Active control of the H-mode transition on MAST

In the presence of strong ?-particle heating, active control of the formation or the properties of the edge transport barrier (ETB) might be necessary to meet the confinement and exhaust needs of burning fusion plasmas. In particular, the sensitive dependences on the magnetic configuration and divertor geometry are suitable candidates for controlling the ETB. Using both methods on the Mega Amp?re Spherical Tokamak (MAST) short L-mode periods of a few milliseconds duration have been introduced into strongly heated H-modes.In double null configuration a vertical plasma shift by ?Z < 2.5?cm opposite to the ion ?B-drift direction led to L-mode periods of 15?ms < ?tL < 20?ms length. The peak power load to the targets at the H?L transition was a factor of 2 below that measured during edge localized modes (ELMs). At the H?L transition a rapid change in toroidal He?II velocity indicating a fast collapse of the radial electric field within was observed spanning a wider radial region than the collapse during ELMs. In single null configuration the change in the connection length of the outer divertor leg by 2?m < ?Lc < 4?m led to L-mode periods of 2?ms < ?tL < 5?ms duration. In both cases fluid modelling indicates a more negative Er and consequently an increase in the E ? B shear in the L-mode just inside the last closed flux surface due to the changes to the magnetic geometry, which lead to improved H-mode access.The active control techniques demonstrated here may help to develop suitable scenarios for future devices and improve the understanding of the physics determining the ETB.