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 E r of several tens of V cm−1; (ii) a negative sign for the plasma potential at central line-averaged electron densities larger than , with comparable values in both machines, even when using different heating methods; (iii) with increasing electron density n e or energy confinement time τ E , the potential evolves in the negative direction; (iv) with electron cyclotron resonance heating and associated increase in the electron temperature T e, τ E degrades and the plasma potential evolves in the positive direction. We generally find that the more negative potential and E r values correspond to higher values of τ 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 Alfvén eigenmodes in TJ-II.

The velocity of plasma rotation and the potential distributions are measured in the TM-4 device in the Ohmic-heating regime. The potential is negative at the centre of the column, and its magnitude is significantly larger than the ion temperature. At the edge, the potential is positive while the rotation velocities are considerably lower than their neoclassical values.

Impurity transport in the T-10 tokamak plasma with ohmic heating is studied in this paper. The values of various impurities densities, measured with the use of passive spectral diagnostics in the visible (Z eff ), active charge exchange measurements (He, C, O), and integral bolometric measurements with absolute extreme ultraviolet detectors (Fe, W) are shown. The experimental data show that accumulation level is growing with impurity nuclear charge and determined by the parameter1.5 , which is common for all sorts of impurities. Accumulation process is determined by neoclassical processes and begins with the increase of impurity content in the plasma and ends with the formation of density profiles more peaked than the n e (r). In discharges with low γ anomalous transport completely dominates. So it prevents the impurity accumulation and flattens their density profiles down to the n e (r). These observations correlates with measured negative (positive) plasma potential in discharges with high γ (low γ). 1D modelling using ASTRA and STRAHL transport codes is performed to describe the behaviour of impurities in a wide range of T-10 ohmic regimes. It is shown that the coefficients of anomalous transport D an and V an established in Krupin et al (1983 Sov. J. Plasma Phys. 9 529-36) and Krupin et al (1985 12th EPS Conf. on Plasma Physics) by describing the density dynamics of injected argon and potassium ions are applicable for the modelling of the He, C, O, W impurity density profiles and their sources. The analysis of the obtained results allows us to state the existence of a common dependence of the anomalous transport for all ions (impurities and deuterons) on the discharge parameters in the T-10 ohmic regimes.

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