Drop In Electron Density Research Articles

Self‐organized pattern formation and plasma diagnosis in atmospheric pressure helium glow discharges with a liquid anode

AbstractA negative DC high voltage is used to generate plasma at ambient condition. Self‐organized patterns (SOPs) are formed on the surfaces of the liquid anodes. With increased current or gap distance or reduced gas flow rate, the diameters of SOPs increase and SOPs become more complex, coinciding along with an increases of gas temperature and plasma radius and a decrease of electron density. Both plasma expansion and liquid evaporation in terms of gas temperature, lead to a drop of electron density and pattern formation. Our research elucidates the correlation between SOPs evolution and plasma properties and clarifies the physical process of SOPs formation based on the interaction between atmospheric pressure plasma and liquid anodes.

Alfvén velocity sudden increase as an indicator of the plasmapause

A radial distribution of ion mass density in the Earth’s magnetosphere may be different from an electron one when local narrow enhancement of heavy ions flux is presented, e.g. oxygen torus. This difference may be important for ultra-low-frequency (ULF) waves research that includes Alfvén waves whose eigenfrequency is dependent on Alfvén velocity and thus on mass density. One of consequences is a shift of the plasmapause location that is thought as a resonator and generation area for ULF waves. In the present study we found an Alfvén velocity increase by a factor of 1.65 or more within a radial distance of 0.5 RE to be a threshold that corresponds to the classic plasmapause definition that is the electron density drop by a factor of 5 or more within a radial distance of 0.5 RE. This result was obtained under all-proton plasma assumption to be confident that the Alfvén velocity may be used to detect the plasmapause with the same accuracy as by electron density. We wish to use this threshold to reveal the plasmapause location when heavier ions will be included and direct comparison with electron density is not possible.

Electron density and electron temperature measurements in an atmospheric pressure plasma interacting with liquid anode

Plasma driven solution electrochemistry has received increasing attention during the last decade for a variety of applications including nanomaterial synthesis. We report the temporal and spatial resolved electron density and temperature for a negative pulsed DC discharge in helium with N2 shielding gas impinging on a liquid anode as measured by Thomson scattering spectroscopy. A stable radial plasma contraction and significant plasma-enhanced N2 mixing was found for the longest investigated pulse width (9 μs). It was found that the plasma enhanced N2 mixing significantly impacts the plasma morphology and electron properties. In addition, we observed a significant increase in electron temperature coinciding with a drop in electron density near the liquid anode surface, which is attributed to electron attachment and electron-water ion cluster recombination enhanced by plasma-induced water evaporation. This near anode surface phenomenon is argued to be responsible for the discharge stabilization by preventing the development of a thermal instability in spite of the significant gas heating. This increase in electron temperature near the anode suggests the presence of a significant flux of hot electrons into solution which might enable non-equilibrium electron-driven reactions in the liquid phase.

Observation of a significant drop of electron density in cascaded arc argon plasma doped with oxygen gas using laser Thomson scattering

In this work, the electron density (n e) and electron temperature (T e) of cascaded arc argon plasma regulated by adding electronegative oxygen gas have been investigated using laser Thomson scattering diagnostic technique. The results indicate that the addition of O2 gas causes a significant decrease of n e, which drops from 1020 m−3 to 1017 m−3. This is mainly attributed to the dissociative recombination reaction between electrons and O2 + molecular ions. Meanwhile, the formation of negative ions, O2 − and O−, consumes electrons and further makes n e decrease. But, T e remains nearly unchanged with the increase of O2 ratio from 0% to 10%. This is probably due to that the electron energy loss by the electron collisions with O2 molecules in the ground state balances the electron heating induced by the super-elastic collisions with the highly vibrational excited O2 molecules.

