S. Fiedler

Edge density measurements with a fast Li beam probe in tokamak and stellarator experiments

Abstract High energy neutral Li beam probes have advanced to the point where they are a standard diagnostic tool on W7-AS and ASDEX-Upgrade, both in terms of the Li beam injector and the reconstruction algorithm to arrive at densities ne(z) along the beam from the Li[2p-2s] resonance line profile. With beam energies in the range 30–70 keV and neutral equivalent currents greater than 1 mA, it is possible to derive ne(z) profiles for line densities n e z 14 cm−2, with a radial resolution of about 0.5 cm and time response of less than 0.2 ms. By way of example, the diagnostic layout on the W7-AS stellarator is sketched and salient results from experiments discussed. Densities over the range 1012–1014 cm−3 are accessible, permitting full coverage of the core density gradient region on W7-AS. Examples from the ASDEX tokamak that involve the H-mode and pellet injection are presented to exemplify the time response.

Two-dimensional density and density fluctuation diagnostic for the edge plasma in fusion devices

A technique is described for the two-dimensional measurement of electron density profile and fluctuations in edge regions of magnetically confined fusion plasmas. The method is based on existing lithium beam beam emission spectroscopy technique, two-dimensional resolution is achieved by electrostatically scanning the beam. If scanning is performed faster than the lifetime of the turbulent structures in the plasma, the diagnostic is capable of measuring the structure of electron density fluctuations as well. The beam strength of currently available beams makes the detection of single fluctuation events impossible, but the full two-dimensional spatial structure of correlations can still be determined. The article describes the technique and fast beam deflection tests up to 250kHz. The capabilities of such a diagnostic for fluctuation measurement are explored by simulating measurement signals. Measurement of both the two-dimensional density profile, fluctuation correlation function and poloidal flow velocity...

determination of Electron Density Fluctuation Correlation Functions via Beam Emission Spectroscopy

Measurement of electron density fluctuations in magnetically confined fusion plasmas by the beam emission spectroscopy (BES) method is discussed and an algorithm is given for the correct reconstruction of correlation functions of the electron density fluctuation from measured correlation functions of beam light fluctuations. The technique is generally applicable to all kinds of neutral beams, provided an appropriate model for beam light emission is available and the beam penetration time through the observation volume is shorter than the characteristic times of the plasma turbulence. Light from a 48 keV Lithium beam on W7-AS plasmas is analysed in detail, whereby electron density fluctuations in the scrape-off layer and in the edge plasma are analysed for a low-density electron cyclotron resonance heated (ECRH) heated discharge. Characteristic changes in the autocorrelation function are found at the last closed flux surface. Limitations of the technique are discussed.

Modelling of fast neutral Li beams for fusion edge plasma diagnostics

We present an experimental and theoretical study on the modelling of fast Li beams which are used for diagnostics of fusion edge plasmas. The atomic collision database utilized for modelling the Li beam attenuation has been revised and extended by detailed investigations of populations resulting from ion impact excitation. We obtain good agreement between measured and calculated Li(nl) populations for an Li beam passing the edge plasma region of the ASDEX Upgrade tokamak and WENDELSTEIN 7 AS stellarator, respectively, at IPP Garching.

Core-edge studies with boundary island configurations on the W7-AS stellarator

Core, edge and scrape-off-layer plasma behaviour is studied principally under conditions of an a = 5/9 boundary island configuration - which is relevant for the upcoming W7-AS divertor campaign - but for now with ten inboard sector limiters. The major focus is on compatibility between good core confinement and attainment of high recycling at the limiter. At low input power Pin0.4 MW, operation at densities necessary to attain effective divertor action in the future invariably leads to a transition to the ELM-free H-mode accompanied by lower edge densities and increased core radiation until radiation collapse ensues. Thereby, enhancement factors in E of nearly two above the international stellarator confinement scaling are transiently achieved. The threshold density ethr, necessary to attain the H-mode increases with heating power, such that at 2 MW NBI heating power the H-mode is completely suppressed and peak densities at the limiter exceeding 1.5 × 1020 m-3 are realized. The efficacy of newly-installed control coils designed to manipulate the island geometry is tested. Their influence on the core plasma is verified. Due to geometrical effects associated with the mutual shadowing of the inboard limiters, statements regarding the influence on island physics must await the divertor configuration.

