Doppler Reflectometer Research Articles

Experimental observation of micro-tearing modes in the tokamak pedestal

A micro-tearing mode has been observed in electron temperature fluctuations and magnetic fluctuations with the frequency range of 80–120 kHz in the pedestal of the Experimental Advanced Superconducting Tokamak (EAST) using Electron Cyclotron Emission Imaging (ECEI) and Mirnov magnetic coil measurement. It appears in the type-I ELMs period after the L-H transition. The measured poloidal phase velocity of the electron temperature fluctuations is ∼40 km/s (f = 80–120 kHz) in the electron diamagnetic direction in the laboratory frame with the poloidal wave number around kΘ ∼0.15 /cm. The two-dimensional structure of MTMs is directly observed using two-dimensional Electron cyclotron emission imaging. The experiment confirmed that the electron temperature (ECEI) and radial electric field (Backward Doppler reflectometer) during MTM meet the pressure balance equation. Until now, the theory and simulation results showed that transport is in heat flux, small particle flux, while the experimental results of this paper indicate that the particle flux is small.

Comparison of methods for turbulence Doppler frequency shift calculation in Doppler reflectometer

The Doppler reflectometer (DR), a powerful diagnostic for the plasma perpendicular velocity (u ⊥) and turbulence measurement, has been widely used in various fusion devices. Many efforts have been put into extracting the Doppler shift from the DR signal. There are several commonly used methods for Doppler shift extraction, such as the phase derivative, the center of gravity, and symmetric fitting (SFIT). However, the strong zero-order reflection component around 0 kHz may interfere with the calculation of the Doppler shift. To avoid the influence of the zero-frequency peak, the asymmetric fitting (AFIT) method was designed to calculate the Doppler shift. Nevertheless, the AFIT method may lead to an unacceptable error when the Doppler shift is relatively small compared to the half width at half maximum (HWHM). Therefore, an improved method, which can remove the zero-frequency peak and fit the remaining Doppler peak with a Gaussian function, is devised to extract the Doppler shift. This method can still work reliably whether the HWHM is larger than the Doppler shift or not.

On the interaction between the island divertor heat fluxes, the scrape-off layer radial electric field and the edge turbulence in Wendelstein 7-X plasmas

The formation of the radial electric field, in the scrape-off layer (SOL) has been experimentally studied for attached divertor conditions in stellarator W7-X. The main objective of this study is to test the validity in a complex three-dimensional (3D) island divertor of simple models, typically developed in tokamaks, relating in the SOL to the sheath potential drop gradient at the target. Additionally, we investigate the effect of the edge shear on the reduction of density fluctuation amplitude, a well-established phenomenon according to the existing bibliography. The main diagnostic for measurements in the SOL is a V-band Doppler reflectometer that can provide the measurement of the and density fluctuations with good spatial resolution. 3D measurements of divertor parameters have been carried out using infrared cameras, with the exponential decay length of the divertor heat flux () resulting in a suitable proxy for the model-relevant , the exponential decay length of the temperature at the divertor. In the investigated attached regimes, it is shown for the first time that the formation of the in the SOL depends on parameters at the divertor, following a qualitatively similar to that found in a tokamak. Then, from the analyzed plasmas, the observed shear at the edge is linked to a moderate local reduction of the amplitude of density fluctuations.

Ku-band multichannel frequency comb Doppler reflectometer on the GAMMA 10/potential control and divertor simulating experiment (PDX) tandem mirror.

A Ku-band (12-18GHz) multichannel Doppler reflectometer (DR) has been developed in the GAMMA 10/potential control and divertor simulating experiment (PDX) tandem mirror device to improve the applicability of DR measurement for simultaneous monitoring of velocity of electron density turbulence at different locations. Our previous single-channel DR circuit has been replaced by the multichannel microwave system using a nonlinear transmission line based comb generator with heterodyne technique. The multichannel DR system has been installed in the central cell of GAMMA 10/PDX. Initial results of application to GAMMA 10/PDX plasma are presented, showing Doppler frequency shifts during an additional ion cyclotron resonance frequency heating and gas-puffing experiment.

3D metal powder additive manufacturing phased array antenna for multichannel Doppler reflectometer.

