Properties of ubiquitous modes induced by a positive electron temperature gradient

Abstract The drift wave instability in conditions of hollow temperature profiles is studied. It is found that the ubiquitous mode (UM) is triggered by a positive electron temperature gradient (ETG), while the trapped electron mode (TEM) is triggered by a negative ETG. The threshold of ETG amplitude for UM excitation is lower than that for TEM, and the UM threshold is consistent with recent experimental results from EAST. Compared with TEM, UM exhibits several distinct properties: it becomes more important and dominant in the short-wavelength regime; its excitation depends more on the ion and electron density gradient; and it is significantly more stabilized by ion temperature gradients. Additionally, the effects of magnetic shear and the fraction of trapped electrons are also studied.