising model. Received 1 October 1999     

Suppression of runaway electrons by resonant magnetic perturbations in TEXTOR disruptions

The generation of runaway electrons in the international fusion experiment ITER disruptions can lead to severe damage at plasma facing components. Massive gas injection might inhibit the generation process, but the amount of gas needed can affect, e.g., vacuum systems. Alternatively, magnetic perturbations can suppress runaway generation by increasing the loss rate. In TEXTOR disruptions runaway losses were enhanced by the application of resonant magnetic perturbations with toroidal mode number n=1 and n=2. The disruptions are initiated by fast injection of about 3x10{21} argon atoms, which leads to a reliable generation of runaway electrons. At sufficiently high perturbation levels a reduction of the runaway current, a shortening of the current plateau, and the suppression of high energetic runaways are observed. These findings indicate the suppression of the runaway avalanche during disruptions.     

Preparation and characterization of polydimethylsiloxanes with narrow molecular weight distribution

The preparation of polydimethylsiloxanes (PDMS) of narrow molecular weight distribution (MWD) by anionic polymerization of hexamethylcyclotrisiloxane (D₃) in an improved polymerization apparatus is described. Using (CH₃)₂Si(OLi)₂ as initiator and (CH₃)₃SiCl (TMCS) as terminating agent, polymers with only methyl groups were obtained with molecular weights ranging from 2 × 10³ to 2 × 10⁶. Kinetic investigations were performed only so far as necessary for controlling the polymerization under the chosen experimental conditions (solvent: n-hexane, solvating agent: hexamethylphosphortriamide (HMPT), polymerization temperature: 25°). The molecular weights of the polymers were determined by light-scattering and, after calibration, by viscometry and GPC. The non-uniformities of the samples with symmetrical MWD were estimated using the 4σ-method. The GPC apparatus had been calibrated with polystyrene and poly-α-methylstyrene samples of extremely small non-uniformity.     

Toroidal plasma rotation induced by the dynamic ergodic divertor in the TEXTOR tokamak

The first results of the Dynamic Ergodic Divertor in TEXTOR, when operating in the m/n=3/1 mode configuration, are presented. The deeply penetrating external magnetic field perturbation of this configuration increases the toroidal plasma rotation. Staying below the excitation threshold for the m/n=2/1 tearing mode, this toroidal rotation is always in the direction of the plasma current, even if the toroidal projection of the rotating magnetic field perturbation is in the opposite direction. The observed toroidal rotation direction is consistent with a radial electric field, generated by an enhanced electron transport in the ergodic layers near the resonances of the perturbation. This is an effect different from theoretical predictions, which assume a direct coupling between rotating perturbation and plasma to be the dominant effect of momentum transfer.     

Size-exclusion chromatography of technical lignins in dimethyl sulfoxide/water and dimethylacetamide

Well defined spent sulfite liquor samples and lignosulfonate fractions obtained by ultrafiltration were analyzed using size-exclusion chromatography. Two different eluent systems (dimethyl sulfoxide/water/lithium bromide; dimethylacetamide/lithium chloride) were compared regarding their suitability for lignin analysis. The differences of the elution profiles and calculated molar masses were discussed using conventional and universal calibration. For further validation four technical lignins from a Round Robin test were included into the study. The results indicated that both analytical systems under investigation were well suited for the analysis of technical lignins.