Electron Density Profile Measurements Research Articles

Development of ultra-short pulse reflectometry on the Experimental Advanced Superconducting Tokamak (EAST).

Microwave reflectometry is an invaluable diagnostic tool for measuring electron density profiles in large fusion devices. Density fluctuations near the plasma cutoff layer, particularly those that are time-varying on the timescale of the reflectometry measurement, can result in distortions in phase and/or amplitude of the reflected waveform, which present challenges to the accuracy of the reconstructed profile. The ultra-short pulse reflectometry (USPR) technique eliminates the time-varying issue in that reflectometry data are collected on a nanosecond timescale, essentially freezing the fluctuations in place. An X-mode dedicated 32-channel USPR system has been developed and installed on the EAST, covering the operation frequency range from 52 to 92GHz. This system enables high-resolution density profile measurements in the plasma pedestal and scrape-off layer, with resolutions reaching 5mm and 1 μs, respectively. Laboratory testing of the system performance has been conducted, demonstrating the potential of the USPR technique to provide accurate and high-temporal-resolution density profiles in challenging plasma environments.

Design of line array detection scheme for Polarimeter-Interferometer system on EAST

A multi-channel POlarimeter-INTerferometer (POINT) system has been constructed in the core region of Experimental Advanced Superconducting Tokamak (EAST) plasma for electron density profile and current density profile measurements, with a spatial resolution of 8.5 cm. To explore the small spatial scale density fluctuation phenomena in the core plasma induced by MHD instabilities effects, a higher spatial resolution and higher responsivity detection is required.A novel line array detection scheme of POINT is proposed. By replacing the single Schottky mixer with a highly integrated line array HEMT detector at the central channel position, the existing single channel can be extended to 5 channels for spatial continuous measurement in the core plasma, which has a significantly simplified structure design compared with the discrete upgrade scheme. Related optical design, component selection, and bench tests have been carried out to validate the feasibility and reliability of the novel scheme.The IF signals are detected by an AlGaN/GaN line array high electron mobility transistors (HEMT) detector based on five silicon lenses integrated. The average responsivity of the line array detector elements is 40 kV/W. Compared with the existing planar diode Schottky mixer with a sensitivity of 750 V/W, the line array detector can also meet the essential requirements of coherent signal measurement. Benefiting from the highly integrated array detector, the spatial resolution will be improved to 4.5 mm in the novel scheme. Additionally, a circular Plano cylindrical lens is designed as a beam-shaping component to compress the Gaussian beam into a stripe. Bench test results displayed the excellent performance of the cylindrical lens, which is consistent with the simulation results. Furthermore, normalized responsivity coefficients of array elements are determined by test results.

Density limits as disruption forecasters for spherical tokamaks

Fusion power output from spherical tokamaks would benefit from increased confined plasma density, but there exists a limit on the density before confinement is lost and the plasma current is disrupted. This density limit has long been characterized by a simple, global Greenwald limit proportional to the plasma current and inversely proportional to the cross sectional area of the plasma. It is shown that in the database of discharges from the National Spherical Tokamak Experiment (NSTX) and Mega Ampere Spherical Tokamak (MAST) spherical tokamaks, the likelihood of disruption does increase above the Greenwald limit, and especially in the plasma current rampdown phase. The physics of the density limit has been recently theoretically explored through local criteria. Several of these are tested using the disruption event characterization and forecasting (DECAFTM) code for their potential effectiveness as disruption warning signals. For a limited set of NSTX discharges, a local island power balance criteria was found to be less reliable, presently, than the Greenwald limit. An empirical critical edge line density and a boundary turbulent transport limit were both tested for MAST-U, which has an electron density profile measurement with high spatial resolution in the outer part of the plasma. Both were found to have similar dependencies on key plasma parameters. In a limited set of MAST-U discharges that appear to disrupt due to rising density at values under the Greenwald limit, crossing of the boundary turbulent transport limit occurred close to the time of disruption. Finally, these limits were evaluated for their potential use in real-time, and it was found that with the necessary real-time inputs and with refinement through further testing, these limits could be implemented in a real-time disruption forecasting system.

