Localized electromagnetic modes in MHD stable regime of the TJ-II Heliac

Abstract

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 (Te ∼ Ti ∼ 100 eV, n ∼ 6 × 1012 cm−3, where Ti, Te 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 = reff/ap, where reff and ap 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.