Profile Reflectometer Research Articles

Design of a low frequency, density profile reflectometer system for the MAST-U spherical tokamak.

Validated and accurate edge profiles (temperature, density, etc.) are vitally important to the Mega Ampere Spherical Tokamak Upgrade (MAST-U) divertor and confinement effort. Density profile reflectometry has the potential to significantly add to the measurement capabilities currently available on MAST-U (e.g., Thomson scattering and Langmuir probes). This work presents the diagnostic requirements, problems, and solutions facing profile reflectometry in spherical tokamaks and MAST-U in particular. Requirements include density measurements near zero electron density in the scrape off layer region, coverage for a broad range of MAST-U plasma parameters, high time (≤10microseconds) and spatial resolutions (≤1cm), reliability, and identification of the plasma start frequency.

Observation of local density increase during pellet homogenization on EAST

By combining the X-mode polarized lower and upper cut-off reflections obtained from the density profile reflectometer, we have successfully attained a comprehensive density profile spanning from the edge to the core region in pellet injection (PI) experiments on EAST. During the homogenization process after PI on EAST, an innovative method was introduced to quantify the local density increase. This approach employed the distinctive ‘dual-reflection’ phenomenon observed in the EAST microwave reflectometer, encompassing measurements of both the X-mode lower and upper cut-off frequencies. Furthermore, experimental investigations were carried out on EAST to comprehensively explore the parallel and poloidal expansion of the high-density pellet cloud. Notably, this study marks the first instance of measuring expansion velocities of pellet materials in both parallel and poloidal directions on EAST. A comparative analysis was performed initially between these experimental measurements and simulation results obtained from the HPI2 code, marking a pivotal stride towards enhancing its applicability in EAST.

Observation of enhanced pedestal turbulence during ELM mitigation with resonant magnetic perturbation on EAST

Edge localized mode (ELM) mitigation accompanied by density pump-out has been achieved during the application of resonant magnetic perturbation (RMP) with a toroidal mode number of n = 4 on EAST recently. The mean ELM frequency increases by a factor of 2.7 from 86 to 235 Hz with a decreased reduction in ELM loss. The evolution of pedestal electron density measured by a profile reflectometer before and after turning on the RMP current is presented. Both the pedestal density and density gradient show a decrease with application of RMP. The density fluctuation in the pedestal region has been measured by an O-mode fluctuation reflectometer. The broadband density fluctuation with a frequency range of 20–115 kHz is enhanced at the later period of the inter-ELM phase during ELM mitigation. This phenomenon is also observed for magnetic fluctuation measured by magnetic probes mounted in the vacuum vessel. A further study shows that the enhanced broadband fluctuations lead to a decrease in the growth rate of the pedestal density and an increase in divert or particle flux. This result implies that these enhanced broadband fluctuations could lead to an enhancement of outward particle transport. The possible roles of the enhanced fluctuations observed in ELM mitigation are also discussed.

New methodology for measuring electron density perturbations caused by plasma coherent modes using profile reflectometry: Magnitudes and radial profiles in DIII-D.

New capabilities of fast-sweep frequency-modulated profile reflectometry are explored to measure electron density ne perturbation magnitudes and radial profiles due to plasma coherent modes in DIII-D. The first approach is based on the frequency analysis of phase perturbations associated with high frequency (∼MHz) Alfvén eigenmodes (AEs). The measurement of ∼5.5MHz fast-ion-driven global Alfvén eigenmodes (GAEs) is demonstrated in a neutral beam-heated DIII-D plasma. The GAE induced a broad radial distribution of phase perturbations in the profile reflectometer data. Analysis of these data determined the effective cutoff location displacement and the estimated ne fluctuation profile. In the second approach, high resolution ne profiles are used directly to determine the radial structure of ne perturbations due to a neo-classical tearing mode. These new measurements broaden the application of profile reflectometry and advance the development of AE spectroscopy as a tool for non-invasive diagnosis of fast-ion-driven modes in DIII-D and burning plasmas such as ITER.