Generation of Post Sunset E Region Electron Density Stratifications at the Magnetic Equator: An Analysis Using In Situ Measurements and Theoretical Estimations

AbstractMeasurements of electron density/irregularities and ion drift velocity/neutral wind were undertaken with Electron Density and Neutral Wind (ENWi) payload on‐board an RH300 flight on 6 April 2018 as part of the Sounding Rocket Experiment (SOUREX1). The presence of a clearly structured ENWi measured electron density profile is an interesting feature observed on this day. Concurrently electron density/irregularities are measured by a Langmuir probe payload which confirmed the presence of stratifications in electron density. The neutral wind measurements by ENWi also reveal a structured pattern. The hodograph analysis of the neutral wind components reveals clockwise rotation indicating downward phase propagation characteristics associated with the gravity waves. Analysis of the requisite ion convergence at the magnetic equator for occurrence of electron density stratifications using the observed electron density structures during the post sunset hours in relation to the ion convergence due to gravity wave winds is performed according to earlier theoretical simulations. This analysis reveals that conditions favoring the formation of stratification related ion convergence are present therein. Thus the present study for first time uses actual measurements of neutral winds and electron densities from same instrument to demonstrate the role of gravity wave induced winds in producing ion convergence at the magnetic equatorial region Trivandrum in the post sunset hours.

Radio-frequency plasma to clean ITER front-end diagnostic mirrors in geometry of Edge Thomson Scattering system

The ITER Edge Thomson scattering (ETS) system provides electron temperature and density profile measurements in the ITER tokamak. In collection optics, the front-end metallic first and second mirrors are expected to experience contamination with beryllium, tungsten and construction materials. Plasma cleaning based on a low-pressure radiofrequency discharge is expected to sputter contaminants. In the plasma cleaning system, a water-cooled first mirror is combined with a powered electrode. Water cooling was realized as a notch filter for the driving frequency with the electrode grounded for a DC-voltage. To understand plasma cleaning effects, a new test model reproducing the ETS First and the Second mirror geometries in a vacuum chamber was developed. Ion energies and fluxes were measured for 40–50 MHz discharges in argon and helium at 1–10 Pa with and without the notch filter for various power transmission schemes. Powers in plasma were increased to 300–400 W to achieve ion fluxes suitable for cleaning. 40 MHz discharges were used for cleaning as being more stable. In helium at 5 Pa the ion flux of 1.3·1019 ions·m−2 s−1 and the ion energies of 120–140 eV were considered for cleaning. Sputtering rates of metal layers were measured at 4–5 nm h−1 for W/WO3 films. Sputtering rates were non-uniform over the electrode and were lower than 0.5 nm h−1 at the edges. At 40–50 MHz, two independent discharges could be ignited at the First and the Second mirrors in argon and helium, and were maintained with minimum interference. Redeposition rate on the walls was estimated as 1–1.5 nm h−1, mainly consisting of the chamber construction materials. Parasitic discharges were observed at powers above 200 W in plasma and influenced plasma stability at pressures 1–2 Pa. The results are important for a number of ITER optical diagnostics where plasma cleaning of front-end water-cooled diagnostic mirrors shall be used.

Diffraction enhanced imaging utilizing a laser produced x-ray source.

Image formation by Fresnel diffraction utilizes both absorption and phase-contrast to measure electron density profiles. The low spatial and spectral coherence requirements allow the technique to be performed with a laser-produced x-ray source coupled with a narrow slit. This makes it an excellent candidate for probing interfaces between materials at extreme conditions, which can only be generated at large-scale laser or pulsed power facilities. Here, we present the results from a proof-of-principle experiment demonstrating an effective ∼2 μm laser-generated source at the OMEGA Laser Facility. This was achieved using slits of 1 ×30μm2 and 2 ×40μm2 geometry, which were milled into 30 μm thick Ta plates. Combining these slits with a vanadium He-like 5.2keV source created a 1D imaging system capable of micrometer-scale resolution. The principal obstacles to achieving an effective 1 μm source are the slit tilt and taper-where the use of a tapered slit is necessary to increase the alignment tolerance. We demonstrate an effective source size by imaging a 2 ± 0.2 μm radius tungsten wire.

