2D plasma potential φ distribution was measured in the electron cyclotron resonance heating (ECRH) and neutral beam injection (NBI) plasmas of the TJ-II stellarator with the heavy ion beam probe (HIBP) for the whole radial range and wide area of the poloidal angle, and supported by Langmuir probe data at the edge. The whole operation domain for the on-axis ECRH was explored (n e = 0.45–0.8×1019 m-3, P EC=220–470 kW), ), in addition, NBI plasmas with n e = 0.9–1.3×1019 m-3 and P NBI = 510 kW were studied. In ECRH plasmas the density ramp-up is accompanied by evolution of the potential from the bell-like to Mexican hat profile, while the density profiles were flat or slightly hollow. The potential has the central positive peak, and LFS-HFS (low field - high field sides) and up-down symmetry. Equipotential lines are consistent with vacuum magnetic flux surfaces. In the high-density NBI scenario, the φ profile was fully negative with a minimum up to -300 V at the centre, while at low-density ECRH plasma, φ has a maximum up to +1 kV at the centre. Fluctuations of potential and density are stronger in low-density scenarios and not poloidally symmetric. At the mid-radius (area of the maximum density), RMS of fluctuations were up to φ~15 V at LFS vs ~ 20 V at HFS; RMS ne~2% at LFS vs ~3% at HFS. In the NBI plasmas with the density raise, the asymmetry is decreasing and finally vanishing at n e = 1.2×1019 m-3. 2D distribution of the NBI-induced Alfvén eigenmodes (AEs) shows asymmetric ballooning structure: contrary to broadband turbulence, AE-associated potential perturbation dominates in the LFS with a factor up to 1.7 respect to the HFS. The electrostatic mode, excited in ECRH plasmas by suprathermel electrons also shows an asymmetric structures: density perturbation dominates in the top-bottom direction compared to LFS-HFS direction.

The goal of this article is to describe processes linked to energy confinement in tokamak plasmas from the perspective of self-organization—the main process that determines the behavior of turbulent plasmas. In the paper Razumova et al 2020 Plasma Phys. Rep. 46 337, such an analysis was performed for regimes without transport barriers. The present paper extends this approach to regimes with barriers and magnetic islands. In a shorter version, it was presented in Razumova et al 2020 Entropy 22 53, which showed that the appearance of islands in the inner part of the barrier is directly related to the formation of the barrier and limits its growth. We discuss the structure of the radial heat flux that carries energy from the plasma in such a way that the pressure profile remains close to the self-consistent profile (as observed in the experiment).

The concept of equivalent tokamak and stellarator discharges with the same electron and ion temperatures and with full absorption of the injected ECRH power was introduced in Dnestrovskij Yu N et al 2021 Plasma Phys. Control. Fusion 63 055012. In the present paper, the concept of the discharges equivalence is extended to the case of partial ECRH power absorption. The conditions of discharges equivalence for this case are formulated. It is shown that in equivalent discharges not only the electron temperatures, but also the absorbed powers are the same. Examples of equivalent experimental discharges of the TJ-II stellarator and simulated discharges of the T-10 tokamak with partial ECRH power absorption are studied. Absorbed ECRH power and energy confinement time are found for TJ-II low-density shots heated with ECRH only.