Measuring the time variation of the wavenumber spectrum of turbulence is important for understanding the characteristics of high-temperature plasmas, and the application of a Doppler reflectometer with simultaneous multi-frequency sources is expected. To implement this diagnostic in future fusion devices, the use of a phased array antenna (PAA) that can scan microwave beams without moving antennas is recommended. Since the frequency-scanning waveguide leaky-wave antenna-type PAA has a complex structure, we have investigated its characteristics by modeling it with 3D metal powder additive manufacturing (AM). First, a single waveguide is fabricated to understand the characteristics of 3D AM techniques, and it is clear that there are differences in performance depending on the direction of manufacture and surface treatment. Then, a PAA is made, and it is confirmed that the beam can be emitted in any direction by frequency scanning. The plasma flow velocity can be measured by applying the 3D manufacturing PAA to plasma measurement.

Application of Dual Frequency Comb Method as an Approach to Improve the Performance of Multi-Frequency Simultaneous Radiation Doppler Radar for High Temperature Plasma Diagnostics

A new Doppler radar using millimeter-waves in the Ka-band, named the “dual-comb Doppler reflectometer”, has been developed to measure the turbulence intensity and its velocity in high-temperature plasmas. For the realization of a fusion power generation, it is very important to know the spatial structure of turbulence, which is the cause of plasma confinement degradation. As a non-invasive and high spatial resolution measurement method for this purpose, we apply a multi-frequency Doppler radar especially with simultaneous multi-point measurement using a frequency comb. The newly developed method of synchronizing two frequency combs allows a lower intermediate frequency (IF) than the previously developed frequency comb radar, lowering the bandwidth of the data acquisition system and enabling low-cost, long-duration plasma measurements. In the current dual-comb radar system, IF bandwidth is less than 0.5 GHz; it used to be 8 GHz for the entire Ka-band probing. We applied this system to the high-temperature plasma experimental device, the Large Helical Device (LHD), and, to demonstrate this system, verified that it shows time variation similar to that of the existing Doppler radar measurements.

On the role of density fluctuations in the core turbulent transport of Wendelstein 7-X

A recent characterization of core turbulence carried out with a Doppler reflectometer in the optimized stellarator Wendelstein 7-X (W7-X) found that discharges achieving high ion temperatures at the core featured an ITG-like suppression of density fluctuations driven by a reduction of the gradient ratio (Carralero et al 2021 Nucl. Fusion 61 096015). In order to confirm the role of ITG turbulence in this process, we set out to establish experimentally the relation between core density fluctuations, turbulent heat flux and global confinement. With this aim, we consider the scenarios found in the previous work and carry out power balance analysis for a number of representative ones, including some featuring high ion temperature. We also evaluate the global energy confinement time and discuss it in the context of the ISS04 inter-stellarator scaling. We find that when turbulence is suppressed as a result of a reduction of η i , there is a reduction of ion turbulent transport, and global performance is improved as a result. This is consistent with ITG turbulence limiting the ion temperature at the core of W7-X. In contrast, when turbulence is reduced following a decrease in collisionality, no changes are observed in transport or confinement. This could be explained by ITG modes being combined with TEM turbulence when the latter is destabilized at low collisionalities.

A feasibility study for a Doppler reflectometer system in the JT-60SA tokamak

In this work we present a study on the viability and practicality of installing a Doppler reflectometer (DR) system in the JT-60SA advanced tokamak. First, we discuss its scientific scope in the context of the JT-60SA research plan. We identify a number of fields in which a DR would be very relevant for the accomplishment of said plan and outline a scientific program for the diagnostic. Then, starting from a number of design hypothesis, we use a ray tracing code to carry out a feasibility study for a number of relevant scenarios and identify a geometric solution for the installation of a DR such that both core and edge can be probed in the prescribed wave number range, thus achieving the proposed scientific objectives. Finally, we perform a preliminary discussion on the different possibilities for a conceptual design (including a minimum viable system and a baseline system) and their requirements in terms of components and space. We conclude that a viable conceptual design could be carried out using a small fraction of a horizontal port, leaving room for additional diagnostic systems.

Impact of magnetic islands on plasma flow and turbulence in W7-X

The effect of magnetic islands on plasma flow and turbulence has been experimentally investigated in the stellarator W7-X. Magnetic configurations with the 5/5 magnetic island positioned at the plasma edge, inside the last closed flux surface, are studied. The main diagnostic used in the present work is a V-band Doppler reflectometer that allows the measurement of the perpendicular plasma flow and density fluctuations with good spatial resolution. A characteristic signature of the 5/5 magnetic island is clearly detected in the perpendicular flow profile. The comparison of the experimental flow and the neoclassically driven E × B flow indicates that the island contribution to the flow is maximum at the island boundaries and close to zero at the island O-point. Besides, a reduction in the density fluctuation level is found nearby the island O-point. The similarities between these observations and those found in other devices and in gyrokinetic simulations are discussed.

