Spontaneous transitions to high core-electron temperatures in W7-X low-iota plasmas close to electron-root condition

Transitions to core-electron-root-confinement (CERC) regime are examined in the presence of low-order rational values of the rotational transform (ι) in Wendelstein 7-X. Experiments at densities and heating powers near the bifurcation from pure ion-root to CERC conditions revealed transitions at stationary plasma conditions but evolving plasma currents. Analysis of the effects due to the ramping plasma current indicates a change of the ι profile temporarily crossing low-order rational values. The transition occurs within a narrow range of plasma currents and, supported by simulations of the shielding current evolution, is consistent with the observed rise in central electron temperature. Prior to these transitions, low-frequency fluctuations were detected, ceasing once higher electron temperatures were reached, and the central radial electric field became positive. Experimental and numerical analyses suggest that these fluctuations are not instabilities, but rather sidebands of zonal flow oscillations, Doppler-shifted by the ambient electric field. Although the oscillations are not the trigger, their systematic characterization offers deeper insight into the underlying physics governing the transition and emphasizes the role of the evolving ι.