Performance demonstration of vacuum microwave components critical for the operation of the ITER low-field side reflectometer.

Final design studies in preparation for manufacturing have been performed for functional components of the vacuum portion of the ITER Low-Field Side Reflectometer (LFSR). These components consist of an antenna array, electron cyclotron heating (ECH) protection mirrors, phase calibration mirrors, and vacuum windows. Evaluation of these components was conducted at the LFSR test facility and DIII-D. The antenna array consists of six corrugated-waveguide antennas for simultaneous profile, fluctuation, and Doppler measurements. A diffraction grating, incorporated into the plasma-facing miter bend, provides protection of sensitive components from stray ECH at 170GHz. For in situ phase calibration of the LFSR profile reflectometer, an embossed mirror is incorporated into the adjacent miter bend. Measurements of the radiated beam profile indicate that these components have a small, acceptable effect on mode conversion and beam quality. Baseline transmission characteristics of the dual-disk vacuum window are obtained and are used to guide ongoing developments. Preliminary simulations indicate that a surface-relief structure on the window surfaces can greatly improve transmission. The workability of real-time phase measurements was demonstrated on the DIII-D profile reflectometer. The new automated real-time analysis agrees well with the standard post-processing routine.

Preliminary design overview and performance assessment of the ITER low-field side reflectometer

The design of the ITER low-field side reflectometer (LFSR) has matured to a complete end-to-end preliminary design. LFSR will supply three important plasma measurements: (1) electron density profile, (2) electron density fluctuations, and (3) poloidal rotation. Simultaneous measurements of the three quantities are enabled by an array of six monostatic antennas which inject from an equatorial port on the outboard side of the ITER vessel. Low-loss transmission lines, consisting of corrugated, overmoded waveguide and miter bends, transmit the 30–165 GHz, O- and X-mode signals to and from the ITER plasma. Integrated transmission-line components serve a range of purposes, such as protection from high-power stray radiofrequency radiation, accommodation of transmission-line displacement, and simultaneous measurement of reference and plasma phases during the discharge. Broadband transmission signals are realized by full-band microwave transceivers combined with quasi-optical multiplexing. A field-programmable gate array (FPGA) processor demodulates the profile reflectometer signals, enabling real-time density profile measurements for plasma control system feedback. A full-scale transmission line test facility provides an integrated environment to assess the performance of critical LFSR components. Theoretical modeling together with insertion loss measurements provide the basis for a comprehensive power budget, which accounts for transmitted output power, transmission-line losses, antenna coupling, and plasma effects. Results indicate that high signal-to-noise ratios are achievable with the current design. A synthetic reflectometer model, using real design parameters and baseline ITER profiles, has been developed to estimate the return signal. With evolving microwave and data acquisition technologies, full-band, ultrafast sweeps (<1 μs) will be realizable for ITER.

Predict-first experiments and modeling of perturbative cold pulses in the DIII-D tokamak

Cold pulses are introduced in Ohmic DIII-D tokamak plasmas via injection of impurities with a laser blow-off system, revealing for the first time in this machine a quick increase in core electron temperature shortly after the edge cold-pulse injection at low collisionality. The experimental results are consistent with predict-first simulations of heat transport enabled by the Trapped Gyro-Landau-Fluid transport model. Measurements of electron density evolution during the cold-pulse propagation are enabled by a high time resolution density profile reflectometer. The density evolution reveals the quick propagation of a pulse from edge to core, which is a mechanism to transiently increase core temperature in low-collisionality plasmas. Local transport simulations with measured density evolution demonstrate that the core temperature response can indeed be explained by the stabilization of Trapped Electron Mode turbulence at low collisionality, thus providing confidence that local transport modeling is enough to explain cold-pulse propagation and associated phenomenology.

