Ubiquitous Mode Transport in Reversed Shear Plasmas: A Gyrokinetic Study

Instabilities driven by radial density gradients, such as the Universal Drift Mode (UDM) and the Ubiquitous Mode (UM), are of interest in plasmas with relatively flat temperature profiles, where conventional temperature-gradient-driven modes are expected to be suppressed such as in LTX [D. P. Boyle et al., 2017]. The UM, primarily driven by trapped electrons, has been studied in circular tokamak geometries for LTX-like plasmas in recent work [S. Choudhary et al., 2025] while the effects of reversed shear in electromagnetic instabilities have been explored previously [M Gopal Krishna et., 2025]. However, the role of the kinetic response of passing electrons and the transport behaviour of the UM under reversed magnetic shear remain largely unexplored. In the present work, we investigate the UM in reversed field configurations and examine the effects of kinetic passing electrons. To carry out the work, global gyrokinetic code ORB5 [E. Lanti et al., 2020] is used to perform simulations. A linear analysis is first carried out using global electrostatic simulations to assess the influence of kinetic electrons and magnetic shear reversal on the mode's growth rates. This is followed by nonlinear simulations to study the resulting turbulence and transport characteristics in these configurations.