O. Neubauer

DESIGN FEATURES OF THE TOKAMAK TEXTOR

Abstract TEXTOR is the Tokamak Experiment for Technology Oriented Research in the field of plasma-wall interaction. The scope includes a detailed analysis of particle and energy exchange between the plasma and the surrounding chamber as well as active measures to optimize the first wall and the plasma boundary region. TEXTOR is a medium-sized tokamak belonging to the class of moderate-field but large-volume devices having a circular cross section of the plasma and an iron core. The plasma major radius is 1.75 m, and the minor radius is 0.47 m. The maximum plasma current is 0.8 MA, the maximum field is 3 T, and the maximum pulse length is 10 s. TEXTOR is fed directly from the 110-kV grid using an installed converter power of ~300 MVA. The inner wall of TEXTOR is equipped with several specially shaped limiters being partly remotely movable. Special design features of TEXTOR are excellent access for diagnostics to domains near the wall, large portholes suitable for implementing methods to control the plasma boundary, facilities to heat the vacuum vessel and the liner, and provisions for exchange of the liner. TEXTOR has been upgraded with auxiliary heating systems (neutral beam injection, radio-frequency heating, and microwave heating of 9 MW in total), a toroidal pumped limiter, an upgraded magnetization coil, and recently the dynamic ergodic divertor (DED). The DED is a novel flexible tool to influence transport parameters at the plasma edge and to study the resulting effects on heat exhaust, edge cooling, impurity screening, plasma confinement, and stability. The number of special features and the flexibility of TEXTOR provide excellent opportunities for important contributions to fusion research.

Structural Analysis of a Prototype Fast Shutter for ITER cCXRS Diagnostic

Optical lifetime of the first mirror is a critical issue for the ITER upper-port-plug core-charge-exchange-spectroscopy diagnostic. A fast shutter is engaged to protect the mirror from depositions between measurements. The prototype shutter will be examined in a test vacuum vessel that is now under development in Forschungszentrum Jülich, Germany. Being located near the plasma, the shutter operates under severe thermal and electromagnetic (EM) loads. The multifield analyses conducted for the shutter are presented in this paper. Since the fast shutter can operate within 0.7 s, its static structural analysis should be accompanied by dynamic studies. This paper gives details about the numerical strategy used for a multifield ANSYS modeling of a complex structure. The shutter structural performance under the service, thermal, and EM loadings is in line with the requirements. A solution for a problem of high local thermostresses revealed by the analysis is proposed. Problems connected with other possible port-plug-shutter layouts are discussed.

Study and design of the ion cyclotron resonance heating system for the stellarator Wendelstein 7-X

The current status of the mechanical and electromagnetic design for the ICRF antenna system for W7-X is presented. Two antenna plugins are discussed: one consisting of a pair of straps with pre-matching to cover the first frequency band, 25–38 MHz, and a second one consisting of two short strap triplets to cover a frequency band around 76 MHz. This paper focusses on the two strap antenna for the lower frequency band. Power coupling of the antenna to a reference plasma profile is studied with the help of the codes TOPICA and Microwave Studio that deliver the scattering matrix needed for the optimization of the geometric parameters of the straps and antenna box. Radiation power spectra for different phasings of the two straps are obtained using the code ANTITER II and different heating scenario are discussed. The potential for heating, fast particle generation, and current drive is discussed. The problem of RF coupling through the plasma edge and of edge power deposition is summarized. Important elements of the complete ion cyclotron resonance heating system are discussed: a resonator circuit with tap feed to limit the maximum voltage in the system, and a decoupler to counterbalance the large mutual coupling between the 2 straps. The mechanical design highlights the challenges encountered with this antenna: adaptation to a large variety of plasma configurations, the limited space within the port to accommodate the necessary matching components and the watercooling needed for long pulse operation.

The power supply system of the Dynamic Ergodic Divertor at TEXTOR-94

The Dynamic Ergodic Divertor (DED) consists of four sets of coils on the inboard side of the TEXTOR-94 vessel, each with four helical conductors. The coil sets will be energized by DC or by 4-phase current at selected frequencies up to 10 kHz with amplitudes up to 15 kA. Power supply units of 750 kW feeding two coils each are foreseen. The modular design allows for different patterns of current distribution. The maximum operating frequency led to the choice of IGBT inverters. At AC operation the reactive load will be compensated by capacitor banks connected in series to the load thus forming resonant circuits. Matching transformers are necessary to match the voltage level of IGBTs to that of the load at maximum output power. All load units are inductively coupled resulting in a power transfer between the load units. An additional de-coupling transformer compensates the coupling. A central control unit co-ordinates the feedback control of current amplitudes and of the mutual phase shifts of all power supply units.

