Edge Electron Research Articles

The effect of CD4 injection on WD molecule sputtering at the divertor target in EAST

Tungsten (W) is one of the preferred plasma-facing materials for future fusion reactors because of the advantages of high melting point, low physical sputtering rate and low fuel retention. The lifetime and service performance of W materials are limited by erosion processes. In recent years, tungsten deuteride molecular (WD) sputtering has been discovered in tokamak. Studying WD sputtering helps to better understand the erosion of tungsten materials in tokamak. EAST adopts the method of injecting impurity gas in the divertor to alleviate the particle flux and heat flux bombarding the tungsten divertor, and CD4 is one of the impurities used. CD4 injection experiments on EAST show that after CD4 is injected from the upper outer (UO) divertor target, despite that the electron temperature (Te) around the strike point on the UO divertor target drops to about 8 eV, strong WD sputtering occurs in the far scrape-off layer (SOL) region. The divertor probe data shows that after CD4 injection, the particle flux from the upstream plasma onto the divertor split into two bands, creating a new particle flux strike point in the far SOL region. The energy of incident particles at the original strike point is significantly reduced, while the energy of incident particles at the far SOL region is enhanced. The edge electron density profile at mid-plane indicates that CD4 injection causes an increase of density in the SOL, which could improve the coupling of Lower-Hybrid Wave (LHW) with the edge plasma as evidenced by the decreasing reflected power of LHW. The increase in absorbed power of LHW could change the particle and energy transport in the edge plasma, thus plays an important role in the enhanced WD sputtering in the far SOL region after CD4 injection.

Self–Trapped Exciton versus Band–Edge Electron Transitions: Insights of the Factors Affecting the Optical Properties of Lead–Free Sn–Halide Perovskites

AbstractLead–free Sn–halide perovskites (Sn–HPs) are attractive photomaterials due to their lower toxicity, and some of them with higher stability against moisture and water, compared to their Pb‐based analogous. Interestingly, Sn‐HPs can exhibit two types of optical characteristics: the first scenario is known as band‐edge electron transitions [or band‐to‐band (b‐b) emission], where accumulated electrons in the conduction band recombine with holes in the valence band, providing a close separation between the absorption edge/photoluminescence (PL) peak (small Stokes shift). The second scenario is denominated as self‐trapped exciton (STE), where intraband gap energy states are formed to trap photocarriers generated in the perovskite, producing a broadband PL and a large Stokes shift. These optical features have been suitable for developing prominent devices, but there is no consolidated explanation about the key factors influencing the emergence of b–b emission or STE in Sn‐HPs, mainly the presence of these PL mechanisms in a particular perovskite system. This review highlights how the chemical composition, structural defects, and synthetic procedures are pivotal to producing Sn‐HPs with specific b–b or STE features. This will allow the preparation of Sn‐HPs with better quality/stability, and facile modulation of their PL properties, expanding their future applicability in LCD technologies.

Ge Enrichment of Ge–Sb–Te Alloys as Keystone of Flexible Edge Electronics

Ge Enrichment of Ge–Sb–Te Alloys as Keystone of Flexible Edge Electronics

A tungsten-wall sputtering model for the plasma start-up simulation in tokamaks

Abstract Tungsten (W) is the most probable material for the plasma-facing components of fusion reactors due to its excellent thermal and physical properties. A W-wall sputtering model has been established to simulate the start-up of a tokamak plasma using the 0D simulation code DYON. This model incorporates the revised Bohdansky formula to calculate the sputtering yield and a modified formula for calculating the energy impacting the walls. This formula integrates the temporal behavior of electron and ion temperatures at the plasma edge, which has been partially verified by the Thomson scattering diagnostic data. With the new model in place, predictive simulations were conducted for KSTAR’s Ohmic plasma under two W-wall scenarios: one with the entire wall surface covered by W and the other with 95% coverage of W and 5% coverage of carbon (C). The results indicate that the full-W wall may perform better from the perspective of start-up performance. The disparity can primarily be attributed to impurities generated through sputtering and recycling on the C wall. The validity of this model will be finally confirmed when the Thomson diagnostic system is able to precisely measure the edge electron temperature during plasma start-up.

