ELM-like activity has been recently observed in TJ-II, in plasmas with stored energy above 1 kJ. The plasma is observed to develop bursts of magnetic activity (seen in Mirnov coil signals) which are followed by a large and distinct spike in the Hα signal. An increase in electrostatic and magnetic fluctuations at the plasma edge and a cold pulse towards the plasma centre are also characteristics of these events. In addition, the electron temperature profile locally flattens at the plasma radius where the temperature is in the range 100-200 eV. This flattening can be explained in terms of enhanced electron heat conductivity. Between ELM-like events the electromagnetic turbulence at the edge decreases and the Te profiles recover their former shapes. This activity is probably triggered by a resonant m = 2, n = 3 mode.

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Previously it has been shown that there is no lack of plasma induced magnetic field components that are strongly dependent on the plasma profiles. Here, a new method for collecting and interpreting magnetic data is presented. The properties of the magnetic probe system needed for the determination of both the plasma pressure profile and the current distribution are discussed. Considerable attention has been given to the evaluation of the required accuracy of magnetic measurements. Possible ways to simplify the general technique for the solution of particular problems are also discussed. As an example the Large Helical Device (LHD) configuration is referred to. A brief description of the peculiarities of plasma equilibria in this system relevant to the problem under consideration is presented. Emphasis is given to the cases in which plasma induced destruction of the plasma boundary is important

A method is developed for studying MHD equilibrium and stability of plasmas confined in toroidal systems with a large constant magnetic field and a small field which varies rapidly in space. The method is used to obtain a system of magnetohydrodynamic equations averaged over the period of the rapidly varying field. In this form, the problem is greatly simplified, and the equations become similar to those used for axisymmetric tokamaks, for which methods have now been developed in some detail. -The problem of the limiting plasma pressure for equilibrium and stability in a stellarator is analysed, and it is shown that, within the framework of an ideal MHD model, 0 = 2P/B 2 can be larger than 10% in systems with large shear and partial compensation of the magnetic fields produced by the diamagnetic currents. Also the question of toroidal drift compensation and, as a result, of the decrease in the neoclassical diffusion coefficient is discussed.

The three dimensional (3-D) and axisymmetric (2-D) magnetic fields produced by currents flowing in plasmas that are confined in stellarators with a nearly planar geometrical axis are investigated. Using the Green's function method to solve the 3-D linear equation derived previously, a simple procedure is presented here to determine the structure of plasma induced external magnetic fields in stellarators. A detailed numerical analysis of the values of the external 3-D magnetic fields and their dependences on the plasma profiles is performed. Analytical formulas are derived that could be useful for estimations. The possibility of using the measurements of these fields for the experimental identification of the magnetohydrodynamic (MHD) equilibria is considered. Also discussed are some interesting aspects of the influence of the magnetic hump on the value of the Pfirsch-Schliiter currents (that appear in recent Heliotron-E experiments) and the influence of different features of the vacuum magnetic field on the dipole component of plasma induced field is classified. The results of calculations related to the modern experiments are presented, and the case that may be typical for future experiments is also considered.

Transitions to the regime with better confinement in the L-2M stellarator are presented. Transitions are indicated only at sufficiently high plasma densities, and for a given value of average density they appear only at higher heating powers. Each transition is easily identified by a sudden fast (<200 µs) small drop of total plasma energy fixed by diamagnetic measurements. After that plasma energy steeply regains its value and then slowly monotonically increases up to the end of the active heating phase (just as the line average plasma density n e ). In the bulk of the plasma parameters evolve slowly. Drastic changes are observed in the region close to the plasma boundary where two moderate order rational magnetic surfaces are located with the rotational transform µ taking the values 2/3 and 3/4. Relative values of plasma parameters' fluctuations and their spectrum widths decrease significantly in this region. The region has a definite sandwich structure being subdivided by the above-named moderate order rational magnetic surfaces into three smaller zones with different plasma parameter dynamics. Transition is triggered by local disturbances of plasma parameters that are caused by instabilities in the vicinity of magnetic surfaces where µ is equal to 2/3 or 3/4. Different hypotheses on the nature of the phenomenon are discussed.

The free boundary stellarator equilibria are studied in the framework of averaged equations. The structure of external magnetic fields created by currents flowing inside the plasma column is analysed. In all cases considered the dipole component of the magnetic field (in agreement with previous investigations) can only be used to determine the volume averaged value of the parameter @ if the net toroidal current is known. For zero net current operation it is shown that the quadrupole component of the external magnetic field can be used for an estimate of the plasma pressure distribution, for example, in the framework of a one parametric set of profiles. The dependence of the external magnetic field on the current distribution for the given pressure profile is also investigated, and it is shown that there is a possibility of distinguishing at least strongly different current distributions (such as the peaked and offcentred current profiles considered here) either for a large shear system at high @ or for a device with averaged vertical elongation of the vacuum magnetic surfaces.

Edge localized mode (ELM) events are typically observed in a broad variety of tokamaks and stellarators. In the TJ-II Heliac, ELM-like events have been observed in experiments at low plasma parameters (T e ∼ T i ∼ 100 eV, n ∼ 6 × 10 12 cm −3 , where T i , T e and n are the ion and the electron temperatures and the plasma density, respectively) away from the electron cyclotron resonance heating power deposition region. Moreover, numerical calculations have shown that the experimental values for plasma pressure are more than an order of magnitude lower than the theoretical pressure threshold for both ideal and resistive magnetohydrodynamic modes. These transport events, now called localized electromagnetic modes, are found to be linked to the presence of rational magnetic surfaces in the vicinity of the steep density gradient region located at 0.5 r 0.7, r = r eff /a p , where r eff and a p are, respectively, the average radius of the magnetic surface and the plasma boundary; so, while the morphology of the time signals is similar to that of the type-III ELMs, the mode in TJ-II is localized at the edge of the confinement region. The characteristic frequency of the oscillations observed by the Mirnov coils is f = 15-50 kHz. The position of the rational magnetic surface is determined by the channel of the ECE diagnostic with the highest correlation to Mirnov probes and matches, within the experimental error, the position obtained from numerical calculations when the modification of the rotational transform due to toroidal currents is taken into account. These two coupled observations are attributed to the coupling of drift modes with Alfvèn and acoustic waves.