Correlation Reflectometer Research Articles

Investigation of a low frequency coherent mode in Wendelstein 7-X with island divertor

During the island divertor operation of W7-X, especially in standard magnetic configuration (with 5/5 island chain in the scrape-off-layer (SOL)), quasi-periodic electromagnetic oscillation is observed. It appears in the frequency range of 1 kHz–2 kHz, therefore it is called the low frequency mode (LFM) within this paper. It is observed by multiple diagnostics, amongst them, a poloidal correlation reflectometer allowing radial localization of the LFM. The LFM is localized in the SOL and shows an obvious modulation effect on the plasma perpendicular velocity ( V⊥ ). Furthermore, broadband turbulence is observed in the fluctuation spectra of the electron density and the magnetic field and the perpendicular correlation length of turbulence eddies are modulated. The calculation of the poloidal flow velocity and its oscillation allows us to study the effect of the LFM on the flow. Cross correlation analysis shows that the perpendicular flow oscillation and the turbulence modulation are closely correlated. The application of external control coils to adjust the island size and position result in a strong modification of the magnetic topology at the plasma edge which affects the appearance, amplitude and frequency of the LFM. Bi-coherence analysis indicates that nonlinear interactions among turbulence components is a possible mechanism for the generation of the LFM.

Passive quadrature demodulation of multiplexed interferometric sensors using a CW correlation reflectometer with a single DFB diode laser.

In this Letter, we report a novel, to the best of our knowledge, and simple approach for passive quadrature-phase demodulation of relatively long multiplexed interferometers based on two-channel coherence correlation reflectometry. Two-wavelength channels are generated using a single unmodulated CW-DFB diode laser and an acousto-optic frequency shifter. The introduced frequency shift determines the optical lengths of the interferometers. In our experiments, all interferometers have the same optical length of 32 cm corresponding to the π/2 phase difference between channel signals. An additional fiber delay line was introduced between channels to destroy coherence between initial and frequency-shifted channels. Demultiplexing of channels and sensors was performed using correlation-based signal processing. Amplitudes of cross correlation peaks obtained for both channels were used to extract the interferometric phase for each interferometer. Phase demodulation of relatively long multiplexed interferometers is experimentally demonstrated. Experimental results prove that the proposed technique is suitable for interrogating a serial array of relatively long interferometers dynamically modulated with phase excursions exceeding 2π. Simultaneous interrogation and phase demodulation were experimentally demonstrated using an in-line array of low-finesse Fabry-Perot interferometric sensors.

Application of the elliptic approximation model for the edge turbulence rotation measurement via the poloidal correlation reflectometer in Wendelstein 7-X

The perpendicular velocity (v ⊥) of turbulence is measured by means of a hopping poloidal correlation reflectometer diagnostic for the study of turbulence rotation in the plasma edge region in Wendelstein 7-X. An elliptic approximation (EA) model is applied for calculating the space-time correlation in the edge region. The v ⊥ profile is compared in the high-ι (edge ι ≈ 1.2) and limiter (edge ι < 1) configurations. It is found that the v ⊥ gradient is inversely proportional to the heating power in the high-ι configuration, which could be attributed to the turbulence activities in the plasma edge. In the limiter configuration, the v ⊥ spike is observed near the island boundary, which is corresponded to the plasma current crash. In the standard configuration (edge ι ≈ 1), the v ⊥ profile in the island region distributes as a function of the perpendicular separation. With increasing of the plasma current, the island enlarges the size as is referred from the v ⊥ scope, accompanying with steepening of the v ⊥ gradient at the boundary of the island.

