H. Renner

Final design of W7-X divertor plasma facing components – tests and thermo-mechanical analysis of baffle prototypes

Abstract The plasma facing components (PFCs) of the W7-X are designed in detail. The current design of the target plates, baffle plates and wall protection is presented which takes into account the requirements of the plasma heating, diagnostic systems and mounting. Prototypes of baffle elements were tested with heat loading to investigate the long term behaviour. The experimental results are compared with finite element calculations of the temperature and stress distributions in the elements. Based on these activities, the fabrication of the W7-X divertor PFCs and the graphite covered wall protection for W7-X can be initiated.

Stellarator Wendelstein VII-AS; Physics and engineering design

The advanced stellarator Wendelstein VII-AS is a medium-sized experimental machine with the following properties and aims: a broadly optimized magnetic configuration with improved plasma equilibrium and smaller neoclassical transport losses compared to classical stellarators; operation with net-current-free plasma; field generation with a modular set of coils instead of the conventional helix/toroidal field coil; system; and good access for plasma heating methods, e.g., neutral beam injection, electron cyclotron heating, and ion cyclotron heating. The physics characteristics and the engineering design of the experiment as well as results from the initial operation period are reported.

Physical Aspects and Design of the Wendelstein 7-X Divertor

Abstract For the Wendelstein 7-X stellarator, an “open divertor” was chosen as a first step in divertor development for the expected extended magnetic and plasma parameter range. Particularly, the three-dimensional (3-D) geometry of the boundary and the provided stationary operation are challenges for the design. So far, simplified models have been used to specify the geometry of the divertor and the performance of the high-heat-load surfaces. By applying the 3-D codes that are now available, the results concerning local heat load and particle exhaust can have more detailed evaluation and can be confirmed generally. Together with the development of improved high-heat-load components, a significant reduction of the target area in comparison with the previous design is possible. The new specifications will be characterized.

Divertor operation in stellarators: results from W7-AS and implications for future devices

The research on divertors for stellarators is at the beginning. Extensive studies are being prepared on large helical device (LHD) and W7-X. W7-AS is now being operated with an open island divertor (ID) which serves as a test bed for the W7-X diverter. The divertor enables access to a new NBI-heated, high-density operating regime with improved confinement properties. This regime-the high-density H-mode (HDH)-displays no evident mode activity, is extant above a threshold density and characterized by flat density profiles. high-energy- and low-impurity-confinement times and edge-localized radiation. Impurity accumulation, normally associated with ELM-free H-modes, is avoided. Quasi-steady-state discharges with n e up to 4 x 10 20 m -3 , edge radiation levels up to 90% and plasma partial detachment at the divertor targets can be simultaneously realized. The accessibility to other improved confinement modes in W7-AS (conventional H-mode anti OC-mode) is not restricted by the divertor. The results provide a promising basis for future experiments, in particular on W7-X, and recommend the ID as a serious candidate for solving the plasma exhaust problem in stellarators.

Water-cooled target modules for steady-state operation of the W7-X divertor

The stellarator WENDELSTEIN 7-X (W7-X) includes water-cooled plasma facing components (PFCs) to allow steady-state operation and to provide an efficient particle and power exhaust up to 10 MW for a maximum pulse duration of 30 min. Ten divertor units are arranged along the helical edge of the fivefold periodic plasma column. The three-dimensional shape and positioning of the target surfaces are optimized to address physics issues for a wide range of experimental parameters, which influence the topology of the boundary. The three-dimensional target surfaces are reproduced by a series of consecutive plane target elements as a set of parallel water-cooled elements positioned onto the frameworks of target modules. The design and arrangement of target modules and elements are described.

Divertor concept for the W7-X stellarator and mode of operation

A favourable property of the stellarator concept is the potential of stationary operation within a magnetic configuration maintained by a superconducting coil system. For proof of principle the stellarator Wendelstein 7-X is presently under construction at Greifswald, Germany, and the start of operation is planned for 2007. The magnetic configuration of the confinement is a non-axisymetric three-dimensional configuration with a helix-like magnetic axis and five identical magnetic field periods. As a first-step divertor design, an open divertor structure has been chosen, which benefits from the inherent divertor property of the magnetic configuration. The system will allow an effective particle and energy exhaust for a wide range of plasma and magnetic parameters. Experimental tools, e.g. localized heating, various heating schemas, gas feed and pellet injection, impurity doping and variation of the pumping speed together with appropriate diagnostics are provided. The purpose is to investigate different modes of operation for the divertor system and to evaluate an extended database for further improvement of the divertor. The main heating method will be 140 GHz ECR as a cw heat source of 10 MW. Additional heating schemes are ICRF and NBI.

