Transition dynamics and confinement scaling in COMPASS-D H mode plasmas

Abstract

An experimental analysis is discussed of different phases of ECRH H mode discharges on COMPASS-D, from L-H transition triggering to steady state. Comprehensive high resolution measurements in the transport barrier region have enabled significant progress to be made in assessing H mode transition models, together with additional experiments aimed at resolving controlling trigger mechanisms. The application of four possible transition models to the data indicates that all demonstrate limiting or critical parameter values at the transition. One of them, based on Alfvén drift wave turbulence suppression, exhibits precursor behaviour in the stability regime diagram and predicts the density and magnetic field dependences of the H mode power threshold observed on COMPASS-D. Analysis of the evolution of the local radial electric field and its shear indicates that significant shear only develops after the L-H transition. Stationary ELMy H modes are achieved at high ρ* with good confinement, and dimensionless scaling over a range of ν* has been carried out, providing valuable confinement data in a regime where heat is deposited primarily to the electrons.