Technical lay-out of the dynamic ergodic divertor

Abstract A description of the layout for the dynamic ergodic divertor (DED) is presented. It is proposed to install a perturbation coil system consisting of four quartets of coils and two compensation coils helically wound on the high field side of TEXTOR-94. The windings follow the magnetic field lines at q =3 surface for one toroidal turn. To obtain a static solution and a rotating field solution, the system can be operated at DC, 50 Hz and in the band between 1 and 10 kHz. As power supplies, convertor units using 4-quadrant bridges with IGBTs (insulated gate bipolar transistor) are foreseen. The required frequency synchronization of the units will be enforced by a central control unit.

Status of the diagnostics development for the first operation phase of the stellarator Wendelstein 7-X

An overview of the diagnostics which are essential for the first operational phase of Wendelstein 7-X and the set of diagnostics expected to be ready for operation at this time are presented. The ongoing investigations of how to cope with high levels of stray Electron Cyclotron Resonance Heating (ECRH) radiation in the ultraviolet (UV)/visible/infrared (IR) optical diagnostics are described.

Approaches to numerical modeling in development process of complex structures for fusion devices

Development of components for fusion device normally comprises numerous simulations to validate their design compatibility with the specified loads according to applicable codes and standards. A strategy of multifield finite element analyses is determined by an iterative nature of design process, and a level of a structure complexity dictates analysis approaches and modeling technique. Nowadays powerful software exploiting the strength of modern computers allows simulating structures in a very detailed way, even for some types of coupled field analyses. As some drawback of this tendency, many efficient numerical tools, used in the past, are not frequently engaged. Sometimes huge numerical resources are spent to solve trivial problems. Direct CAD-analysis meshing of structures in all their complexity may mix effects of different importance and may lead to overlooking some key structure features. Computational problems with convergence in nonlinear analyses often make modeling less flexible to design changes. This paper gives examples of engineering simplified modeling made with clear understanding of an analysis goal and a role of simulations in design process, like for the W7-X stellarator busbar system. Engineering methods to perform multifield analysis including electromagnetic transients for some ITER diagnostics are also presented. An engineering material model to calculate steel superconducting structures working in inelastic range is discussed.

Removable samples for ITER—a feasibility and conceptual study

The control of the radioactive inventory in the vacuum vessel of ITER is a main safety issue. Erosion of activated plasma-facing components (PFC) and co-deposition of tritiated dust on PFC and in areas below the divertor constitute the main sources of in-vessel radioactive inventory mobilizable in the case of an accident and also during venting of the vessel. To trace the dust and tritium inventory in the machine, the use of collectors in the form of removable samples was evaluated, beside other techniques, since it provides a reliable way to follow the history of the deposits and check critical areas. Four types of removable probes and two optional active diagnostics were selected out of about 30 different options. For all four probes, a conceptual design was worked out and the feasibility was checked with preliminary estimations of thermal and electromagnetic loads, as well as remote handling paths. The highest temperature estimated for the front face of all probes lies in the range 300–500 °C, which is tolerable. Installed in representative places, such removable samples may provide information about the dust and tritium distribution inside the vacuum vessel.

Investigation of turbulence rotation in limiter plasmas at W7-X with newly installed poloidal correlation reflectometer

For the first operation phase of the optimized stellarator W7-X, a heterodyne poloidal correlation reflectometry diagnostic is installed and put into operation. The system is intended to measure the perpendicular (with respect to the magnetic field) turbulence rotation and turbulence properties, such as the decorrelation time and correlation length at the plasma edge. Furthermore, it can give information on the magnetic field line pitch angle. The system consists of an array of microwave antennas distributed in the poloidal and toroidal directions. The frequency range of 22 GHz–40 GHz allows us to access local plasma densities of m−3– m−3. During the first operation phase the turbulence rotation is measured in almost all plasmas. In addition, the radial electric field is estimated and compared to that in neoclassical theory. The relatively low plasma density allows us to cover up to 80% of the plasma radius during OP1.1. The obtained data cover various experimental programs and are partly presented in the paper.

Strategies of Numerical Modeling in Component Development for Fusion Devices

The development of components for fusion devices includes numerous simulations to validate their design compatibility with specified loads according to applicable codes and standards. The strategy of multifield finite element analyses is determined by the iterative nature of the design process. The level of structural complexity dictates the analytical approaches and modeling techniques. Even though nowadays powerful software exploiting the performance of modern computers allows structures to be simulated in a very detailed way, direct meshing of CAD models with all their complexity is often not beneficial for nonlinear and multifield analyses. Reasonable modeling simplifications, submodeling, and integration with analytical modeling and testing can optimize the analysis process. This paper gives examples of simplified modeling for engineering purposes with a clear understanding of the analysis goal and the role of simulations in the design process, such as for the W7-X stellarator busbar system. Engineering methods to perform multifield analyses including electromagnetic transients for ITER diagnostic port plug components are also presented. An engineering material model is discussed for calculating the superconducting coil steel structures operating in the inelastic range.