Tracking magnetic resonances in the effective electron heat diffusivity of ECH plasmas of the TJ-II Heliac

Low order rational numbers of the rotational transform (magnetic resonances, in short) have been considered deleterious for magnetic confinement in toroidal devices. The design and operation of stellarators has been based on the notion that either the resonances should be relegated to small volumes with high magnetic shear, or they had to be simply avoided. Low density, low collisionality discharges of the low magnetic shear TJ-II device show that these resonances can benefit electron heat confinement. Only in conditions of very low shear over an extended radial portion of the plasma, do the resonances have a detrimental effect on confinement. The experiments consist of Electron Cyclotron Heated (ECH) plasmas where a moderated Ohmic current is induced. A distinctive feature of the TJ-II Heliac is its high rotational transform.