Temporal and Spatial Variations of Storm Time Midlatitude Ionospheric Trough Based on Global GNSS‐TEC and Arase Satellite Observations

AbstractTemporal and spatial variations of the midlatitude ionospheric trough during a geomagnetic storm on 4 April 2017 have been investigated using Global Navigation Satellite System total electron content data together with Arase satellite observations. After the geomagnetic storm commencement, the trough minimum location moves equatorward from 60 to 48° in geomagnetic latitude within 4 hr. The trough minimum location identified from the Global Navigation Satellite System total electron content data is located near the footprint of an abrupt drop of electron density detected by the Arase High‐Frequency Analyzer instrument. The longitudinal variation of the trough minimum location shows a significant variation with a scale of 1,000–2,500 km during both storm and quiet times. This phenomenon has not yet been reported by previous studies. After the onset of the storm recovery phase, the trough minimum location rapidly moves poleward back to the quiet time location within 4 hr.

Influence of nanoparticle formation on discharge properties in argon-acetylene capacitively coupled radio frequency plasmas

This contribution presents experimental results regarding the influence of nanoparticle formation in capacitively coupled radio frequency (13.56 MHz) argon-acetylene plasmas. The discharge is studied using non-invasive 160 GHz Gaussian beam microwave interferometry and optical emission spectroscopy. Particularly, the temporal behavior of the electron density from microwave interferometry is analyzed and compared with the changing plasma emission and self-bias voltage caused by nanoparticle formation. The periodic particle formation with a cycle duration between 30 s and 140 s starts with an electron density drop over more than one order of magnitude below the detection limit (8 × 1014 m−3). The electron density reduction is the result of electron attachment processes due to negative ions and nanoparticle formation. The onset time constant of nanoparticle formation is five times faster compared to the expulsion of the particles from the plasma due to multi-disperse size distribution. Moreover, the intensity of the argon transition lines increases and implies a rising effective electron temperature. The cycle duration of the particle formation is affected by the total gas flow rate and exhibits an inverse proportionality to the square of the total gas flow rate. The variation in the total gas flow rate influences the force balance, which determines the confinement time of the nanoparticles. As a further result, the cycle duration is dependent on the axial position of the powered electrode, which also corresponds to different distances relative to the fixed optical axis of the microwave interferometer.

Optimization of plasma parameters with magnetic filter field and pressure to maximize H⁻ ion density in a negative hydrogen ion source.

Transverse magnetic filter field as well as operating pressure is considered to be an important control knob to enhance negative hydrogen ion production via plasma parameter optimization in volume-produced negative hydrogen ion sources. Stronger filter field to reduce electron temperature sufficiently in the extraction region is favorable, but generally known to be limited by electron density drop near the extraction region. In this study, unexpected electron density increase instead of density drop is observed in front of the extraction region when the applied transverse filter field increases monotonically toward the extraction aperture. Measurements of plasma parameters with a movable Langmuir probe indicate that the increased electron density may be caused by low energy electron accumulation in the filter region decreasing perpendicular diffusion coefficients across the increasing filter field. Negative hydrogen ion populations are estimated from the measured profiles of electron temperatures and densities and confirmed to be consistent with laser photo-detachment measurements of the H(-) populations for various filter field strengths and pressures. Enhanced H(-) population near the extraction region due to the increased low energy electrons in the filter region may be utilized to increase negative hydrogen beam currents by moving the extraction position accordingly. This new finding can be used to design efficient H(-) sources with an optimal filtering system by maximizing high energy electron filtering while keeping low energy electrons available in the extraction region.

Investigation of sudden electron density depletions observed in the dusk sector by the Poker Flat, Alaska incoherent scatter radar in summer

AbstractThis paper investigates unusually deep and sudden electron density depletions (troughs) observed in the Poker Flat (Alaska) Incoherent Scatter Radar data in middle summer of 2007 and 2008. The troughs were observed in the premidnight sector during periods of weak magnetic and solar activity. The density recovered to normal levels around midnight. At the time when the electron density was undergoing its steep decrease, there was usually a surge of the order of 100 to 400 K in the ion temperature that lasted less than 1 h. The Ti surges were usually related to similar surges in the AE index, indicating that the high‐latitude convection pattern was expanding and intensifying at the time of the steep electron density drop. The convection patterns from the Super Dual Auroral Radar Network also indicate that the density troughs were associated with the expansion of the convection pattern to Poker Flat. The sudden decreases in the electron density are difficult to explain in summer because the high‐latitude region remains sunlit for most of the day. This paper suggests that the summer density troughs result from lower latitude plasma that had initially been corotating in darkness for several hours post sunset and brought back toward the sunlit side as the convection pattern expanded. The magnetic declination of ~22° east at 300 km at Poker Flat greatly facilitates the contrast between the plasma convecting from lower latitudes and the plasma that follows the high‐latitude convection pattern.

