Publication
A global, gyro-kinetic nonlinear study of the ubiquitous mode (UM) for Lithium Tokamak eXperiment (LTX)-like profiles, under flat electron and ion temperature scenarios is conducted using a global gyrokinetic model. Exact MHD equilibria with circular shapes are considered. In contrast to conventional trapped electron modes (TEM), where the mode rotates in the electron diamagnetic drift direction, the collision-less TEM branch associated with UMs exhibits a mode rotation in the direction of ion diamagnetic drift. The dispersion relations and mode structures for both UMs and conventional TEMs are computed through linear analysis, revealing that UMs become unstable for toroidal mode numbers n > 17 ( k θ ρ s > 1.14 ). Nonlinear simulations are performed to study the turbulence driven by UMs. The results show weak zonal flow (ZF) excitation, suggesting that the ZF shearing rate plays a limited role in the saturation mechanism of microturbulence. ZFs are further examined for the effects of temperature gradients ( η i , e < η crit ITG ), however, only an insignificant change is observed. One interesting observation is a natural shift of the dynamics from the UM dominated linear and quasilinear regime to the TEM dominated nonlinear regime, with characteristic high-n to low-n ‘inverse cascade’ typical of the quasi-2D nature of tokamak turbulence. Mode coupling and inverse cascading are identified as the dominant mechanisms driving turbulence saturation.