First dynamic magnetic configuration scans in ECRH plasmas of the TJ-II Heliac

Abstract

The configurational flexibility of the TJ-II Heliac has been upgraded with the commissioning of a mode of operation that allows changing the magnetic configuration dynamically: the currents feeding the different coil sets can be ramped during the discharge, which allows for, for example, moving up or down the offset of the rotational transform profile. The first discharges in this operation mode have been designed to investigate the effect of low order rational values of the rotational transform, , in low magnetic shear plasmas created and sustained with electron cyclotron resonance heating. The main magnetic resonances (8/5 and 5/3 in this work) do not deteriorate confinement except occasionally in a short, transient manner that is accompanied by a magnetic event with a clear frequency splitting.

Influence of Low‐Order Rational Surfaces on the Radial Electric Field of TJ‐II ECH Plasmas

“…These experimental results could explain the local transport changes associated with low order rational surfaces reported in Refs. [4] [10]. In those experiments the lowest values of the effective electron thermal diffusivity are found in regions around the position of the lowest order rational surfaces…”

“…Besides, the ohmic transformer is used to compensate induced toroidal plasma currents -as explained in Ref. [10]-to keep the rotational transform profiles as close as possible to the profiles in conventional shot by shot magnetic configuration scans. In these experiments, clear changes in the Doppler reflectometer spectra are detected at given time instants indicating changes in both, the Doppler peak frequency and its amplitude, and therefore in the perpendicular rotation velocity and in the amplitude of the density fluctuations.…”

“…Recently, a new mode of operation has been implemented on TJ-II in which the currents in the coils can be ramped during the discharge in order to change the magnetic field configuration [10]. This mode of operation allows for a dynamic scan of the rotational transform in a wide range during a single shot and can reveal effects that are difficult to notice in scans performed on a shot to shot basis.…”

The role of magnetic islands in modifying long range temporal correlations of density fluctuations and local heat transport

“…In the experiments presented here, the position of the rational surfaces is slowly scanned outwards by means of the external induction of Ohmic current [10], allowing the exploration of various quantities as a function of the magnetic flux by diagnostics observing at a fixed position. From Doppler reflectometer (DR) measurements, it is found that a rational surface belonging to a low order rational (3/2) is associated with a localized variation of the radial electric field.…”

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

“…In the TJ-II Heliac,ῑ values ranging from 1 to 2 can be easily accessed in low vacuum magnetic shear conditions, |ŝ| = | − (ρ/ι)dι/dρ| < 0.1, for a given average radial coordinate ρ. The development of tools to change dynamically the magnetic configuration in Heliac devices [5,6] and its practical realization [11,12], proves that magnetic resonances are a viable external control of confinement properties. More than blindly putting resonances away, either avoiding them entirely or imposing large magnetic shear, we suggest that their physics be known and investigated in detail so they can eventually be used as one more beneficial feature of stellarator devices.…”