Speaker
Description
One of the biggest challenges of a reliable fusion reactor is the handling of large heat and particle loads on the divertor wall. Key to reducing these loads is plasma detachment, in which a large range of processes occur between the plasma and the neutral background [1,2].
Molecular processes dominate the plasma dynamics, which is why molecules are often studied in divertor research [1,2]. However, reliable rovibrational measurements on the electronic ground state are lacking for divertor-relevant plasmas, causing for example MAR (Molecularly Assisted Recombination) to be poorly understood. We will use the active laser spectroscopy technique named CARS (Coherent Anti-Stokes Raman Scattering) to measure these hydrogen populations.
This contribution will focus on two parts. First, an investigation on the optimal form of CARS is performed for use in the low-pressure hydrogen environments of the divertor region, using ns-pulsed lasers. To this end, three distinct CARS schemes are tested in the lab: collinear CARS, BOXCARS, and collinear polarization CARS. The investigation focuses on the lowest pressure each scheme could measure, including the sensitivity of each scheme when measuring with a large nitrogen background pressure. The first results of these tests will be presented. Most sensitive at low pressures are collinear CARS and collinear polarization CARS, enabling density measurements in the order of 10^{18} m^{-3}. Collinear polarization CARS is the most sensitive diagnostic when detecting species at a large background pressure.
The second part of this contribution shows results of collinear CARS applied to the linear plasma device Magnum-PSI, which is able to generate divertor-relevant plasma conditions (ne ≈ 10^{21} m^{-3}, Te < 5 eV). Spatially-resolved measurements of the lower rovibrational population are ongoing and the first preliminary results will be presented.
[1] A. Loarte, et al., Nucl. Fusion 47 S203 (2007). DOI: 10.1088/0029-5515/47/6/S0
[2] S.I. Krasheninnikov and A.S. Kukushkin, J. Plasma Phys. 83 (2017). DOI:10.6100/IR58304