Variation Of Electron Number Density Research Articles

Portable Thomson scattering system for temporally resolved plasma measurements under low density conditions.

We present the development of a portable Thomson scattering diagnostic system allowing simultaneous spatially and temporally resolved plasma property measurements for low density plasmas. The setup uses a compact pulsed Nd:YAG laser (532nm) as the light source with suppression by two volume Bragg grating notch filters and dispersion with a single-stage spectrometer before measurement with an intensified camera. A key issue is the detailed light collection and how it impacts the sensitivity and elastic light suppression, for which we have investigated two optical configurations, one based on a 7 × 1 linear fiber bundle and the other based on a slit spatial-filter. We find that the configuration with the slit spatial-filter provides a higher sensitivity by a factor of ∼2 along with more uniform spatial response. We have developed a custom pulsed-plasma setup with a modulation at 20kHz, representative of the Hall thruster breathing mode oscillation, to show the possibility of temporally resolved measurements for electric propulsion applications. We have successfully recorded the variations in electron number density and temperature with sub-mm spatial resolution and capturing ten temporal points over the 50 µs modulation period. The detection limit of electron density (with the spatial-filter configuration) is ∼1.6 × 1017m-3, which is ∼1/10 of the plasma density in the acceleration channel of Hall thrusters.

Experimental and Numerical Investigations on Microwave Absorption by the Cold Collisional Capacity-Coupled-Plasma

The absorption of electromagnetic (EM) wave in the cold collisional capacity-coupled-plasma (CCP) is studied in this paper. First, the time-dependent fluid model is used to gain the spatial variation of electron number density when discharge tends to be in steady state in CCP. The pure argon gaseous electronics conference cell reactor type is used in the model where the diameter is 4 cm, the interelectrode distance is 2.5 cm, the operating frequency is 13.56 MHz, the gas pressure is 1-2 torr, and the applied voltage is 500-1500 V. Second, EM attenuation of 7-12 GHz is studied theoretically for CCP using the plasma parameter obtained from the first step. When the EM wave passes through the plasma, the transmission attenuation is -4.5 dB at 7 GHz, which deduces while EM frequency raises. In addition, the transmission attenuation increases with the applied voltage and gas pressure, respectively. Finally, microwave measurement system is used to measure the transmission attenuation of CCP in anechoic chamber. This paper shows that the theoretical and experimental results have a good agreement on the transmission attenuation.

Effects of cold electron number density variation on whistler-mode wave growth

Abstract. We examine how the growth of magnetospheric whistler-mode waves depends on the cold (background) electron number density N0. The analysis is carried out by varying the cold-plasma parameter a = (electron gyrofrequency)2/(electron plasma frequency)2 which is proportional to 1/N0. For given values of the thermal anisotropy AT and the ratio Nh/N0, where Nh is the hot (energetic) electron number density, we find that, as N0 decreases, the maximum values of the linear and nonlinear growth rates decrease and the threshold wave amplitude for nonlinear growth increases. Generally, as N0 decreases, the region of (Nh/N0, AT)-parameter space in which nonlinear wave growth can occur becomes more limited; that is, as N0 decreases, the parameter region permitting nonlinear wave growth shifts to the top right of (Nh/N0, AT) space characterized by larger Nh/N0 values and larger AT values. The results have implications for choosing input parameters for full-scale particle simulations and also in the analysis of whistler-mode chorus data.