Assessment of ECH stray radiation levels at the W7-X Michelson Interferometer and Profile Reflectometer

Electron Cyclotron Heating and Electron Cyclotron Current Drive are key components for heating and control in magnetically confined fusion plasmas. The high power microwaves are not always completely absorbed leading to stray radiation [1], [2]. At W7-X, the total injected microwave power can be up to 7.5 MW @140 GHz while the entire Electron Cyclotron Emission picked-up by an observer at the edge of the plasma is a fraction of a mW. In the situation of a Michelson Interferometer, the principle measurement is the entire ECE spectrum. Thus, any stray radiation is bound to enter the spectrum. In this work initial stray radiation measurements without filters at the location of two microwave receivers -the Michelson Interferometer and the Profile Reflectometer -are discussed. The data is used to dimension a notch filter to be used with the broad band Michelson Interferometer.

A free-standing wire scattering technique to monitor calibration variations of the DIII-D density profile reflectometer.

Real-time phase calibration of the ITER profile reflectometer is essential due to the long plasma duration and expected waveguide path length changes during a discharge. Progress has been recently made in addressing this issue by employing a phase calibration technique on DIII-D that monitors calibration variations that occur during each plasma discharge. By installing a thin free-standing metallic wire (1 mm diameter) near the end of the overmoded waveguide transmission system (oriented perpendicular to the waveguide axis), the round-trip phase shift from the wire is detected simultaneously with the plasma phase shifts. Variations in the reflectometer round trip path length (∼26 m) are then calculated after each DIII-D plasma discharge, allowing the calibration phase to be accurately monitored and updated. The round-trip reflectometer path length is observed to vary by ∼3 mm (root mean square value) during a typical DIII-D discharge. Using the variations in calibration phase, the density profile measurement accuracy can be improved. Since the wire retro-reflected power is ∼0.01 of the plasma signal, minimal effect is observed on the reflected signal from the plasma. Importantly, through a suitable choice in wire diameter, the calibration signal can be made approximately independent of the V-band reflectometer launch polarization. This is particularly important on DIII-D since orthogonal X- and O-mode polarized beams are coupled into the same transmission waveguide and launch antenna.

An eight-channel Doppler backscattering system in the experimental advanced superconducting tokamak.

Doppler backscattering system can measure the perpendicular velocity and fluctuation amplitude of the density turbulence with intermediate wavenumber. An eight-channel Doppler backscattering system has been installed in the Experimental Advanced Superconducting Tokamak (EAST), which can probe eight different radial locations simultaneously by launching eight fixed frequencies (55, 57.5, 60, 62.5, 67.5, 70, 72.5, 75 GHz) into plasma. The quasi-optical system consists of circular corrugated waveguide transmission, a fixed parabolic mirror, and a rotatable parabolic mirror which are integrated with quasi-optics front-end of the profile reflectometer inside the vacuum vessel. The incidence angle can be chosen from 5° to 12°, and the wavenumber range is 2-15/cm with the wavenumber resolution Δk/k≤0.21. Ray tracing simulations are used to calculate the scattering locations and the perpendicular wavenumber. The dynamic range of this new eight-channel Doppler backscattering system can be as large as 40 dB in the EAST. In this article, the hardware design, the ray tracing, and the preliminary experimental results in the EAST will be presented.

A novel technique for real-time estimation of edge pedestal density gradients via reflectometer time delay data.

A new model-based technique for fast estimation of the pedestal electron density gradient has been developed. The technique uses ordinary mode polarization profile reflectometer time delay data and does not require direct profile inversion. Because of its simple data processing, the technique can be readily implemented via a Field-Programmable Gate Array, so as to provide a real-time density gradient estimate, suitable for use in plasma control systems such as envisioned for ITER, and possibly for DIII-D and Experimental Advanced Superconducting Tokamak. The method is based on a simple edge plasma model with a linear pedestal density gradient and low scrape-off-layer density. By measuring reflectometer time delays for three adjacent frequencies, the pedestal density gradient can be estimated analytically via the new approach. Using existing DIII-D profile reflectometer data, the estimated density gradients obtained from the new technique are found to be in good agreement with the actual density gradients for a number of dynamic DIII-D plasma conditions.

Performance and data analysis aspects of the new DIII-D monostatic profile reflectometer system.

