Accessibility and properties of ELMy H-mode and ITB plasmas in TCV

Abstract

This paper presents experimental results on the accessibility and the properties of plasmas with improved confinement in TCV. First, the H-mode threshold power is measured in Ohmic plasmas. Above an Ohmic threshold density, the threshold power increases with the density. A lower threshold density is found when additional electron cyclotron heating (ECH) is applied. At these low densities, the threshold power increases dramatically with decreasing density. Only a small fraction of the wide operational domain leading to the Ohmic H-mode is found to lead to a stationary regime with edge localized modes (ELMs). The ELMs have an irregular frequency, but in TCV they can be triggered by an external magnetic perturbation that induces a rapid vertical movement of the plasma. With this perturbation, the ELM frequency can be increased. The ELM triggering mechanism is provided by the vertical movement of the plasma away from the X-point of a single null configuration. This movement induces a positive current at the plasma edge, and we deduce that the ELMs are being controlled by this modification of the plasma edge current.Electron internal transport barriers (eITBs) are produced deep in the plasma during the stationary phase of TCV discharges. Different scenarios of ECH or electron cyclotron current drive (ECCD) at different radial locations have been used to obtain eITBs with and without inductively driven current. The eITBs are characterized by steep electron temperature gradients, high confinement improvement and a large fraction of bootstrap current. In plasmas with fully non-inductively driven current the size and the strength of the eITB are controlled by the location of the power deposition and by the co- or counter-direction of the central ECCD. Finally, a small inductive perturbation of an otherwise non-inductively driven plasma current profile progressively shrinks the eITB, confirming the link between current profiles and eITBs.