Experimental power balance study on turbulent heat transport at Wendelstein 7-X

Power balance analysis is used to compile an experimental database of turbulent heat transport in various plasma scenarios at the stellarator Wendelstein 7-X. As figures of merit for turbulence suppression, turbulent transport coefficients , and are defined. The database spans a broad parameter range covering different fueling schemes, heating power values and sources as well as different magnetic configurations of W7-X. This paper seeks out correlations with main turbulence drivers to identify modes responsible for turbulent transport. ITG modes are thought to dominate the turbulent transport in W7-X. These modes are driven by a steep ion temperature gradient and suppressed by a density gradient, quantified by the parameters and (Beurskens et al 2021 Nucl. Fusion61 116072). Experiments with neutral beam or hydrogen pellet injection allow to increase the central density and thereby create steep density gradients. The database unveils a characteristic dependence of the turbulent transport coefficients on and , which is evidence for the prevalence of ITG modes. In plasmas with suppressed ITG modes, turbulent transport is reduced to near zero in the ion channel, but a residual level remains in the electron channel. Possible responsible modes are discussed. Global confinement time normalized to ISS04 is calculated independently from the power balance analysis, using diamagnetic loops and interferometry. It decays with increasing turbulent transport coefficients in the database and shows only minimal scatter. This serves as a cross-check for the power balance analysis. All plasmas achieving a normalized confinement time over one feature an increased density gradient. The modular coil system of W7-X opens up a wide space of magnetic configurations, including low and high magnetic mirror, low and high rotational transform ι as well as low shear. The configuration dependence of turbulent transport is discussed.