Particle confinement control with resonant magnetic perturbations at TEXTOR

Two very contrary particle confinement stages were obtained at TEXTOR-DED by application of resonant magnetic perturbations. On the one hand a spontaneous build up of the total number of particles N tot with correlated increase in the particle confinement time τ p was observed and on the other hand a controlled decrease of N tot and τ p – the so called stochastic particle pump out is seen. Numerical analysis of the perturbed magnetic field topology shows that both domains can be distinguished by the ratio of short connection length field lines touching a specific resonant flux surface (here the q = 5 / 2 surface) to the complete perturbed layer width. During improved particle confinement, the hyperbolic fixed points (X-points) of the pitch resonant islands are directly connected to the DED target followed by an ≲ 40 % increase in τ p . The subsequent increase in the E × B shear rate Ω E × B at the q = 5 / 2 surface and a steepening of ∇ n e ( r ) suggests a reduction of the radial particle transport. On the opposite, complete stochastisation of this island chain, i.e. a predominant diffusive field line characteristics, causes a ≲ 30 % decrease of τ p with a reduction in Ω E × B at the q = 5 / 2 surface and ∇ n e ( r ) indicating enhanced effective outward particle transport.