Island divertor transport modelling and comparison with experiment

A realistic modelling of island divertor transport, taking into account the complex 3D island structures and the discontinuous divertor plates and baffles, has become possible by the development, of the 3D Monte Carlo code EMC3-EIRENE [1,2], which has been already used to predict basic features of the W7-AS island divertor [3,4]. Recently the code has been extended to include a self-consistent treatment of impurity transport [5]. This paper presents first applications of the extended code to W7-AS for different edge densities up to detachment conditions and a discussion of the related physics. In addition, a new mapping technique is presented, which extends the application range of the EMC3-EIRENE code to open magnetic field structures like "open" islands and arbitrary stochastic fields

Divertor transport study in the large helical device

The edge transport properties in LHD have been investigated in order to clarify divertor/SOL functions of heliotron type device. The momentum loss, mainly through friction of counter-flows induced by ergodic field lines, breaks the pressure conservation along flux tubes. This prevents high recycling regime even at high density operation, (n) over bar similar to 7 x 10(19) m(-3). The momentum loss is found to be larger than in W7-AS. This is because of the higher ratio of perpendicular and parallel transport scale length, similar to 10(-4), in the ergodic layer, which enhances the friction between counter-flows more than in the island divertor. In the heliotron configuration, a large temperature drop from LCFS to divertor by an order of magnitude is easily realized due to the long connection length in the ergodic layer. This is certainly a favourable feature for future reactors in terms of reduction of damage on the divertor plate. (c) 2007 Elsevier B.V. All rights reserved

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 proportional to 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 proportional to 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 q(95). 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 q(95) dependence of the transition from Type I to Type III ELMs. Experimental evidence common to Type III ELMs in standard ELMy H-modes and in plasmas with ITBs indicates that they are driven by the same instability