Description
For the neoclassically optimized stellarator W7-X, impurity transport is governed by three mechanisms—classical, neoclassical, and turbulent transport—where the classical contribution, usually negligible in tokamaks, may become relevant. Understanding impurity transport and developing effective impurity control strategies are essential for achieving long-pulse, steady-state high-performance (HP) plasma operation. Impurity accumulation is routinely monitored using bolometer tomography, where it is indicated by pronounced core radiation, typically originating from carbon and metal ions. This behaviour is commonly observed in turbulence-reduced HP plasmas with improved energy confinement and elevated ion temperatures. In representative HP scenarios achieved by pellet injection and combined ECRH–NBI heating, steep density gradients comparable to ion temperature gradients and strong negative radial electric fields (Eᵣ < 0, ion root) are present, a condition under which neoclassical transport typically drives inward impurity convection.
Recently, a new operational regime has been identified in which strong core impurity radiation is observed despite only modest energy confinement. This regime is sustained solely by ECRH at very high core plasma densities (nₑ(0) > 1.0 × 10²⁰ m⁻³) and appears either prior to the HP phase established by pellet fuelling or after the HP phase achieved with combined ECRH-NBI heating — when NBI is switched off. This regime differs both from conventional gas-fuelled ECRH plasmas, which typically exhibit flat and low core density profiles with modest particle and energy confinement, and from low-ECRH-power, low-edge-density HP plasmas obtained after gas puff termination.
Compared to the HP phase, these high-density ECRH plasmas are characterized by steeper core density gradients, reduced ion temperature and energy confinement, and weaker negative radial electric fields. These conditions significantly increase impurity collisionality, indicating a potentially important role of classical transport. Calculations reveal a strong inward classical convection driven mainly by the bulk plasma density gradient and further enhanced for highly charged metal ions. Simulations show that this mechanism is sufficient to explain the observed impurity accumulation and the radiation profiles measured by bolometers. For comparison with neoclassical transport, numerical investigations using the SFINCS code are planned. Detailed experimental results will be presented together with numerical modelling and simulations of impurity radiation.