The cross-polarization scattering system for the magnetic fluctuation measurement in the Experimental Advanced Superconducting Tokamak.

The cross-polarization scattering (CPS) system for magnetic fluctuation measurements in the Experimental Advanced Superconducting Tokamak (EAST) has been designed and installed. Different from the Doppler reflectometer (DR) system, the CPS system detects the perpendicular polarization of the electromagnetic wave induced by magnetic fluctuations B̃. The CPS system in the EAST has been developed from the existing Doppler reflectometer system, and they are integrated together for simultaneous measurement of magnetic and density fluctuations. Ray-tracing simulations are used to calculate the scattering locations and the wavenumber coverage of the magnetic fluctuation for CPS. In the experiments, the CPS and DR system data were different in Doppler shift, amplitude, and spectrum broadening. In this article, the hardware design, the ray tracing, and the preliminary results of the system in the EAST are presented.

Development of a tunable multi-channel Doppler reflectometer on J-TEXT tokamak.

Doppler reflectometer is a powerful diagnostic tool to study the turbulence for tokamak plasmas. It can provide information on the density fluctuation, the poloidal rotation, the radial electric field, its shear, etc. A tunable multi-channel V-band (50-75GHz) Doppler reflectometer system has been developed on the J-TEXT tokamak for the measurement under various toroidal magnetic fields. A universal serial bus controlled synthesizer is used as a source that can adjust the probing frequency remotely. This Doppler reflectometer can measure the plasma in 0.3 < ρ < 1 . Its radial resolution is <2 cm, and k⊥ is ∼ 4-12 cm-1. Based on the Doppler reflectometer, the perpendicular turbulence propagation velocity, the profile of the radial electric field, the geodesic acoustic mode, and some other phenomena have been observed on J-TEXT.

Investigations of Alfvén Modes at the Globus-M2 Tokamak Using a V-Band Multifrequency Doppler Reflectometer

We present new information on the development of Alfven modes on the Globus-M2 spherical tokamak. Data have been obtained using a V-band Doppler reflectometer with probing radiation frequencies from 50 to 75 GHz. Аs a multifrequency source of probing radiation, a microwave harmonic synthesizer has been used. As a result of using the new reflectometer, the toroidal Alfven mode localization in a magnetic field of 0.5 T has been obtained. The spectral components of the Alfven mode with Doppler frequency shifts caused by toroidal plasma rotation have been registered. In the central regions of the discharge, so-called “Alfven cascades” have been investigated.

Исследования альфвеновских мод на токамаке Глобус-М2 с использованием многочастотного допплеровского рефлектометра V-диапазона

New information on the development of Alfvén modes in the Globus-M2 spherical tokamak is presented. The data were obtained using a V-band Doppler reflectometer with probing radiation frequencies from 50 to 75 GHz. A microwave harmonic synthesizer was used as a multifrequency probing source. As a result of using a new reflectometer, the localization of the toroidal Alfvén eigenmode has been determined at a magnetic field of 0.5 T. The spectral components of the Alfvén mode with Doppler frequency shifts due to toroidal plasma rotation have been recorded. The so-called Alfvén cascades have been investigated in the central regions of the discharge.

Characterization of the radial electric field and edge velocity shear in Wendelstein 7-X

In this work we present the first measurements obtained by the V-band Doppler reflectometer during the second operation phase of Wendelstein 7-X to discuss the influence in the velocity shear layer and the radial electric field, Er, of several plasma parameters such as magnetic configuration, rotational transform or degree of detachment. In the first place, we carry out a systematic characterization of the turbulence rotation velocity profile in order to describe the influence of density and heating power on Er under the four most frequent magnetic configurations. The E value in the edge is found to increase with configurations featuring higher ι, although this does not apply for the high mirror configuration, KJM. As well, the Er value in the SOL and the velocity shear near the separatrix are found to display a clear dependence on heating power and density for all configurations. For a number of relevant cases, these results are assessed by comparing them to neoclassical predictions obtained from the codes DKES and KNOSOS, finding generally good agreement with experimental results. Finally, the evolution of Er at the edge is evaluated throughout the island-divertor detachment regime achieved for the first time in the 2018 campaign. After detachment, E is reduced both at the SOL and edge, and the plasma column shrinks, with the shear layer seemingly moving radially inwards from the separatrix.

