Negative viscosity from negative compressibility and axial flow shear stiffness in a straight magnetic field

Negative compressibility ion temperature gradient (ITG) turbulence in a linear plasma device controlled shear de-correlation experiment can induce a negative viscosity increment. However, even with this negative increment, we show that the total axial viscosity remains positive definite, i.e., no intrinsic axial flow can be generated by pure ITG turbulence in a straight magnetic field. This differs from the case of electron drift wave turbulence, where the total viscosity can turn negative, at least transiently. When the flow gradient is steepened by any drive mechanism, so that the parallel shear flow instability (PSFI) exceeds the ITG drive, the flow profile saturates at a level close to the value above which PSFI becomes dominant. This saturated flow gradient exceeds the PSFI linear threshold, and grows with ∇Ti0 as |∇V∥|/|k∥cs|∼|∇Ti0|2/3/(k∥Ti0)2/3. This scaling trend characterizes the effective stiffness of the parallel flow gradient.