Status of edge electron density and temperature measurements with Helium Beam Emission Spectroscopy (He-BES) on EAST

Helium Beam Emission Spectroscopy (He-BES) diagnostic has been developed on EAST, which is able to measure the edge electron density and temperature profiles simultaneously using a helium line intensity ratio method. The diagnostic includes the beam injector and the detection system. There are 20 observation channels within an observation range of 80 mm in the detection system at the low filed side, which can cover the whole scrape-off layer (SOL) and part of the pedestal region of EAST. The beam injector system has been upgraded to Supersonic Molecular Beam Injector (SMBI) system to realize deeper helium injection since the 2021 campaign. Four spectral lines at wavelengths of 728.1 nm, 706.5 nm, 667.8 nm and 656.3 nm are detected by the He-BES. The first three spectral lines, including 728.1 nm, 706.5 nm, 667.8 nm, are measured for calculating edge n e and T e profiles based on the collisional-radiative model (CRM) model, and the last spectral line (656.3 nm) is used for the measurement of D α emission. The edge electrostatic fluctuations can be obtained from the power spectrum of D α emission. The electron density and temperature profiles calculated from the 667.8/728.1 and 728.1/706.5 nm line ratios are in good agreement with those from other diagnostics in the edge region of plasma. The self-consistency of He-BES diagnostic is also verified, such as the density pump out caused by LHW and the lower edge temperature caused by the lower heating power.

Experimental investigation of PDI bifurcation of lower hybrid waves during electron density ramp-up in EAST

The effect of parametric decay instability (PDI) on the current drive efficiency of 4.6 GHz lower hybrid (LH) waves in EAST is investigated experimentally, showing the PDI channel bifurcation of LH waves for the first time in EAST. First, experiments with three platforms of LH power were performed, achieving the LH power required for the PDI occurrence. Second, PDI bifurcation experiments were further carried out by ramping up the plasma electron density. The loop voltage increases with an increase in density, implying a decrease in the current driven by the LH wave. PDI bifurcation during electron density ramp-up was studied by analyzing the parallel refractive index ( n∥ ) and the frequency spectrum broadening, which is measured with a radio frequency magnetic probe array recently installed close to the LH antenna. It is observed for the first time that they both first increase with density, then there is not much variation and a clear sideband in the frequency spectrum is also observed when the density is up to 4×1019 m−3 , suggesting a change in the PDI channel. Calculation of the mode growth rate driven by PDI shows that when the edge electron density is up to 1.9×1018 m−3 , the growth rate of the ion cyclotron quasi-mode (ICQM) will exceed that of the ion sound quasi-mode (ISQM), quantitively explaining that with an increase in density, the PDI channel partly transits from the ISQM to the ICQM channel. Studies provide a possible way to reduce the power deposition in the edge region and improve drive capability by means of mitigating PDI behavior.

Decoupling of peeling and ballooning thresholds for pedestal stability and reduction in ELM frequency via enhanced turbulence with edge electron cyclotron heating in DIII-D

The edge localized mode (ELM) frequency (f ELM) decreased by 63% when electron cyclotron heating (ECH) deposition location is shifted from ρ = 0.4 to ρ = 0.8 in DIII-D discharges where the power ratio between neutral beam injection (NBI) and ECH (P NBI/P ECH) is kept at ∼1. The performance of the pedestal in the ECH heated case is compared with a pure NBI reference discharge while keeping the total input power constant. All these discharges are performed at balanced input torque conditions. Furthermore, in the pure NBI discharge a strong decoupling of the peeling–ballooning (PB) thresholds is observed. The PB decoupling is preserved when the ECH is deposited at ρ = 0.8 and P NBI/P ECH ∼ 1, while the thresholds manifest a closed stability boundary when the ECH is deposited at ρ = 0.4. The inter-ELM pedestal recovery time is considerably larger for the ECH at ρ = 0.8 case. Increased pedestal turbulence is observed in beam emission spectroscopy (BES), Doppler backscattering and magnetic diagnostics for the ECH at the ρ = 0.8 case. Strong growth of a TEM-like mode is observed in BES and the mode growth is correlated with the decrease in f ELM. In view of these observations, the increased pedestal turbulence seems to be the plausible reason behind the delayed pedestal recovery following an ELM event in the ECH at ρ = 0.8 case, and the preservation of PB decoupling through temperature pedestal profile widening. TRANSP interpretative simulations show that the ECH at the ρ = 0.8 case is more susceptible to ITG/TEM turbulence.