The Evolution of Free Electron Density during the Interaction between Femtosecond Pulse and Lithium Niobate Crystal

The nonlinear ionization process was discussed in detail induced by femtosecond pulse in Lithum Niobate (LN) crystal. The MPI provides “seed” electrons for avalanche ionization process, and avalanche becomes significant when the density of electron created by MPI over 1020 cm-3. Recombination process leads to an obvious drop of electron density and then a dynamic equilibrium is achieved between the generation of electron and the recombination process, and the density is dependent on the incoming field.

Response of the equatorial lower ionosphere to the total solar eclipse of 22 July 2009 during sunrise transition period studied using VLF signal

Changes in equatorial D‐region electron density are studied using subionospherically propagating VLF signal at 18.2 kHz over a distance of 2200 km during the total solar eclipse of 22 July 2009. There are very few studies about the eclipse's effects on the equatorial lower ionosphere in the scientific literature. In the light of that, the objective of the present work is to study the effects of the eclipse on the dynamics of the equatorial lower ionosphere during ionospheric sunrise transition period. In the present case, great circle path between VLF transmitter and receiver falls totally in partial eclipse zone, having a maximum solar obscuration of 90% and an average obscuration of 74%. Results show an average decrease of 3.2 dB in signal strength compared to control days during peak solar obscuration over the path. A comparison with previous studies shows an increase both in lower ionosphere virtual reflection height (H′) and Wait inverse scale height parameter (β); the values estimated are 74.5 km and 0.46 km−1 compared to unperturbed ionosphere values of 71 km and 0.43 km−1, respectively. During maximum eclipse over the path, the model profile shows an average 80% drop in electron density at a height of 71 km at equatorial lower ionosphere. A nonlinear variation of lower ionosphere electron density with solar radiation is found as opposed to the model study proposed by previous workers.

Physical Conditions in Quasar Outflows: Very Large Telescope Observations of QSO 2359–1241

We analyze the physical conditions of the outflow seen in QSO 2359-1241 (NVSS J235953–124148), based on high-resolution spectroscopic VLT observations. This object was previously studied using Keck HIRES data. The main improvement over the HIRES results is our ability to accurately determine the number density of the outflow. For the major absorption component, the populations from five different Fe II excited levels yield a gas density nH = 104.4 cm−3 with less than 20% scatter. We find that the Fe II absorption arises from a region with roughly constant conditions and temperature greater than 9000 K, before the ionization front where temperature and electron density drop. Further, we model the observed spectra and investigate the effects of varying gas metallicities and the spectral energy distribution of the incident ionizing radiation field. The accurately measured column densities allow us to determine the ionization parameter (log UH ≈ − 2.4) and total column density of the outflow [log NH(cm −2) ≈ 20.6]. Combined with the number density finding, these are stepping stones toward determining the mass flux and kinetic luminosity of the outflow, and therefore its importance to AGN feedback processes.

Compensation for transient chamber wall condition using real-time plasma density feedback control in an inductively coupled plasma etcher

