T. Andreeva

Plans for the first plasma operation of Wendelstein 7-X

Wendelstein 7-X (W7-X) is currently under commissioning in preparation for its initial plasma operation phase, operation phase 1.1 (OP1.1). This first phase serves primarily to provide an integral commissioning of all major systems needed for plasma operation, as well as systems, such as diagnostics, that need plasma operation to verify their foreseen functions. In OP1.1, W7-X will have a reduced set of in-vessel components. In particular, five graphite limiter stripes replace the later foreseen divertor. This paper describes the expected machine capabilities in OP1.1, as well as a selection of physics topics that can be addressed in OP1.1, despite the simplified configuration and the reduced machine capabilities. Physics topics include the verification and adjustment of the magnetic topology, the testing of the foreseen plasma start-up scenarios and the feed-forward control of plasma density and temperature evolution, as well as more advanced topics such as scrape-off layer (SOL) studies at short connection lengths and transport studies. Plasma operation in OP1.1 will primarily be performed in helium, with a hydrogen plasma phase at the end.

Design and Analysis of Divertor Scraper Elements for the W7-X Stellarator

A set of new water-cooled divertor components is being designed for the Wendelstein 7-X stellarator to protect the edges of the primary plasma facing components during the bootstrap current evolution (~ 40 s). These new components, referred to as scraper elements (SEs), will intercept field lines and associated heat flux that would otherwise overload the main target edges in certain operational scenarios. The SEs are calculated to experience peak heat fluxes ~15-16 MW/m2 and will be constructed from carbon fiber reinforced composite monoblocks of a type that has been qualified for ITER. The heat flux distribution and magnitude is calculated from field line following in a 3-D magnetic field that includes the contribution from plasma currents. The heat flux calculations are coupled with an engineering design in an iterative process to generate SEs that meet the design criteria while reducing the geometric complexity of the elements.

Specific Features of Wendelstein 7-X Structural Analyses

The Wendelstein 7-X modular stellarator is in the final assembly phase at the Max Planck Institute for Plasma Physics in Greifswald, Germany. The design and assembly of the basic machine, that is, without in-vessel components, diagnostics and periphery, is completed. Structural parameters such as bolt preload, initial gap widths for contacts between structure elements, final magnet module positions, etc., were specified on the basis of detail numerical modeling and are now implemented. The focus of the numerical analysis has been shifted toward fast consideration of nonconformities and changes in assembly procedures, to preparation of commissioning, assessment of possible field disturbances, and exploration of operational limits. In parallel the analyses of in-vessel components, diagnostics, and periphery are being continued. This paper focuses on the specific features in the development, evolution, and realization of analysis strategies, implemented numerical approaches. Further specific subjects are standards and codes, safety margins in relation to expected tolerances and uncertainties, and the confirmation of analysis results by tests as well as their benchmarking with alternative models in different numerical codes. Finally, some lessons learned so far which might be relevant for other large fusion machines are highlighted, and a brief outlook on future work is given.

Tracking of the magnet system geometry during Wendelstein 7-X construction to achieve the designed magnetic field

Wendelstein 7-X, currently under commissioning at the Max-Planck-Institut fur Plasmaphysik in Greifswald, Germany, is a modular advanced stellarator, combining the modular coil concept with optimized properties of the plasma. Most of the envisaged magnetic configurations of the machine are rather sensitive to symmetry breaking perturbations which are the consequence of unavoidable manufacturing and assembly tolerances. This overview describes the successive tracking of the Wendelstein 7-X magnet system geometry starting from the manufacturing of the winding packs up to the modelling of the influence of operation loads. The deviations found were used to calculate the resulting error fields and to compare them with the compensation capacity of the trim coils.

Analysis of the Magnetic Field Perturbations During the Assembly of Wendelstein 7-X

Abstract The magnetic configurations of the Wendelstein 7-X (W7-X) stellarator are sensitive to perturbations of the magnetic field resonant with ι/2π = 1. Such perturbations can be caused by deviations of the current filament positions of the real coil system from the design due to the accuracy achievable during the manufacture of the coils and assembly of the magnet system. The sensitivity of the magnetic field to the different types of error has been investigated by introducing randomly distributed errors to the coil shapes and positions within the given tolerances. A statistical analysis of these error distributions was performed. This procedure will be used to assess the magnetic configuration of W7-X before the completion of each assembly step.