Investigation on the edge cooling threshold of the density limit in the J-TEXT tokamak with limiter and divertor configurations

Cooling of the plasma edge is widely considered to be a key element in the density limit (DL) of tokamaks. This paper investigates the critical edge cooling threshold of the DL by exploring various plasma configurations in the joint-Texas experimental tokamak. Notably, significant differences in the edge electron temperature in the vicinity of the last closed flux surface were observed between the limiter and divertor configurations. However, the electron temperature drops to a similar level in the vicinity of q= 3 surface close to DL, independent of the magnetic field configuration. In addition, to evaluate the reliability of the critical edge cooling threshold, experiments were conducted by increasing the carbon impurity content to enhance the edge cooling rate. These experiments involved two approaches to increase the carbon impurity content: methane injection and penetration of a graphite solid source. Results from these experiments indicate that the temperature threshold of the q = 3 surfaces remains consistent even with a stronger edge cooling rate. The consistency observed in the electron temperature threshold near the q = 3 surface, regardless of magnetic configuration or edge cooling rate, could help refine existing theoretical and simulation works and improve the prediction accuracy of the DL disruption.

Highly efficient spin field-effect transistor based on nanographene and hBN heterostructures: spintronic and quantum transport properties

Intense efforts are being undertaken to design atomic-scale nanodevices in response to the demand for novel electronic/spintronic devices to address the problems facing the current devices. In this work, we construct new spintronic nanodevices based on 2D heterostructures from graphene and hBN quantum dots. The heterostructures are ferromagnetic spin-ordered semiconductors with nonzero spin originating from the unpaired edge electrons of the graphene quantum dots. The net spin and its distribution on different layers of the heterostructures are controllable. For instance, doping the hBN layer in the lateral graphene-hBN-graphene structure with C-atoms can boost the net spin and facilitate its distribution on the three layers. The electronic properties are also affected where half metallicity has been achieved in this system. The I-V characteristics of the lateral G-hBN heterostructure show an oscillating spin-up current that is significantly higher than the regular spin-down current. Doping G-hBN with C-atoms significantly enhances the current magnitude and removes the oscillation due to the decreased net spin by the dopants. We demonstrate a field effect transistor from the lateral G-hBN-G heterostructure that operates based on quantum tunneling from the G-layer through the hBN-layer. The device works as a spin filter at zero gate voltage. A significant enhancement of the drain current is achieved by applying a positive gate voltage. This device shows an ON/OFF ratio of up to 1010 making it an ideal applicant for future spintronic nanodevices.

Cu(I/II)-Co(II/III) photocatalyst with intrinsic electron transport centre for photoreduction of chromium(VI) and photodegradation of methyl violet

A redox switchable photocatalyst consisting of mixed valent copper (CuI/II) sulfide (CuxS) and cobalt (CoII/III) oxide (CoyOz) designated as CuxS-CoyOz (x=1,2, y=1,3 and z=1,4) was synthesized to execute the Z-scheme photocatalysis process for photoreduction of chromium(VI), and photodegradation of Methyl Violet (MV) dye. The synthesized photocatalyst with a band gap of 3.3 eV was found to be highly effective to reduce 99.62% Cr(VI) and 99.32% of MV dye under UV-light irradiation. The possibility of switching the oxidation state in Cu and Co centre favored the band edge electron transfer on light irradiation and supported the Z-scheme electron transport chain. The photocatalytic reduction and degradation process was dependent on various reaction conditions like catalyst amount, pH of the reaction mixture, irradiation time. The reduction of Cr(VI) occurred with high efficacy at pH=2 while the MV dye degradation proceeded at high pH=10. The catalyst could reduce maximum concentration of Cr(VI) upto 250 ppm and MV dye upto 10−3 M. The mechanism of the photocatalytic processes were well studied by different spectroscopic and electrochemical analysis. The photocatalytic activity of synthesized CuxS-CoyOz catalyst was compared with five other transition metal sulfides M-S (MnS, FeS, CoaSb, NiS and ZnS, where a=3, 4; b=3, 4) combined with cobalt oxide and it was found that CuxS-CoyOz exhibited the highest activity in both the photocatalytic process.

