Structure of pellet cloud emission and relation with the local ablation rate

Abstract

The experimental reproduction of the conditions of pellet injection expected in future large devices being
not possible in present day machines, it is mandatory to validate as thoroughly as possible the available abla-
tion models. Among the different points still under discussion, there is the relation between the spectroscopic
measurement of the ablation clouds and the local ablation rate. This relation is investigated by coupling an
emission model to the time-dependent simulation of ablation clouds with the HPI2 pellet ablation/deposition
code. The simulated quantities are the time-evolution of the cloud visible spectrum ( λ = 400 − 700 nm ) and
images in different wavelength domains (e.g. Hα, Hβ or the continuum centered at λ = 576 nm ). It is found
that the cloud emission is anisotropic, this is particularly the case for Hα and Hβ lines, and that the relation
between the cloud emission and the ablation rate depends not only on the conditions of pellet injection, but also
on the direction of observation. It follows that, in general, it is not possible to estimate the ablation profile from
that of an emission line (Hα or Hβ ). The code predictions are compared with corresponding measurements for
a welldocumented pellet injected in LHD, showing a good agreement for global values and main trends. The
reasons for observed discrepancies are discussed.