E. Harmeyer

Physics and engineering design for Wendelstein VII-X

AbstractThe future experiment Wendelstein VII-X (W VII-X) is being developed at the Max-Planck-Institut fur Plasmaphysik. A Helical Advanced Stellarator (Helias) configuration has been chosen because of its confinement and stability properties. The goals of W VII-X are to continue the development of the modular stellarator, to demonstrate the reactor capability of this stellarator line, and to achieve quasi-steady-state operation in a temperature regime >5 keV. This temperature regime can be reached in W VII-X if neoclassical transport plus the anomalous transport found in W VII-A prevail. A heating power of 20 MW will be applied to reach the reactor-relevant parameter regime.The magnetic field in W VII-X has five field periods. Other basic data are as follows: major radius R0 = 6.5 m, magnetic induction B0 = 3 T, stored magnetic energy W ≈ 0.88 GJ, and average plasma radius a = 0.65 m. Superconducting coils are favored because of their steady-state field, but pulsed water-cooled copper coils are also bei...

Improved support concept for the Helias reactor coil system

For the Helias reactor (HSR) a system of non-planar superconducting coils has been developed. The distributions of the magnetic field and the forces are calculated. In order to support the magnetic forces a scheme of mutual support of the coils is applied. The common vault support concept of earlier studies has been modified: Two toroidal support rings, connected to each other, are introduced at the inner part of the coil system. These new rings connect the coils and carry the main part of the centripetal forces. Finite element computations are done with half a field period, taking into account the stellarator symmetry. The mechanical stress and strain distributions inside the coil support structure are analyzed.

Superconducting coil system for a stellarator fusion reactor

A superconducting coil system has been developed for the stellarator fusion reactor. The distributions of the magnetic field and the forces are calculated. In order to support the magnetic forces a scheme of mutual support of the coils is applied. The pancakes of the coil winding pack of each individual coil are embedded in a strong coil housing of stainless steel. Linear and nonlinear finite element computations are done with half a field period, taking into account the stellarator symmetry. The mechanical stress and strain distributions inside the winding pack and the coil structure are analyzed. Furthermore, for the superconducting coil system the quench protection system is addressed.

Structural analysis of the Helias reactor coil system

For the modular non-planar coil system of a Helias reactor (HSR) the distributions of the magnetic field and the coil forces are calculated by means of the EFFI code. In order to support the magnetic forces a scheme of mutual support of the coils is applied. The pancakes of the coil winding pack of each individual coil are embedded in a strong coil housing of stainless steel. Linear and nonlinear finite element computations are done with half a field period, using the ANSYS code and taking into account the stellarator symmetry. The mechanical stress and strain distributions inside the winding pack and the coil structure are analyzed.

On Mechanical Boundary Conditions in Large Helias Coil Systems and the Influence of Sliding on Stresses

The WENDELSTEIN 7-X stellarator experiment is developed as a Helias (Heli cal A dvanced S tellarator) configuration. The superconducting modular coils with nonplanar shape introduce a magnetic field with a maximum net coil force of about 4 MN. The resultant force vector of a whole field period is directed towards the torus centre. In order to support the magnetic forces, a scheme of mutual support has been developed. Each coil is enclosed by an individually adapted stainless steel housing. The coils of a field period are connected to a module by a pair of reinforcements inside the torus. For reasons of accuracy calculations are made within one period. The investigated stress levels in the coils and support depend on the mechanical contact between winding pack and coil housing. The nonplanar coils lead to difficulty in defining the bearing conditions at the ends of a period. The assumptions for prescribing these boundary conditions, which are needed for the FE analysis, are presented.

The Helias Reactor Concept: Comparative Analysis of Different Field Period Configurations

Abstract The Helias reactor (HSR) is an upgraded version of the Wendelstein 7-X (W7-X) experiment. A straightforward extrapolation of W7-X leads to a five-period configuration with a major radius of 22 m. To reduce the size of the reactor, another option with four periods has been investigated. Recent studies have focused on a three-period Helias configuration (HSR3/15i) (major radius 15 m, plasma radius 2.5 m, B = 5 T), which presents a more compact option than the five- and four-period configurations. In HSR3/15i, the resulting magnetic configuration is consistent with the island divertor concept. The stochastic region outside the last magnetic surface is imposed by the remnants of the 3/4 islands and the plasma flows along distinct channels toward the plates. The main problem is due to the high value of the bootstrap current (~1 MA) and alpha-particle losses (estimated as 6%). Further optimization of HSR3/15i can cause the maximum value of the magnetic field at the superconductive coils to be exceeded. There is a trade-off between physics goals (alpha-particle confinement and small bootstrap current) and technical realization (NbTi technology). The comparative analysis of different period configurations will be presented.

Power Supply Optimization for the Superconducting Coil System of HELIAS Fusion Reactor

A. power supply system for feeding the superconducting coils of the Helias reactor, an upgraded system of the WENDELSTEIN-7X device, has been investigated. This multi- converter supply system has been optimized, in view of low losses in the components and only small negative impact to the power grid. The design of the optimized multi-converter supply system was studied by means of computer simulations, using the SIMPLORER code. The influence of the passive structures on operation of the power supply system was taken into account. The computation of induced eddy currents in the coil structure during transient processes are transformed into electric network analyses using the inductance and resistance data of the nonplanar coils and their coil housings, by means of the finite element network (FEN) method. This approximation allows the investigation of the whole coil system including power supplies and passive structures.

Investigation and Optimization on Auxiliary System Operation of HELIAS Fusion Reactor

Investigations on operation of the power supply for the auxiliary system of the fusion power plant were made using the NEPLAN code. The new approach was to use an additional 80 MW, 1.2 GJ energy storage unit with the optimized magnetic confinement system (superconducting magnet system, multiconverter unit and STATCOM) as a SMES or modular flywheel storage unit (magnetodynamic storage MDS) to reduce network loadings in the power plant start-up phase during plasma heating.

Investigation on power supply operation for the helias stellarator fusion reactor

Investigations on operation of the power supply for the auxiliary system of the fusion power plant were made using the SIMPLORER, EFFI and NEPLAN codes. The new approach was to use an additional 80 MW, 1.2 GJ energy storage unit with the magnetic confinement system (superconducting magnet system, multiconverter unit and STATCOM) as a SMES to reduce network loadings to 80 MW instead of 160 MW in the power plant start-up operation during plasma heating. This reduction of the active power pulsed loads reduces the risk of oscillating power frequency changes. These oscillations produce transient power flows which can danger the grid stability.

Magnetic Field, Force and Stress Calculations for Modular Helias Coil Systems

At IPP Garching, the future experiment WENDELSTEIN VII-X is being developed. A Helias configuration ( Helical A dvanced S tellarator) has been chosen because of its confinement and stability properties. Superconducting coils are favoured because of their steady state field, but pulsed water-cooled copper coils could be used also. Two limiting cases are studied for WVII-X with the major radius between R 0 = 5 m and R 0 = 6.5 m, the magnetic induction between B 0 = 4 T and B 0 = 3 T at a stored magnetic energy of W ≈ 0.66 GJ and W ≈ 0.88 GJ. The stress and strain distributions caused by the electromagnetic forces have been investigated by finite-element computations. A mutual support scheme is used between adjacent coils. For a typical load case, the equivalent stress reaches a peak value of about 110 MPa in the composite conductor.