M. A. Ochando

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)

Multi-scale physics mechanisms and spontaneous edge transport bifurcations in fusion plasmas

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.

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.

First dynamic magnetic configuration scans in ECRH plasmas of the TJ-II Heliac

The configurational flexibility of the TJ-II Heliac has been upgraded with the commissioning of a mode of operation that allows changing the magnetic configuration dynamically: the currents feeding the different coil sets can be ramped during the discharge, which allows for, for example, moving up or down the offset of the rotational transform profile. The first discharges in this operation mode have been designed to investigate the effect of low order rational values of the rotational transform, , in low magnetic shear plasmas created and sustained with electron cyclotron resonance heating. The main magnetic resonances (8/5 and 5/3 in this work) do not deteriorate confinement except occasionally in a short, transient manner that is accompanied by a magnetic event with a clear frequency splitting.

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 effects on the TJ-IU torsatron plasma edge turbulence

A study of plasma edge turbulence carried out in the ECRH heated TJ-IU torsatron is presented. Radial profiles of ion saturation current and floating potential, together with the fluctuation levels of these have been evaluated in the plasma edge by means of Langmuir probe arrays. The existence of two different propagation modes in the proximity of the velocity shear layer has been observed. In the plasma bulk side of the limiter radius, high-frequency fluctuations are negligible and only one propagation mode stands. A detailed examination of the data shows the existence of a quasi-coherent mode probably related to the local magnetic configuration. A radial probe scan reveals an increase in the turbulent particle flux for the location of a rational surface as calculated by the VMEC free-boundary 3D equilibrium code.

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.

UP-DOWN AND IN-OUT ASYMMETRY MONITORING BASED ON BROADBAND RADIATION DETECTORS

Abstract Understanding the origin of asymmetries, either in plasma flows reaching the divertor region or in plasma emissivity from the confined volume, is a question of great interest in magnetic confined plasma research. In the TJ-II stellarator, a set of bolometer systems distributed around the vacuum chamber has been installed to monitor up-down and in-out radiation asymmetries that are observed in transitions to different confinement regimes. Although plasma emissivity is a complex magnitude, a comparison of signals from twin detectors placed at selected positions may be of help to distinguish between the different causes of asymmetry. Moreover, this is a nonperturbing method that is especially appropriate when plasma wall clearance is small.