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In this paper, we present the design and commissioning results of the upgraded collective Thomson scattering diagnostic at the Wendelstein 7-X stellarator. The diagnostic has a new radiometer designed to operate between the second and third harmonics of the electron cyclotron emission from the plasma at 171–177 GHz, where the emission background has a minimum and is of order 10–100 eV. It allows us to receive the scattered electromagnetic field with a significantly improved signal-to-noise ratio and extends the set of possible scattering geometries compared to the case of the original instrument operated at 140 GHz. The elements of the diagnostic are a narrowband notch filter and a frequency stabilized probing gyrotron that will allow measuring scattered radiation spectra very close to the probing frequency. Here, we characterize the microwave components applied to the radiometer and demonstrate the performance of the complete system that was achieved during the latest experimental campaign, OP2.1. [ABSTRACT FROM AUTHOR]

A Collective Thomson Scattering (CTS) diagnostic is installed at Wendelstein 7-X for ion temperature measurements in the plasma core. The diagnostic utilizes 140 GHz gyrotrons usually used for electron cyclotron resonance heating (ECRH) as a source of probing radiation. The CTS diagnostic uses a quasi-optical transmission line covering a distance of over 40 m. The transmission line is shared between the ECRH system and the CTS diagnostic. Here we elaborate on the design, installation, and alignment of the CTS diagnostic and present the first measurements at Wendelstein 7-X. [ABSTRACT FROM AUTHOR]

The scalings of correlation lengths and times with the drift-scale parameter ρs and other dimensionless parameters are investigated. Using a novel 8×8 Langmuir probe matrix, the two-dimensional structure of drift-wave turbulence has been measured inside the confinement region of the toroidal low-temperature plasma in the TJ-K torsatron [N. Krause et al., Rev. Sci. Instrum. 73, 3474 (2002)]. Using five different gases from hydrogen to argon, ρs could be varied by a factor of 10. For small ion masses, the scalings of the microscopic parameters turn out to be close to the predictions from drift-wave turbulence, which lead to a gyro-Bohm scaling of the diffusivity. Including heavier ions turns the scaling to more Bohm-like. It is shown that ρs scaling studies carried out on the diffusivity can be misleading if residual dependencies on other parameters are present and the cross phase between poloidal electric field and density fluctuations is not constant as in the present case. The measured turbulent diffusivity is consistent with typical values reported for the edge of fusion plasmas. [ABSTRACT FROM AUTHOR]

Turbulent transport in magnetized plasmas has an intermittent nature. Peaked probability density functions and a 1/frequency decay of the power spectra have been interpreted as signs of self-organized criticality generated, similar to a sand pile, by the critical gradients of ion- (ITG) or electron-temperature-gradient (ETG) driven instabilities. In order to investigate the degree of intermittency in toroidally confined plasmas in the absence of critical pressure or temperature gradients, data from the drift-Alfvén-wave turbulence code DALF3 [B. Scott, Plasma Phys. Controlled Fusion 39, 1635 (1997)], running with a fixed background pressure gradient, and from a weakly driven low-temperature plasma are analyzed. The intermittency is studied on different temporal scales, which are separated by a wavelet transform. Simulated and experimental data reproduce the results on intermittent transport found in fusion plasmas. It can therefore be expected that in fusion plasmas, too, a substantial fraction of the bursty nature of turbulent transport is not related to avalanches caused by a critical gradient as generated by ITG or ETG turbulence. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Comparative studies between a toroidal low-temperature plasma and drift-Alfvén-wave simulations were carried out in order to investigate the microscopic structure of turbulence. The dimensionless plasma parameters in the TJ-K torsatron [N. Krause et al., Rev. Sci. Instrum. 73, 3474 (2002)] are similar to those in the edge of a fusion plasma. At the same time the fluctuations can be fully diagnosed by probe arrays. Fluctuation spectra are analyzed by wavelet techniques indicating a large amount of intermittency in both numerical and experimental data. Since in both cases no critical gradient is present, the intermittency is not due to a state in self-organized criticality (SOC). The spectral density P(ω,k) of the turbulence was measured with a 64-tip Langmuir probe array. A broad spectrum indicates fully developed turbulence. The wave-number spectrum of the density fluctuations decays with a power law with an exponent of 23. The experiments confirm predictions from the turbulence code. The cross-phase between potential and density fluctuations is close to zero on all scales and the spectra shift to smaller wave numbers when the drift scale ρs is increased by changing the ion mass from hydrogen to helium and argon. The ρs scaling is confirmed by correlation measurements within the tips of the poloidal array and an 8×8 probe matrix. The results point to the drift-wave mechanism being responsible for the drive of the turbulence in the low-β plasma of TJ-K. [ABSTRACT FROM AUTHOR]

The Torsatron TJ-K is a toroidal device operated with a magnetically confined low-temperature plasma to study turbulence and wave propagation. The magnetic configuration is the same as for a fusion plasma device. However, the plasma parameters are such that probes can be used in the entire plasma cross section to measure the detailed spatiotemporal structure of the turbulence. At the same time, the dimensionless parameters governing turbulence are in the same range as those of the fusion plasma edge. The objective of the experiment is to obtain data, which allow a close comparison with drift-wave turbulence simulations. Furthermore, the propagation of the helicon heating wave in a toroidal geometry as well as excited gap Alfvén modes will be studied. The article presents the experimental setup and the diagnostics. It is demonstrated that the TJ-K plasma has parameters and profiles relevant to achieve the objectives of the project. First results of turbulence and wave studies are presented. [ABSTRACT FROM AUTHOR]

Baffles are obstacles that shield plasma electrons from being collected by a floating probe by virtue of a small electron gyroradius compared to a large ion gyroradius. A baffled probe floats at space potential if the baffling results in equal electron and ion saturation currents. With such a probe, potential fluctuations can be monitored locally with minimal plasma perturbation. The performance is documented in the edge of a stellarator plasma. [ABSTRACT FROM AUTHOR]

Experimental investigations and simulations of loop antenna excited whistler waves in a cylindrical low temperature plasma are presented. Experiments are performed in the VINETA [Franck et al., Plasma Sources Sci. Technol. 14, 226 (2005)] device and simulations are generated using IPF-FD3D [C. Lechte, IEEE Trans. Plasma Sci. 37, 1099 (2009)], an implementation of the Yee-algorithm. A good agreement of the experiment and the simulation is found. The simulation is used to predict the spatial structure of the wave, the dominant source of the electric field, the relative contribution of electron, and displacement current as well as parallel and perpendicular currents to the magnetic field of the wave. [ABSTRACT FROM AUTHOR]

A passive phased array Doppler reflectometry system has recently been installed in the Wendelstein-7X stellarator. In contrast to conventional Doppler reflectometry systems, the microwave beam can be steered on short time scales in the measurement plane perpendicular to the magnetic field in the range of ±25° without mechanical steering components. This paper characterizes the design and properties of the phased array antenna system and presents the first measurement results from the latest OP1.2a campaign.