Effect of poloidal asymmetry on the impurity density profile in tokamak plasmas

Abstract

The effect of poloidal asymmetry of impurities on impurity transport driven by electrostatic turbulence in tokamak plasmas is analyzed. It is found that if the density of the impurity ions is poloidally asymmetric then the zero-flux impurity density gradient is significantly reduced and even a sign change in the impurity flux may occur if the asymmetry is sufficiently large. This effect is most effective in low shear plasmas with the impurity density peaking on the inboard side and may be a contributing factor to the observed outward convection of impurities in the presence of radio frequency heating. ͓doi:10.1063/1.3569841͔The presence of impurities in fusion plasmas has a significant effect on fusion performance. Accumulation of impurities in the core would have detrimental effect on fusion reactivity due to increased radiation losses and plasma dilution. On the other hand the presence of impurities at the edge can be beneficial, and seeded impurities are often used to create a radiative belt in order to reduce the heat loads on the walls. Therefore, it is of great value to understand the physics mechanisms governing the impurity transport, and to identify plasma conditions in which impurity accumulation in the core can be avoided.Experimental observations on several tokamaks show that the impurity distribution over the flux surface is often poloidally asymmetric. [1][2][3][4][5] The asymmetries are usually most pronounced for heavy impurities. In the plasma core, where the collisionality is low, in-out asymmetries can arise due to toroidal rotation or the presence of radio frequency ͑RF͒ heating. In the case of rotating plasmas, the poloidal asymmetry of the impurity ion distribution is due to the centrifugal force, which causes the impurities to accumulate on the outboard side of flux surfaces. 1,2 In the case of RF heated plasmas, the asymmetry is a result of the increase of the hydrogen-minority density on the outboard side. These particles tend to be trapped on the outside of the torus and the turning points of their orbits drift toward the resonance layer due to the heating. The poloidal asymmetry in the hydrogenminority density gives rise to an electric field that pushes the other ion species to the inboard side. In the case of highlycharged impurities, this effect is amplified by their large charge Z ͑Ref. 3͒. In the tokamak edge, where the plasma is sufficiently collisional, steep radial pressure or temperature gradients give rise to an in-out asymmetry. 6 Neoclassical theory also predicts an up-down asymmetry, which is caused by the ion-impurity friction. 7 Recent experiments have shown that auxiliary heating can influence the impurity convective flux. For example in JET, it has been observed that with ion cyclotron resonance heating ͑ICRH͒ the accumulation of high-Z impurities can be avoided. 8,9 The physical mechanism by which the change of the direction of the impurity convective velocity occurs has not yet been clearly identified, in spite of the various efforts that have been made. [9][10][11] I...

Analytic formulae for centrifugal effects on turbulent transport of trace impurities in tokamak plasmas

“…While these effects can remain moderate on the main ion species, they can be very significant for heavy impurities. Similarly, recent theoretical [7][8][9] and experimental 10 works have pointed out that poloidal asymmetries in the density of non-thermal minority species can cause poloidally asymmetric background potentials which can have a nonnegligible impact on the transport of heavy impurities, due to their large charge.…”

Effect of poloidal asymmetries on impurity peaking in tokamaks

“…One of the ways to influence the impurity transport is to provide additional central heating which has been shown to give a flattening effect on the density profiles of impurities in the core, [1][2][3][4][5] for reasons that are still not properly understood. Recent work noted that the impurity cross-field transport driven by electrostatic turbulence depends on the poloidal asymmetry of the impurities, 6,7 and this, along with other effects, may be a contributing factor to the avoidance of accumulation of impurities with high charge numbers.…”

Radio Frequency Induced and Neoclassical Asymmetries and their Effects on Turbulent Impurity Transport in a Tokamak

“…A bit further out from the center, changes in the turbulence properties can also play an important role. Recent studies pointed towards a possible role of poloidal asymmetries which can be generated in RF heated plasmas [9][10][11][12][13]. In plasmas lacking strong toroidal rotation the densities of the different species are usually assumed to be approximately flux functions.…”