Digital single-photon-avalanche-diode arrays for time-of-flight Thomson scattering diagnostics.

The collection optics of Thomson scattering systems for plasma devices are designed with maximum possible étendue to keep the required laser energy low. If the spatial resolution along the laser beam is performed by a time-of-flight method, then the detectors, in addition to a large sensitive area, must offer a high frequency bandwidth. Up until now, only microchannel-plate photomultipliers meet these requirements. Here, we investigate the potential use of digital avalanche photodiode arrays operated in the Geiger mode as alternative detectors. In this mode of operation, each array will serve as a fast, sensitive detector. The use of these detectors will lead to significant improvements of the Thomson scattering diagnostic. Most important of these will be a better spatial resolution, down to about 2cm without deconvolution. Furthermore, the lifetime of the detectors will be increased; the detectors will cover the whole blue wing of the scattered spectrum when using a single wavelength laser, and this will enable measurements of electron temperature and density profiles at kHz repetition rates.

Feasibility of applying the lower cut-off frequency for the density radial coverage extension in EAST reflectometry measurement

The extraordinary mode (X-mode) lower cut-off frequency is proposed for use in the reflectometry diagnostic on ITER for the electron density profile measurement, which is a trade-off between extreme plasma parameters and the accessible probing frequency. In contemporary experiments, the lower cut-off frequency can be identified at the probing frequency below the electron cyclotron frequency (f ce) under certain plasma conditions. We provide here, for the first time, the experimental validation of the use of the lower cut-off frequency for the density profiles via the reflectometry measurement on EAST. The corresponding group delay of the lower cut-off frequency evolves continuously with the upper one, revealing a reasonable radial coverage extension of reflectometry measurement toward the plasma core. It is concluded that the lower cut-off frequency can be used as a supplement to the upper one in the density profile inversion process, which is of particular interest in the high magnetic field and/or density discharge to extend the radial coverage of reflectometry measurement.

Role of NBI fuelling in contributing to density peaking between the ICRH and NBI identity plasmas on JET

Density peaking has been studied between an ICRH and NBI identity plasma in JET. The comparison shows that 8 MW of NBI heating/fueling increases the density peaking by a factor of two, being R/L n = 0.45 for the ICRH pulse and R/L n = 0.93 for the NBI one averaged radially over ρ tor = 0.4, 0.8. The dimensionless profiles of q, ρ *, υ *, β n and T i/T e ≈ 1 were matched within 5% difference except in the central part of the plasma (ρ tor < 0.3). The difference in the curvature pinch (same q-profile) and thermo-pinch (T i = T e) between the ICRH and NBI discharges is virtually zero. Both the gyro-kinetic simulations and integrated modelling strongly support the experimental result where the NBI fuelling is the main contributor to the density peaking for this identity pair. It is to be noted here that the integrated modeling does not reproduce the measured electron density profiles, but approximately reproduces the difference in the density profiles between the ICRH and NBI discharge. Based on these modelling results and the analyses, the differences between the two pulses in impurities, fast ions (FIs), toroidal rotation and radiation do not cause any such changes in the background transport that would invalidate the experimental result where the NBI fuelling is the main contributor to the density peaking. This result of R/L n increasing by a factor of 2 per 8 MW of NBI power is valid for the ion temperature gradient dominated low power H-mode plasmas. However, some of the physics processes influencing particle transport, like rotation, turbulence and FI content scale with power, and therefore, the simple scaling on the role of the NBI fuelling in JET is not necessarily the same under higher power conditions or in larger devices.

