M. Kick

Ion temperature and beam-driven plasma waves from collective scattering of gyrotron radiation in W7-AS

Collective scattering of gyrotron radiation and its potential for plasma diagnostics was investigated using the W7-AS stellarator. The ion temperature was determined from thermal spectra in neutral-beam-heated plasmas. The excitation of a narrow band plasma wave by a non-thermal ion component was observed and the wave characteristics were investigated experimentally. The results constitute an important step toward alpha -particle diagnostic via scattering of powerful microwave radiation.

Electron Cyclotron Resonance Heating in the Wendelstein VII-A Stellarator

Plasma build-up and heating of net-current-free plasmas in W VII-A was investigated by ECRH. Experiments were performed at two ECR-frequencies (28 and 70 GHz) and different heating scenarios were investigated such as first harmonic ordinary mode heating and second harmonic extraordinary mode heating. The basic effects predicted by theory, i.e. localized wave absorption and optical thickness of the plasma were verified. The electron heat conduction was found to be governed by neoclassical losses in the plasma core for high enough temperatures, whereas enhanced losses have to be assumed in the outer plasma regions. Generation of a target plasma with sufficient parameters to allow further heating by NBI was successfully demonstrated. Configuration studies showed a beneficial influence of small shear on the confinement, where internal currents have to be taken into account.

Measurement and calculation of the radial electric field in the stellarator W7-AS

At the advanced stellarator W7-AS, active charge exchange recombination spectroscopy (CXRS) is used to measure the toroidal and poloidal impurity rotation velocity. The radial electric field is determined experimentally from the radial force balance equation. In parallel, is calculated by means of the neoclassical drift kinetic equation solver (DKES) transport code. Neoclassical calculation and measurement are, in general, in good agreement. A selection of discharges is presented in this paper which shows typical properties of at W7-AS, and which highlights the mutual relationship between and transport.

Transport in stellarators

The local electron and ion heat transport as well as the particle and impurity transport properties in stellarators are reviewed. In this context, neoclassical theory is used as a guideline for the comparison of the experimental results of the quite different confinement concepts. At sufficiently high temperatures depending on the specific magnetic configuration, neoclassical predictions are confirmed by experimental findings. The confinement properties in the LMFP collisionality regime are discussed with respect to the next stellarator generation, for which at higher temperatures the neoclassical transport is expected to become more important.

Collective Thomson scattering at W7-AS

Collective Thomson scattering (CTS) of electromagnetic radiation from thermal plasma fluctuations in principle allows the velocity distribution of plasma ions and its composition in the plasma to be measured. The use of powerful microwave radiation from gyrotrons opens new perspectives for the application of CTS, which is considered to be a promising candidate for alpha-particle diagnostics in reactor-size tokamaks with D/T operation. We have performed the first experiments at W7-AS with different scattering geometries to prove the applicability of gyrotrons for CTS. The experiments were performed with a 140 GHz gyrotron which is routinely used for ECRH, delivering a power of 0.45 MW. The receiver antenna and detection system for the registration of CTS spectra were especially designed for the scattering experiment. In backscattering experiments, which have inherently no spatial resolution, we have measured a transversely propagating, non-thermal lower-hybrid turbulence, which is driven by perpendicularly injected fast particles from a diagnostic neutral beam. The instability is excited by the beam ions under double-resonance conditions, where the LH frequency coincides with some harmonic of the beam ion gyrofrequency. For scattering geometries with the scattering wavevector not perpendicular to the magnetic field, thermal density fluctuations in the plasma were experimentally detected. The ion temperatures derived from these thermal spectra agree well with other diagnostics. A modified scattering geometry ( scattering) allows local measurements of the ion temperature and is considered a prototype for the design of a routine diagnostic for ion-temperature measurements.

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.

Diagnostic strategy of the W7-X stellarator

The Wendelstein 7-X experiment is a concept test for properties of reactor relevant plasmas in advanced stellarators. Prominent features include a modular superconducting coil assembly, a fivefold toroidal symmetry, and a helical magnetic axis. Due to the optimization process, W7-X is characterized by a vacuum magnetic field configuration with smooth magnetic surfaces, improved equilibrium properties with a weak dependence of rotational transform and shear on the plasma pressure β, good magneto-hydrodynamic stability properties due to magnetic well stabilization, reduced neoclassical transport losses and negligible bootstrap current in the long mean-free-path regime, good collisionless α-particle confinement in an equivalent reactor, and, as a technical aspect, good feasibility of the superconducting modular coils. W7-X will be heated by continuous electron cyclotron resonance heating and pulsed neutral beam injection and ion cyclotron resonance heating. The envisaged parameters are Te⩽10 keV, Ti⩽6 keV ce...

High-confinement NBI discharges in the W7-AS stellarator

In W7-AS, the longest energy confinement times were achieved in neutral beam injection heated (NBI-heated) discharges under low wall-recycling conditions. Low recycling is needed to control the density at line-averaged values of . Under these conditions, confinement was improved by a factor of two above the common scaling estimate. The reduction of radial transport is concentrated into a layer at about two-thirds of the plasma radius. In this region steep pressure gradients and a strong gradient in the radial electric field develop. Specific for the discharges is the slow transition to improved confinement, lasting up to three energy confinement times. Since the measured electric field is consistent with the neoclassical ambipolar field, this high-confinement mode could be an example where sheared plasma flow as created by the neoclassical radial electric field leads to a suppression of anomalous transport.

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.

High power heating experiments on WENDELSTEIN 7-AS stellarator

The upgraded NB heating power on W7-AS allowed extension of the accessible parameter space. In first experiments, a remarkable increase of the ion temperatures was obtained. The results are discussed on the basis of a detailed transport analysis which confirms the neoclassical nature of the ion transport up to the 1 mfp regime. The maximum NB heating power was applied to investigate beta limits. Although beta values close to the predicted stability limit could be reached, no clear indication of a limit has been found so far. Several experimental observations on W7-AS point to the necessity of a divertor. The feasibility of a divertor making use of the natural islands in an optimized stellarator was investigated, and a possible solution for an island divertor on W7-AS is outlined.