Modeling of sub-ionospheric VLF signal perturbations associated with total solar eclipse, 2009 in Indian subcontinent

During the total solar eclipse of 2009, a week-long campaign was conducted in the Indian sub-continent to study the low-latitude D-region ionosphere using the very low frequency (VLF) signal from the Indian Navy transmitter (call sign: VTX3) operating at 18.2kHz. It was observed that in several places, the signal amplitude is enhanced while in other places the amplitude is reduced. We simulated the observational results using the well known Long Wavelength Propagation Capability (LWPC) code. As a first order approximation, the ionospheric parameters were assumed to vary according to the degree of solar obscuration on the way to the receivers. This automatically brought in non-uniformity of the ionospheric parameters along the propagation paths. We find that an assumption of 4km increase of lower ionospheric height for places going through totality in the propagation path simulate the observations very well at Kathmandu and Raiganj. We find an increase of the height parameter by h′=+3.0km for the VTX-Malda path and h′=+1.8km for the VTX-Kolkata path. We also present, as an example, the altitude variation of electron number density throughout the eclipse time at Raiganj.

A simplified spectrum standardization method for laser-induced breakdown spectroscopy measurements

Relatively high uncertainty (or low repeatability) is one of the main bottlenecks for wide application of LIBS quantitative measurements. The change of plasma temperature and electron number density from pulse to pulse weakens the correlation between the ablation mass and total or part of the spectral area for the same sample, making the normally applied normalization method not effective enough for uncertainty reduction. In the present work, it was assumed that there existed a standard state for samples with similar matrix, where there is a standard plasma temperature, electron number density, and total number density of the element of interest. Therefore, Taylor expansion can be applied near the standard plasma condition to obtain the standard state value of the characteristic line intensity from theory. The temperature variation was regarded to be proportional to the variation of the logarithm of the ratio of two spectral line intensities of the interested element, the variation of electron number density was regarded to be proportional to the variation of the full width at half maximum (FWHM), and the variation of total number density was regarded to be proportional to the variation of the sum of the multiple spectral line intensities of the measured element. Based on these assumptions, the calibration model was established. The results show that measurement precision and accuracy can be greatly improved by the application of this normalization method in measuring the Cu concentration for 29 brass alloy samples. The average relative standard deviation (RSD) value, the coefficient of determination (R2), the root mean square error of prediction (RMSEP), and average value of the maximum relative error were 2.92%, 0.99, 1.46%, 8.42%, respectively, while the values for normalization with the whole spectrum area were: 8.61%, 0.95, 3.28%, 29.19%, respectively, showing significant improvement.

Simulation of Resonant Interaction between Energetic Electrons and Whistler-Mode Chorus in the Outer Radiation Belt

We construct a realistic model to evaluate the chorus wave-particle interaction in the outer radiation belt L = 4.5. This model incorporates a plasmatrough number density model, a field-aligned density model and a realistic wave power and frequency model. We solve the 2D bounce-averaged momentum-pitch-angle Fokker–Planck equation and show that the Whistler-mode chorus can be effective in the acceleration of electrons, and enhance the phase space density for energies of ∼1 MeV by a factor from 10 to 103 in about two days, consistent with the observation. We also demonstrate that ignorance of the electron number density variation along field line and magnetic local time in the previous work yields an overestimate of energetic electron phase space density by a factor 5∼10 at large pitch-angle after two days, suggesting that a realistic plasma density model is very important to evaluate the evolution of energetic electrons in the outer radiation belt.

Spatial and temporal studies of laser ablated ZnO plasma

Gallium doped zinc oxide was ablated using the third harmonics of Nd:YAG laser at various laser fluences and nitrous oxide ambient gas pressures. Optical emission spectroscopic technique was used to determine the plasma parameters. Spatial variation of electron number density (Ne) was determined along the direction normal to the target surface. The electron density obtained was of the order of 1017 cm−3, for the laser fluence in the range of 1.27–6.35 J cm−2. The influence of the substrate temperature on the plasma plume was studied by keeping an Al2O3 (alumina) substrate at a distance of 5 cm distance from the target at various temperatures. The increase in the substrate temperature enhanced the electron number density and intensity of spectral emissions of various species in the plume. The time of flight transients of specific emissions from the plume were recorded. The velocity of neutral gallium decreased from 6.45 to 3.87 km/s at 4 mm distance from the target when the ambient gas pressure was increased from 0.0001 to 0.1 mbar. The velocities of the species increased considerably with an increase in laser fluences. ZnO thin films were deposited on alumina substrates. The plasma plume kinematics were used to corroborate the nature of thin film deposition.

