I. Pastor

Sheared flows and transition to improved confinement regime in the TJ-II stellarator

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 Er 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. (Some figures in this article are in colour only in the electronic version)

Threshold for sheared flow and turbulence development in the TJ-II stellarator*

The influence of plasma density and edge gradients on the development of perpendicular sheared flow has been investigated in the plasma edge region of the TJ-II stellarator. The development of the naturally occurring velocity shear layer requires a minimum plasma density. Experimental findings have shown that there is a coupling between the onset of sheared flow development and an increase in the level of plasma edge turbulence; once sheared flow is fully developed the level of fluctuations and turbulent transport slightly decreases whereas edge gradients and plasma density increases. Electron density profiles show a broadening evolution as density increases above the critical value where sheared flow is developed, while the temperature profile remains similar, reflecting the strong impact of plasma density in the global confinement scaling. Furthermore, the shearing rate of the spontaneous sheared flow turns out to be close to the one needed to trigger a transition to improved confinement regimes. Density ramp experiments show, within the experimental uncertainty, no evidence of hysteresis during the spontaneous shear development. Power modulation, in the proximity of the critical plasma density, allows the characterization of plasma potential and electric field relaxation during the transition. The present results have a direct impact on the understanding of the physics mechanisms underlying the generation of critical sheared flow, pointing to the important role of turbulent driven flow.

Enhanced heat confinement in the flexible heliac TJ-II

Recent experimental results show that the core electron temperature in the TJ-II stellarator almost doubles previously obtained values for the same heating power. These plasmas, heated with electron cyclotron waves, are characterized by their low density, and by having highly peaked electron temperature profiles and flat, or even hollow, density profiles. The conditions for obtaining these high electron temperature discharges regarding their density, injected power and dependence on plasma species are described. Neoclassical and experimental transport analyses are performed for these discharges, showing a reduction in the electron heat conductivity at the plasma core. The relations of this observed confinement enhancement to the CHS internal transport barrier and the W7-AS neoclassical electron root feature are discussed.

Plasma potential and turbulence dynamics in toroidal devices (survey of T-10 and TJ-II experiments)

A direct comparison of the electric potential and its fluctuations in the T-10 tokamak and the TJ-II stellarator is presented for similar plasma conditions in the two machines, using the heavy ion beam probe diagnostic. We observed the following similarities: (i) plasma potentials of several hundred volts, resulting in a radial electric field Er of several tens of V?cm?1; (ii) a negative sign for the plasma potential at central line-averaged electron densities larger than 1\times 10^{19}\,{\rm m}^{-3} SRC=http://ej.iop.org/images/0029-5515/51/8/083043/nf381326in001.gif/>, with comparable values in both machines, even when using different heating methods; (iii) with increasing electron density ne or energy confinement time ?E, the potential evolves in the negative direction; (iv) with electron cyclotron resonance heating and associated increase in the electron temperature Te, ?E degrades and the plasma potential evolves in the positive direction. We generally find that the more negative potential and Er values correspond to higher values of ?E. Modelling indicates that basic neoclassical mechanisms contribute significantly to the formation of the electric potential in the core. Broadband turbulence is suppressed at spontaneous and biased transitions to improved confinement regimes and is always accompanied by characteristic changes in plasma potential profiles. Various types of quasi-coherent potential oscillations are observed, among them geodesic acoustic modes in T-10 and Alfv?n eigenmodes in TJ-II.

Improved confinement regimes induced by limiter biasing in the TJ-II stellarator

The influence of limiter biasing on plasma confinement, turbulence and plasma flows is investigated in the TJ-II stellarator. Experimental results show that it is possible to modify global confinement and edge plasma parameters with both positive and negative biasing. Significant and minor modifications in the structure of plasma fluctuations have been observed during the transition to improved confinement regimes induced by limiter biasing. These results show evidence of electric field induced improved confinement via multiple mechanisms. The investigation of the relaxation of plasma potential and electric fields shows evidence of two different characteristic decay times.

