Routes to high-performance operation in Wendelstein 7-X (W7-X): turbulence suppression with shaping of the density profile

Density gradients lead to reduced heat transport and improved energy confinement. This is explained by the gyrokinetic theory by suppression of the ITG turbulence and generally weak TEM in the W7-X geometry. Experimentally, the gradients can be created with two main methods that can be scaled: continuous pellet fueling and NBI heating. Good confinement can be maintained for long periods with a proper choice of the ECRH heating in both cases: tens of seconds for the continuous pellet fueling and 4 seconds with the NBI heating. The condition for the stability of the density profile is the match between core particle sources and the reduced particle transport. If the heating is raised too strongly, the particle transport increases and the improved performance is lost or becomes transient. We find that the normalized temperature gradient should be kept at the level between 1 - 1.2 of the density gradient for the optimum stable operation. Control of this operational point with the use of real time density profile measurements, that are under development, and available heating and fueling actuators will allow even further extension of the improved confinement regime in time.