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A conceptual design for a 2D beam emission spectroscopy diagnostic system to measure ion gyro-scale plasma turbulence at Wendeslstein 7-X is described. The conceptual design identifies field-aligned viewing geometries and ports for cross-field turbulence measurements in the neutral beam volume. A 2D sightline grid covers the outer plasma region, and the grid configuration provides sufficient k-space coverage in radial and poloidal directions for ion temperature gradient and trapped-electron mode turbulence measurements. Emission intensity estimates, optical transmission losses, and detector noise levels indicate that the measurements will be sensitive to plasma density fluctuations as small as δn/n ≈ 0.5% with a bandwidth of 1 MHz. Implementation challenges include a small beam emission Doppler shift due to nearly radial heating beams and reduced optical throughput due to collection aperture limitations. [ABSTRACT FROM AUTHOR]

The statistical characteristics of the scrape-off layer (SOL) turbulence have been investigated in the first divertor plasma operation of W7-X using a reciprocating probe. The turbulence spectra, auto-correlation time and the statistical parameters are analyzed in three magnetic configurations. In standard and high mirror configurations, the SOL turbulence can be classified into four patterns from the outer SOL to the inner SOL, and each pattern is characterized by the poloidal cross correlation spectrum and the turbulence propagation property. A strong dependence of the SOL turbulence on the magnetic topology is demonstrated experimentally. In high iota configuration, the SOL island is relatively narrow and consequently the fluctuation level of turbulence is weak in the far SOL but enhanced significantly in the near SOL. In the reversed field case, the SOL turbulence reveals similar propagation properties and statistical parameters with those in the normal field case, demonstrating the same turbulence structure in both toroidal field directions. [ABSTRACT FROM AUTHOR]

A significant improvement of plasma parameters in the optimized stellarator W7-X is found after injections of frozen hydrogen pellets. The ion temperature in the post-pellet phase exceeds 3 keV with 5 MW of electron heating and the global energy confinement time surpasses the empirical ISS04-scaling. The plasma parameters realized in such experiments are significantly above those in comparable gas-fuelled discharges. In this paper, we present details of these pellet experiments and discuss the main plasma properties during the enhanced confinement phases. Local power balance is applied to show that the heat transport in post-pellet phases is close to the neoclassical level for the ion channel and is about a factor of two above that level for the combined losses. In comparable gas-fuelled discharges, the heat transport is by about ten times larger than the neoclassical level, and thus is largely anomalous. It is further observed that the improvement in the transport is related to the peaked density profiles that lead to a stabilization of the ion-scale turbulence. [ABSTRACT FROM AUTHOR]

In the first experimental campaign of Wendelstein 7-X (W7-X), a combined probe head mounted on the multi-purpose manipulator has been used to measure the scrape-off layer (SOL) turbulence characteristics. The preliminary experimental results are summarized to illustrate the SOL turbulence properties in the limiter configuration on W7-X. In a standard limiter configuration, significant electrostatic fluctuations can be found in the near SOL, and the dominant frequency of fluctuation power is below 100 kHz. The auto-correlation spectrum power law decay factor is α ≈ −1 below 40 kHz and α ≈ –2 between 50 and 200 kHz. A broadband spectrum appears between 240 and 380 kHz with a low spectral power density, but a high cross-correlation coefficient. The statistical characteristics of turbulence are calculated by the two-point cross-correlation technique. A clear poloidal dispersion relation is found in the spectrum S(kθ, f), propagating along the ion diamagnetic drift direction with a group velocity (below 100 kHz) about 0.56 km/s in the near SOL in the laboratory frame. The poloidal correlation length is around 5–10 mm in SOL. The turbulence phase velocity is about 0.5–1 km/s when close to the last closed flux surface, which is comparable with the poloidal E × B drift speed. [ABSTRACT FROM AUTHOR]

Abstract: The control of the dynamics of drift waves is investigated experimentally in a linear laboratory plasma device. The control parameter is a mode selective drive of currents parallel to the magnetic field using an arrangement of eight azimuthally arranged electrodes and eight saddle coils, respectively. The driven currents resemble the intrinsic drift mode currents. Single coherent drift modes are synchronized in frequency by both control setups only if the control signal has the same mode structure and direction of phase velocity as the drift mode. The frequency range of synchronization depends nonlinearly on the amplitude of the driven current. In drift wave turbulence the turbulent fluctuations are synchronized to a single coherent drift mode to the expense of the energy of the broadband turbulent fluctuations. As a result the phase shift of density and potential fluctuations decreases considerably, thereby reducing the cross-field transport. [Copyright &y& Elsevier]

