Title:“Boundary, SOL, and Divertor Physics Studies on TCV”
Dr. E.S. Marmar, Plasma Science and Fusion Center, MIT, (Principal Investigator)
Dr. J.L. Terry, Plasma Science and Fusion Center, MIT, (Co-Investigator)
Dr. O. Schmitz, Univ. of Wisconsin at Madison (Co-Investigator)
The quest to command the hot and dense plasma boundary a fusion grade plasma requires advanced diagnostics. In this work, two U.S. Universities team up to conduct detailed studies of key boundary, scrape-off layer, and divertor physics issues. The central element is operation and upgrade of two advanced diagnostic systems, presently deployed on the versatile TCV Tokamak. On this device, the plasma boundary shape can be adjusted across a wide variety, which provides unparalleled flexibility to address the most challenging questions of boundary physics.
One of the diagnostic systems is Multi-Spectral Imaging (MSI). It is a key diagnostic for studying divertor-plasma dynamics under the wide range of configurations accessible in TCV, including studying divertor detachment in multiple advanced divertor configurations and studying the effects of different connection lengths under both open and baffled conditions. Additionally, one of the most compelling goals of MSI is to obtain 2D maps of electron temperature and density in the divertor and X-point regions using multiple high-resolution images of emission line intensities whose intensity-ratios are sensitive to Te(r,?) and/or ne(r,?). This constitutes the first application of helium line-ratio spectroscopy for 2D imaging, and combines the strong expertise in that field at UW Madison with the highly versatile MSI system operated by MIT. As part of the program to refine and benchmark the He line-ratio spectroscopy, we will exploit TCV’s installation of a system that places a He gas puff near the X-point and within the field-of-view of the MSI. This provides a local emission source and simplifies the analysis that obtains the 2D emissivities. The 2D imaging of the cold, high density plasmas obtained in some of TCV’s divertor regimes represents new territory for the diagnostic technique. Hence, this project combines diagnostic innovation with immediate application in the extremely versatile divertor environment at TCV.
The second diagnostic system is Gas-Puff-Imaging (GPI), recently commissioned at TCV as part of an MIT/TCV collaboration. GPI has proven to be valuable for understanding a number of key physics issues of both the boundary and the SOL, e.g. blobs, SOL fluctuation statistics, and coherent edge modes. Its deployment on TCV allows continued investigation of these issues, but also provides the opportunity to use GPI to study, for the first time, additional key edge physics issues. For example, TCV has long established the merits of negative triangularity, which is now being considered as a serious candidate configuration for a test reactor. GPI will be used to study the response of the main plasma boundary to variation of the plasma triangularity between ?0.4 ? ? ? 0.4. Furthermore it will be used to study the response of the plasma boundary to divertor dynamics (e.g. detachment) in advanced divertor configurations and to compare boundary turbulence between the “closed” and “open” divertor configurations that are planned for TCV’s 2019/20 run campaigns. A key component of GPI is the localized gas puff. Our investigations will include the use of innovative nozzles for GPI. The nozzles and their emission patterns will be characterized using an existing test-facility at UW Madison.
Funding Information: funded by the Department of Energy, Office of Fusion Energy Science, DE-SC00020425