Impact of surface treatments on the electron affinity of nitrogen-doped ultrananocrystalline diamond

In recent years, various forms of nanocrystalline diamond (NCD) have emerged as an attractive group of diamond/graphite mixed-phase materials for a range of applications from electron emission sources to electrodes for neural interfacing. To tailor their properties for different uses, NCD surfaces can be terminated with various chemical functionalities, in particular hydrogen and oxygen, which shift the band edge positions and electron affinity values. While the band edge positions of chemically terminated single crystal diamond are well understood, the same is not true for nanocrystalline diamond, which has uncontrolled crystallographic surfaces with a variety of chemical states as well as graphitic grain boundary regions. In this work, the relative band edge positions of as-grown, hydrogen terminated, and oxygen terminated nitrogen-doped ultrananocrystalline diamond (N-UNCD) are determined using ultraviolet photoelectron spectroscopy (UPS), while the band bending is investigated using photoelectrochemical measurements. In contrast to the widely reported negative electrode affinity of hydrogen terminated single crystal diamond, our work demonstrates that hydrogen terminated N-UNCD exhibits a positive electron affinity owing to the increased surface and bulk defect densities. These findings elucidate the marked differences in electrochemical properties of hydrogen and oxygen terminated N-UNCD, such as the dramatic changes in electrochemical capacitance.

Estimates of global recycling coefficients for LTX-β discharges

We report the first observation of global recycling coefficient R near 0.5 in the Lithium Tokamak eXperiment-β (LTX-β), significantly below the minimum R previously reported in other devices. In a series of experiments with varied Li wall conditioning, estimates of the recycling coefficient have been made using a Lyman-α array and DEGAS2 modeling. A progressive reduction in Lyman-α emission with increased lithium and an increase in edge electron temperature are observed. It is also observed that with increasing Li coating thickness, the effective particle confinement time τp* is reduced and approaches TRANSP calculated energy confinement time (τE), with τp* near τE,TRANSP for the lowest recycling coefficients. Edge temperatures approaching core plasma temperatures, first reported in LTX, can now be directly connected to estimates of the recycling coefficient and qualitatively agree with previous UEDGE simulations. The particle flux to the limiting surfaces appears to be significantly reduced in comparison with fluid scrape-off layer (SOL) models, indicating that a large fraction of the SOL ions are mirror trapped. SOL collisionality drops more than an order of magnitude below the banana regime boundary, indicating the importance of kinetic effects. Full-f 1x2v gyrokinetic simulations of SOL field lines with the GKEYLL code indicate that the fraction of ions trapped along field lines increases as collisionality drops, as a result of increased lithium evaporation.

Time-resolved mid-infrared photoluminescence spectroscopy of an undoped InAs substrate

Time-resolved mid-infrared photoluminescence (PL) spectroscopy of an undoped InAs substrate has been achieved with wavelength upconversion and time-correlated single photon counting methods. The substrate exhibits multiple PL peaks at photon energies of around 0.415 eV, and the peak positions and intensities change as the temperature is varied from 3.7 to 80 K. The dominant PL peaks are attributed to free and donor-bound excitons and radiative recombination between electrons at the Fermi edge in the conduction band and holes in the valence band edge. The PL lifetime of the excitons is 12 ns, which is four times longer than that of GaAs. The band edge electron–hole recombination has a longer PL lifetime of 60 ns at 20 K. The unveiling of luminescence dynamics in narrow bandgap semiconductors will contribute to the development of mid-infrared light-emitting devices.

Fe Kα XANES, Fe Kβ HERFD XANES and EPMA flank method determinations of the oxidation state of Fe in garnet

