G. Kühner

First island divertor experiments on the W7-AS stellarator

1. Abstract In the past, under limiter conditions, it has been impossible to produce high-power, highdensity, quasi-stationary neutral beam injection (NBI) discharges in W7-AS. Such discharges tended to evince impurity accumulation, lack of density control and subsequent radiation collapse (Normal Confinement). Presently, W7-AS is operating with a modular, open island divertor similar to that foreseen for W7-X. The divertor enables access to a new NBI heated, high density (ne up to 4·10 20 m -3 ) operating regime (High Density H-mode). It is extant above a threshold density, and is characterized by flat density profiles, high energyand low impurity confinement times and edge-localized radiation. The HDH-mode shows strong similarity to ELM-free H-mode scenarios previously observed in W7-AS, but in contrast to these avoids impurity accumulation. These new features enable full density control and quasi steady-state operation over many confinement times (at present only technically limited by the availability of NBI) also under conditions of partial detachment from the divertor targets. In HDH-mode, even in attached discharges, the divertor target load is considerable reduced. This is mainly due to favourable upstream conditions (higher nes), edge localized radiation and increased power deposition width. The benefits of the HDH-mode do not restrict only to hydrogen plasmas. They also occur ‐ albeit in a modified manner ‐ in deuterium plasmas. Undoubtedly, there are clear isotope effects between hydrogen and deuterium discharges. The results obtained in W7-AS render good prospects for W7-X and support the island divertor concept as a serious candidate for devices with magnetic islands at the edge. 2. Results Fig. 1 summarizes the behaviour of the energy confinement time E =W/Pabs, the normalized radiated power Prad/Pabs, and separatrix density nes obtained from quasi-stationary discharges with Pabs=1.4 MW as a function of the line-averaged density ne. E-values in NC follow the scaling E ISS95 =0.26· a 0.4 ·Bt 0.83 ·a 2.21 ·R 0.65 ·ne 0.51 ·Pabs -0.59 , [2], whereas for the HDH-mode one finds E ~ 2· E ISS95 . P rad /P abs grows smoothly with ne until partial plasma detachment, where a jump in the normalized radiated power occurs. The separatrix density n es increases sharply at the NC HDH-mode transition point, then continues to climb with ne and saturates

Experimental and neoclassical electron heat transport in the LMFP regime for the stellarators W7‐A, L‐2, and W7‐AS

The electron energy balance is analyzed for equivalent low‐density electron cyclotron resonance heated (ECRH) discharges with highly peaked central power deposition in the stellarators W7‐A [Plasma Phys. Controlled Fusion 28, 43 (1986)], L‐2 [Proceedings of the 6th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Berchtesgaden, 1976 (International Atomic Energy Agency, Vienna, 1977), Vol. 2, p. 115] and W7‐AS [Proceedings of the 9th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Baltimore, 1982 (International Atomic Energy Agency, Vienna, 1983), Vol. 3, p. 141]. Within the long mean‐free path (LMFP) collisionality regime in stellarators, the neoclassical electron heat diffusivity χe can overcome the ‘‘anomalous’’ one. The neoclassical transport coefficients are calculated by the dkes code (Drift Kinetic Equation Solver) [Phys. Fluids 29, 2951 (1986); Phys. Fluids B 1, 563 (1989)] for these configurations, and the particle and energy flu...

Kinetic modelling of the ECRH power deposition in W7-AS

Kinetic effects are important in low-density high-power ECRH discharges, and the electron distribution function can significantly deviate from Maxwellian. The ECRH power deposition is analysed for perpendicular on-axis heating in W7-AS, with different magnetic configurations characterized by either a minimum or a maximum of B on the plasma axis in the RF injection plane. The different heating scenarios are modelled by means of a new bounce- averaged Fokker-Planck code, well suited for the magnetic field geometry close to the plasma axis of W7-AS. The power deposition profile is estimated from the analysis of heat wave propagation stimulated by ECRH power modulation. In general, peaked deposition profiles as predicted from a ray-tracing code are obtained, but with an additional much broader contribution. The broadening of the thermal power deposition profile is assumed to be related to the radial transport by the rB drift of locally trapped suprathermal electrons. This is simulated by means of a simple convective Fokker-Planck model. The theoretical predictions are shown to be consistent with the experimental findings. Kinetic effects on the determination of the temperature both by Thomson scattering and by ECE diagnostics are briefly discussed.

