Anomalously Rapid Current Penetration in a Toroidal Turbulent Heating Experiment

The behavior of the plasma-current penetration in a toroidal device has been investigated for a wide range of plasma density (${10}^{11}\lesssim n\lesssim {10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$). The observed rapid penetration cannot be attributed to the enhancement of plasma resistivity. A possible explanation is proposed in terms of electrostatic instabilities driven by electron velocity gradients.