Optimization of neoclassical transport

The viability of the stellarator programme as an alternative to tokamaks in the path towards the fusion reactor relies on our capability to design magnetic configurations whose neoclassical radial energy transport is as low as that of tokamaks. Stellarator configurations are typically designed including the minimization of the effective ripple [Nemov-99] as an optimization criterion. However, according to the experimental data existing in the International Stellarator/Heliotron Confinement DataBase (ISHCDB), the correlation between effective ripple and device performance is not satisfactory [Yamada-05]. In principle, this could appear to be at odds with the fact that neoclassical theory is able to provide reasonably good (although improvable) quantitative predictions for ion radial energy transport of stellarators for fusion-relevant plasmas, as the ones we characterized in [Dinklage-13]. One of the reasons is that the effective ripple provides the level of transport in the 1/nu neoclassical regime, but it does not carry information about other relevant regimes (and, in particular, of the transport associated with the theory refinements discussed in this project.

The objective of this activity will be:

  • Use KNOSOS to characterize more accurately the energy confinement of several devices of the ISHCDB. In particular, we will assess which of the refinements developed by the research team (and not included [Dinklage-13]) are more relevant. We will also explore the possibility of using a figure of merit for stellarator optimization that works better than the effective ripple.

Scenarios free of impurity acccumulation

The analytical expressions of [Calvo-2018] have been combined with the equations solved by KNOSOS for the main ions. This combination provides fast calculations of the radial flux of collisional impurities as a function of, among other things, the background plasma.

The objective of this activity will be:

  • Use KNOSOS to explore systematically the parameter space (including magnetic configuration, collisionality and shape of the background profiles) in order to find scenarios in which impurities do not accumulate.

 

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