Dynamics of Turbulence Suppression in a Helicon Plasma

Experiments are currently being conducted in the the Helicon-Cathode Device (HelCat) at the University of New Mexico. The goal is to the study in detail the transition from a turbulent to a non-turbulent state in the presence of flow shear. HelCat has intrinsic fluctuations that have been identified as drift-waves. Using simple electrode biasing, it has been found that these fluctuations can be completely suppressed. In some extreme cases, a different instability, possibly the Kelvin-Helmholtz instability, can be excited. Detailed studies are underway in order to understand the characteristics of each mode, and to elucidate the underlying physics that cause the change between an unstable plasma, and an instability-free plasma. Dynamics being observed include changes in flow profiles, both azimuthal and parallel, as well as changes in potential and temperature gradients. Further understanding is being sought using several computer codes developed at EPFL: a linear stability solver (LSS,\footnote{P. Ricci and B.N. Rogers (2009). Phys Plasmas 16, 062303.} a one-dimensional PIC code/sheath solver, ODISEE,\footnote{J. Loizu, P. Ricci, and C. Theiler (2011). Phys Rev E 83, 016406} and a global, 3D Braginski code, GBS.\footnote{Ricci, Rogers (2009)} A basic overview of results will be presented.