Showing papers by "R. C. Wolf published in 2016"

The Thomson Scattering System at Wendelstein 7-X

Abstract: This paper describes the design of the Thomson scattering system at the Wendelstein 7-X stellarator. For the first operation campaign we installed a 10 spatial channel system to cover a radial half profile of the plasma cross section. The start-up system is based on one Nd:YAG laser with 10 Hz repetition frequency, one observation optics, five fiber bundles with one delay line each, and five interference filter polychromators with five spectral channels and silicon avalanche diodes as detectors. High dynamic range analog to digital converters with 14 bit, 1 GS/s are used to digitize the signals. The spectral calibration of the system was done using a pulsed super continuum laser together with a monochromator. For density calibration we used Raman scattering in nitrogen gas. Peaked temperature profiles and flat density profiles are observed in helium and hydrogen discharges.

From W7-X to a HELIAS fusion power plant: motivation and options for an intermediate-step burning-plasma stellarator

Abstract: As a starting point for a more in-depth discussion of a research strategy leading from Wendelstein 7-X to a HELIAS power plant, the respective steps in physics and engineering are considered from different vantage points. The first approach discusses the direct extrapolation of selected physics and engineering parameters. This is followed by an examination of advancing the understanding of stellarator optimisation. Finally, combining a dimensionless parameter approach with an empirical energy confinement time scaling, the necessary development steps are highlighted. From this analysis it is concluded that an intermediate-step burning-plasma stellarator is the most prudent approach to bridge the gap between W7-X and a HELIAS power plant. Using a systems code approach in combination with transport simulations, a range of possible conceptual designs is analysed. This range is exemplified by two bounding cases, a fast-track, cost-efficient device with low magnetic field and without a blanket and a device similar to a demonstration power plant with blanket and net electricity power production.

Forward Modeling of X-Ray Imaging Crystal Spectrometers Within the Minerva Bayesian Analysis Framework

Abstract: Two X-ray imaging crystal spectrometer systems are currently being prepared for commissioning at the stellarator Wendelstein 7-X (W7-X). Both are expected to be ready for the first plasma operation...

Wendelstein 7-X Program—Demonstration of a Stellarator Option for Fusion Energy

Abstract: The superconducting stellarator Wendelstein 7-X is currently being commissioned. First plasmas are expected for the second half of 2015. W7-X is designed to overcome the main drawbacks of the stellarator concept and simultaneously demonstrate its intrinsic advantages relative to the tokamak—i.e., steady-state operation without the requirement of current drive or stability control. An elaborate optimization procedure was used to avoid excessive neoclassical transport losses at high plasma temperature, while simultaneously achieving satisfactory equilibrium and stability properties at high $\beta $ in combination with a viable divertor concept. In addition, fast-ion confinement must be consistent with the requirements of alpha-heating in a power plant. Plasma operation of Wendelstein 7-X follows a staged approach following the successive completion of the in-vessel components. The main objective of Wendelstein 7-X is the demonstration of steady-state plasma at fusion relevant plasma parameters. Wendelstein 7-X will address major questions for the extrapolation of the concept to a power plant. These include divertor operation at high densities, plasma fueling at high central temperatures, avoiding impurity accumulation, and an assessment of the effect of neoclassical optimization on turbulent transport and fast-ion confinement. A power plant concept based on an extrapolation from Wendelstein 7-X, the helical advanced stellarator, has been developed.

Methods for measuring 1/1 error field in Wendelstein 7-X stellarator

Abstract: Wendelstein 7-X is an optimized helical axis stellarator that came into operation at the end of 2015. A m/n = 5/5 island chain is used in most of its configurations to form a divertor. This island chain at is sensitive to symmetry-breaking error fields, with the resonant 1/1 field being of particular concern because of its influence on the divertor heat flux distribution. Measurement and compensation of the 1/1 mode is therefore necessary. Experimentally, vacuum error fields in W7-X will be studied with a flux surface mapping diagnostic. In this paper numerical simulations for planning and analysing such measurements are presented. Two methods for determining the 1/1 mode are considered: measurement of the island width and measurement of a helical shift of the magnetic axis. Measurement of the resonant island width is a sensitive technique, but the island structure is also affected by other co-resonant components. A complementary method is to measure a helical shift of the magnetic axis in a configuration close to the resonance. This method has a simple interpretation and isolates the 1/1 error field from higher order resonant modes.

