Doppler Backscattering System Research Articles

An E-band multi-channel Doppler backscattering system on EAST.

An E-band (60-90 GHz) multi-channel Doppler backscattering (DBS) system with X-mode polarization has been installed on the Experimental Advanced Superconducting Tokamak (EAST), which can measure the turbulence at five different radial locations simultaneously. This system can launch 31 fixed microwave frequencies in the range of 60-90 GHz with a 1GHz interval into the plasma, and five probing signals are selected by employing a reference signal and multiple filters. During experiments, the frequency of the reference signal is tunable in the E-band, and the selected probing signals can be changed as needed without any other adjustments, which can be performed in one shot or between shots. Furthermore, the incident angle can be adjusted from -10° to 20°, and the wavenumber range is 4-25cm-1 with a wavenumber resolution of Δk/k ≤ 0.35. Ray tracing simulations are employed to calculate the scattering locations and the perpendicular wavenumber. In this article, the hardware design, ray tracing, and initial results obtained from the EAST plasma will be presented.

First data and preliminary experimental results from a new Doppler backscattering system on the MAST-U spherical tokamak

A new Doppler backscattering (DBS) system, consisting of Q-band and V-band, has been installed and achieved its first data on the MAST-U spherical tokamak. The Q-band and V-band have separate microwave source systems, but share the same optical front-end components. The Q-band and V-band sources simultaneously generate eight (34, 36, 38, 40, 42, 44, 46 and 48 GHz) and seven (52.5, 55, 57.5, 60, 62.5, 65 and 67.5 GHz) fixed frequency probe beams, respectively. These frequencies provide a large range of radial positions from the low-field-side edge plasma to the core, and possibly to the high-field-side edge, depending on the plasma conditions. The quasi-optical system consists of a remotely-tunable polarizer, a focusing lens and a remotely-steerable mirror. By steering the mirror, the system provides remote control of the probed density fluctuation wavenumber, and allow the launch angle to match the magnetic field. The range of accessible turbulence wavenumbers (kθ ) is reasonably large with normalized wavenumber kθρs ranging from <0.5 to 9. The first data acquired by this DBS system is validated by comparing with the data from the other DBS system on MAST-U (introduced in ref. [21]). An example of measuring the velocity profile spanning from the edge to the center in a high-density plasma is presented, indicating the robust capabilities of the integrated Q-band and V-band DBS systems.

A novel Doppler backscattering (DBS) system to simultaneously measure radio frequency plasma fluctuations and low frequency turbulence.

A novel quadrature Doppler Backscattering (DBS) system has been developed and optimized for the E-band (60-90GHz) frequency range using either O-mode or X-mode polarization in DIII-D plasmas. In general, DBS measures the amplitude of density fluctuations and their velocity in the lab frame. The system can simultaneously monitor both low-frequency turbulence (f < 10MHz) and radiofrequency plasma density fluctuations over a selectable frequency range (20-500MHz). Detection of high-frequency fluctuations has been demonstrated for low harmonics of the ion cyclotron frequency (e.g., 2fci ∼ 23MHz) and externally driven high-frequency helicon waves (f = 476MHz) using an adjustable frequency down conversion system. Importantly, this extends the application of DBS to a high-frequency spectral domain while maintaining important turbulence and flow measurement capabilities. This unique system has low phase noise, good temporal resolution (sub-millisecond), and excellent wavenumber coverage (kθ ∼ 1-20cm-1 and kr ≲ 30cm-1). As a demonstration, localized internal DIII-D plasma measurements are presented from turbulence (f ≤ 5MHz), Alfvenic waves (f ∼ 6.5MHz), ion cyclotron waves (f ≥ 20MHz), as well as fluctuations around 476MHz driven by an external high-power 476MHz helicon wave antenna. In the future, helicon measurements will be used to validate GENRAY and AORSA modeling tools for prediction of helicon wave propagation, absorption, and current drive location for the newly installed helicon current drive system on DIII-D.