Experimental study of core and edge fluctuations by reflectometry on EAST tokamak

An eight-channel poloidal correlation reflectometer (PCR) with O-mode polarization has been installed in the EAST tokamak to measure the fluctuations from core to edge. The PCR launches eight different frequency microwaves (20.4, 24.8, 33, 40, 42.4, 48, 52.6, 57.2 GHz) into the plasma from the low field side and two poloidally separated antennae are used to receive the reflected waves. As a result, the diagnostic can measure fluctuations in eight (radial)× two (poloidal) spatial positions. The diagnostic has been applied to study the core and edge pedestal fluctuations during an inter-ELM phase in H-mode plasma. This inter-ELM phase can be divided into two stages. In the first stage, a low frequency (<50 kHz) broadband fluctuation dominates in the pedestal gradient region. In the second stage, this fluctuation is strongly suppressed and quasi-coherent fluctuations (QCFs) appear. The QCF’s amplitude increases with the pedestal density gradient, implying density gradient driven instabilities. But the core fluctuations inside the pedestal show no evident changes during the inter-ELM phase.

Investigation of turbulence rotation in the SOL and plasma edge of W7-X for different magnetic configurations

The W7-X stellarator is optimized with respect to neoclassical transport. Therefore turbulent transport plays an important role. It is equipped with an inertial cooled graphite divertor which intersects the island chain at the plasma edge depending on the magnetic configuration. Additional control coils and the plasma current modify the iota profile at the plasma edge and shift the position of the island chain. To monitor the effects on the poloidal propagation velocity in the scrape-off layer (SOL) and the plasma edge, an O-mode Poloidal Correlation Reflectometer (PCR) is used which simultaneously monitors the propagation of low-k turbulence. Operating in the density range of 0.6 × 1019 m−3 to 2 × 1019 m−3 it covers a large part of the SOL and the plasma edge and allows for the experimental determination in the last closed flux surface (LCFS) and the associated shear layer in low to mid-density discharges. In this paper it is shown that the propagation in the shear layer and its vicinity is determined best, when based on an elliptical model. Different magnetic configurations with magnetic edge topology of five independent islands for ι = 1 and six linked islands for ι = 0.81 are investigated. Also the effects of the plasma current and additional control coils on the edge magnetic topology are studied. The coherence spectra of antenna pairs for different poloidal separations is investigated. Using a decomposition method for the measured coherence spectra the characterization of turbulence spectra is possible with respect to e.g. broad band turbulence and quasi coherent modes. A strong reduction of the broad band turbulence is observed in the vicinity of the LCFS which is evidence for the suppression of low-k turbulence at the shear layer.

Experimental characterization of a quasi-coherent turbulent structure in the edge plasma in Wendelstein 7-X

A quasi-coherent mode (QCM) near the plasma edge is characterized experimentally by measuring the density fluctuation via a poloidal correlation reflectometer (PCR) in Wendelstein 7-X. This QCM frequency descends from 25 kHz near the separatrix to 10 kHz as the cutoff position penetrates toward the core. A similar mode structure is also observed in the spectra of electron cyclotron emission edge channels and Mirnov coil signals, which shows a clear correlation with respect to the PCR measurement. The mode evaluation yields a range for the poloidal mode number of , and for the normalized mode scale , which suggest a drift type instability. The QCM structure appears in the standard and narrow-mirror configurations (both with the rotational transform in the boundary), but is absent in the high- configuration (boundary ). The impact of plasma parameters on the QCM is discussed. It is observed that the QCM frequency and amplitude can be affected by the edge magnetic topology and plasma parameters. The shifts upward during the negative and non-stellarator-symmetric control coil current scans, which may be attributed to the effect of the shear, Er, and ne in the plasma edge region.

Investigation of turbulence rotation and radial electric field in the island divertor and plasma edge at W7-X

Diverted plasmas in the stellarator W7-X are investigated using a hopping poloidal correlation reflectometer and a set of Langmuir probes to investigate the plasma edge and the scrape off layer at two different toroidal positions and poloidal cross sections, respectively. The properties of the scrape off layer are studied as function of two magnetic configurations and as function of plasma parameters. The experimentally determined radial position of the inversion point of the radial electric field is compared with calculations. For certain magnetic configurations a remnant island in the scrape off layer is observed and studied as function of the plasma current. Different methods to determine the radial electric field are applied which allow to estimate the phase velocity of the turbulence. The measured phase velocity is small and shows some evidence for the existence of resistive ballooning modes in the scrape off layer. Spectrum decomposition of coherence spectra is applied for a radial localization of modes and to determine their properties. An estimate of the radial correlation length of the broad band (BB) turbulence in the vicinity of the last closed flux surface is calculated from the decrease of BB turbulence amplitude as obtained from the coherence spectra. The obtained radial correlation length is found to be .

