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07 mars 2014
Core plasma instability efficiently flushes impurities in magnetic fusion devices

Heavy impurities, when present in the plasma core, degrade the energy confinement in a tokamak, and thus the fusion performance. Means of evacuating them towards the edge must be investigated. The non-linear magnetohydrodynamic code XTOR-2F, jointly developed by Ecole Polytechnique and CEA, now simulates the impurity transport in presence of core instability, and clearly show that impurities may be efficiently flushed from the core.

 

The presence of heavy impurities in the core plasma of tokamaks is one potential limitation on the way towards production of fusion electricity by magnetic fusion. The ITER tokamak, currently under construction in Cadarache, will make use of large amounts of Tungsten (atomic number Z=74) for its plasma facing components, exposed to energy fluxes of the order of 10 MW/m2. The material erosion caused by such fluxes may be followed by the penetration of the Tungsten impurity in the hot core plasma, where the impurity will radiate large amounts of power away from the plasma, thus degrading the overall plasma performance.. In other words, the penetration of impurities in the hot core provides an undesired channel to evacuate the energy of the fusion mixture, thus impeding the good confinement of the plasma. Hence the question of the transport of such impurities in the plasma is particularly important, in order to better control them

It is known that magneto-hydrodynamic instabilities can lead to bursts of heat and particle transport in a fusion plasma. In particular, the instability called « saw-tooth », characterized by periodic, fast redistributions of the internal density and temperature, is known to flush the plasma in flattening its profiles from the very core up to approximately mid-radius. It thus potentially represents a way to wash the core from its impurity content. However, contrary to the wellknown behaviour of temperature during such saw-tooth events, the density behaviour was not well understood so far, and recent measurements on the electron density behaviour on Tore Supra and JET suggested that the flushing was not as efficient as initially expected.

 By carrying simulations with the non-linear tri-dimensional magneto-hydrodynamic code XTOR-2F, we have recovered the experimental results of the electron density behaviour, and by applying the code to impurities, we have shown that the small deviation from flatness observed after the crash of the instability was actually not significant. We have compared the results to a one dimensional model widely used in the community, called the Kadomtsev model, and we have found that the differences between the two, regarding the prediction of the post-crash profiles, are not significant either.

The general conclusion is that saw-teeth indeed provide an interesting mechanism for washing the core from its impurity content, and that one dimensional models are able to predict quite reliably the impact of the crash on impurity density profiles.

 

 

Phys. Plasmas 21, 012507 (2014)

 

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