Characterization of mesoscopic turbulent transport events with long-radial-range correlation in DIII-D H-mode plasmas

A dimensionless collisionality scan has been performed in H-mode plasmas on DIII-D tokamak, with detailed measurements of intermediate-to-high wavenumber turbulence using Doppler backscattering systems. It is found that the shorter wavelength turbulence develops into spatially asymmetric turbulent structures with a long-radial-range correlation (LRRC) in the mid-radius region of high collisionality discharges. Linear cgyro simulations indicate that the underlying turbulence is likely driven by the electron-temperature-gradient mode. The LRRC transport events are highly intermittent and show a power spectrum of Sñ(k⊥)∝k⊥−1 for density fluctuations, which is often associated with self-organized criticality. The magnitude and the radial scale of those turbulent structures increase significantly when the Er×B mean flow shearing rate decreases at higher collisionality. The enhanced LRRC transport events appear to be correlated with the degraded energy confinement time. The emergence of such LRRC transport events may serve as a candidate explanation for the degrading nature of H-mode core plasma confinement at high collisionality.

First Results of the Implementation of the Doppler Backscattering Diagnostic for the Investigation of the Transition to H-Mode in the Spherical Tokamak Globus-M2.

This paper presents the first results of a study of the LH transition on the new spherical Globus-M2 tokamak using the Doppler backscattering (DBS) diagnostic. New data characterizing the H-mode of discharges with higher values of the plasma parameters, such as magnetic field Bt up to 0.9 T and plasma current Ip up to 450 kA, were collected and analyzed. An upgraded neutral beam injection (NBI) system was used to initiate the LH transition. DBS allows the measurement of the poloidal rotation velocity and the turbulence amplitude of the plasma. The multi-frequency DBS system installed on Globus-M2 can simultaneously collect data in different areas spanning from the separatrix to the plasma core. This allowed for the radial profiles of the rotation velocity and electric field to be calculated before and after the LH transition. In addition, the values and temporal evolution of the velocity shear were obtained. The associated turbulence suppression after the transition to the H-mode was investigated using DBS.

Design elements and first data from a new Doppler backscattering system on the MAST-U spherical tokamak.

A new Doppler backscattering (DBS) system has been installed and tested on the MAST-U spherical tokamak. It utilizes eight simultaneous fixed frequency probe beams (32.5, 35, 37.5, 40, 42.5, 45, 47.5, and 50GHz). These frequencies provide a range of radial positions from the edge plasma to the core depending on plasma conditions. The system utilizes a combination of novel features to provide remote control of the probed density wavenumber, the launched polarization (X vs O-mode), and the angle of the launched DBS to match the magnetic field pitch angle. The range of accessible density turbulence wavenumbers (kθ) is reasonably large with normalized wavenumbers kθρs ranging from ≤0.5 to 9 (ion sound gyroradius ρs = 1cm). This wavenumber range is relevant to a variety of instabilities believed to be important in establishing plasma transport (e.g., ion temperature gradient, trapped electron, electron temperature gradient, micro-tearing, kinetic ballooning modes). The system is specifically designed to address the requirement of density fluctuation wavevector alignment which can significantly reduce the SNR if not accounted for.

Receiver circuit improvement of dual frequency-comb ka-band Doppler backscattering system in the large helical device (LHD).

Doppler-backscattering (DBS) has been used in several fusion plasma devices because it can measure the perpendicular velocity of electron density perturbation v⊥, the radial electric field Er, and the perpendicular wavenumber spectrum S(k⊥) with high wavenumber and spatial resolution. In particular, recently constructed frequency comb DBS systems enable observation of turbulent phenomena at multiple observation points in the radial direction. A dual-comb microwave DBS system has been developed for the large helical device plasma measurement. Since it is desirable to control the gain of each frequency-comb separately, a frequency-comb DBS system was developed with a function to adjust the gain of the scattered signal intensity of each channel separately. A correction processing method was also developed to correct the amplitude ratio and the phase difference between the in-phase and quadrature-phase signals of the scattered signals. As a result, the error in Doppler-shift estimation required to observe vertical velocity and the radial electric field was reduced, which enables more precise measurements.

Evaluation of a new DIII-D Doppler backscattering system for higher wavenumber measurement and signal enhancement.