The ferric to total iron ratios (Fe3+/∑Fe) of garnets can be paired with thermodynamic mineral activity models to quantify the oxygen fugacity of garnet-bearing rocks. However, techniques with a high analytical and spatial resolution are necessary to distinguish differences in garnet Fe3+/∑Fe ratios at the percent level and to accurately measure garnets that are zoned or contain inclusions. We acquired conventional Fe Kα and high-resolution energy fluorescence detection (HERFD) Fe Kβ X-ray absorption near edge structure (XANES) spectra and electron microprobe flank method analyses on a suite of 27 peridotitic and eclogitic garnets with Fe3+/∑Fe ratios previously determined by Mössbauer spectroscopy to evaluate the precision of each technique. We examined variations in the energy and intensity of three XANES spectral features as a function of Fe3+/∑Fe ratios: 1) the intensity ratio of two-post edge features (I-ratio; Fe Kα only); 2) the energy of the Fe edge at 90% normalized intensity (E0.9; Fe Kα only) and 3) the pre-edge centroid energy (Fe Kα and HERFD Fe Kβ). In accordance with previous work, we find the energies of garnet pre-edge centroids are relatively insensitive to Fe3+/∑Fe ratios. The I-ratios of peridotitic and eclogitic garnets are offset from each other at low Fe3+/∑Fe ratios (≤0.13); I-ratio garnet XANES calibrations are composition-specific. The E0.9 feature is independent of garnet major element composition in spectra that have been corrected for the effects of self-absorption. We produce two Fe Kα garnet XANES calibrations based on variations in the E0.9 feature; one calibration with all garnet reference materials included (Fe3+/∑Fe up to 1.0; “all garnet calibration”) and another calibration specific to garnets with low Fe3+/∑Fe ratios (“low ferric calibration”). Fe3+/∑Fe ratios calculated from the mean of up to 25 flank method measurements on eight garnet reference materials fall within 4% absolute of a one-to-one correlation with Fe3+/∑Fe ratios measured by Mössbauer. The standard error of the mean Fe3+/∑Fe ratio calculated from flank method approaches the Mössbauer-determined Fe3+/∑Fe ratio within estimated error (3%) after three analyses. Flank method precision is enhanced at higher beam current; however, the precision of the flank method does not approach the precision of XANES under any microprobe analytical condition tested here. Garnet reference materials detailed here are available by request to the Smithsonian Institution.

Investigation on edge defect characteristics and electronic transport characteristics of graphene nano cutting

The effects of cutting crystal direction and speed on edge morphology, defects and electron transport characteristics were studied by molecular dynamics from the distribution state of defect atoms, the number of defect atoms, cutting force and radial distribution function. The edge defects of zigzag graphene nanoribbons were extracted, and the difficulty of forming different kinds of defects and the influence of different defects on band gap were studied by density functional theory. The results indicate that cutting graphene along the [010] (zigzag) direction has a smaller variance and smoother cutting. The obtained graphene nanoribbons have fewer defects and good edge quality. And the higher the cutting speed, the fewer defects of the graphene nanoribbons formed, resulting in smaller damage. The typical defects at the edges include 5–8–5 defect (double-vacancy defect), 5–9 SV defect (single-vacancy defect), stone wales (SW) defect, chain defect, crack defect and hole defect. The relationship between the magnitude of forming energy values produced by different defect types is as follows: crack defect > chain defect > SW defect > 5–9 SV defect > 5–8–5 defect > hole defect. Hole defect is the most difficult to form. The band gap width of the cut edge containing defects is smaller than that of the perfect graphene nanoribbon, resulting in the increase of the conductivity of the graphene nanoribbon in the direction of metal characteristics. The presence of defects can open the band gap with of intrinsic graphene.

Improving Performance of Impact Point Determination Algorithms of Gamma Interaction in a Monolithic Scintillator Using ML and Numerical Methods

Two algorithms for classification of the number of interaction points of gamma-ray in a Compton camera with scintillator crystal are developed, taking into account the limited processing power of the edge electronics in the space environment. One of the algorithms is relaying on a machine learning image classification convolution network, and the other is a numerical method, searching for multiple maxima peaks in the light intensity over the detector plane. Preliminary results of the accuracy of the two approaches are provided and discussed.

Development of dual-path multi-pass Thomson scattering system in GAMMA 10/PDX

A dual-path Thomson scattering (DPTS) system was developed to measure electron temperature and density in a GAMMA 10/PDX central cell (CC) and an end cell (EC). The DPTS comprises CC Thomson scattering system (CC-TS) and an EC Thomson scattering system (EC-TS). In the CC-TS, we developed a multipass system to improve the signal intensities and time resolution. In the EC-TS, a double-pass system was developed as a preliminary step in developing a multipass system. In a previous EC-TS, the optical components for the double-pass configuration were placed in a vacuum vessel. To reduce the influence of stray light and facilitate alignment, a hole for the double-pass laser path was made on the partition wall in the vacuum chamber such that the probe laser could be extracted into the atmosphere and the double-path configuration could be easily adjusted. We applied DPTS to detached plasma experiments and successfully measured the core and edge electron temperatures and densities with the detached plasma experiments with the additional application of supersonic molecular beam injection and central electron cyclotron heating in the core plasma to simultaneously introduce intermittent particle flux.

