Impurity accumulation

In magnetic confinement fusion, atomic species other than the fusion reactants (e.g. deuterium and tritium for the fuel mix envisaged for the first demonstration reactors) are termed ‘impurities’. The presence of even small concentration of impurities (especially those of high charge number Z ) in the confinement volume has deleterious consequences on plasma performance, due to radiation power loss and fuel dilution. Furthermore, their accumulation in the core region can ultimately preclude the steady state operation of a fusion reactor. On the grounds of the standard neoclassical theory, such accumulation is expected to occur for medium and high-Z impurities. This is particularly the case of stellarator-type reactors, for which a negative radial electric field (i.e. directed towards the core) is a natural condition, and no ion temperature screening effect is expected unlike in the tokamak configuration. Finally, the use of heavy species such as tungsten as the divertor and first wall material is, to date, the preferred option to meet the requirements of heat exhaust, material erosion, high radiation fraction and tritium retention.

In view of these facts, a robust strategy for the control of core impurity accumulation is needed as part of an integral solution to the several requirements to achieve magnetic confinement fusion. The two fundamental approaches are (1) to control the source of impurities from the divertor and plasma facing components by tailoring of the scrape-off layer (SOL) regime and (2) to act on the radial transport of impurities in the confined region. However, it should be noted that stringent conditions on SOL and core regimes are imposed by detachment and fusion performance respectively.

Plasma discharge scenarios with controlled core impurity concentration have been demonstrated in several devices. Central heating with microwaves in the electron and ion cyclotron frequencies has been shown to be instrumental for controlling impurities in tokamaks. In helical devices, experimental conditions with low impurity confinement time and very low impurity concentration levels in the core have been documented: the high density H (HDH) mode in the stellarator Wendelstein 7-AS and the impurity hole in high- temperature low-density plasmas of the large helical device. However, these scenarios are not understood yet.

An overview of theoretical expectations and numerical modelling capabilities can be found in the following presentation::

In particular, we have studied the neoclassical transport of impurities and compared it with experimental observations, with the goal to find and understand stellarator scenarios of low impurity content.

Other related results can be found in:

  • J A Alonso, J L Velasco, I Calvo, T Estrada, J M Fontdecaba, J García-Regaña, J Geiger, M Landreman, K McCarthy, F Medina, B Ph Van Milligen, M A Ochando, and F I Parra. Parallel impurity dynamics in the TJ-II stellarator. Plasma Physics and Controlled Fusion, 58(7):074009, 2016. arxivPDF
  • B Zurro, J L Velasco, E M Hollmann, A Baciero, M A Ochando, K J McCarthy, F Medina, I Pastor, R J Hajjar, J M García-Regaña, A Melnikov, L Eliseev, HIBP team, and TJ-II team. “Transport analysis of impurities injected by laser blow-off in ECRH and NBI-heated plasmas of TJ-II”. In 42th European Physical Society Conference on Plasma Physics, Lisbon, Portugal, 2015. PDF

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