On the determination of ionospheric electron density profiles using multi-frequency riometry

Abstract. Radio wave absorption in the ionosphere is a function of electron density, collision frequency, radio wave polarisation, magnetic field and radio wave frequency. Several studies have used multi-frequency measurements of cosmic radio noise absorption to determine electron density profiles. Using the framework of statistical inverse problems, we investigated if an electron density altitude profile can be determined by using multi-frequency, dual-polarisation measurements. It was found that the altitude profile cannot be uniquely determined from a “complete” measurement of radio wave absorption for all frequencies and two polarisation modes. This implies that accurate electron density profile measurements cannot be ascertained using multi-frequency riometer data alone and that the reconstruction requires a strong additional a priori assumption of the electron density profile, such as a parameterised model for the ionisation source. Nevertheless, the spectral index of the absorption could be used to determine if there is a significant component of hard precipitation that ionises the lower part of the D region, but it is not possible to infer the altitude distribution uniquely with this technique alone.

Design of an edge TV Thomson scattering diagnostic system on EAST

The study of the edge plasma has significant effect for the performance improvement of magnetic confinement. However, the Thomson scattering system, which has been used to provide electron temperature and electron density on EAST, do not meet the requirements for the edge plasma measurements in measurement region, spatial and spectral resolution. Herein, a TV Thomson scattering diagnostic system has been constructed for the measurements of edge electron temperature and electron density profiles on EAST. A new laser system, optical collection system and data acquisition system are designed to enable the TV Thomson system operate at the measurement region of 0.65–1.1 ρ (“ρ” is the unitary radius in flux coordinate) with spectral resolution of 5 nm and spatial resolution as low as 2 mm. And the theoretical analysis proves that the system has the measurability at electron temperature of 20–1000 eV and density beyond 0.2 × 1019 m-3, which fulfills research of the pedestal microstructure on EAST. The establishment of the edge TV Thomson system can create the conditions for the edge physical study on EAST.

On the Cause of the Post‐Sunset Nocturnal OI 630 nm Airglow Enhancement Over Low‐Latitude Thermosphere

AbstractAirglow emissions serve as good tracers of the altitudinal regions at which they originate. OI 630 nm (red line) nocturnal emissions originate from around 250 km altitude. A total of 142 nights of data corresponding to the months of January, February, and March in the years 2013, 2014, and 2016, obtained from Mt. Abu (24.6°N, 72.7°E, 16°N Mag), Gurushikhar, India, a low‐latitude location, are investigated. These are compared with the column integrated emission rates calculated using, as inputs, the measured electron density profiles obtained from a digisonde from Ahmedabad (AMD, 23.0°N, 72.6°E, 15°N Mag), India. Following the expected monotonic decrement in the emissions after sunset, an enhancement is observed on several nights that peaks at around 20–21 local time (LT). The cause for this enhancement has been investigated in detail and it is found that the neutral winds, as obtained using digisondes at two locations, show almost a very good correlation between a poleward directed wind or cessation of equatorward wind over AMD and the observed airglow emission enhancement in the post‐sunset time. Further, the percentage enhancement in emissions also shows a decrease in magnitudes from January to March which has a broad similarity to the decrease in the model climatological meridional wind magnitudes in the same duration. Based on the data spanning over different years, it is inferred that, during geomagnetic quiet periods, almost all of the nocturnal variability in OI 630 nm emissions is due to the variations in the neutral wind.

Implementation and data processing of a five-channel microwave interferometer with high temporal resolution and low noise on Sino-UNIted Spherical Tokamak.

A five-channel microwave interferometer with high temporal resolution and high phase resolution has been developed for measuring electron density profiles and fluctuations on the Sino-UNIted Spherical Tokamak. A correction algorithm, based on the low signal amplitude detection, is proposed to eliminate the fringe jump errors. The correction algorithm achieves an accuracy of 92%. The density profile is reconstructed through the Park-matrix method, with the geometry of magnetic surfaces calculated by the equilibrium fitting. The reconstructed density profiles for discharges with supersonic molecular beam injection are in good agreement with the results of another 94GHz single-channel horizontal interferometer and the Langmuir probes. The temporal resolution of the system is 0.5 µs and the line-integrated electron density resolution is 1 × 1015m-2, which benefits from the single sideband modulation technique. Therefore, the multichannel interferometer system is capable of studying the details of the high-frequency (up to 200 kHz) density fluctuation such as that in the minor disruption event.

