Description
Radial transport processes in the hot plasma are one of the main topics in nuclear fusion research. As related transport coefficients are expected to depend not only on the gradient of the quantity for which the flux is evaluated but also on other plasma parameters, the most common way to describe the transport is by using a transport matrix with substantial off-diagonal terms. Experiments to determine all components of this matrix at once are very difficult if not to say impossible. Most experiments conduct therefore specialized measurements of one or two quantities like particle or heat flux. Commonly employed are so-called perturbative measurements, perturbing one quantity by a small, hence linearizable, amount while trying to keep the other quantity constant. By this method, one can determine how the local flux (transport) changes if the gradient is varied by an increment. Harmonic repetitions of the perturbation allows for statistical noise reduction of the experimental data.
In this work, perturbative gas-puff-modulation experiments are applied for the first time to the core plasma of the Wendelstein 7-X (W7-X) stellarator. The unique capability of W7-X to alter the magnetic field allows for transport studies in different kinds of magnetic
configurations with varying transport behavior. The transport of hydrogen gas-puffs into the core originating from the divertor gas inlet is evaluated using Thomson scattering radial density profiles. The diffusive and convective components of the particle flux are compared with each other and with those determined in recent laser blow-off impurity measurements. Different parameter sets of densities and temperatures are evaluated aiming to complete the image of the core transport behavior in optimized stellarators.