Stationary magnetic entropy in ohmic tokamak plasmas: Experimental evidence from the TCV device

Abstract

Electron temperature, pressure and density profiles from a large variety of ohmically heated plasmas in the TCV tokamak(BT<1.5 T, R0 = 0.88 m, a<0.25 m) are compared with theoretical predictions based on the assumption that the magnetic entropy is constant in time. These discharges include limited and diverted discharges with elongations in the range 1-2.54, triangularities in the range 0.5-0.72 and plasma currents in the range 0.1-1 MA. Over the entire range of quasi-stationary ohmic conditions it is observed that the sawtooth inversion radius and the electron temperature in the confinement region (that outside the inversion radius) depend solely on the parameter ⟨j⟩/(q0j0), where ⟨j⟩is the cross-sectionally averaged current density. These observed sawtooth inversion radii and electron temperature profiles are in excellent agreement with the predictions. The stiffness of the temperature profiles implies a correlation between density and pressure profiles, which is observed in experiment. The observed electron pressure profiles are consistent with poloidal equilibria derived by combining the requirement that magnetic entropy be constant in time with the Grad-Shafranov-Schlüter equation. The observed range of pressure profiles is however smaller than the theoretically accessible one.