Imaging motional Stark effect measurements at ASDEX Upgrade

Abstract: This paper presents an overview of results from the Imaging Motional Stark Effect (IMSE) diagnostic obtained during its first measurement campaign at ASDEX Upgrade since installation as a permanent diagnostic. A brief overview of the IMSE technique is given, followed by measurements of a standard H-mode discharge, which are compared to equilibrium reconstructions showing good agreement where expected. The development of special discharges for the calibration of pitch angle is reported and safety factor profile changes during sawteeth crashes are shown, which can be resolved to a few percent due to the high sensitivity at good time resolution of the new IMSE system.

ECE measurements in WENDELSTEIN 7-X plasmas

Abstract: The optimized stellarator Wendelstein 7-X aims at ECRH-heated high-density steady-state discharges at reactor relevant collisionality regimes and beta. During its first operation phase, OP1.1, dedicated to integrated commissioning, tests of components and diagnostics, the device was equipped with five uncooled inboard carbon limiters, which restricted the magnetic configuration. To avoid uncontrolled overheating the overall energy input for a single experiment scenario had been set to < 4MJ. For the maximum available heating power, PECRH= 4 MW provided by 6 gyrotrons, these conditions allowed a discharge duration of 1.2s, equivalently a stationary discharge of 6s could be realized at reduced power.

Stellarator fusion power plants

Abstract: In the stellarator community, a broad variety and diversity of three-dimensional magnetic configurations exist. In this chapter, the respective power plant studies of the major helical magnetic confinement concepts are presented, namely for the heliotron, the advanced stellarator and the compact stellarator concept. Since the diversity of the magnetic configurations is also reflected in the proposed technological solutions, the individual engineering concepts (in particular for blanket and divertor) are presented and brought in context by their advantages and drawbacks. Further, an outlook is given regarding the current developments and future prospects of stellarator fusion power plants.

Power Balance Analysis of Wendelstein 7-X Plasmas Using Profile Diagnostics

Abstract: Using Profile Diagnostics S.A. Bozhenkov1, G. Fuchert1, H. Niemann1, M. Beurskens1, Y. Feng1, O.P. Ford1, J. Geiger1, M. Hirsch1, U. Höfel1, M.W. Jakubowski1, J. Knauer1, P. Kornejew1, A. Langenberg1, H.P. Laqua1, H. Maassberg1, N.B. Marushchenko1, D. Moseev1, N. Pablant2, E. Pasch1, K. Rahbarnia1, T. Stange1, J. Svensson1, H. Trimino Mora1, P. Valson1, G. Wurden3, D. Zhang1, R.C. Wolf1 and W7-X team 1 Max-Planck-Intitut für Plasmaphysik, Greifswald, Germany 2 Princeton Plasma Physics Laboratory, Princeton, NJ, USA 3Los Alamos National Laboratory, Los Alamos, USA

First Results from the Thomson Scattering System at the Stellarator Wendelstein 7-X

Abstract: -up A multi-pulse Nd-YAG laser Thomson scattering system (TS) has been installed on W7-X as a diagnostics for electron temperature and density profile measurements. For the first operation phase (OP1.1) the last closed flux surface W7-X was defined by five graphite limiters mounted at the inboard side. The main objective of OP 1.1 was the integrated operation and components test of W7-X, combined with a physics program. For OP 1.1 the TS system was equipped with a reduced set of 10 spatial points to cover a half profile (from core to edge region) of the outboard side of the plasma cross section. One spatial point was located inside the scrape-off layer (SOL) in the limiter shadow. In OP1.1 the Thomson scattering system provides electron temperature Te and density ne measurements every 100 ms with a 2.5 cm spatial resolution at the core and 4 cm at the edge. The measured core temperatures during OP1.1 were up to 10 keV and the edge temperature at the SOL was less than 15 eV. Typically, core densities were 2-6·10 m.The density calibration of the TS system utilizes anti-Stokes Raman scattering from nitrogen gas and by cross-calibration with a CO2 dispersion interferometer with a close line of sight [1]. In this paper we also present the comparison of the TS temperature profiles with electron-cyclotron emission (ECE) and X-ray spectroscopy (XICS) measurements.