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Two reproducible regimes of tokamak operation, with excitation or suppression of MHD activity can be obtained using a voltage-biased electrode inside the edge of the TCABR tokamak. The experiment was carried out adjusting the tokamak parameters to obtain two types of discharges: with strong or weak MHD activity, without biasing in both cases. The plasma current was adjusted to cover a range of safety factor from 2.9 up to 3.5, so that when biasing was applied the magnetic island (3,1) could interact with the edge barrier. The application of biasing in subsequent discharges of each type resulted in excitation or suppression of the MHD activity. The results show that the dominant modes are m = 2, n = 1 and m = 3, n = 1 for excitation and partial suppression, respectively. In both regimes a strong decrease in the radial electric field is detected with destruction of the transport barrier and of the improved confinement caused by different mechanisms. The measurements include temporal behaviour of edge transport, turbulence, poloidal electric and magnetic fields, edge density, radial electric fields and radial profile of Hα line intensity. The explanation of the excitation and suppression processes is discussed in the paper.

Analytical tokamak plasma equilibria, with non-monotonic plasma current profile and perturbed by ergodic magnetic limiters, are described by non-twist conservative maps. Examples are given of concentration of magnetic field lines escaping to the tokamak wall. The robustness of invariant curves on the shearless region is also observed.

An ergodic magnetic structure is generated in the TBR-1 Tokamak boundary to handle heat flux and minimize impurity generation. In this work, the effect of this structure on the plasma edge turbulence is investigated. Thus, spectral alterations driven by perturbing magnetic fields created by resonant helical windings are observed in the small Tokamak TBR-1. The investigated oscillations are detected with an array of Langmuir and magnetic probes. Spectral and bispectral analyses revealed significant changes induced by the resonant helical windings, mainly in the MHD frequency band. Thus, whereas the known Mimov oscillation amplitude reduction is observed, the density and potential oscillation spectra are slightly affected. However, the bispectral analysis shows a clear reduction of the wave-wave mode coupling in the MHD frequency range, while the high frequency turbulent coupling is steadily reduced

We applied a non-integrable drift-kinetic model, valid for large aspect ratio tokamaks, to investigate plasma edge particle transport driven by drift waves. Particle transport is obtained from the chaotic trajectories obtained by numerically integrating the canonical equations of motion, for the total flow formed by the equilibrium sheared flow and few dominant resonant drift waves propagating in the sheared equilibrium magnetic field. Thus, we investigate the transport dependence on the radial profiles of the electric and magnetic fields and show that radial particle transport at the plasma edge can be reduced by properly modifying the electric and magnetic shear profiles. For non-monotonic radial electric fields, we also observe non-twist transport barriers with shearless invariants identified by extremum values of the rotation number profiles of the invariant curves. The observed non-twist barriers are modified by the magnetic shear and persist for magnetic shear variations expected in present tokamaks.

The existence of primary shearless tori is a distinguishing feature of nontwist maps. However, secondary shearless tori have been identified in the phase space of twist maps, in the neighbourhood of peculiar bifurcations of elliptic fixed points. In this paper, we report secondary shearless bifurcations in a twist symplectic map that describes chaotic field lines in tokamaks with an ergodic limiter. We identify the onset of secondary shearless tori, around elliptic fixed points within the island of stability, by examining numerical profiles of the internal rotation number. We present examples of field line transport barriers, associated with secondary shearless tori and their rupture, which reduce the usual magnetic field line escape at the tokamak plasma edge.

Long-distance correlations (LDCs) of plasma potential fluctuations in the plasma edge have been investigated in the TCABR tokamak in the regime of edge biasing H-mode using an array of multi-pin Langmuir probes. This activity was carried out as part of the scientific programme of the 4th IAEA Joint Experiment (2009). The experimental data confirm the effect of amplification of LDCs in potential fluctuations during biasing recently observed in stellarators and tokamaks. For long toroidal distances between probes, the cross-spectrum is concentrated at low frequencies f f f = 13–15 kHz and f ∼ 40 kHz and low wave numbers with a maximum at k = 0. The effects of MHD activity on the LDCs in potential fluctuation are investigated.

We explore a method for constructing two-dimensional area-preserving, integrable maps associated with Hamiltonian systems, with a given set of fixed points and given invariant curves. The method is used to find an integrable Poincaré map for the field lines in a large aspect ratio tokamak with a poloidal single-null divertor. The divertor field is a superposition of a magnetohydrodynamic equilibrium with an arbitrarily chosen safety factor profile, with a wire carrying an electric current to create an X-point. This integrable map is perturbed by an impulsive perturbation that describes non-axisymmetric magnetic resonances at the plasma edge. The non-integrable perturbed map is applied to study the structure of the open field lines in the scrape-off layer, reproducing the main transport features obtained by integrating numerically the magnetic field line equations, such as the connection lengths and magnetic footprints on the divertor plate.

For tokamak models using simplified geometries and reversed shear plasma profiles, we have numerically investigated how the onset of Lagrangian chaos at the plasma edge may affect the plasma confinement in two distinct but closely related problems. Firstly, we have considered the motion of particles in drift waves in the presence of an equilibrium radial electric field with shear. We have shown that the radial particle transport caused by this motion is selective in phase space, being determined by the resonant drift waves and depending on the parameters of both the resonant waves and the electric field profile. Moreover, we have shown that an additional transport barrier may be created at the plasma edge by increasing the electric field. In the second place, we have studied escape patterns and magnetic footprints of chaotic magnetic field lines in the region near a tokamak wall, when there are resonant modes due to the action of an ergodic magnetic limiter. A non-monotonic safety factor profile has been used in the analysis of field line topology in a region of negative magnetic shear. We have observed that, if internal modes are perturbed, the distributions of field line connection lengths and magnetic footprints exhibit spatially localized escape channels. For typical physical parameters of a fusion plasma, the two Lagrangian chaotic processes considered in this work can be effective in usual conditions so as to influence plasma confinement. The reversed shear effects discussed in this work may also contribute to evaluate the transport barrier relevance in advanced confinement scenarios in future tokamak experiments.