Plasma potential and turbulence dynamics in toroidal devices (survey of T-10 and TJ-II experiments)

Abstract

A direct comparison of the electric potential and its fluctuations in the T-10 tokamak and the TJ-II stellarator is presented for similar plasma conditions in the two machines, using the heavy ion beam probe diagnostic. We observed the following similarities: (i) plasma potentials of several hundred volts, resulting in a radial electric field Er of several tens of V cm−1; (ii) a negative sign for the plasma potential at central line-averaged electron densities larger than , with comparable values in both machines, even when using different heating methods; (iii) with increasing electron density ne or energy confinement time τE, the potential evolves in the negative direction; (iv) with electron cyclotron resonance heating and associated increase in the electron temperature Te, τE degrades and the plasma potential evolves in the positive direction. We generally find that the more negative potential and Er values correspond to higher values of τE. Modelling indicates that basic neoclassical mechanisms contribute significantly to the formation of the electric potential in the core. Broadband turbulence is suppressed at spontaneous and biased transitions to improved confinement regimes and is always accompanied by characteristic changes in plasma potential profiles. Various types of quasi-coherent potential oscillations are observed, among them geodesic acoustic modes in T-10 and Alfvén eigenmodes in TJ-II.