Development of an ordinary mode multi-channel correlation reflectometer on EAST tokamak.

An ordinary-mode polarized multi-channel correlation reflectometer has been developed on the Experimental Advanced Superconducting Tokamak (EAST). The system with four different probing frequencies (i.e., 20.4 GHz, 24.8 GHz, 33 GHz, and 40 GHz) and two poloidally spaced receiving antennas can realize both the radial correlation measurement and the poloidal correlation measurement. These diagnostics focus on the measurement of density fluctuation in the pedestal region to investigate the turbulence transport and H-mode physics on EAST. In this article, the system hardware design, the key component tests, and the system performance are shown in detail.

Density and magnetic fluctuations in type III-ELM pedestal evolution in JET: experimental and numerical characterization

Density and magnetic fluctuation measurements in low-β type-III ELM discharges are obtained in the Joint European Torus (JET). They are observed during the inter-ELM pedestal evolution, after the LH transition phase, at about 60–70 kHz. Density fluctuations are measured with a correlation reflectometer system installed on the low-field side and they are localized at the pedestal top. Magnetic fluctuations with a spatial scale are measured through a high resolution coil array. The main features and the relations with local plasma parameters are presented. The nature of these fluctuations is discussed along with linear gyrokinetic simulations. Ion temperature gradient (ITG) modes are the dominant instabilities in the frequency range of interest. In terms of radial localization, typical oscillation frequency and qualitative relation with the possible linear drive, ITG modes are consistent with the experimental density fluctuations measurements. Micro-tearing modes (MTMs), found unstable with a lower growth rate, appear a possible explanation for magnetic fluctuations in terms of typical wavenumbers and direction of propagation.

Diagnostic set-up and modelling for investigation of synergy between 3D edge physics and plasma-wall interactions on Wendelstein 7-X

A group of edge diagnostics and modelling has been developed for investigation of synergy between 3D edge physics and plasma-wall interactions on Wendelstein 7-X (W7-X). Two endoscopes have been designed for visible and ultraviolet spectroscopy and tomography of the plasma edge, along with infrared thermography of the divertor tiles. 2D profiles of impurities (e.g. helium, carbon) will be measured by two endoscopes viewing the island divertor region in the plasma edge with a spatial resolution of <2 mm. A multipurpose manipulator, which is used as the carrier either of the probe head for measuring the plasma edge profiles or of samples for plasma exposure studies, was installed at the outside mid-plane on W7-X in 2015. A poloidal correlation reflectometer has also been installed at W7-X. The system consists of an antenna array observing the propagation of turbulent phenomena in the mid-plane. The EMC3-EIRENE code package has been adapted for plasma edge transport in helium plasma at W7-X using a hybrid fluid–kinetic approach by enabling EMC3 to treat non-hydrogen isotopes and extending the usage of EIRENE features within EMC3-EIRENE.

Investigation of turbulence rotation in limiter plasmas at W7-X with newly installed poloidal correlation reflectometer

For the first operation phase of the optimized stellarator W7-X, a heterodyne poloidal correlation reflectometry diagnostic is installed and put into operation. The system is intended to measure the perpendicular (with respect to the magnetic field) turbulence rotation and turbulence properties, such as the decorrelation time and correlation length at the plasma edge. Furthermore, it can give information on the magnetic field line pitch angle. The system consists of an array of microwave antennas distributed in the poloidal and toroidal directions. The frequency range of 22 GHz–40 GHz allows us to access local plasma densities of m−3– m−3.During the first operation phase the turbulence rotation is measured in almost all plasmas. In addition, the radial electric field is estimated and compared to that in neoclassical theory. The relatively low plasma density allows us to cover up to 80% of the plasma radius during OP1.1. The obtained data cover various experimental programs and are partly presented in the paper.