Plasmaspheric scale height modeling based on COSMIC radio occultation data

The scale height in the plasmasphere (HP) is an important factor to present the dynamic nature and variations of the plasmasphere. It is challenging to build reliable plasmaspheric scale height model because of poor coverage and limited continuity of plasmaspheric sounder data. In this paper, an empirical orthogonal function (EOF) analysis method is applied to organize the HP obtained from total electron content (TEC) measurements of precise orbit determination (POD) antennas and electron density profiles of Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission, from the year 2007–2014, to reconstruct a global plasmaspheric scale height model. The model depends on parameters including the month of the year, the local time, the geographic latitude and the solar radio flux index F10.7. Comparison of this model and the scale height deduced from the International Satellites for Ionospheric Studies (ISIS) data shows remarkable consistency in HP values and variations.

A Novel Method for Deriving the Vary‐Chap Scale Height Profile in the Topside Ionosphere

AbstractThe modeling of topside ionospheric electron density using height‐varying scale height methods have received increasing attention. The modified Chapman function with a variable scale height, known as Vary‐Chap function, can well represent the topside electron density profiles, and is considered as a promising direction to improve the International Reference Ionosphere model. In order to facilitate modeling, a new formula with four unknown coefficients is proposed to fit the Vary‐Chap scale height, and these unknowns can be determined by the least squares technique. The approximated electron density profile is reconstructed by using the fitted scale height, and its average percentage error relative to the measured electron density profile is quantitatively analyzed by using the Alouette/ISIS topside sounder data sets and COSMIC radio occultation databases, and the accuracies of five other approaches (i.e., Nava et al., 2008, https://doi.org/10.1016/j.jastp.2008.01.015; Nusmei et al., 2012, https://doi.org/10.1029/2012RS004989; Porl et al., 2019, https://doi.org/10.1007/s10712-019-09521-3; Reinisch et al., 2007, https://doi.org/10.1016/j.asr.2006.05.032; Wu et al., 2020, https://doi.org/10.1029/2019JA027637) to approximate the topside electron density profiles are also compared. From the perspective of average percentage error, our formula performs well to reconstruct the topside electron density profiles, especially at the altitudes between hmF2 and hmF2 + 1,200 km.

A proposed method for improving the IRI2016 model by means of Swarm over the American Sector during the event of 5–11 September 2017

A new method is proposed for improving the IRI2016 vertical electron density profile by ingesting the in situ Ne values from Swarm A, B and C. The method is validated by comparing the improved electron density profiles with measured full height profiles over three Incoherent Scatter Radar stations in the American sector (Jicamarca, Millstone Hill and Sondrestrom) during the period 5–11 September 2017 covering both quiet and storm days. We considered 90 epochs corresponding to times when one of the Swarm satellites passed near an Incoherent Scatter Radar at a time when the Incoherent Scatter Radar was measuring electron density profiles.

Characterizing Average Electron Densities in the Martian Dayside Upper Ionosphere

AbstractWe use more than 10 years of the Martian topside ionospheric data measured by the Mars Advanced Radar for Subsurface and Ionosphere Sounding radar sounder on board the Mars Express spacecraft to derive an empirical model of electron densities from the peak altitude up to 325 km. Altogether, 16,044 electron density profiles obtained at spacecraft altitudes lower than 425 km and at solar zenith angles lower than 80° are included in the analysis. Each of the measured electron density profiles is accurately characterized by the peak electron density, peak altitude, and three additional parameters describing the profile shape above the peak: (i) steepness at high altitudes, (ii) main layer thickness, and (iii) transition altitude. The dependence of these parameters on relevant controlling factors (solar zenith angle, solar irradiance, crustal magnetic field magnitude, and Sun‐Mars distance) is evaluated, allowing for a formulation of a simple empirical model. Mars Atmosphere and Volatile EvolutioN Extreme Ultraviolet monitor data are used to show that the solar ionizing flux can be accurately approximated by the F10.7 index when taking into account the solar rotation. Electron densities predicted by the resulting empirical model are compared with electron densities locally evaluated based on the Mars Advanced Radar for Subsurface and Ionosphere Sounding measurements, with the Langmuir Probe and Waves electron density measurements on board the Mars Atmosphere and Volatile EvolutioN spacecraft, and with electron densities obtained by radio occultation measurements. Although the electron densities measured by the Langmuir Probe and Waves instrument are systematically somewhat lower than the model electron densities, consistent with former findings, the model performs reasonably well.