Optimum Confinement in the Wendelstein 7-AS Stellarator

Optimum confinement is realized in Wendelstein 7-AS by wall conditioning and by properly adjusting the parameters determining the magnetic field configuration. The effective heating of net current free plasmas by ECRF and neutral beam injection (NBI) involves different plasma parameters and transport regimes. Stationary plasmas are generally produced by ECRF, whereas density and impurity control is a severe problem during NBI. This has initiated different kinds of impurity and particle control scenarios. An extended parameter range with electron temperatures of 200 eV<or=Te<or=3 keV, ion temperatures of 100 eV<or=Ti <or= 0.7 keV and electron densities of 1019 <or= ne<or=3.1020 m-3 was accessible. The characteristics of the energy confinement and the particle and impurity transport are described and related to the specific heat and particle sources. The investigations comprise the analysis of electron and ion heat conductivity, particle transport modelling and impurity transport studies by laser blow-off-experiments. The influence of the ambipolar electric field is discussed.

Confinement studies on the Wendelstein VII-AS stellarator

ECR heating at B0=2.5T has been extensively used in the 1990 experimental period of the W VII-AS stellarator. As it is a low-shear experiment the magnetic configuration (especially details of the rotational transform profile) depends sensitively on plasma currents (pressure driven, ohmic, EC driven, Ohkawa current) which in turn have a strong influence on energy and particle confinement properties. For the stationary phase a transport analysis has been performed, yielding the profiles of the electron heat conduction and the ion particle diffusion coefficients. The former was subjected to a statistical analysis resulting in phenomenological expressions for chi e and tau E. First experiments using neutral beam injection (ECRH target plasma) as well as combined heating (NBI+ECRH) are also discussed.

Divertor for W7-X : II. Comparison of island and axisymmetric divertor

The need for sufficient power and particle exhaust is common to all magnetic confinement concepts. For the new stellarator W7-X, making use of the intrinsic island structure at the edge, an island divertor is planned. The concept of the W7-X island divertor (as a prototype of general island divertors) and its specific design criteria will be presented. A comparison of the basic physics guidelines for divertor design with axisymmetric divertors (specifically with the ASDEX-Upgrade Div-II configuration) will be made. The common elements (general scrape-off layer physics due to the dominant parallel transport and atomic physics effects through neutrals) and possible differences (three-dimensional effects, differences in field-line geometries, additional flexibility through divertor coils, possible ergodicity in some configurations, core baffling of neutrals) will be discussed.

Divertors for Helical Devices: Concepts, Plans, Results, and Problems

Abstract With Large Helical Device (LHD) and Wendelstein 7-X (W7-X), the development of helical devices is now taking a large step forward on the path to a steady-state fusion reactor. Important issues that need to be settled in these machines are particle flux and heat control and the impact of divertors on plasma performance in future continuously burning fusion plasmas. The divertor concepts that will initially be explored in these large machines were prepared in smaller-scale devices like Heliotron E, Compact Helical System (CHS), and Wendelstein 7-AS (W7-AS). While advanced divertor scenarios relevant for W7-X were already studied in W7-AS, other smaller-scale experiments like Heliotron-J, CHS, and National Compact Stellarator Experiment will be used for the further development of divertor concepts. The two divertor configurations that are being investigated are the helical and the island divertor, as well as the local island divertor, which was successfully demonstrated on CHS and just went into operation on LHD. At present, on its route to a fully closed helical divertor, LHD operates in an open helical divertor configuration. W7-X will be equipped right from the start with an actively cooled discrete island divertor that will allow quasi-continuous operation. The divertor design is very similar to the one explored on W7-AS. For sufficiently large island sizes and not too long field line connection lengths, this divertor gives access to a partially detached quasi-steady-state operating scenario in a newly found high-density H-mode operating regime, which benefits from high energy and low impurity confinement times, with edge radiation levels of up to 90% and sufficient neutral compression in the subdivertor region (>10) for active pumping. The basic physics of the different divertor concepts and associated implementation problems, like asymmetries due to drifts, accessibility of essential operating scenarios, toroidal asymmetries due to symmetry breaking error fields, etc., are discussed.