Perturbative particle transport studies in the W7-AS stellarator

Electron transport coefficients have been determined in the W7-AS stellarator. The diffusion and convection coefficients were obtained by modulating the gas feed into the plasma and by measuring the propagation of the density perturbation. The experiments were carried out at a variety of plasma densities, heating powers and magnetic fields. The results are summarized in the form of a scaling expression for the diffusion coefficient. Transient inward convection was found in the boundary plasma. This convection plays a minor role in the core plasma except with higher heating power, where outward convection was observed. The results are compared with earlier W7-AS transport results.

Operational range and transport barrier of the H-mode in the stellarator W7-AS

In W7-AS the H-mode is characterized by an edge transport barrier localized in the first 3-4 cm inside the separatrix. In the ELMy H-mode preceding the quiescent state ELMs appear as a sudden breakdown of the edge transport barrier in coincidence with bursts of fluctuations. Between ELMs fluctuations are identical to those of the quiescent H-mode. The operational range of the quiescent H-mode is determined by narrow windows of the edge rotational transform and a threshold edge electron density. In contrast, ELM-like events are observed for a variety of plasma conditions by far exceeding the narrow operational windows for the quiescent state.

Stellarator optimization studies in W7-AS

Wendelstein 7-AS is the first large-scale modular stellarator, partially optimized with respect to reduced equilibrium currents . It therefore plays an important role for the optimized next step stellarator W7-X as a pilot experiment for manufacturing techniques of coils and vessel and for equilibrium and neo-classical transport properties of the Wendelstein stellarator line of IPP. Experimental results from W7-AS of importance for the continuation and optimization of the advanced stellarators are summarized: with respect to magnetic optimization we address improved equilibrium, stability and neo-classical transport and the feasibility of the modular coil system. Regarding operational enhancements we focus on demonstrated high-density operation and the soft edge density limit, improved confinement, verification of the 3D plasma edge topology, stability of island structures for exhaust properties and demonstrated stable divertor high recycling in the island divertor configuration.

Radiation measurements and modeling of the density limit on the W7-AS stellarator

Density limit discharges in the W7-AS stellarator with a strong density ramp were compared to a series of discharges with constant line integrated density approaching the maximum value achieved in the density ramp. The physics of the density limit in stellarators was demonstrated to be consistent with the predictions of the two-point model, indicating that this model successfully describes the density limit process in both stellarators and tokamaks. The discharges with a strong density ramp were found to have broader density profiles than those discharges with constant line integrated density. The latter discharges had the electron density profile form found in the improved confinement H-NBI mode on W7-AS. Modeling of the radiation profile, to simultaneously match the measured bolometer and soft X-ray radial profiles of radiated power, implies that impurity density profiles were peaked and continuously increased during the discharge. The increase in radiated power decreased the net deposited power to the plasma and the diamagnetic energy fell. The aim of producing steady-state discharges at the highest possible density is aided by the reduction of impurity sources by helium glow discharge cleaning.

Development of a compact 2.45 GHz ECR ion source

ECR‐heated plasmas as produced by microwave discharges in magnetic fields are now routinely utilized for production of intense singly and multiply charged ion beams. Such ECR ion sources (‘‘ECRIS’’) can be operated during extended lifetimes even with reactive working gases. Our requirement for a simple, long‐lived, high current Li+ ion source for Li‐CXS tokamak plasma diagnostics led to a study on plasma production with a coaxial 2.45 GHz microwave discharge (23 mm outer diameter) embedded in a variable magnetic field configuration. The chosen geometry is compatible with Li vapor production from heat pipe ovens or externally heated LiAl alloy liners, and also permits incorporation of a permanent magnet quadrupole structure for enhancing the ion confinement in the discharge region. As a first step, with a simple magnetic mirror field generated by two axially adjustable solenoids the microwave discharge properties and extractable ion beam currents have been investigated with H2, He, and Ar as feeding gases.