A phenomenological model of non-ambipolar particle transport to understand the SOL currents in Wendelstein 7-X (W7-X)

In the typical standard magnetic field configuration of W7-X (edge iota = 5/5), it is observed that in attached plasma condition, the divertor receives a significant non-ambipolar particle flux as parallel current in the long connection length region. In contrast, the currents become very small when the plasma is detached. Two arrays of 10 flush mounted Langmuir probes are used for collecting these currents in the lower and upper divertors (10 at each location). The spatio-temporal profiles of these parallel currents are not explained by only considering the thermoelectric current contribution. Therefore, it becomes imperative to take into account the contribution from the non-ambipolar perpendicular drifts in the SOL, hence opening the possibility to understand the non-ambipolar SOL drifts by analyzing the SOL currents. A phenomenological model has been developed to understand the contribution of perpendicular fluid drifts to the parallel non-ambipolar fluxes to the target. Due to the long connection lengths and small pitch angles of the magnetic field lines in the boundary islands of W7-X, even a gentler parallel pressure gradient leads to a steep poloidal pressure gradient, which in combination with the radial pressure gradient in the SOL, drive the perpendicular diamagnetic current. The total perpendicular diamagnetic current is calculated by the model. A non-vanishing perpendicular divergence of the diamagnetic current would force a parallel current towards the strike point on the target to make the total current divergence free.