Diagnostic setup for investigation of plasma wall interactions at Wendelstein 7-X

Wendelstein 7-X being the most advanced stellarator is currently prepared for commissioning at Greifswald. Forschungszentrum Jülich is preparing a research programme in the field of plasma wall interactions (PWI) by developing a dedicated set of diagnostic systems. The specific interest at Wendelstein 7-X is to understand PWI processes in presence of a 3D plasma boundary of an island divertor. Furthermore, for the first time steady state plasma at high density and low temperature in the divertor region will be available. Since PWI only could be understood in conjunction with the edge plasma properties the aim of the setup is to observe both the edge plasma as well as surface processes. For optimum combination of different diagnostic methods the edge diagnostic systems are aligned toroidally along one out of five magnetic islands. Main systems are a multipurpose fast probe manipulator, two gas boxes in opposite divertor modules together with two endoscopes each observing the divertor regions, a poloidal correlation reflectometer, a dispersion interferometer in the divertor, and VUV and X-ray spectroscopy in the plasma core. The concept of the diagnostic setup is presented in this paper.

Exploiting Graphic Processing Units Parallelism to Improve Intelligent Data Acquisition System Performance in JET's Correlation Reflectometer

The performance of intelligent data acquisition systems relies heavily on their processing capabilities and local bus bandwidth, especially in applications with high sample rates or high number of channels. This is the case of the self adaptive sampling rate data acquisition system installed as a pilot experiment in KG8 B correlation reflectometer at JET. The system, which is based on the ITMS platform, continuously adapts the sample rate during the acquisition depending on the signal bandwidth. In order to do so it must transfer acquired data to a memory buffer in the host processor and run heavy computational algorithms for each data block. The processing capabilities of the host CPU and the bandwidth of the PXI bus limit the maximum sample rate that can be achieved, therefore limiting the maximum bandwidth of the phenomena that can be studied. Graphic processing units (GPU) are becoming an alternative for speeding up compute intensive kernels of scientific, imaging and simulation applications. However, integrating this technology into data acquisition systems is not a straight forward step, not to mention exploiting their parallelism efficiently. This paper discusses the use of GPUs with new high speed data bus interfaces to improve the performance of the self adaptive sampling rate data acquisition system installed on JET. Integration is sues are discussed and performance evaluations are presented.

Observation of turbulence suppression after electron-cyclotron-resonance-heating switch-off on the HL-2A tokamak

The formation of a transient internal transport barrier (ITB) is observed after the electron-cyclotron-resonance-heating (ECRH) switch-off in the HL-2A plasmas, characterized by transient increase of central electron temperature. The newly developed correlation reflectometer provided direct measurements showing reduction of turbulence in the region of steepened gradients for the period of ITB formation triggered by the ECRH switch-off. Furthermore, the reduction of core turbulence is correlated in time with the appearance of a low-frequency mode with a spectrally broad poloidal structure that peaks near zero frequency in the core region. These structures have low poloidal mode number, high poloidal correlation, and short radial correlation and are strongly coupled with high-frequency ambient turbulence. Observation indicates that these structures play important roles in the reduction of the core turbulence and in improvements of the core transport after the off-axis ECRH is turned off.

Density Fluctuation Measurements Using a Frequency Hopping Reflectometer in JT-60U

In JT-60U, a frequency hopping reflectometer has been developed to evaluate the poloidal rotation velocity as a Doppler reflectometer and the radial correlation of density fluctuations in plasmas. The system can measure the radial profile of density fluctuations or the radial correlation profile at two spatial points within 250 ms by combining with the other fixed frequency reflectometer. The radial profiles of the poloidal rotation velocity evaluated from the Doppler-shifted frequency spectrum of density fluctuations show a positive radial electric field in co-rotating plasmas and a negative radial electric field in counter-rotating plasmas. Density fluctuation measurement at the internal transport barrier (ITB) using a correlation reflectometer revealed that long-range correlation increased when ITB was degraded owing to the central heating by electron cyclotron waves.