A new frequency-modulated profile reflectometer system, featuring a monostatic antenna geometry (using one microwave antenna for both launch and receive), has been installed on the DIII-D tokamak, providing a first experimental test of this measurement approach for profile reflectometry. Significant features of the new system are briefly described in this paper, including the new monostatic arrangement, use of overmoded, broadband transmission waveguide, and dual-polarization combination/demultiplexing. Updated data processing and analysis, and in-service performance aspects of the new monostatic profile reflectometer system are also presented. By using a raytracing code (GENRAY) to determine the approximate trajectory of the probe beam, the electron density (ne) profile can be successfully reconstructed with L-mode plasmas vertically shifted by more than 10 cm off the vessel midplane. Specifically, it is demonstrated that the new system has a capability to measure ne profiles with plasma vertical offsets of up to ±17 cm. Examples are also presented of accurate, high time and spatial resolution density profile measurements made over a wide range of DIII-D conditions, e.g., the measured temporal evolution of the density profile across a L-H transition.

X-Mode Frequency Modulated Density Profile Reflectometer on EAST Tokamak

An extraordinary-mode (X-mode) frequency-modulated continuous-wave (FMCW) profile reflectometer has been built on EAST. In the reflectometer, continuous waves with frequency sweeping from 12.5 GHz to 18 GHz were generated through a Hyperabrupt Tuned-varactor Oscillator (HTO) source and a four times active multiplier was used to increase the frequency to V-band (50 GHz to 72 GHz). The polarization of horn lens antenna is perpendicular to the magnetic field line at the edge plasmas. According to the V-band frequency range and polarization, the system cover density range from 0.5 × 1019m−3 to 3.0 × 1019m−3 (when toroidal magnetic field is 1.8 T), with time resolution of 12.5 ∼ 50 μs. The density profile could be calculated by assuming the edge profile through an empirical equation. The maximum spatial error deduced by the method is about 4 cm. This reflectometer has been successfully applied in 2010 autumn EAST campaign, the temporal evolution of density profiles was acquired during the low confinement mode to high confinement mode transition. The density pedestal of EAST Tokamak was observed and the top value and gradient of the density pedestal were estimated.

Development of the W-band density profile and fluctuation reflectometer on EAST

A X-mode polarized W-band reflectometer for plasma density profile and fluctuation measurement is designed and installed on EAST. In measuring the density profile, a voltage controlled oscillator (VCO) is used as the source, allowing a high temporal resolution measurement. The density profile in a plasma with high magnetic field (3.0T) has been measured by combination of V- and W-band reflectometers. For fluctuation measurements, a frequency synthesizer is used instead of the VCO as a microwave source. The core density fluctuations during sawtooth activity are measured and analyzed.

SOL and pedestal density profile evolution during DIII-D ELMy and ELM-suppressed H-mode operation

A fast profile reflectometer has been used to study the detailed characteristics of scrape-off-layer (SOL) and pedestal density profile evolution in DIII-D ELMy H-mode plasmas with high temporal (⩾10μs) and spatial (∼4mm) resolution. Results show a significant enhancement in radial particle transport to the outer vessel wall during Type-I ELMs. The radial particle transport time (∼140μs) becomes comparable to the parallel transport time, as a result of a high radial velocity (∼620m/s) of SOL profile expansion. The dynamics can vary from ELM-to-ELM. Scalings with pedestal density have been obtained, indicating that ELM is less affected as the pedestal density increases. Finally, during ELM-suppressed H-mode operation with a stochastic magnetic boundary, the radial extent of observed SOL profile modifications do not extend to the vessel wall, and profiles are steeper, substantially different from the SOL changes associated with Type-I ELMs.

