Resonant pedestal pressure reduction induced by a thermal transport enhancement due to stochastic magnetic boundary layers in high temperature plasmas.

Good alignment of the magnetic field line pitch angle with the mode structure of an external resonant magnetic perturbation (RMP) field is shown to induce modulation of the pedestal electron pressure p(e) in high confinement high rotation plasmas at the DIII-D tokamak with a shape similar to ITER, the next step tokamak experiment. This is caused by an edge safety factor q95 resonant enhancement of the thermal transport, while in contrast, the RMP induced particle pump out does not show a significant resonance. The measured p(e) reduction correlates to an increase in the modeled stochastic layer width during pitch angle variations matching results from resistive low rotation plasmas at the TEXTOR tokamak. These findings suggest a field line pitch angle resonant formation of a stochastic magnetic edge layer as an explanation for the q95 resonant character of type-I edge localized mode suppression by RMPs.

Note: Arc discharge plasma source with plane segmented LaB6 cathode.

A plane cathode composed of close-packed hexagonal LaB6 (lanthanum hexaboride) segments is described. The 6 cm diameter circular cathode is heated by radiation from a graphite foil flat spiral. The cathode along with a hollow copper anode is used for the arc discharge plasma production in a newly developed linear plasma device. A separately powered coil located around the anode is used to change the magnetic field strength and geometry in the anode region. Different discharge regimes were realized using this coil.

Model for the transition to the radiatively improved mode in a tokamak

A model for the transition to the radiatively improved (RI) mode triggered in tokamaks by seeding of impurities is proposed. This model takes into account that with increasing plasma effective charge the growth rate of the toroidal ion temperature gradient (ITG) instability, considered nowadays as the dominant source of anomalous energy losses in low-confinement (L) mode, decreases. As a result the plasma density profile peaks due to an inward convection generated by trapped electron turbulence. This completely quenches ITG induced transport and a bifurcation to the RI mode occurs. Conditions necessary for the L-RI transition are investigated.

Quasistationary high confinement discharges with trans-greenwald density on TEXTOR-94

Confinement quality as good as ELM-free H-mode at densities substantially above the Greenwald density limit ( &nmacr;(e,0)/n(GW) = 1.4) has been obtained in discharges with a radiative boundary under quasistationary conditions for 20 times the energy confinement time. This is achieved by optimizing the gas-fueling rate of RI-mode discharges which tailors their favorable energy confinement and leads to discharges with beta values just below the operational limit beta(n) = 2 of TEXTOR-94, thereby effectively avoiding confinement back transitions or disruptions. In addition, this high-density regime is favorable for helium removal and results in figures of merit tau(*)(p,He)/tau(E) approximately 10-15, relevant for a future fusion power reactor.

Improved confinement due to open ergodic field lines imposed by the dynamic ergodic divertor in TEXTOR.

The ergodization of the magnetic field lines imposed by the dynamic ergodic diverter (DED) in TEXTOR can lead both to confinement improvement and to confinement deterioration. The cases of substantial improvement are in resonant ways related to particular conditions in which magnetic flux tubes starting at the X points of induced islands are connected with the wall. This opening process is connected with a characteristic modification of the heat deposition pattern at the divertor target plate and leads to a substantial increase and steepening of the core plasma density and pressure. The improvement is tentatively attributed to a modification of the electric potential in the plasma carried by the open field lines. The confinement improvement bases on a spontaneous density built up due to the application of the DED and is primarily a particle confinement improvement.

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.

Influence of the static Dynamic Ergodic Divertor on edge turbulence properties in TEXTOR.

Systematic measurements on the edge turbulence and turbulent transport have been made by Langmuir probe arrays on TEXTOR under various static Dynamic Ergodic Divertor (DED) configurations. Common features are observed. With the DED, in the ergodic zone the local turbulent flux reverses sign from radially outwards to inwards. The turbulence properties are profoundly modified by energy redistribution in frequency spectra and suppression of large scale eddies. The fluctuation poloidal phase velocity changes direction from electron to ion diamagnetic drift, consistent with the observed reversal of the Er x B flow. In the laminar region, the turbulence is found to react to an observed reduced flow shear.

Nature of the isotope effect on transport in tokamaks.

The reduction of energy and particle losses with the increasing mass of the hydrogen isotope is more pronounced under conditions of improved confinement when the dominant ion temperature gradient instability is suppressed and other channels of anomalous transport are of importance. In this Letter, we reconsider the dissipative trapped electron (DTE) instability by taking into account finite Larmor radius effects in the analysis of the ion response to perturbations. By applying the improved mixing length approximation in order to estimate the transport coefficients, it is demonstrated that DTE contribution is intrinsically dependent on the isotope mass and provides a plausible explanation for the isotope effect. Contrary to the common belief, it is shown that the DTE turbulence may be of importance for reactor plasmas of low collisionality.