H. Dreier

Development of a multichannel dispersion interferometer at TEXTOR

The design and main characteristics of 14-channel dispersion interferometer for plasma profile measurement and control in TEXTOR tokamak are presented. The diagnostic is engineered on the basis of modular concept, the 10.6 microm CO(2) laser source and all optical and mechanical elements of each module are arranged in a compact housing. A set of mirrors and retroreflectors inside the TEXTOR vacuum vessel provides full coverage of the torus cross section with 12 vertical and two diagonal lines of sight, no rigid frame for vibration isolation is required. Results of testing of the single-channel prototype diagnostic and the pilot module of the multichannel dispersion interferometer are presented.

Design of multichannel laser interferometry for W7-X

An eight channel interferometer is developed for density feedback control and the continuous measurement of electron density profiles in the stellarator W7-X. An additional sightline is launched in the geometry of the Thomson scattering for cross calibration. Due to the W7-X coil geometry access is strongly restricted. This motivates the optimization of the sightline geometry and design studies for supplementary chords. In-vessel retroreflectors will be used and inserted in the first wall elements. To cope with associated mechanical vibrations and thermal drifts during the discharges with envisaged duration of 30min either two-color or second harmonic interferometry techniques must be applied. Optimum wavelengths are found to be about 10 and 5μm. A CO2∕CO interferometer (10μm∕5μm) will be tested and compared with an existing CO2∕HeNe test interferometer. A special difficulty of remotely operated diagnostics is the need of long transmission lines with a path length of about 60m required from the diagnostic...

First results from the modular multi-channel dispersion interferometer at the TEXTOR tokamak

At the TEXTOR tokamak in Jülich, Germany, a modular dispersion interferometer was installed and operated for the first time. Equipped with four lines of sight, the line-integrated density could be measured in parallel at different major radii with a resolution of better than 3 × 10(17) m(-2). This paper will describe the setup and show the first measurement results. Among others, it was possible to detect the evolution of a disruption with a time resolution of 4 μs. The movement of the runaway beam following the disruption could be resolved spatially and temporarily.

Bayesian experimental design of a multichannel interferometer for Wendelstein 7-X

Bayesian experimental design (BED) is a framework for the optimization of diagnostics basing on probability theory. In this work it is applied to the design of a multichannel interferometer at the Wendelstein 7-X stellarator experiment. BED offers the possibility to compare diverse designs quantitatively, which will be shown for beam-line designs resulting from different plasma configurations. The applicability of this method is discussed with respect to its computational effort.

Diagnostic developments for quasicontinuous operation of the Wendelstein 7-X stellarator

The stellarator Wendelstein 7-X will allow for quasicontinuous operation with the duration only being limited to two 30 min discharges per day, at a continuous heating power of 10 MW electron cyclotron resonance heating (ECRH) at 140 GHz, by the capacity of the cooling water reservoir. This will result in high thermal loads on all plasma facing components of 50-100 kW/m(2) from radiation alone and of up to about 500 kW/m(2) on components additionally exposed to convective loads. In high density scenarios toroidally varying ECRH stray radiation levels of 50-200 kW/m(2) need to be coped with, requiring careful material selection and different shielding and hardening techniques. Furthermore, a gradual buildup of coatings on plasma facing optical components, which without any measures being taken, would lead to high transmission losses already within a few days of long pulse operation (equivalent to about 1 year of operation in pulsed devices like JET or ASDEX-upgrade) and therefore needs to be prevented as much as possible. In addition in situ cleaning as well as absolute calibration techniques need to be developed for all plasma facing optical systems. Here we report about some of our efforts to find, for various types of diagnostics, ways to cope with these adverse effects. Moreover, we give a few examples for individual diagnostic specific issues with respect to quasicontinuous operation, such as the development of a special integrator for the magnetic diagnostics as well as special interferometer types which can cope with unavoidable vibrations and slow path length changes due to, e.g., thermal expansion of the plasma vessel.

Bayesian Design of Diagnostics: Case Studies for WENDELSTEIN 7-X

Abstract A general method for the design of diagnostics by means of Bayesian probability theory is outlined. Case studies are discussed for two diagnostics planned for Wendelstein 7-X (W7-X). First, line-of-sight optimization for interferometry meeting with physical optimization targets is investigated. The filter transmission bandwidth for Thomson scattering in W7-X is discussed as a second example.

Integrated Bayesian Experimental Design

Any scientist planning experiments wants to optimize the design of a future experiment with respect to best performance within the scheduled experimental scenarios. Bayesian Experimental Design (BED) aims in finding optimal experimental settings based on an information theoretic utility function. Optimal design parameters are found by maximizing an expected utility function where the future data and the parameters of physical scenarios of interest are marginalized. The goal of the Integrated Bayesian Experimental Design (IBED) concept is to combine experiments as early as on the design phase to mutually exploit the benefits of the other experiments. The Bayesian Integrated Data Analysis (IDA) concept of linking interdependent measurements to provide a validated data base and to exploit synergetic effects will be used to design meta‐diagnostics. An example is given by the Thomson scattering (TS) and the interferometry (IF) diagnostics individually, and a set of both. In finding the optimal experimental des...

Data adaptive control parameter estimation for scaling laws for magnetic fusion devices

Data Adaptive Planning determines the expected utility of a single new measurement using existing data and a data descriptive model. The method can be used for experimental planning. It is applied to scaling laws for magnetic fusion devices. Explicitly, the scaling of the stellarator W7-AS is examined for a subset of ι= 1/3 data. In control parameter space regions of high utility are identified and serve for fixing discharge and machine parameters for upcoming discharges. It will be shown that a skillful analysis of experimental uncertainties is of utmost importance for significant results.

Integrated approaches in fusion data analysis

The concept of integrated data analysis in nuclear fusion requires the linkage of data and physical information. Summarizing the key steps for the analysis of transport in the core plasma, benefits of probabilistic modelling of single diagnostics are discussed. Concepts for full diagnostics models consisting of several diagnostics modules and linkage through mapping procedures are given in figures of Bayesian graphical models. Coupling to theory codes is demonstrated by the error estimation of neoclassical error analysis allowing a quantitative physical model validation. As an inverted use of the integrated data analysis approach, goals for the design of diagnostics and sets of diagnostics (meta‐diagnostics) are outlined.

Diagnostic design for steady-state operation of the W7-X stellarator

The status of the diagnostic developments for the quasistationary operable stellarator Wendelstein 7-X (maximum pulse length of 30 min at 10 MW ECRH heating at 140 GHz) will be reported on. Significant emphasis is being given to the issue of ECRH stray radiation shielding of in-vessel diagnostic components, which will be critical at high density operation requiring O2 and OXB heating.