Electron thermal conductivity from heat wave propagation in Wendelstein 7-AS

Heat wave propagation experiments have been carried out on the Wendelstein 7-AS stellarator. The deposition of electron cyclotron resonance heating power is highly localized in the plasma centre, so that power modulation produces heat waves which propagate away from the deposition volume. Radiometry of the electron cyclotron emission is used to measure the generated temperature perturbation. The propagation time delay of the temperature perturbation as a function of distance to the power deposition region is used to determine the electron thermal conductivity χe. This value is then compared with the value determined by global power balance. In contrast to sawtooth propagation experiments in tokamaks, it is found that the value of χe from heat wave propagation is comparable to that calculated by power balance. In addition, inward propagating waves were produced by choosing a power deposition region away from the plasma centre. Experiments were carried out at 70 GHz in the ordinary mode and at 140 GHz in the extraordinary mode. Variations of the modulation power amplitude have demonstrated that the inferred value of χe is independent of the amplitude of the induced temperature perturbations

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.

Confinement studies on the Wendelstein VII-AS stellarator

ECR heating at B0=2.5T has been extensively used in the 1990 experimental period of the W VII-AS stellarator. As it is a low-shear experiment the magnetic configuration (especially details of the rotational transform profile) depends sensitively on plasma currents (pressure driven, ohmic, EC driven, Ohkawa current) which in turn have a strong influence on energy and particle confinement properties. For the stationary phase a transport analysis has been performed, yielding the profiles of the electron heat conduction and the ion particle diffusion coefficients. The former was subjected to a statistical analysis resulting in phenomenological expressions for chi e and tau E. First experiments using neutral beam injection (ECRH target plasma) as well as combined heating (NBI+ECRH) are also discussed.

Cutting edge concepts for control and data acquisition for Wendelstein 7-X

Wendelstein 7-X is intended to demonstrate steady state high performance plasma operation and to explore the physics basis of the Helias reactor concept. From there, the W7-X CoDaC concepts aim for steady state plasma stabilization at favorite operation points and largely or even fully documented plasma experiments for achieving efficiently scientific results. Several concepts are being employed that are not common to the fusion research so far. The plasma control system is based on segments consisting of control parameters and transition conditions to other segments. The data acquisition system aims at streaming all acquired data for archiving, control and monitoring purposes and tries to prevent early data reduction. This leads to data streaming rates of up to 30 GBytes/s during plasma operation of up to half an hour. The data analyses framework is intended to support largely automation with analyses chains based on service oriented architectures. A coupling to real time systems is envisaged later in the project for solving complex control problems. The envisaged model based data analyses rely on the existence of unanalyzed raw data in combination with a systematic documentation of the experimental setup, in particular for the diagnostics.

Low- and high-mode separation of short wavelength turbulence in dithering Wendelstein 7-AS plasmas

In this article measurements of small scale electron density fluctuations in dithering high confinement~H!-mode plasmas obtained by collective scattering of infrared light are presented. A scan of the fluctuation wavenumber was made in a series of similar discharges in the Wendelstein 7-AS ~W7-AS! stellarator @H. Renner et al., Plasma Phys. Control. Fusion 31, 1579 ~1989!#. The experimental setup and discharge properties are described. Ha-light observing an inner limiter was used to separate low confinement ~L!- and H-mode phases of the plasma; the separated density fluctuations are characterized. It was found that L- ~H-! mode fluctuations dominate at high ~low! frequencies, respectively, and that they possess well-defined and distinguishable scaling properties. Wavenumber spectra for L- and H-mode measurements are calculated and fitted by power-laws and exponential functions. The separated measurements can be fitted with the same exponents in L- and H-mode. Correlations between the density fluctuations, the H a-signal and magnetic fluctuations as measured by Mirnov coils were analyzed. Correlation calculations using 50 ms time windows ~several dithering periods! with time lag steps of 100 ms showed that all the fluctuating quantities are highly correlated and that the maximum correlation occurs for high frequency density fluctuations. Performing separate L- and H-mode correlations on a 20 ms time scale between magnetic and density fluctuations leads to the result that the minimum correlation time scale in L-mode is of order 100 ms, while no correlation exists for H-mode.

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.

Island divertor investigations on the W7-AS stellarator

Abstract The effectiveness of 10 newly installed control coils, which had already been implemented in preparation for the upcoming divertor experiments, has been demonstrated in several important areas. Most importantly we have been able to show that the control coils can be used to influence the ratio of parallel to cross-field transport within an island by nearly a factor 2. High recycling conditions have been attained at the inboard limiters for ι ( a )=5/9 but there was no indication of detachment. Excellent data consistency has been achieved during the high density phase of the high recycling discharges between the saturation current I sat measured by the target Langmuir probes and the equivalent values derived from the new absolutely calibrated signal of the 2D H α CCD camera. Poloidal as well as toroidal asymmetries caused considerable problems when we tried to relate measurements of different edge/target diagnostic signals, taken at equivalent positions around the machine. These asymmetries could with high probability be attributed to toroidally non-symmetrically distributed components like the diamagnetic loop (module 5) or the MINISOX camera (module 3).