Localized Electron Density Regulation Effect for Promoting Solid-Liquid Ion Adsorption to Enhance Areal Capacitance of Micro-Supercapacitors.

The development of flexible microelectronic systems requires the construction of high-energy-output planar micro-supercapacitors (MSCs). Herein, the localized electron density, by introducing graphene quantum dots (GQDs) on the surface of electrodes, is regulated. The enhanced local field intensity promotes ion electrostatic adsorption at the solid-liquid interface, which significantly improves the energy density of MSCs in the confined space. Local electronic structure has been investigated from the perspective of the topological analysis of the electron localization function (ELF) and the electron density. Impressively, the edges of the simulated structure exhibit a higher electron density distribution than the CC skeleton. This finding indicates that the introduced GQDs reinforce the intrinsic electrical double-layer capacitance (EDLC) and the oxygen-bearing functional groups at the edge, further increasing the pseudocapacitance performance. Moreover, the edge electron aggregation effect enables the all-carbon-based symmetric MSCs to exhibit ultra-high areal capacitance (21.78mFcm-2 ) and excellent cycle stability (86.74% retention after 25000 cycles). This novel surface local charge regulation strategy is also applied for intensifying ion electrostatic adsorption on Zn-ion hybrid MSCs (polyvalent metal ions) and ion-gel electrolyte MSCs (non-metallic ions). With excellent planar integration, this device demonstrates excellent flexibility and has potential applications in timing and environmental monitoring.

Flavor symmetry breaking in spin-orbit coupled bilayer graphene

Recent experimental discovery of flavor symmetry breaking metallic phases in Bernal-stacked bilayer graphene points to the strongly interacting nature of electrons near the top (bottom) of its valence (conduction) band. Superconductivity was also observed in between these symmetry breaking phases when the graphene bilayer is placed under a small in-plane magnetic field or in close proximity to a monolayer WSe$_2$ substrate. Here we address the correlated nature of the band edge electrons and obtain the quantum phase diagram of their many-body ground states incorporating the effect of proximity induced spin-orbit coupling. We find that in addition to the spin/valley flavor polarized half and quarter metallic states, two types of intervalley coherent phases emerge near the phase boundaries between the flavor polarized metals. Both spin-orbit coupling and in-plane magnetic field disfavor the spin-unpolarized valley coherent phase. Our findings suggest possible competition between intervalley coherence and superconducting orders, arising from the intriguing correlation effects in bilayer graphene in the presence of spin-orbit coupling.

Extending the low-recycling, flat temperature profile regime in the lithium tokamak experiment-β (LTX-β) with ohmic and neutral beam heating

Recent experiments in the lithium tokamak experiment-β (LTX-β) have extended the duration, performance, operating conditions, and diagnosis of the flat-temperature profile, low-recycling regime first observed in LTX. As expected, Li retains hydrogen and suppresses edge neutral cooling, allowing increased edge electron temperature, roughly equal to the core T e. Flat temperature profiles had been obtained transiently in LTX, as the plasma density decayed following the cessation of edge gas puffing. Careful control over the fueling in LTX-β has now been shown to sustain the flat T e profile and hot edge unique to the low-recycling regime for multiple confinement times in high performance discharges with decaying or steady density. With low density, the flat T e profile is also seen to extend into the scrape-off layer. Neutral beam heating is observed in target discharges with relatively flat electron temperature profiles (T edge ∼ T core/2), though beam heating is stronger in discharges with higher fueling, higher density, and depressed edge T e. Beam heating produces additional peaking of the T e profile, without degradation of the energy confinement time. Neutral beam heating of target discharges with relatively flat electron temperature profiles similarly results in broad beam heated temperature profiles. Energy confinement in LTX-β generally compares favorably to ohmic and H-mode scalings, frequently exceeding them by factors of 2–4. New and improved diagnostics in LTX-β enable better characterization of this unique regime, including measurements of ion temperature and high field side Thomson scattering profiles. As an initial step toward characterizing turbulence with no T e gradient and roughly equal density and pressure gradient, core fluctuation spectra have been measured in peaked T e discharges using far-forward scattering and fluctuation reflectometry.