Fluid Turbulence in Quantum Plasmas

Nonlinear fluid simulations are developed by us to investigate the properties of fully developed two-dimensional (2D) electron fluid turbulence in a very dense Fermi (quantum) plasma. We find that a 2D quantum electron plasma exhibits dual cascades, in which the electron number density cascades towards smaller turbulent scales, while the electrostatic potential forms larger scale eddies. The characteristic turbulent spectrum associated with the nonlinear electron plasma oscillations (EPO) is determined critically by a ratio of the energy density of the EPOs and the electron kinetic energy density of quantum plasmas. The turbulent transport corresponding to the large-scale potential distribution is predominant in comparison with the small-scale electron number density variation, a result that is consistent with the classical diffusion theory.

Time-resolved measurements of electron number density and collision frequency for a fluorescent lamp plasma using microwave diagnostics

Microwave interferometry is an established nonperturbing plasma diagnostic technique to measure plasma number density that is simple, accurate, robust, and reliable. This paper reports a related microwave diagnostic method that measures both the electron number density and the electron-neutral collision frequency, which are crucial to understanding the behavior and transport coefficients of plasma. This method measures the attenuation and phase shift of a microwave signal propagating through a plasma with a network analyzer. These measured quantities are related to the real and imaginary parts of the plasma index of refraction by Appleton's equations, which contain the electron number density and collision frequency. Since the electron number density and collision frequency can be obtained directly from measured quantities, one need not know the electron energy distribution function, the electron kinetic temperature, or the electron energy-dependent cross section for the collision process to determine the electron collision frequency. The experimental measurements used to illustrate the method are a paired comparison of the time-resolved electron number density and collision frequency of two types of commercial fluorescent lamps: the standard mercury-based lamp, and the recently introduced green low mercury lamp. Since the plasma properties are periodic at 60 Hz, time-resolved measurements could be made by using the external triggering feature of the network analyzer. Data were taken to illustrate the variation of electron number density and collision frequency during one 60-Hz cycle of the fluorescent lamp plasmas.

Combined Effect of Secondary Electron Emission, Plasma Ion and Electron Number Density Variation due to Dust Charging and Ionization-Recombination Processes on Dust Ion Acoustic Wave Propagation

In a dusty plasma, in addition to influencing the dust charge state the secondary electron emission also leads to an enhancement of the electron number density. For proper accounting for secondary electron emission effect one has therefore, for the sake of consistency, also to take into account the ambient electron and ion loss due to charging collisions. The existence of a steady state for the dusty plasma is possible only through compensation of such particle losses or gain – a likely candidate for which are ionization and recombination processes in dusty plasma. Taking into account the combined effects all these processes and assuming that the frequencies of the various types of interparticle collisions associated with the above-mentioned processes are much less than the dust ion acoustic (DIA) frequency it is shown that a small perturbation of the steady state of the dusty plasma may result in DIA wave growth provided the ionization rate is sufficiently high. Threshold values of the ratio of the ionization rate to recombination rate depending on different plasma parameters are shown graphically. The analysis presented is restricted to the case when the dust charge remains negative. It is seen that instability persists even when there is no secondary emission. The threshold for instability is lower for smaller values of the dimensionless charge parameter z0 = Zd0e2/aTe when secondary emission takes place, compared to the case when it is absent. The situation tends to get reversed as z0 increases. The possibility of the existence of such instability was not noted earlier.

