Study of Type III ELMs in JET
This paper presents the results of JET experiments aimed at studying the operational space of plasmas with a Type III ELMy edge, in terms of both local and global plasma parameters. In JET, the Type III ELMy regime has a wide operational space in the pedestal n(e)�T(e) diagram, and Type III ELMs are observed in standard ELMy H-modes as well as in plasmas with an internal transport barrier (ITB). The transition from an H-mode with Type III ELMs to a steady state Type I ELMy H-mode requires a minimum loss power, P(TypeI). P(TypeI) decreases with increasing plasma triangularity. In the pedestal n(e)�T(e) diagram, the critical pedestal temperature for the transition to Type I ELMs is found to be inversely proportional to the pedestal density (T(crit ) 1/n) at a low density. In contrast, at a high density, T(crit) , does not depend strongly on density. In the density range where T(crit) 1/n, the critical power required for the transition to Type I ELMs decreases with increasing density. Experimental results are presented suggesting a common mechanism for Type III ELMs at low and high collisionality. A single model for the critical temperature for the transition from Type III to Type I ELMs, based on the resistive interchange instability with magnetic flutter, fits well the density and toroidal field dependence of the JET experimental data. On the other hand, this model fails to describe the variation of the Type III n(e)�T(e) operational space with isotopic mass and q95. Other results are instead suggestive of a different physics for Type III ELMs. At low collisionality, plasma current ramp experiments indicate a role of the edge current in determining the transition from Type III to Type I ELMs, while at high collisionality, a model based on resistive ballooning instability well reproduces, in term of a critical density, the experimentally observed q95 dependence of the transition fromType I toType III ELMs.
Bibliographic Reference: An article published in: Plasma Physics and Controlled Fusion 46 (2004) 723�750
Availability: This article can be accessed online by subscribers, and can be ordered online by non-subscribers, at: stacks.iop.org/PPCF/46/723
Record Number: 200518266 / Last updated on: 2005-07-26
Original language: en
Available languages: en