Description
Small-scale instabilities induce turbulence, which significantly increases the transport of heat and particles. This greatly shortens the confinement time and hampers the development of a self-sustained plasma reaction in magnetic confinement devices. Systematic experimental investigations of turbulence, especially with the goal of validating first-principle gyrokinetic codes, thus remain of paramount importance. The "Tokamak à Configuration Variable" (TCV), located at the Swiss Plasma Center in Lausanne, is a versatile and compact tokamak designed to explore various plasma shapes and configurations. Among the advanced diagnostic tools used on TCV, the Tangential Phase Contrast Imaging (TPCI) system provides a unique capability to investigate plasma density fluctuations with high spatial and temporal resolution. TPCI is a diagnostic method based on the transmission of laser light, designed to produce an image of density fluctuations. This method has been successfully employed over a number of TCV campaigns to measure ion-scale density fluctuations such as those induced by the ion temperature gradient (ITG) and trapped-electron mode (TEM).
This diagnostic has been recently upgraded to access and map electron-scale fluctuations for the first time. In particular, it now features a more powerful laser and a 64-element photovoltaic detector capable of a sampling frequency of at least 10 MHz. The diagnostic setup will be presented in detail along with first data.
Recent results will also be discussed in association with a study of the effect of plasma fluctuations on electron-cyclotron (EC) beam broadening. Turbulence levels were measured in several plasma scenarios (in particular positive vs negative triangularity), EC injection angle configurations, and heating power distributions. The results show a clear link between the injection geometry (on-axis versus off-axis), the level of transport related to the sign of the triangularity and the appearance of transport barriers, and the resulting turbulence level.