The factors determining the evolution of edge-localized modes in plasmas driven by lower hybrid currents

Abstract

Modulated lower-hybrid waves (LHWs) are injected into the Experimental Advanced Superconducting Tokamak to determine the physical principles responsible for the suppression or mitigation of edge-localized modes (ELMs). There are two cases of modulated-ELM evolution (stable and unstable cases), because of two different modulated pedestal densities. They can be attributed to additional magnetic perturbations induced by the LHWs, similarly to the effect of resonant magnetic perturbations. As regards the case of unstable modulated ELM evolution, the plasma stored energy increases as the LHWs turn on. In contrast, the central line-averaged electron density decreases, which is different from the case of ELM suppression or from the stably modulated case. The effect of LHWs or density ‘pump-out’ effect can pass across the top of the pedestal region and enter the interior of the density pedestal, causing a decrease in the electron density gradient and its value at the top of the pedestal. Simultaneously, the pressure gradient and edge bootstrap current density increase. For ELM suppression (or for the stable) case, LHWs can couple only with the plasma outside the top region of pedestal, because of the higher top value of density pedestal. Thus, LHWs can pump out the electron density significantly only in the pedestal foot region, producing a larger gradient of electron density pedestal. Statistical analysis of the data indicates that there is a threshold value of the central line-averaged electron density for each of the two modulated ELM cases. Furthermore, the ELM amplitude is modulated by LHWs with a time delay of hundreds of microseconds, which may be further evidence that LHWs have a significant impact on the evolution of ELMs and pedestal structures. All these results imply that there is a significant correlation between the ELM behavior and the electron density profiles modulated by LHWs.