Measurement of Radial Electric Field Using Doppler Reflectometer in High-Density Plasma of Heliotron J

Measurement of Radial Electric Field Using Doppler Reflectometer in High-Density Plasma of Heliotron J

Coherent modes and turbulences observations with multi-channel Doppler reflectometer on experimental advanced superconducting tokamak

With a newly installed 5-channel W-band Doppler reflectometer (DR) in EAST tokamak, many interesting coherence modes and turbulences have been observed. In this paper, the tearing mode (TM), a novel inter-edge localized mode (ELM) electromagnetic mode, and the eigenmode geodesic acoustic mode (GAM) accompanied by a broadband quasi-coherent mode (QCM) during H-mode stage have been reported. The TM could be directly observed in both density fluctuation and perpendicular velocity fluctuation at the DR channel localized in the magnetic island region; moreover, many high-order TM frequency harmonics, dual turbulence peaks inside and outside the island separatrix, and strong flow shear across the island separatrix could also be observed. A high-frequency (∼230 kHz) electromagnetic mode (toroidal mode number n = 1) could be identified. The mode is generated dozens of milliseconds before the ELM crash and may affect the ELM dynamic process. The eigenmode GAM excited during the H-mode stage is generated near the top of the density pedestal and then propagates radially inward with a radial wavenumber of about 1 cm−1. A broadband QCM with a frequency range of 20–100 kHz is always accompanied by eigenmode GAM and nonlinearly coupled with both GAM and the background turbulence from the bispectrum analysis. All these results convince the validity of the Doppler reflectometer in multi-scale measurement from the mesoscale to microscale.

Spatial structure of low-frequency fluctuations throughout the transition of poloidal flow velocity in edge plasmas of LHD

It is observed that a low-frequency (∼2 kHz) density fluctuation is excited in a transition of the poloidal flow velocity (Vθ) in the edge magnetic stochastic region of the Large Helical Device plasmas. Furthermore, it is found that the propagation velocity becomes approximately zero in the proximity of the appearance region of the low-frequency fluctuation by using the edge multichannel microwave Doppler reflectometer system. In particular, the low-frequency fluctuation is considered to be transmitted in both directions (inward and outward) away from the excitation position of the fluctuation, which behaves as the precursor of a magnetic fluctuation burst. Afterward, the edge Hα signal intensity is sharply increased.

Exhaust of turbulence cloud at the tongue shaped deformation event

Exhaust of turbulence cloud at the tongue-shaped deformation which triggers MHD bursts is observed in the Large Helical Device in the low density plasma with significant contribution of trapped particles injected by perpendicular neutral beam injection. The exhaust of turbulence cloud is characterized by the abrupt large increase of turbulence amplitude in the frequency range of 150–500 kHz measured with Doppler reflectometer at the edge region of the plasma (). The increase of turbulence amplitude is significantly large and is by one order of magnitude. This abrupt increase of turbulence level is transient and disappears within one milli-second (typically ∼600 μs). In contrast, the turbulence level slightly inside the plasma edge () decreases by a factor of 2 after the MHD bursts.

Microwave frequency comb Doppler reflectometer applying fast digital data acquisition system in LHD.

We succeeded in increasing the radial observation points of the microwave frequency comb Doppler reflectometer system from 8 to 20 (or especially up to 45) using the high sampling rate of 40 GS/s digital signal processing. For a new acquisition system, the estimation scheme of the Doppler shifted frequency is constructed and compared with the conventional technique. Also, the fine radial profile of perpendicular velocity is obtained, and it is found that the perpendicular velocity profile is consistent with the E × B drift velocity one.

Application of the Haar Wavelet to the Analysis of Plasma and Atmospheric Fluctuations

The parameters of turbulence measured by means of a Doppler reflectometer at the plasma periphery in an L-2M stellarator and in atmospheric vortices (typhoons and tornadoes) are investigated using the wavelet methods with involvement of the Haar function. The periods of time taken for the transition (a bound of parameters) to occur in the L-2M stellarator plasma and in atmospheric processes are estimated. It is shown that high-and low-frequency oscillations of certain parameters, in particular, pressure, that occur in atmospheric vortices decay or increase at different moments of time, whereas the density fluctuation amplitudes that occur in plasma at different frequencies vary in a synchronous manner.