G.W. Pacher

Analysis of performance of the optimized divertor in ITER

The paper describes the results of a physics analysis of a modified divertor cassette for ITER. The issues addressed are the impact on the operational window, the effect of gas leaks through the broader gaps between the divertor cassettes and radiation power loading of different components of the cassettes. The analysis shows that the new design ensuring more flexibility for ITER operation remains acceptable within the framework of the usual trade-off between the target power loading and helium removal efficiency. The radiation load on the side walls of the cassette structures in the inter-cassette gaps is identified as a design constraint not previously considered.

Scaling laws for edge plasma parameters in ITER from two-dimensional edge modelling

Results of a detailed study of the parameter space of the ITER divertor with the B2-Eirene code are presented. Relations between plasma parameters at the separatrix, the interface between the core and edge plasma, are parametrized to provide a set of boundary conditions for the core models. The reference ITER divertor geometry is compared with the straight target option, and the possibility of controlling the edge density by shifting the plasma equilibrium in ITER is explored.

Effect of neutral transport on ITER divertor performance

This paper describes the present state of the development of the computational model of the ITER edge plasma. Neutral–neutral collisions and molecular dynamics are introduced into the self-consistent scheme. First results of ITER modelling including these effects indicate that the operational window for the ITER divertor shifts towards higher neutral pressure in the private-flux region, retaining the operational flexibility determined in the previous analyses.

Divertor issues on ITER and extrapolation to reactors

The current status of divertor modelling for ITER is presented, and major physics and technology constraints on the divertor operation are discussed in the paper. Extensive exploration of the operational window of the ITER divertor has lead to the emergence of simple scalings of the divertor plasma parameters with input power and plasma density, which are used here to make an educated guess on the divertor performance in a commercial reactor. The impact of fast transient events (ELMs), causing significant variation of the power loading, on the divertor operation and design in ITER is discussed and their implications for a reactor are shown. The issue of tritium co-deposition via hydrocarbons inside the vacuum vessel and in the pumping ducts is considered for ITER and projected to a reactor.

ITER operation window determined from mutually consistent core–SOL–divertor simulations: definition and application

An operating window for ITER is defined based on mutually consistent core–SOL–divertor modelling, in which the core turbulent transport is based on the Weiland formulation as incorporated into the multi-mode model. The window consists of five limits, one of which is the edge-based density limit based on divertor detachment. The predicted operating space is ample for ITER to fulfil its mission, reaching a maximum Q ≈ 60 at Palpha ≈ 150 MW (65% of the edge-based density limit, 1.1 times the Greenwald limit), and a maximum Palpha of 220 MW at Q ≈ 15 (90% of the edge-based density limit, 1.45 times the Greenwald limit). This operating window takes into account physics constraints and the technical constraints imposed by the divertor system, i.e. peak power load and attainable pumping speed, but does not include further constraints arising from other technological aspects of the ITER design, such as first wall cooling or shielding, which may further limit operation at high fusion power. The operating window is still compatible with the ITER mission if the magnetic field were reduced by 5% or if the underlying core transport were GLF-like rather than Weiland-like. A moderate reduction in helium exhaust or in pumping speed could be accommodated. Other changes in the operating window resulting from different technical or physical hypotheses are also evaluated.

Modelling of DEMO core plasma consistent with SOL/divertor simulations for long-pulse scenarios with impurity seeding

The integrated core-pedestal-SOL model is applied to the simulation of a typical DEMO operation. Impurity seeding is used to reduce the power load on the divertor to acceptable levels. The influence on long-pulse operation of impurity seeding with various impurities is investigated. DEMO operation at acceptable peak power loads and long-pulse lengths is demonstrated.

Scaling of ITER divertor parameters – interpolation from 2D modelling and extrapolation

Detailed modelling studies of the divertor plasma for ITER have been carried out. Using these results, scaling relationships are developed linking SOL power, density, throughput, pumping speed, peak divertor power load, and helium density for ITER conditions in order to systematise the results and to extrapolate them beyond the range presently covered by the simulations. The key parameter for the scalings is the neutral pressure in the divertor. Both peak power load and helium density vary as the square of the power at constant pressure. The inclusion of helium elastic collisions reduces the helium density and leads to a steeper reduction with increasing pressure. Variants of the input conditions, i.e. different geometry, no helium elastic collisions, carbon walls, are also discussed, the consistency of the edge modelling with conditions required in the core is treated, and extrapolation to higher power operation is carried out.

Combined model for the H-mode pedestal and ELMs

A model is developed for use in integrated modelling codes to predict the AQ1 height, width and shape of the H-mode pedestal as well as the frequency and width of edge localized modes (ELMs). The model for the H-mode pedestal in tokamak plasmas is based on flow shear reduction of anomalous transport, while the periodic ELM crashes are triggered by MHD instabilities. The formation of the pedestal and the L–H transition in this model are the direct result of � Er × � B flow shear suppression of transport. Suppression of the anomalous transport enhances the role of neoclassical transport in the pedestal region. The ratio of suppression of anomalous thermal transport in electron and ion channels controls the ratio of electron to ion temperature at the top of the pedestal. Two mechanisms for triggering ELMs are considered. ELMs are triggered by ballooning modes if the pressure gradient exceeds the ballooning limit or by peeling modes if the edge current density exceeds the peeling mode criterion. The models for the pedestal and ELMs are used in a predictive integrated modelling code to follow the time evolution of tokamak discharges from L-mode through the transition from L-mode to H-mode, with the formation of the H-mode pedestal, and, subsequently, the triggering of ELMs. The objective is to produce self-consistent predictions of the width, height and shape of the H-mode pedestal and the frequency of ELMs. The dependencies of pedestal temperature, pedestal width and ELM frequency as a function of plasma heating power, magnetic field and density are discussed.

Chemical Impurity Production in the Tokamak de Varennes

Plasma contamination due to the generation of impurity molecules has been studied by mass spectrometry and by visible emission spectroscopy in the Tokamak de Varennes. The dominant effects are carbon monoxide formation, which is correlated with the residual water vapor pressure in the vacuum chamber, and the formation of C{sub 1}, C{sub 2}, and C{sub 3} hydrocarbons. The measured molecular fluxes are sufficient to account for a large part of the plasma impurity content. Visible spectroscopy indicates that the plasma is significantly affected by these chemical impurity sources. The molecules appear to originate mainly from the stainless steel walls rather than from the graphite limiters.

Impact of potential narrow SOL heat flux on H-mode access in ITER

The paper presents results of a first analysis of the divertor performance during the L–H transition in ITER. The integrated model consists of the SOLPS4.3 code suite for the SOL and divertor, and the ASTRA code for the core and pedestal regions. The results of SOLPS4.3 are parametrized and used as the boundary conditions for ASTRA, ensuring a consistent description of the plasma core and the edge. Boundary conditions switch from those for wide (L-mode) to narrow (H-mode) SOL once the transition criterion is met. The results show that, for conditions for which a full-power operational space with acceptable power loading of the targets exists, a transition from the initial L-mode operation to H-mode can be found for the same assumptions, i.e. the full-power H-mode regime is accessible.