Ballistic transport phenomena in TJ-II

Perturbative transport experiments have been performed at the stellarator TJ-II. Both the inward propagation of edge cooling pulses induced by the injection of nitrogen, and the outward propagation of heat pulses due to spontaneous spikes of the central temperature have been analysed. It has been found that the observed propagation is incompatible with diffusive transport models. Simultaneous inward and outward propagation eliminates an explanation in terms of a pinch. A numerical simulation with a resistive interchange turbulence model suggests that the observed propagation is related to the successive destabilizations of pressure gradient driven modes associated with rational surfaces.

Plasma Potential Evolution Study by HIBP Diagnostic During NBI Experiments in the TJ-II Stellarator

Abstract The heavy ion beam probe diagnostic is used in the TJ-II stellarator to study directly the plasma electric potential with good spatial (up to 1 cm) and temporal (up to 2 μs) resolution. Singly charged heavy ions, Cs+, with energies of up to 125 keV are used to probe the plasma column from the edge to the core. Both electron cyclotron resonance heating (ECRH) and neutral beam injection (NBI)-heated plasmas (PECRH = 200 to 400 kW, PNBI = 200 to 400 kW, ENBI = 28 keV) have been studied. Low-density ECRH [[over bar]n = (0.5 to 1.1) × 1019 m-3] plasmas in TJ-II are characterized by positive plasma potential on the order of 1000 to 400 V. A negative electric potential appears at the edge when the line-averaged density exceeds 0.5 × 1019 m-3. Further density rises are accompanied by a decrease in the core plasma potential, which becomes fully negative for plasma densities [over bar]n ≥ 1.5 × 1019 m-3. The NBI plasmas are characterized by a negative electric potential across the whole plasma cross section from the core to the edge. In this case, the absolute value of the central potential is on the order of -500 V. These results show a clear link between plasma potential and density in the TJ-II stellarator.

Magnetic configuration and plasma parameter dependence of the energy confinement time in ECR heated plasmas from the TJ-II stellarator

In this work we study the influence of magnetic configuration?rotational transform and minor radius?and plasma parameters?mainly plasma density?on the confinement of low plasma pressure, electron cyclotron resonance (ECR) heated TJ-II plasmas, taking advantage of the TJ-II remarkable magnetic configuration flexibility. Previous discharges in all-metal wall conditions showed a positive exponential dependence of the energy confinement time on the rotational transform, with exponent 0.6, higher than the one deduced from the ISS95 database (0.4). A set of recent plasma discharges, produced in boronized wall conditions, yields different dependences on rotational transform and, above all, on plasma density. The rotational transform-dependence of the boronized data set, with exponent 0.35, might still be considered marginally compatible with the ISS95 prediction, but this is not the case with the density dependence. In this paper we describe the similarities and differences observed between all-metal and boronized data sets and we discuss their possible physical origins.

Plasma Potential Measurements by the Heavy Ion Beam Probe Diagnostic in Fusion Plasmas: Biasing Experiments in the TJ-II Stellarator and T-10 Tokamak

Abstract The effect of edge biasing on plasma potential was investigated in the TJ-II stellarator and the T-10 tokamak. The Heavy Ion Beam Probe (HIBP) diagnostic, a unique tool for studying the core potential directly, was used in both machines. Experiments in TJ-II show that it is possible to modify the global confinement and edge plasma parameters with limiter biasing, illustrating the direct impact of radial electric fields on TJ-II confinement properties. For the first time it was shown that the plasma column in a stellarator can be charged as a whole for a hot, near-reactor-relevant plasma. The plasma potential and electric fields evolve on two different characteristic time scales. Although the experimental conditions in the two machines have many important differences, the basic features of plasma potential behavior have some similarities: The potential response has the same polarity and scale as the biasing voltage, and the fluctuations are suppressed near the electrode/limiter region. However, whereas both edge and core plasma potential are affected by biasing in TJ-II, the potential changes mainly near the biased electrode in T-10.

Review of confinement and transport studies in the TJ-II flexible heliac

TJ-II is a four period, low magnetic shear stellarator (R = 1.5 m, a < 0.22 m, B0 ≤ 1.2 T) which was designed to have a high degree of magnetic configuration flexibility. In the most recent experimental campaign, coupling of the full ECRH power (PECRH ≤ 600 kW) to the plasma has been possible using two ECRH transmission lines which have different power densities. Both helium and hydrogen fuelled plasmas have been investigated. The article reviews the latest physics results in particle control, configuration effects, and transport and fluctuation studies.