Abstract
A spectroscopic method for spatial resolution measurement in fuel pellet ablation clouds is being developed in the Large Helical Device (LHD). Spatial resolution is obtained thanks to optics that have a narrow, band-shaped field-of-view. The Stark-broadened Hβ emission line of a deuterium pellet ablation cloud is isolated and analyzed with a spectral lineshape code. The electron density profile of the ablation cloud along its direction of elongation is derived through least squares fitting. The obtained profile is peaked and has a dip at its center which confirms what can be found in simulations. Moreover, the order of magnitudes for the derived electron densities are in agreement with what has already been found in the LHD.
Highlights
Most fusion reactor designs rely on pellet injection, requiring the need to understand the ablation process perfectly
A measurement is made on a discharge (#135454) in which a series of six deuterium pellets have been successively injected into the plasma of Large Helical Device (LHD)
The plasma was initiated and sustained through a so-called balanced neutral beam injection (NBI) pattern where neutral beams were injected from opposite directions
Summary
Most fusion reactor designs rely on pellet injection, requiring the need to understand the ablation process perfectly. A way to proceed is to study the ablation cloud that forms around a pellet when it interacts with the background plasma. The authors attempted to derive the plasma parameters inside an ablation cloud with a dented intensity profile by using a simulation model. Results derived from the model suggested the presence of a dip at the center of the electron density profile. Significant discrepancies between the experimentally obtained intensity profile and the synthetically reconstructed one were reported. These discrepancies have raised the need for a way to directly measure the plasma parameters distribution inside ablation clouds
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