Impurity transport

Click on the figure to see the temporal evolution.

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 high- Z ones) 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. The achievement of long pulse operation without impurity accumulation in the high-heat flux divertor phase of W7-X (2019) is one of the milestones of the stellarator mission in the European fusion roadmap.

On the grounds of the standard neoclassical theory, such accumulation is expected to occur for medium and high-Z impurities, in the presence of a negative radial electric field (i.e. directed towards the core), see e.g. [Zurro-15, Zurro-15b]. A negative radial electric field is a natural condition of a high density fusion-grade plasma with strong ion-electron thermal coupling. This is particularly the case of stellarator-type reactors, for which no ion temperature screening effect is expected [Dinklage-13, García-Regaña-13] as 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 [Neu-02, Doyle-07]. In stellarators, experimental conditions with low impurity confinement time and very low impurity concentration levels in the core have been documented in W7-AS [McKormick-02] and LHD [Yoshinuma-09] respectively. However the physics behind these impurity control techniques and regimes remain poorly understood and their extrapolation to fusion-relevant conditions uncertain.

As a physical ingredient of core impurity transport, experimental and theoretical efforts have been invested in understanding impurity density variations along the magnetic field lines (i.e., variations on magnetic flux surfaces, often called “asymmetries”) [Ingesson- 00, Fülop-11, Viezzer-13, García-Regaña-13, Arévalo-14, Angioni-14, Rozhansky-15]. The specific dynamics of high charge and mass species can lead to large density asymmetries which can, in turn, alter their radial transport and accumulation. This is in particular the case when coupled to variations of electrostatic potential on flux surfaces, which have been recently diagnosed in the TJ-II stellarator [Alonso-14, Pedrosa-15]. Such coupling can potentially result in an outward convection of impurities like that observed in LHD’s “impurity hole” [Ida-09, Alonso-15]. Therefore, the investigation of the physical causes of those variations and their relationship with the magnetic geometry, radial electric field and collisionality is of great interest so as to assess their possible instrumentality for the much needed control of impurity accumulation in stellarators.

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