Reactive ion etch processing is known to exhibit significant variability in final etch performance due to wall condition. Previous studies have shown that neutral species transients depend strongly upon chamber seasoning. In this article, we show simultaneous measurements of wall-state-induced changes in plasma density and poly-Si etch rate, and demonstrate a real-time feedback control system that corrects for the plasma density variation. We demonstrate that controlling the plasma density to constant value eliminates the classic “first wafer effect” in Cl2 etching of poly-Si in a Lam 9400 transformer coupled plasma (TCP). Chamber conditions studied include fluorination/cleaning by C2F6 plasmas compared to chlorination/deposition from Cl2 plasmas. Transient density changes due to wall condition were measured using a microwave resonance cavity technique called broadband rf. Following chamber fluorination, broadband data show a significant drop in electron density (∼39%) from nominal levels. This is followed by subsequent partial recovery of the nominal density during a 60 s Cl2 etch. Independent measurements correlate strongly with the broadband signals; particularly real-time poly-Si etch rate and SiCl4 etch product concentration. Observed real-time variations in all signals were then compensated using a single input/single output proportional-integral (PI) feedback control algorithm, in which the broadband peak frequency is the system output variable and TCP power is the system actuator. Such PI control not only stabilizes broadband peak frequencies, but also steadies poly-Si real-time etch rate and SiCl4 etch product concentration. This compensation scheme is then applied over multiple runs to reduce etch depth variability due to chamber condition. Results of this control system are shown to attenuate first wafer effects by a factor of 3 and reduce overall etch depth variation from run to run by an additional 33% compared to standard manufacturing practice. When applied to patterned wafer etches, feedback control of plasma density with an oxide hardmask is found not to alter critical dimensions or profile in any observable way. Only total etch depth appears to be affected using density control, as the real-time etch rate is in general increased in comparison to open-loop etching.

Variations in ionospheric parameters during periods of minimum and maximum solar activity

Ionospheric investigations in Ukraine are being carried out with uniquely developed incoherent scatter radars. Data on the variations in the electron density and the electron and ion temperatures during periods of low (1994) and high (1989 - 1990) solar activity are presented. The F-layer peak height ranges from 450 at night to 240 km in the daytime; the electron density maximum varies approximately from over a cycle of solar activity throughout the winter period. The electron temperature has a maximum afternoon value as the solar activity is high in summer. In the periods of magnetic disturbances, a drop in is most frequently observed. In winter, the influence of the sunrise and sunset at the magnetic conjugate point is also observed. An increase in electron temperature, a drop in electron density, and the displacement of the heated region along the magnetic field lines are clearly seen in the ionosphere irradiated by high-power, HF radiation. Longitudinal effects in the ionospheric behaviour were observed in simultaneous measurements in Kharkiv and Millstone Hill (USA).

Microstructure and phase characterization of diamond-like amorphous hydrogenated carbon films using STM/STS

Scanning tunneling microscopy and spectroscopy (STM/STS) are used to obtain nanoscale information on morphological and electronic properties of the surface of diamond-like amorphous hydrogenated carbon (a-C:H) films. The films are prepared by r.f. plasma decomposition of methane CH 4. A two phase model of a-C:H involving sp 2 clusters, embedded in a sp 3-bonded matrix, is suggested. A new approach to a detection of graphite-like clusters at the surface of a-C:H films is proposed. An overlayer of indium tin oxide (ITO) which helps to detect graphite-like clusters is used. The ITO deposition is performed in the conditions which routinely cause ITO to grow as a good conductor with high electron density. The shape of current-voltage ( I- V) characteristics obtained on the ITO/a-C:H, however, indicates nanoclusters of insulator within the matrix of the conductor. To explain the observed phenomenon the following results are considered. First, I- V characterization hints that thin films of ITO grown on weakly textured graphite normally have reduced electron density. Second, X-ray photoelectron spectroscopy measurements show that weakly textured graphite adsorbs oxygen much stronger than a-C:H. It is suggested, therefore, that it is oxygen, adsorbed by graphite-like clusters at the surface of a-C:H, which causes local drop of electron density in the ITO. As a consequence, I- V characterization of ITO/a-C:H can be used for obtaining a high resolution map of the location of graphite-like clusters over the a-C:H surface.

Negative ion measurements and etching in a pulsed-power inductively coupled plasma in chlorine

Anomalous side wall etching, called `notching' in gate poly-Si etching, is suppressed in a pulsed-power chlorine inductively coupled plasma (ICP). To understand the mechanism, comprehensive time-resolved measurements were performed on such key parameters as chlorine negative ion density, electron density, electron temperature and plasma potential. Comparison of these data with argon afterglows reveals a rapid electron cooling and a remarkable electron density drop which are caused by electron dissociative attachment forming abundant negative ions. The measurements of RF bias and plasma potential suggest a new mechanism of notch-free etching. Namely, the substrate potential in the positive RF phase instantaneously exceeds the plasma potential in the afterglow by a considerable amount, . Then electrons are accelerated through a sheath and neutralize positive charges on the gate oxide layer. Finally, the poly-Si etching process utilizing abundant is examined, focusing on the bias-frequency-dependence.