The high density fluctuation poloidal wavenumber, kθ (kθ > 8cm-1, kθρs > 5, ρs is the ion gyro radius using the ion sound velocity), measurement capability of a new Doppler backscattering (DBS) system at the DIII-D tokamak has been experimentally evaluated. In DBS, wavenumber (k) matching becomes more important at higher wavenumbers, owing to the exponential dependence of the measured signal loss factor on wave vector mismatch. Wave vector matching allows for the Bragg scattering condition to be satisfied, which minimizes the signal loss at higher k's. In the previous DBS system, without toroidal wave vector matching, the measured DBS signal-to-noise ratio at higher kθ (>8cm-1) is substantially reduced, making it difficult to measure higher kθ turbulence. The new DBS system has been optimized to access higher wavenumber, kθ ≤ 20cm-1, density turbulence measurement. The optimization hardware addresses fluctuation wave vector matching using toroidal steering of the launch mirror to produce a backscattered signal with improved intensity. The probe's sensitivity to high-k density fluctuations has been increased by approximately an order of magnitude compared to the old system that has been in use at DIII-D. Note that typical measurement locations are above or below the tokamak midplane on the low field side with normalized radial ranges of 0.5-1.0. The new DBS probe system with the toroidal matching of fluctuation wave vectors is thought to be critical to understanding high-k turbulent transport in fusion-relevant research at DIII-D.

Validating and optimizing mismatch tolerance of Doppler backscattering measurements with the beam model (invited).

We use the beam model of Doppler backscattering (DBS), which was previously derived from beam tracing and the reciprocity theorem, to shed light on mismatch attenuation. This attenuation of the backscattered signal occurs when the wavevector of the probe beam's electric field is not in the plane perpendicular to the magnetic field. Correcting for this effect is important for determining the amplitude of the actual density fluctuations. Previous preliminary comparisons between the model and Mega-Ampere Spherical Tokamak (MAST) plasmas were promising. In this work, we quantitatively account for this effect on DIII-D, a conventional tokamak. We compare the predicted and measured mismatch attenuation in various DIII-D, MAST, and MAST-U plasmas, showing that the beam model is applicable in a wide variety of situations. Finally, we performed a preliminary parameter sweep and found that the mismatch tolerance can be improved by optimizing the probe beam's width and curvature at launch. This is potentially a design consideration for new DBS systems.

Doppler backscattering systems on the Globus-M2 tokamak

Doppler backscattering (DBS) was successfully previously used on the Globus-M tokamak. The diagnostic was utilised in the form of either a single-frequency or a four-frequency dual homodyne system. It was used primarily for the study of zonal flows, filaments and Alfvén eigenmodes. These phenomena are worth being studied both on the periphery and in the core region of the plasma in a tokamak. For this specific reason two multifrequency DBS systems were installed on the upgraded Globus-M2 tokamak. The first four-frequency system with dual homodyne detection had already been used on the Globus-M tokamak and has lower probing frequencies which provide measurements from the periphery plasma. The second and new six-frequency DBS system was installed with a non-linear transmission line that was adapted to generate probing signals at frequencies 50, 55, 60, 65, 70 and 75 GHz. In general, the range of probing frequencies corresponds to the region of critical plasma densities from 5 × 1018 to 7 × 1019 m−3 at normal incidence. The pyramidal horn antennas are located inside the vacuum vessel with a special cardan-like rotator outside the camera so as to tilt antennas in the toroidal and poloidal directions. A previously developed code was applied to simulate 3D raytracing for all frequency channels. Calculations were carried out for different angles of incidence and for different electron density distributions in order to investigate the possibilities of the implementation of radial and poloidal correlation Doppler reflectometry. Examples of the DBS system application for study of plasma properties in the Globus-M2 tokamak are presented.

Formation of the radial electric field profile in the WEST tokamak

Sheared flows are known to reduce turbulent transport by decreasing the correlation length and/or intensity of turbulent structures. The transport barrier that takes place at the edge during improved regimes such as H mode, corresponds to the establishment of a large shear of the radial electric field. In this context, the radial shape of the radial electric field or more exactly of the perpendicular E × B velocity appears as a key element in accessing improved confinement regimes. In this paper, we present the radial profile of the perpendicular velocity measured using Doppler back-scattering system at the edge of the plasma, dominated by the E × B velocity, during the first campaigns of the WEST tokamak. It is found that the radial velocity profile is clearly more sheared in lower single null configuration (with the B × ∇B magnetic drift pointing toward the active X-point) than in upper single null configuration for ohmic and low current plasmas (B = 3.7 T and q 95 = 4.7), consistently with the expectation comparing respectively ‘favourable’ versus ‘unfavourable’ configuration. Interestingly, this tendency is sensitive to the plasma current and to the amount of additional heating power leading to plasma conditions in which the E × B velocity exhibits a deeper well in USN configuration. For example, while the velocity profile exhibits a clear and deep well just inside the separatrix concomitant with the formation of a density pedestal during L–H transitions observed in LSN configuration, deeper E r wells are observed in USN configuration during similar transitions with less pronounced density pedestal.

