Characteristics of the TFTR limiter H-mode: The transition, ELMs, transport and confinement

Abstract

H-Modes obtained through transitions from the supershot regime have been studied on TFTR. The characteristics of these H-modes are similar to those found on other tokamaks with one main exception, the density profiles can be highly peaked. In the best cases the enhanced confinement in the core of the initial supershot is retained in the H-mode phase, while the confinement in a broad edge region is enhanced. Thus in TFTR, ali of the important physics of H-modes such as transitions, enhanced edge confinement, ELMs and other phenomena are studied in a large circular limiter tokamak with the added feature of centrally peaked density profi!es and the advantage of an extensive set of diagnostics. The threshold power for the transition is found to be a linear function of plasma current. "Transitions and ELMs are affected by the mix of co-and counter-neutral beam injection (NBI) and by perturbations introduced by pellet injection, gas puffing, and current ramping before and during NBI. Fluctuations near both transition and ELM events have been • characterized. High frequency magnetic fluctuations in the range > 100-250 kHz usually decrease during the transition. Microwave scattering spectra of density fluctuations in the plasma edge show a feature at high frequency ( > 1.5 MHz at k.t.= 5.5 cm -1) during the Hmode, which is not observed in the plasma core and which is consistent with an edge poloidal rotation velocity, V0, of~104 m/s. The fluctuations begin at the transition, propagate in the direction of electron diamagnetic drift, and have modulation correlated with ELMs. Several TFqTRH-modes showed a modest improvement in confinement over that of the supershots from which they originated, and an understanding of these may eventually lead to a plasma with the combined advantages of both the supershot and the Hmode. The characteristics and physics of the _ H-modes are considered relative to other tokamaks and in light of various theoretical studi,ls.