A quantitative study of the high latitude ionospheric trough using EISCAT's common programmes

Eighteen days of EISCAT data were used in a systematic study of the high latitude trough. Apart from a few days at midwinter, the pattern was the same in all cases. Near midnight the reversal of plasma flow from westward to eastward caused significant frictional heating of the ion population. At the same time a strong plasma velocity was observed upwards along the magnetic field line. This was the result of 1. (i) a southward neutral wind 2. (ii) a vertical wind driven by Joule heating 3. (iii) diffusion. Both enhanced recombination—associated with the increase in ion temperature—and the escape of plasma along the field line contribute to the drop in electron density.

Electron density measurements in vacuum arcs at anode spot formation threshold

CO2 laser interferometry was used to determine electron density profiles in copper vacuum arcs just before and after the transition from the low-voltage quiescent mode to the high-voltage mode associated with anode-spot formation. Just before transition, the electron-density profiles are peaked on axis, with maxima of 9×1014 cm−3 and 6×1014 cm−3 in the cathode and anode regions, respectively. At about 0.1 ms after transition, the electron-density profile in the cathode region remains unchanged, while the central electron density near the anode drops be a factor of about 2. No constriction in the density profile was observed at any axial position. Previously reported anode surface temperature measurements are used to attribute the sudden drop in electron density near the anode just after transition to a local increase in the electron-ion recombination rate caused by a sudden influx of relatively cold neutral atoms from the anode surface. The implications of these observations with regard to a mechanism for transition are discussed.

Topside sounder observations of equatorial bubbles

Large scale regions of depleted equatorial ionospheric plasma, called equatorial bubbles, are investigated using topside sounder data. The sounder’s unique remote measuring capability enables the magnetic field‐aligned nature of the bubbles to be investigated. A search of all available Alouette 2 and ISIS 1 ionograms during nighttime perigee passes near the magnetic equator has revealed a variety of echo signatures associated with bubbles. In addition to a sudden drop in electron density, these signatures usually include in situ spread F and ducted traces. The ducted traces have been used to determine the electron density distribution and to infer changes in ion composition along the magnetic field line within the duct associated with the bubble. In some cases it can be determined that the bubble is asymmetric with respect to the magnetic equator. Even though such features require 3 dimensional models for their explanation, the great field‐aligned extent of the bubbles (relative to their cross section) suggests that current theories, which ignore variations along the magnetic field, are still applicable.

A model of ionospheric F-2 region storms in middle latitudes

The proposed ionospheric storm model is based on a heat source located at magnetic noon on Feldstein's auroral oval. The rotation of the Earth produces an apparent motion of the source which is greater than the speed of the disturbance. This gives rise to a wake or front which sweeps over the globe and determines the onset time of the negative phase which results from a change in chemical composition. At the front, focussing will occur which accounts for the sudden drop in electron density (or contents) sometimes observed. The calculated onset times of the negative phase are compared with observations for a number of storms. The local onset times vary from 12 at the latitude of the source to around 24 at 10° geomagnetic latitude. This model predicts that the onset of the negative phase at a given location, for storm which commence between about 2000 LT to about 1000 LT, is independent of the time of storm commencement.

Studies of ionospheric storms using a simple model

The ionospheric storm model is based on a heat source located at magnetic noon on Feldstein's auroral oval. The rotation of the earth produces an apparent motion of the source that is greater than the speed of the disturbance. This gives rise to a wake or front that sweeps over the globe and determines the onset time of the negative phase, which results from a change in chemical composition. At the front focusing will occur, which accounts for the sudden drop in electron density (or content) sometimes observed. Calculated diurnal variations of maximum electron density and height are in general agreement with the positive and negative phases of storms.