Axial variation of electron number density in thermal plasma spray jets

The electron number density has been measured in a plasma spray torch using Stark broadening of H $_{\beta}$ and Ar-I (430 nm) line. A small amount of hydrogen (1% by volume in argon gas) was introduced to study the H $_{\beta}$ line profile. Axial variation of electron number density has been determined up to a distance of 20 mm from the nozzle exit point of spray torch. The plasma torch was operated at 5 and 10 kW power level and flow of argon was kept at 25 liters per minute. Using the measured excitation temperature data under same experimental conditions, the electron number density has also been calculated using Saha equation. Comparison of electron number densities measured from Stark broadening with those derived from excitation temperature measurements under the assumption of local thermodynamic equilibrium (LTE) in thermal plasma jets indicate about the deviation from LTE in thermal plasma jets. The electron number density measurement using Stark broadening of Ar-I (430 nm) line will be particularly useful when only argon gas is used in thermal plasma jets.

Relations between small scale electron number density fluctuations, radar backscatter, and charged aerosol particles

During several campaigns during the last 10 years, detailed in situ studies of electrons, positive ions, and charged aerosols have been performed by means of rocket borne instruments in the presence of Polar Mesophere Summer Echoes (PMSE) and Noctilucent Clouds (NLC). We have studied the correlation between the amount of charged aerosols present in the mesopause region and the PMSE echo power. We have also correlated the PMSE echo strength with the small‐scale structure of electrons at the radar Bragg scale that are responsible for the echoes. We find that PMSE occur for rather small amounts of charged aerosols, with the number of electrons exceeding the number of charged aerosols. This is in contradiction with previous, mainly theoretical, studies predicting that PMSE only occur when the ratio between the aerosol charge number density and the number density of electrons is larger than about 1. We also find that there is a high degree of correlation between the PMSE echo power and the fluctuation intensity of electrons at scales comparable to half the radar wavelength. This confirms that variations in electron number density are responsible for the echoes, but does not explain the mechanism that creates the fluctuations.

Transport coefficients in arc plasma of SF6–N2 mixtures

We compute thermal and electrical conductivity coefficients as a function of temperature for different compositions of the N2–SF6 mixture. The method we used was derived from Chapman–Enskog’s. The composition of the plasma is calculated assuming the hypothesis of local thermodynamic equilibrium. Calculations were made for temperatures between 1000 and 15 000 K. We show that the densities of the compounds specifically due to the SF6–N2 mixture (i.e., NF, NS, N2F4, NF3, etc....) are negligible compared with the densities of the products due to SF6 alone and N2 alone. The characteristics of the transport coefficients of the mixture are therefore a combination (depending on the proportions in the mixture) of the characteristics of pure N2 and SF6. These theoretical values allow us to interpret the curves obtained experimentally by measuring, using laser interferometry at two wavelengths, the variation of electron number density in a decaying arc discharge in a SF6–N2 mixture. By substituting the values of the transport coefficients into an energy balance equation, we also determined, in the stationary state the profiles of the temperature and the electric field and the variation of the electron density with the discharge current.

Determination of plasma density and temperature profiles using magneto-acoustic oscillations

This paper reports on theoretical and experimental refinements in the use of forced radial magneto-acoustic oscillations of a magnetized plasma as a diagnostic technique. The theory of such oscillations is extended to include radial variations in electron number density, neutral atom density and plasma temperature. In experiments carried out in an argon afterglow plasma, observations of the radial variation of the oscillation amplitude, and of the total flux in the plasma, are combined with theory to yield radial density and temperature profiles. These profiles are in good agreement with independent measurements made using laser interferometry and spectroscopic techniques.

Four Models of Electron Distribution in the Orbitron

As a guide to the design of orbitron pumps and gauges, the radial variation of electron number density is calculated for four models of angular and axial momentum distributions. Uniform angular momentum distribution (for fixed axial momentum) gives an almost constant density. Angular momentum distribution that varies as sin2 gives two peaks. Adding a uniform distribution of axial momentum decreases the density near the cathode such that the outer peak is removed from the two-peak model. Also, the density is approximately proportional to the electron speed near the cathode in the model with uniform distribution of angular momentum.