High-resolution dual-polarization frequency modulated reflectometer density profile measurements on DIII-D

A novel dual-polarization technique has been applied to the frequency-modulated electron density profile reflectometer on DIII-D. In this application, both O- and X-mode waves are simultaneously launched from a common high-performance solid-state source, and simultaneous measurements of both O- and X-mode reflections from corresponding cutoffs serve to expand the density coverage. The system consists of both Q-band (33–50 GHz) and V-band (50–72 GHz) subsystems, their combination covers a density range from 0 to 6.4×1019 m−3, with a time resolution of ⩾10 μs and spatial resolution of ⩾4 mm. The dual-polarization technique was implemented successfully in the Q-band subsystem during 2003 and detailed information for H-mode pedestal physics studies was provided. It has also recently been applied to the V-band subsystem, and extended contributions to H-mode pedestal physics and particle transport studies are expected during the 2004 DIII-D campaign.

X-mode reflectometry for magnetohydrodynamic activity associated with q=1 surface measurements on Tore Supra

Tore Supra is equipped with two 20 μs fast sweep X-mode reflectometers operating between 50–110 GHz dedicated to density profile determination and an X-mode fixed frequency reflectometer operating between 105–155 GHz for density fluctuation measurements. Heterodyne and sine–cosine detection provide measurements of the reflected signal with high sensitivity. Operating profile reflectometer in burst mode (5 μs dead time between two consecutive sweeps) allows quasi-simultaneous measurements at fixed frequency over a broad frequency band. Thus, information on plasma fluctuations, such as magnetohydrodynamic (MHD) activity, up to 20 kHz as well as a radial localization of the modes is accessible. The temporal evolution of the q=1 rational surface during sawtooth crash activity has been recorded in the plasma center with high spatial resolution. In addition, a direct comparison between signals associated with a central MHD mode from both profile and fluctuation reflectometers, positioned at different toroidal angles, allows one to determine the plasma toroidal velocity.

Application of fast reflectometer density profile measurements to investigate plasma instabilities in DIII-D

The high spatial (⩾2 mm depending upon plasma conditions) and temporal (10 μs) resolution possible with the DIII-D solid state profile reflectometer system makes it possible to investigate profile modifications associated with plasma instabilities. In order to take full advantage of this fast sweep and high resolution capability, multiple issues had to be addressed, including fast data acquisition, large data acquisition memory depth, improved signal to noise, accurate profile start location, and finally fast accurate automatic data analysis. Improvements in all these areas make the profile reflectometer ready for plasma instability studies. In a demonstration of this capability, fast, high spatial resolution reflectometer measurements have been successfully used to investigate the edge harmonic oscillation (EHO) in the quiescent double barrier regime, as well as the edge localized modes (ELMs) on DIII-D. It is found that the scrape-off layer density profile is modulated at the EHO fundamental frequency. The edge density profile evolution is also tracked during ELMs, showing that the density profile expands to the vessel wall at the onset of Type 1 ELMs.

Improved reflectometer electron density profile measurements on DIII-D

Details are presented of recent improvements to hardware and analysis code components of the DIII-D density profile reflectometer system. The improvements to the hardware increased the system signal-to-noise ratio and reduced spurious reflections. An improved automated method is developed to accurately determine the zero density plasma start position. With these improvements the uncertainty in the zero density plasma start position is reduced from ∼7 to ∼2 mm, the position uncertainty with a fixed mirror target is reduced from ∼4 to ∼2 mm, and the time resolution is improved to 10 μs (previously ∼100 μs).

Implementation of reflectometry as a standard density profile diagnostic on DIII-D

The profile reflectometer system on the DIII-D tokamak has been significantly upgraded in order to improve time coverage, data quality, and profile availability. The performance of the reflectometer system, which utilizes continuous frequency modulated (FMCW) radar techniques, has been improved as follows: First, a new PC-based data acquisition system has been installed, providing higher data sampling rates and larger memory depth. The higher sampling rate enables use of faster frequency sweeps of the FMCW microwave source, improving time resolution, and increasing profile accuracy. The larger memory depth enables longer data records, so that profiles can now be obtained throughout 5 s discharges at 100 Hz profile measurement rates, while continuous sampling at 10 MHz is available for 1 s for high time resolution physics studies. Second, an initial automated between-shots profile analysis capability is now available. Third, availability of the profiles to end users has been significantly improved.