Successful improvement in temperature and density measurements

The TCV tokamak was recently shut down to upgrade its Thomson Scattering (TS) diagnostic. Right after its commissioning, the first measurements have demonstrated impressive improvement in the diagnostic capabilities. As can be seen on the figure, the electron density profile obtained by the new TS system (blue) contains many more points than that obtained with the previous setup (red). This increased resolution enables a better characterization of fine structures that may occur along the profile such as the shoulder around the vertical position of -0.2m called ‘pedestal’.

The upgrade was driven by the recent interest in plasma edge phenomenon requiring enhanced spatial resolution in the so-called plasma pedestals. It has been partly funded by EUROfusion, the European consortium for fusion research. Its primary objectives were to almost double the number of installed polychromators passing from 47 to 89 and to completely redesign the fibre optic system collecting the scattered light to cope with the increase of polychromators.

Very early in tokamak fusion research, laser diffusion onto the plasma electrons, called Incoherent Thomson Scattering, has provided crucial spatial and temporal electron temperature and density measurements. The principle is to send a powerfull laser beam through the plasma and to collect the laser light scattered by plasma electrons. The scattered light is then analysed spectrally by polychromators which allows precise measurements of local electron temperature and density. Such system has been working on the TCV tokamak since its beginning. However, multiplying the number of collection volumes and corresponding polychromators led to a much better spatial resolution.