Interplay between intrinsic plasma rotation and magnetic island evolution in disruptive discharges

The behavior of the intrinsic toroidal rotation of the plasma column during the growth and eventualsaturation of m/n = 2/1 magnetic islands, triggered by programmed density rise, has been carefully investigatedin disruptive discharges in TCABR. The results show that, as the island starts to grow and rotate at aspeed larger than that of the plasma column, the angular frequency of the intrinsic toroidal rotation increasesand that of the island decreases, following the expectation of synchronization. As the island saturates at alarge size, just before a major disruption, the angular speed of the intrinsic rotation decreases quite rapidly,even though the island keeps still rotating at a reduced speed. This decrease of the toroidal rotation is quitereproducible and can be considered as an indicative of disruption

Bistability of the Nuclear Polarisation created through optical pumping in InGaAs Quantum Dots

We show that optical pumping of electron spins in individual InGaAs quantum dots leads to strong nuclear polarisation that we measure via the Overhauser shift (OHS) in magneto-photoluminescence experiments between 0 and 4T. We find a strongly non-monotonous dependence of the OHS on the applied magnetic field, with a maximum nuclear polarisation of 40% for intermediate magnetic fields. We observe that the OHS is larger for nuclear fields anti-parallel to the external field than in the parallel configuration. A bistability in the dependence of the OHS on the spin polarization of the optically injected electrons is found. All our findings are qualitatively understood with a model based on a simple perturbative approach.Comment: Phys Rev B (in press