Sheared flows have been experimentally studied in TJ-II plasmas. In lowdensity ECH plasmas, sheared flows can be easily controlled by changing the plasma density, thereby allowing the radial origin and evolution of the edge velocity shear layer to be studied. In high density NBI heated plasmas a negative radial electric field is observed that is dominated by the diamagnetic component. The shear of the negative radial electric field increases at the L-H transition by an amount that depends on the magnetic configuration and heating power. Magnetic configurations with and without a low order rational surface close to the plasma edge show differences that may be interpreted in terms of local changes in the radial electric field induced by the rational surface that could facilitate the L-H transition. Fluctuation measurements show a reduction in the turbulence level that is strongest at the position of maximum E r shear. High temporal and spatial resolution measurements indicate that turbulence reduction precedes the increase in the mean sheared flow, but is simultaneous with the increase in the low frequency oscillating sheared flow. These observations may be interpreted in terms of turbulence suppression by oscillating flows, the so-called zonal flows.
The photocurrent-time dependence observed during experiments of water photo-oxidation at an n-TiO2 single crystal, as a function of the applied potential and illumination intensity, has been analyzed on the basis of a kinetic model previously proposed. In a basic medium, the model assumes as a first step the photogeneration of surface-bound OH~ radicals from Ti coordinated OH-ions of basic character. OH~ radicals further recombine on the TiO2 surface at a second step, leading to the formation of surface peroxo complexes (chemisorbed HO2-species). Finally, photogenerated hydrogen peroxide species become photo-oxidized and O2 evolves. At small bandbending the decay time constant, ~, characterizing the photocurrent transients is very sensitive to the applied potential, but does not depend on the illumination intensity. The photocurrent-time dependence is dominated by e--h § surface recombination, mainly via photogenerated OH~ radicals. _The pseudo first-order rate constant of this reaction shows a gaussian distribution around a main value k = k3nb exp (-q~JkT), with kz = 10 -1~ cm 3 s -1, nb and r representing the bulk electron density and the bandbending, respectively. This behavior indicates the existence of a heterogeneous distribution of Ti~OH-surface sites mediating the process. In the high bandbending region x is not sensitive to the applied external bias, but strongly depends on the light intensity. The transient is then due to the surface accumulation of positive charge at photogenerated OH~ radicals, which produces a slow bandbending decrease and a concomitant diminution of the photocurrent. These effects are controlled by the rate, ks, of hydrogen peroxide formation, which is a second-order reaction in the surface concentration of photogenerated OH~ radicals. A k2 ~ 1014 cm 2 s -1 can be estimated.Photostable oxides with large bandgaps, like n-TiO2, have been widely employed as photoelectrodes for water splitting in a photoelectrochemical cell (1). However, the mechanisms of charge transfer at the oxide-electrolyte interface involved in photo-oxidation of water are still obscure. Of special interest are the kinetics of the intermediate steps involved in the reaction and the nature of the photogenerated species.The technique of optically induced transients has proved to be an interesting tool for the kinetic research of photoreactions occurring at the semiconductor/electrolyte interface in photoelectrochemical cells (PEC) (2). In this type of time-resolved technique the transient can be induced by perturbing the system, either with a very short light pulse (laser flash), Ref. (2j-s), or with a continuous illumination source and a shutter, . In the first case time resolution is, in principle, limited by the width of the light pulse (from ps to ns) and kinetic information can be obtained about the ver~ fast processes following illumination of the system. In the second case time resolution is restricted to the range of ms because of the slow aperture speed of the shutter placed between the light source and the...
In the last campaign, the TJ-II heliac has been operated under lithium-coated walls, representing the first stellarator ever working under these boundary conditions. Enhanced density control and discharge reproducibility, leading to the drastic enlargement of the operational window, have been obtained. A strong decrease in recycling together with changes in the shot by shot fuelling characteristics and in the wall particle inventory have been recorded. These changes, associated with the new wall scenario, had led to a long-lasting good density control. The new conditions were also mirrored in the plasma profiles under NBI heating scenarios with increased peaking of the electron density profiles. Fuelling rates corresponding just to the nominal beam current were obtained for the first time, and transitions from bell to dome-type plasma profiles, with different collapsing limits, were observed and tentatively ascribed to changes in the local edge power balance. ELM-type activity was observed in concomitance to reduced fluctuation levels and confinement improvement. Record values of plasma energy content were measured at central densities up to 8 × 10 19 m −3 under Li-coated walls.
The magnitude of radial transport in magnetic confinement devices for controlled nuclear fusion suffers spontaneous bifurcations when specific system parameter values are exceeded. Here we show, for the first time, that the correlation length of the plasma potential becomes of the order of the machine size during the edge bifurcation itself, quite unlike the density fluctuations. The mechanism governing the development of this bifurcation, leading to the establishment of an edge transport barrier, is still one of the main scientific conundrums facing the magnetic fusion community after more than twenty years of intense research. The results presented here show the dominant role of long-range correlations when approaching the Low to High confinement edge transition in fusion plasmas. This is in line with the expectation that multi-scale interactions are a crucial ingredient of complex dynamics in many non-equilibrium systems.
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