Progress in the design and development of a test divertor (TDU) for the start of W7-X operation
Introduction
A new fusion experiment of the Stellarator type, Wendelstein 7-X, is currently being built at the Max Planck Institute for Plasma Physics, Greifswald. Over the coming years this experiment will be completed and start operation. The goal of this experiment is to prove the suitability of the Stellarator concept as a desirable alternative to the Tokamak for a future fusion power plant.
The W7-X machine has a divertor [1] consisting of 10 divertor units installed inside the vacuum vessel along the helical edge of the plasma contour, see Fig. 1. This divertor system is used to control the energy and particle flow from the plasma confinement region at the plasma boundary. The components of the divertor that have to withstand high-heat fluxes of power and particles are called the target plates. The target plates of the HHF divertor and the TDU consist of a series of target elements each with a 3D plasma facing surface.
Under stationary operation the heat radiated and conducted to the divertor requires that it is actively water cooled. Given the technically challenging nature of some of the HHF divertor components it has been decided not to install all of them at the start of W7-X operation but to start instead with an adiabatically cooled divertor known as the test divertor (TDU). This TDU will be replaced, after about two years of operation, with the actively cooled high-heat flux (HHF) divertor.
The TDU therefore acts as the divertor which will be used during machine, plasma and diagnostic commissioning and for the initial development of a range of plasma configurations and modes of operation up to a significant fraction of the final machine performance but for shorter plasma pulse lengths.
To make the experience gained during the TDU phase as relevant as possible for the later HHF divertor phase, the geometry of the TDU follows as closely as possible the geometry of the HHF, especially the plasma facing surface. In addition the TDU will provide valuable information for the alignment of the divertor target surfaces to the magnetic confinement surfaces which will enable the optimum installation of the HHF divertor for long pulse operation.
Section snippets
Design specification of the TDU
An outline study was performed at the end of 2007 for the scenario with the intermediate phase of operation of the W7-X machine and a design concept for the TDU was developed and studied. The project agreed a definition of the operating requirements of the TDU and these were expanded into a detailed list of requirements, outlining as comprehensively as possible the requirements of the TDU. The requirements related to plasma performance are given in Table 1. In addition the list of requirements
Description of the TDU
The HHF divertor of W7-X is an open structure designed to achieve an effective power and particle exhaust for a wide range of operational magnetic parameters under steady state conditions. The amount, location and type of energy that will fall on the divertor units depend heavily on the power injected into the plasma, the magnetic configuration and the sharing of the incident energy between the various divertor units.
The TDU replicates many of the geometrical features, in particular the plasma
Materials and testing
The materials used for the TDU are standard, readily available materials commonly found in many fusion experiments. The tiles are made of fine grain graphite, a low Z material, to minimise radiation losses in the plasma and are attached from the rear for the three main target areas to ensure that no holes are present in the tiles on the plasma facing surface. No special restrictions are placed on the Co content of the steels used as no operation with deuterium fuelling is foreseen in the TDU
Thermal performance of the TDU
FEM analysis has been performed to demonstrate the development of the temperature of the TDU after repeated pulsing. The results of some these calculations are shown in Fig. 4. In this case with a 10 min interval between pulses the surface temperature of the divertor reaches in excess of 1900 °C. To avoid the temperature exceeding 1800 °C a duty cycle of greater than 10 min needs to be established, however, the specified value of 20 min for the inter-pulse period can be achieved, for full
Tolerance and installation requirements
The requirements for installation of the TDU inside the W7-X machine are as follows:
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The TDU must fit with the already designed and completed in-vessel components; see [5]. This significantly limits the methods available for accessing the adjustment features.
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The TDU must have sufficient adjustment possibility to take into account the potential inaccuracy of the plasma vessel mounting brackets of up to ±6 mm in all three directions
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The 10 individual units of the TDU must in addition be able to be
Conclusions
The design of the TDU is underway together with a parallel programme to demonstrate the ability of the designed components to meet the challenging requirements for this component. These are:
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the ability to withstand the heat loads and particle fluxes
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simple and quick to install
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adjustable after operation
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plasma facing surfaces well aligned to magnetic field surfaces.
Much work is still to be done before the final planned installation of the main TDU components in 2012/2013 but sufficient progress
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Divertor operation in stellarators: results from W7-AS and implications for future devices
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