A Ka-band tunable direct-conversion correlation reflectometer for NSTX

The recent availability of broadband microwave quadrature mixers in the Ka-band (28-40 GHz) of frequencies has allowed the fabrication of low-cost direct-conversion detection circuits for use in the variable-frequency correlation reflectometer on the National Spherical Torus eXperiment (NSTX). The quadrature receiver in this case can be implemented as a simple homodyne circuit, without the complication of a single-sideband modulator or a feedforward tracking circuit present in more typical designs. A pair of direct-conversion receivers is coupled with broadband microwave voltage-controlled oscillators to construct a flexible dual-channel radar system with a fast frequency settling time of ∼160 μs. A detailed description of the design and a full characterization of the hardware are provided. Examples of turbulence measurements from radial and poloidal correlation reflectometry on NSTX using a poloidal array of antennas (oriented normal to the magnetic flux surfaces for conventional reflectometry) are presented.

Self-adaptive sampling rate data acquisition in JET’s correlation reflectometer

Data acquisition systems with self-adaptive sampling rate capabilities have been proposed as a solution to reduce the shear amount of data collected in every discharge of present fusion devices. This paper discusses the design of such a system for its use in the KG8B correlation reflectometer at JET. The system, which is based on the ITMS platform, continuously adapts the sample rate during the acquisition depending on the signal bandwidth. Data are acquired continuously at the expected maximum sample rate and transferred to a memory buffer in the host processor. Thereafter the rest of the process is based on software. Data are read from the memory buffer in blocks and for each block an intelligent decimation algorithm is applied. The decimation algorithm determines the signal bandwidth for each block in order to choose the optimum sample rate for that block, and from there the decimation factor to be used. Memory buffers are used to adapt the throughput of the three main software modules (data acquisition, processing, and storage) following a typical producer-consumer architecture. The system optimizes the amount of data collected while maintaining the same information. Design issues are discussed and results of performance evaluation are presented.

Reflectometry measurements during operation of the dynamic ergodic divertor at TEXTOR

The O-mode correlation reflectometer at TEXTOR is used for the investigation of the propagation and the properties of the turbulence at the plasma edge. The multi-horn antenna set-up allows the investigation of the correlation properties of the turbulence and its rotation. Turbulence and plasma rotation for different plasma conditions are found to be in agreement and based on this comparison the radial electric field is estimated. In addition the density fluctuation level is estimated from the standard deviation of the phase of the reflected signal.The influence of a magnetic perturbation field, generated by the dynamic ergodic divertor (DED), on the turbulence and transport properties is studied and compared with plasmas without such a field perturbation. The strength and radial range of the perturbation field can be widely varied. Together with tangential neutral beam injection in co- and counter-current directions, the turbulent transport is investigated. The combination of neutral beam injection and the DED enables the modification of the plasma rotation profile. It can lead either to the generation of a locked mode or the formation of a transport barrier.

Properties of Geodesic Acoustic Modes and the Relation to Density Fluctuations

The geodesic acoustic mode (GAM) is a high frequency branch of zonal flows, which is observed in toroidal plasmas. Because of toroidal curvature effects, density fluctuations are excited, which are investigated with the O-mode correlation reflectometer at TEXTOR. This Letter reports on the poloidal distribution of GAM induced density fluctuation and compares them with theoretical predictions. The influence of the GAM flows on the ambient turbulence is studied, too.

Microwave Correlation Reflectometry for Tokamak CASTOR

A correlation reflectometer in 8-mm wavelength range has been constructed and installed on CASTOR tokamak. The reflectometer can operate on three fixed frequencies (29, 33, and 35 GHz). This is universal correlation sine-cosine reflectometer, which can work in poloidal or toroidal correlation regime and in ordinary mode (O-mode) and/or extraordinary mode (X-mode). A computational method of a direct evaluation of the phase fluctuations as well as absolute value of the phase from sine-cosine data has been developed. Using this method the relative plasma density fluctuations in the CASTOR core plasma has been determined in the range of few percent under assumption of parabolic density distribution.