Concept planning

The initial planning for Wendelstein 7-X was aimed at finding the optimum magnetic field.


Computer graphic: Magnetic coils and plasma of Wendelstein 7-X. The shape of the plasma is governed by the optimised magnetic field: It features fivefold symmetry, i.e. viewed from above, the plasma is not circular but resembles a pentagon. Other features are the spiral shape of the magnetic axis and the varying – triangular to bean-shaped – cross-section of the plasma.

The structure of Wendelstein 7-X composed of single coils allows the magnetic field to be shaped in detail. A great deal of theory and computation effort was invested to optimise the magnetic field for Wendelstein 7-X so as to overcome the disadvantages of previous classical stellarators. Its predecessor, Wendelstein 7-AS (1988 - 2002), the first device of this new generation of Advanced Stellarators, had already subjected elements of the concept to first experimental testing.

 

The further developed successor, Wendelstein 7-X, is now to investigate the new stellarator’s suitability for a power plant. The optimised field has to meet seven conditions deriving from the power plant requirements:

  1. As with predecessor Wendelstein 7-AS, small feedback of the plasma pressure to the confining magnetic field is needed, but also
     
  2. good quality of the magnetic field and robustness to possible field perturbations and
     
  3. the plasma energy density required for economic power plant operation at not too high a magnetic field.
     
  4. The heat losses of the plasma should be of the right amount. In the previous stellarator concepts the heat losses would have been unacceptably high.
     
  5. The bootstrap current must be negligibly small. This ring current arises from the radial density and temperature gradients and could distort the magnetic field.
     
  6. Fast particles also have to be well confined, this being a particularly weak aspect of classical stellarators. For in a future power plant the fast helium nuclei produced in fusion have to keep the plasma at the fusion temperature when the external heating is switched off.
     
  7. Lastly, the magnetic field cage should be manufactured as simply and inexpensively as possible by means of a system of modular superconducting coils.


These seven criteria required formulation of new complex computer codes. Moreover, development of appropriate computing methods was a prerequisite to rush the large codes at reasonable speed through the computer.

All in all, optimisation only became possible with the supercomputer generation of the 1980s.

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