Observation of quasi-coherent density fluctuation in scrape-off layer enhancing boundary transport in high- hybrid plasmas on DIII-D

We report the observation of a quasi-coherent fluctuation (QCF) by the Doppler backscattering system in the scrape-off layer (SOL) region of the DIII-D tokamak. This QCF is observed in high power, high performance hybrid plasmas with near double-null divertor shape during the electron cyclotron heating period. This mode is correlated with a steepened SOL density profile, and leads to significantly elevated particle and heat fluxes between edge localized modes. The SOL QCF is a long-wavelength ion-scale fluctuation (–0.4 and ), and propagates in the ion diamagnetic direction in the plasma frame. Its radial expanse is about 1.5–2 cm, well beyond the typical width of heat flux λ q on DIII-D. Also, the SOL QCF does not show any clear dependence on the effective SOL collisionality, and thus may raise issues on the control of plasma-material interactions in low collisionality plasmas in which the blob-induced transport is reduced. A linear simulation using BOUT++ with a five-field reduced model is performed and compared with experimental observations. In simulation results, an interchange-like density perturbation can be driven by the SOL density gradient, and its peak location and the radial width of the density perturbation are in agreement with the experimental observations.

Doppler backscattering diagnostic with dual homodyne detection on the Globus-M compact spherical tokamak.

This article considers a four-frequency microwave Doppler backscattering (DBS) system in the compact spherical tokamak Globus-M. The hardware was adequate for the purposes of studying the peripheral plasma in the tokamak. The multichannel DBS system is based on duplication of a dual homodyne detection circuit for four incident Ka-band frequencies. The ray tracing results for a spherical torus are described, and specific requirements for the antenna tilt adjustment are defined. Some new experimental results are given for using DBS diagnostics on the Globus-M tokamak in order to illustrate its efficiency.

Recent Doppler backscattering applications in Globus-M tokamak

Recently, a four-frequency Doppler backscattering (DBS) system was installed on the Globus-M tokamak. In addition to the classical DBS application for determining the mean plasma rotational velocities, DBS was used for the investigation of different plasma oscillatory processes in the spherical Globus-M tokamak. Application of various DBS data processing allowed us to reveal and investigate plasma oscillations, such as geodesic acoustic mode, limit cycle oscillations and Alfvén modes. It has been shown that DBS also allows registering filament like structures, which manifest as quasi-coherent oscillations in signals of various DBS channels. The multi-frequency DBS system makes it possible to localize plasma oscillations, their poloidal and radial sizes and mode structure. The main results of the DBS application for the study of these oscillations are presented. A comparison of the DBS data with the data of other diagnostics is also given.

I-mode investigation on the Experimental Advanced Superconducting Tokamak

By analyzing large quantities of discharges in the unfavorable ion drift direction, I-mode operation has been confirmed in the Experimental Advanced Superconducting Tokamak. During the L-mode to I-mode transition, the energy confinement is improvement substantially by the formation of a high-temperature edge pedestal, while the particle confinement remains almost identical to that in the L-mode. Similar with I-mode observations on other devices, the Er profiles obtained by the eight-channel Doppler backscattering system (DBS8) (Hu et al 2017 Rev. Sci. Instrum. 88 073504) show a deeper edge Er well in the I-mode than that in the L-mode. Additionally a weak coherent mode (WCM) with the frequency range of 40–150 kHz is observed at the edge plasma with the radial extent of about 2–3 cm. WCM could be observed in both density fluctuation and radial electric field fluctuation, and the bicoherence analyses showed significant couplings between WCM and high frequency turbulence, implying that the Er fluctuation and the caused flow shear from WCM should play an important role during I-mode operation. In addition, a low-frequency oscillation with a frequency range of 5–10 kHz is always accompanied with WCM, where the geodesic acoustic model intensity decreases or disappears. The evidence shows that the a low-frequency oscillation may be a novel kind of limited cycle oscillation but further investigation is needed to explain the new properties such as the harmonics and obvious magnetic perturbations.