Abstract: The propagation properties of plasma density structures in a turbulent linear cylindrical helicon plasma are investigated. The governing instability is of the drift-wave type. An azimuthal limiter is used to steepen the radial density gradient to achieve a weakly developed turbulent state. The spatiotemporal evolution of turbulent plasma density fluctuations is investigated by Langmuir probe array measurements. In the strong plasma density gradient region the probability distribution function (PDF) of density fluctuations is rather symmetric. Here, the relict of a single m =1 drift mode dominate the correlation of the fluctuations. Radially further outwards the fluctuations are dominated by large positive density bursts, associated with strongly asymmetric PDFs. The spatiotemporal measurements reveal that these intermittent bursts can be ascribed to radially propagating turbulent structures, which originate in the strong plasma gradient region. Their radial velocity reaches 5% of the local ion sound speed. [Copyright &y& Elsevier]

The formation and propagation of spatiotemporal fluctuation structures in weakly developed drift-wave turbulence in a linearly magnetized helicon device is investigated. Turbulent density fluctuations in the far edge plasma display an intermittent character with large-amplitude positive density bursts. Their peak amplitudes correspond to the time-averaged density in the maximum radial plasma pressure gradient. The conditional average technique is applied to reconstruct the dynamics of turbulent coherent structures in the azimuthal plane. The formation of turbulent structures is closely linked to a quasicoherent m=1 drift wave mode, which is generally observed in the radial density gradient region in the weakly developed turbulent state. It is demonstrated that every positive high amplitude density burst in the plasma edge is due to the radial propagation of a turbulent structure. The typical scale size of the turbulent structures is 4ρs and their lifetime exceeds the eddy turnover time by orders of magnitude, thereby characterizing them as coherent structures. Although the turbulent structures propagate mainly azimuthally in the direction of the E×B drift they are observed to have a radial velocity, which is typically 10% of the ion sound speed. [ABSTRACT FROM AUTHOR]

A high speed (250 kHz), 300 frame charge coupled device camera has been used to image turbulence in the Alcator C-Mod Tokamak. The camera system is described and some of its important characteristics are measured, including time response and uniformity over the field-of-view. The diagnostic has been used in two applications. One uses gas-puff imaging to illuminate the turbulence in the edge/scrape-off-layer region, where D2 gas puffs localize the emission in a plane perpendicular to the magnetic field when viewed by the camera system. The dynamics of the underlying turbulence around and outside the separatrix are detected in this manner. In a second diagnostic application, the light from an injected, ablating, high speed Li pellet is observed radially from the outer midplane, and fast poloidal motion of toroidal striations are seen in the Li+ light well inside the separatrix. [ABSTRACT FROM AUTHOR]

Abstract: The influence of an externally generated shear flow on the propagation of structures in drift-wave turbulence is investigated in a linear laboratory plasma device. Spatiotemporal studies of the dynamics of turbulent structures reveal that structures have a radial component of propagation, thereby contributing mainly to the fluctuation induced particle flux and leading to intermittent density fluctuations in the edge plasma. A radially localized radial -velocity shear layer is generated by a biased emissive filament. The shear layer modifies the phase between density and potential fluctuations, which results in a reduction of transport driven by electrostatic turbulence. A shear decorrelation of the structures, i.e. a decrease of their radial correlation length, is not observed. [Copyright &y& Elsevier]

Abstract: Movies of edge turbulence at both the outboard midplane and the region outboard of the typical lower X-point location in C-Mod have been obtained using gas-puff-imaging together with fast-framing cameras. Intermittent turbulence structures, typically referred to as blobs or filaments, are observed in both locations. Near the midplane the filaments are roughly circular in cross-section, while in the X-point region they are highly elongated. Filament velocities in this region are ∼3x faster than the radial velocities at the midplane, in a direction roughly outward across the local flux surfaces. The observations are consistent with the picture that the filaments arise in the outboard region and, as a consequence of the rapid parallel diffusion of the potential perturbations, map along field lines. Results from a 3D BOUT turbulence simulation reproduce many of the spatial features observed in the experiment. [Copyright &y& Elsevier]