Concept of a Helias ignition experiment

The Helias ignition experiment is an upgraded version of the Wendelstein 7-X experiment. The magnetic configuration is a four-period Helias configuration (major radius 18 m, plasma radius 2.0 m, B = 4.5 T), which presents a more compact option than the five-period configuration. Much effort has been focused on two versions of the four-period configuration. One option is the power reactor HSR4/18 providing at least 3 GW of fusion power and the second is the ignition experiment HSR 4/18i aiming at a minimum of fusion power and the demonstration of self-sustaining burn. The design criteria of the ignition experiment HSR 4/18i are the following: The experiment should demonstrate a safe and reliable route to ignition; self-sustained burn without external heating; steady-state operation during several hundred seconds; reliability of the technical components and tritium breeding in a test blanket. The paper discusses the technical issues of the coil system and describes the vacuum vessel and the shielding blanket. The power balance will be modelled with a transport code and the ignition conditions will be investigated using current scaling laws of energy confinement in stellarators. The plasma parameters of the ignition experiment are: peak density 2–3×1020 m−3, peak temperature 11–15 keV, average beta 3.6% and fusion power 1500–1700 MW.

Validation of wendelstein 7-X fabrication and assembly stages by magnetic field calculations

Abstract The Wendelstein 7-X stellarator, which is currently under construction in Greifswald, Germany, is a five-period machine, and many of the planned operational plasma scenarios are characterized by the rotational transform ɩ/2π = 1 at the plasma boundary. Such magnetic configurations are particularly sensitive to the symmetry-breaking perturbations caused by fabrication and assembly errors, which can occur at different stages of the device construction. Analysis of nonplanar and planar winding packs (WPs) fabricated up to the present time has confirmed the existence of a systematical portion in the manufacturing deviations. The level of the magnetic field perturbation due to the statistical part in manufacturing errors can be expected to be of order 1 × 10−4 at the end of the WP production. Validation of different assembly steps and the resulting distortion of the current path will be done on the basis of the reference point measurements. The influence of the assembly errors and corresponding uncertainties on the magnetic field perturbation is estimated for some cases.

Magnetic Field Accuracy and Trim Coils in Wendelstein 7-X

Abstract The superconducting magnet system of Wendelstein 7-X (W7-X) consists of five identical field periods (modules). Magnetic field errors arise if the modules are not exactly identical. Even small deviations in the coil shapes of the same type or misalignments of coils or modules break the periodicity of the system and cause error field components. Simulation of the magnetic field perturbations that are expected has been done by the analysis of existing winding packages and statistical extrapolations of inaccuracies expected during assembly steps. A numerical experiment has shown that assembly errors should contribute significantly more than manufacturing errors of individual coils. Compensation of the magnetic field perturbation can be done with the help of the coil adjustment during the assembly or by the individual adjustment of all five modules. Further compensation of field errors is possible with additional coils. The existing control coils in W7-X can be used for error field compensation; however, their efficacy is limited. Therefore, solutions employing normal-conducting trim coils outside the cryostat vessel are also considered here.

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.

Numerical modelling at the transition from W7-X construction to operation

The Wendelstein 7-X modular advanced stellarator is in the commissioning phase at the Max Planck Institute for Plasma Physics in Greifswald, Germany. The focus of the numerical analysis has been shifted from support of the machine design and assembly towards preparation of commissioning steps, assessment of possible field disturbances under operational loads, and exploration of operational limits. The paper emphasizes on the development, evolution and realization of new analysis strategies, as well as on implemented numerical approaches for electromagnetic, thermal and structural analyses. Remarkable results of first comparisons with measurements from the extended mechanical instrumentation system obtained during evacuation and flooding of the cryostat, the first cool-down of the magnet system and also during first part of the superconducting coil groups commissioning are presented in detail. Finally, some lessons learned during the transition phase are highlighted which might be relevant for other large fusion machines.