Magnetic Resonances in ECR‐Heated Plasmas of the TJ‐II Heliac

Abstract

Low order rational values of the rotational transform -magnetic resonances, for brevity-can be present inside ECR-heated plasmas of the TJ-II Heliac, in low magnetic shear conditions, without causing damage to confinement. Moreover, in agreement with previous experience in the TJ-II, the resonances seem to benefit confinement and are associated to changes in the radial electric field, at least in the density gradient region. These results encourage considering magnetic resonances as possible external actuators on confinement in stellarator/heliotron devices.

Alfvén eigenmodes measured in the TJ-II stellarator

“…This type of calculation allows a quick estimation of the poloidal mode number, and it has been tested in TJ-II dynamic configuration scans, where modes associated with different rational surfaces are detected and identified as they move along the plasma radius [27,28].…”

Overview of TJ-II experiments

“…To give more physical meaning to the results, the profiles of 1/T e are presented in the form of an effective electron-heat diffusivity, χ e = Q e /(nT e V ), where Q e is the volume integral of the ECRH power, n is the plasma density and V is the radial derivative of the plasma volume. The experiments show reduced values of χ e in regions where the lowest order magnetic resonances are located as the configuration is varied during the discharge [22]. This is illustrated in figure 7, where χ e is shown in the form of contour plots as a function of time and minor radius in two symmetric experiments of dynamic configuration scans: in figure 7 It is worth recalling that TJ-II operates nominally in very low magnetic shear magnetic configurations.…”

“…The plasmas behave as if the resonances happened in an environment of 'healed' islands, which is part of the motivation to study the healing of islands in helical devices jointly with other devices [24]. Kinetic calculations [25] in the geometry of the standard TJ-II magnetic configuration, based on collisional transport alone, show 3D plasma current densities able to modify the magnetic structure in the resonant regions, even in the absence of electric fields [22]. Both elements, radial electric field and plasma current in the resonant regions, are also important in other phenomena such as the formation of internal electronheat barriers and have been proposed as tools to control plasma performance.…”

Magnetic configuration sweep control in heliac type stellarators

“…Plasma shape, position and rotational transform profiles could then be swept during a single discharge, and equivalence between discharge time and rotational transform could approximately be made. This allowed the comparison between experiments performed on a shot to shot basis [40] with magnetic configuration sweeps [46], for instance. With the exception of the radial field coil, all TJ-II coil systems are inductively coupled to the plasma, so sweeping of the coil configuration currents results in induced ohmic current in the plasma.…”

“…The approach was, however, not optimal, since bootstrap current [43], [44], non inductive current drive by the neutral beam injectors, and residual electromagnetic induction arising from imperfect sweep compensation affected the plasma in unpredictable ways, resulting in moderate but uncontrolled excursions of the plasma current respect to the desired reference waveforms. Despite its limitations, the previous control system functionality has been successfully used in MHD studies [46], radial electric field [47], [48] zonal flow and turbulence dynamics [49] [50], supra-thermal ion behaviour [51] etc. Section II outlines the 2013 upgrade of the TJ-II control system, which has being designed to improve the experiment reproducibility during magnetic configuration sweeps.…”