The study of filaments by the Doppler backscattering method in the ‘Globus-M’ tokamak

Investigation of filamentary structures was performed on the spherical tokamak Globus-M by the Doppler backscattering (DBS) method. Filaments were detected in the H-mode regime both during the edge localized mode event and in the periods between them. They manifested themselves as bursts of quasi-coherent fluctuations (BQFs) of the output signals of the DBS quadrature detector. To confirm BQF appearing due to backscattering from a moving filament, special experiments were carried out with two DBS systems located at different poloidal angles. Using the new filament research method one could evaluate their velocity, mode number, and radial and poloidal positions. Using this technique, it was possible to estimate the size of the filaments. Experiments have shown that the filaments move predominantly in the direction of the electron diamagnetic drift with a velocity exceeding the velocity of background small-scale fluctuations. Filaments were recorded more frequently when the density gradient on the plasma periphery increased.

Five-channel tunable W-band Doppler backscattering system in the experimental advanced superconducting tokamak.

A 5-channel Doppler backscattering system has been designed and installed in the Experimental Advanced Superconducting Tokamak (EAST). Through an I/Q-type double sideband modulator and a frequency multiplier, an array of finely spaced (Δf = 400 MHz) frequencies that span 1.6 GHz has been created. The center of the array bandwidth is tunable within the range of 75-97.8 GHz, which covers most of the W band (75-110 GHz). The incident angle can be adjusted from -4° to 12°, and the wavenumber range is 4-15 cm-1 with a wavenumber resolution of Δk/k ≤ 0.35. Ray tracing is used to calculate the scattering location and the scattering wavenumber. This article details the hardware design, the ray tracing, and the preliminary experimental results from EAST plasmas.

Investigation of electromagnetic geodesic acoustic mode in EAST RF-heating plasma

The magnetic fluctuations of geodesic acoustic modes (GAMs) have been investigated using a Doppler backscattering system and Mirnov probes during ion cyclotron resonance heating and lower hybrid wave heating in the edge plasma in the experimental advanced superconducting tokamak, and the magnetic component of the GAM was observed. The frequency of the GAM has a clear relationship with the edge electron temperature, and the mode numbers of the poloidal component of the magnetic structure of the GAM are approximately m=2 (sin (2θ)) and n = 0. A more detailed investigation shows that the maximum values of the amplitude of the poloidal magnetic field fluctuations and poloidal electric field fluctuations are approximately 10−6 T and 200 V/m, respectively.

Experimental study on the magnetic coherent mode in the H-mode pedestal of EAST

Here we report the experimental analysis on the low-n (mostly n = 1, sometimes n = 2) magnetic coherent mode (MCM) at a characteristic frequency 20–, which has been frequently observed in various H-mode discharges on EAST. This mode can be easily identified in the magnetic fluctuations measured by the fast Mirnov coils mounted on the vacuum vessel wall, but is detected by the local measurements of edge electrostatic fluctuations only when the mode is sufficiently strong. The apperance of the MCM is summarized covering broad ranges of discharge parameters, in particular, the different heating schemes including pure neutral beams injected in either co- or counter-current direction as well as pure ratio-frequency waves. This may rule out the possibility of fast particle driven modes. Radial distribution and poloidal propagation of the MCM are investigated using the Doppler backscattering system and Langmuir probes inserted at the outer midplane, respectively. Temporal evolution of MCM amplitude during large ELM crashes is evaluated in detail, may suggesting the mode is closely correlated with pedestal buildup. Dedicated experiments reveal the possible correlations of MCM’s frequencies with edge line-averaged density and edge safety factor q95. We also present the observation of multi MCMs at relatively high q95, which are speculated locating at different rational surfaces in the pedestal via analyzing their mode structures and nonlinear interactions. Finally, effect of the MCM on edge particle transport is explored via surveying the correlation between the intermittent events of the mode and the particle fluxes deposited on the divertor target plates, utilizing the conditional analysis method. Corresponding results suggest that the MCM seems to primarily result in a notable poloidal redistribution of the divertor particle flux, rather than a considerable net increase of the total flux.

Observation of geodesic acoustic mode in EAST using Doppler backscattering system

This paper presents an investigation of the geodesic acoustic mode (GAM) using two poloidally separated Doppler backscattering systems in Experimental Advanced Superconducting Tokamak. Each system allows for the simultaneous measurement of turbulence rotation velocity and density fluctuations with high precision. With the Doppler backscattering systems, the GAM frequency, the symmetric feature of poloidal flow fluctuations, and the interaction between GAM and turbulence are surveyed. The results of bispectral analysis show a clear interaction of GAM with the ambient turbulence. A clear measurement of GAM in the envelope of plasma density fluctuations using Doppler backscatter system is shown, and the modulations of the turbulence perpendicular velocity on the density turbulence are correlated at two different poloidal position.