Thomson scattering systems on C-2W field-reversed configuration plasma experiment

TAE Technologies' newly constructed C-2W experiment aims to improve the ion and electron temperatures in a sustained field-reversed configuration plasma. A suite of Thomson scattering systems has been designed and constructed for electron temperature and density profile measurements. The systems are designed for electron densities of 1 × 1012 cm-3 to 2 × 1014 cm-3 and temperature ranges from 10 eV to 2 keV. The central system will provide profile measurements of Te and ne at 16 radial locations from r = -9 cm to r = 64 cm with a temporal resolution of 20 kHz for 4 pulses or 1 kHz for 30 pulses. The jet system will provide profile measurements of Te and ne at 5 radial locations in the open field region from r = -5 cm to r = 15 cm with a temporal resolution of 100 Hz. The central system and its components have been characterized, calibrated, installed, and commissioned. A maximum-likelihood algorithm has been applied for data processing and analysis.

Measurements of electron density profile by frequency modulated continuous wave reflectometer in the T-10 tokamak using high magnetic field side probing and extraordinary mode lower cutoff.

A new, previously unused combination of the high magnetic field side probing and the lower extraordinary mode cutoff was used in the T-10 tokamak for the electron density profile measurements by using a frequency modulation continuous wave reflectometer. This scheme has a significant advantage for large fusion machines where flat electron density profiles and high electron temperatures are expected. The reflectometer design is based on a common scheme with voltage controlled oscillators, active multipliers, and in-phase/quadrature signal detection. Iterative procedures were applied to calibrate a microwave source frequency and compensate the dispersion of probing signals in the conventional rectangular waveguides used as the transmission lines. About 0.2% stability of the beating frequency was reached after the calibration procedure had been finished. An advanced finite-difference scheme was proposed for the reconstruction of the electron density profile to reduce errors and improve the overall performance of the algorithm. Reconstructed electron density profiles demonstrate good agreement with conventional T-10 diagnostic data. Some specific technical and physical aspects of the diagnostic were also discussed on the basis of T-10 operation experience.

Helium line ratio spectroscopy for high spatiotemporal resolution plasma edge profile measurements at ASDEX Upgrade (invited).

The thermal helium beam edge diagnostic has recently been upgraded at the ASDEX Upgrade (AUG) tokamak experiment. Line ratio spectroscopy on neutral helium is a valuable tool for simultaneous determination of the electron temperature and density of plasmas. The diagnostic now offers a temporal resolution of 900 kHz with a spatial resolution of up to 3 mm at 32 lines of sight (LOS) simultaneously. The LOS covers a radial region of 8.5 cm, starting at the limiter radius and reaching into the confined region beyond the separatrix. Two components are of particular importance for the aforementioned hardware improvements. The first is the optical head, which collects the light from the experiment. Equipped with an innovative clamping system for optical fiber ends, an arbitrary distribution pattern of LOS can be achieved to gain radial and poloidal profiles. The second major development is a new polychromator system that measures the intensity of the 587 nm, 667 nm, 706 nm, and 728 nm helium lines simultaneously for 32 channels with filter-photomultiplier tube arrays. Thus, the thermal helium beam diagnostic supplements the AUG edge diagnostics, offering fast and spatially highly resolved electron temperature and density profile measurements that cover the plasma edge and scrape-off layer region. Plasma fluctuations, edge localized modes, filaments, and other turbulent structures are resolved, allowing analysis of their frequency and localization or their propagation velocity.