Effect of radiofrequency on the sheath-plasma resonance in a low-density plasma

Summary Bohm's theory on plasma probes has been modified, by taking into account the effect of the applied RF voltage on the ion saturation current, for explaining the experimentally detected behaviour of the plasma sheath resonance frequency in a low-temperature and low-density plasma

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Riassunto La teoria di Bohm sulle sonde nei plasmi è stata modificata, tenendo conto dell'effetto dovuto alla tensione RF applicata sulla corrente ionica di saturazione. Si spiega cosi l'andamento della frequenza di risonanza plasma-guaina ionica, rilevato sperimentalmente, in un plasma a bassa temperatura e bassa densità.

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Резюме Предлагается модификация теории Бома для плазменных зондов, которая учитывает влияние приложенного радиочастотного напряжения на ионный ток насыщения. Предложенная модификация позволяет объяснить экспериментально обнаруженное поведение резонансной частоты плазменной оболочки при низкой температуре и при низкой плотности плазмы.

     

VUV-spectroscopy of the plasma light emission generated by the pulsed arc cluster ion source (PACIS)

A pinhole grid spectrometer is used to measure the light emission from the plasma of the pulsed arc cluster ion source (PACIS). Spectra of various metals and carbon have been measured between 20 and 100 nm. In the case of carbon the average electron temperature is estimated to about 0.69 eV. Higher temperatures up to 0.79 eV are measured when inserting seeding gas which flushes the discharge volume with approx. one atmosphere of helium. An operation under this source conditions leads to the generation of an intense charged cluster beam. The application of the source as a bright light source in the VUV region is discussed. Received: 26 February 1998 / Revised: 12 May 1998 / Accepted: 14 May 1998     

Langmuir probe and spectroscopic studies of RF generated helium-nitrogen mixture plasma

This paper reports the effect of helium percentage variation in a capacitive RF helium-nitrogen mixture plasma on various plasma parameters and concentration of nitrogen active species (N₂(C³Π _u) and N₂ ⁺(B²Σ _u ⁺)). Langmuir probe is used for determination of electron energy distribution functions, effective electron temperature, plasma potential and electron density. Optical emission spectroscopy is used for determination of electron temperature from Boltzmann's plot of He–I lines and the relative changes in the concentration of active species by measuring the emission intensities of nitrogen (0-0) bands of the second positive and the first negative systems. The results demonstrate that electron temperature, electron density and concentration of active species increase significantly with increase in helium percentage in the mixture and RF power.     

Polarization change of high-frequency electromagnetic waves in magnetically sheared plasmas

Summary The polarization change of an electromagnetic wave passing through a magnetized plasma turns out to be of renewed interest because of the possibility, offered by polarimetry measurements in the infrared range of frequencies, of determining the current density profile for a tokamak plasma. Because of magnetic shear, acting as a coupling parameter between the characteristic modes, the fraction of energy in each mode is not a constant, and a non-WKB treatment is necessary in order to describe the evolution of the wave polarization. The present work shows that this evolution, together with the one of the mode energy, is described by a set of four first-order differential equations. The solutions of this system are discussed in the general case, and in significant limiting cases. Work performed under JET Contract     

Neutron spectrometry in compact ignition experiments

Summary The role of neutron spectrometry is discussed as a diagnostics of plasmas of compact ignition experiments. The situation with regard to instrumentation (including diagnostic capabilities and limitations) is reviewed on the basis of existing experience and new solutions are proposed as required by compact-plasma observations. Especially, the question of time resolution is addressed and a dedicated spectrometer system is devised to make use of the high neutron fluxes expected for the nuclear-burn phase of compact plasmas. It is shown that a system of three neutron spectrometers can provide adequate data on ion temperature of D and DT plasmas over a range of plasma conditions of interest and with the required accuracy. The diagnostic output from this neutron spectrometer system is presented especially with regard to flash ignition studies.     

XUV lines emitted from plasma accelerated during magnetic reconnection

Summary We compute the intensity of the emission in the O VI, Mg X, Si XII, Fe XIII, Fe XVI transitions and the profiles of these spectral lines for a plasma flowing out of reconnecting current sheets that originate in the active region corona either during transient brightenings or in preflare conditions. The characteristic of these lines is a significant non-thermal broadening consistent with plasma non-thermal velocities of the order of 300 km s⁻¹. Hence, it is possible to infer the occurrence of magnetic reconnection in the solar corona by investigating the broadening of transition region and coronal lines in the sites where reconnection is presumed to take place.     

Influence of resonant helical windings on the mirnov oscillations in a small tokamak

Summary The influence of resonant helical perturbations on the plasma confined by the small tokamak TBR-1 was experimentally investigated. Strong attenuations of the Mirnov oscillations could be easily obtained by activating different resonant helical windings. The amplitudes of these oscillations were restored as the helical perturbation ended. Partially supported by CNPq.     

Growth of electron energies with ion beam injection in a double plasma device

This paper reports about the observed energy growth of both high and low energetic electron species in the target plasma region with the increase in plasma potential in the source region of a double plasma device. This situation can be correlated to the injection of an ion beam from source to target plasma region. Plasma is solely produced in the source region and a low-density diffuse plasma is generated in the target region by local ionization between the neutral gas and the high energetic electrons coming from the source region. The growth of electron energy is accompanied by a decrease in diffuse plasma density. It is observed that although energy of high energetic group increases with the injected beam energy, the diffuse plasma density falls due to their decreasing population.     

Atom-sensitive textiles as visual indicators for plasma post-discharges

The efficiency of surface treatments by plasma and post-discharge plasma processes is greatly dependent on the density of active species, such as neutral atoms in post-discharges. Therefore, many diagnostics exist to detect the presence and measure the concentrations of these species, but they often require expensive instrumentation and highly qualified personnel. These conditions are not often met when the process is industrially used and it becomes important to imagine simple indicators allowing to validate that the correct operating conditions are reached. In the present paper, we present the first results on the investigation of an inexpensive and easy to use visual indicator able to quantify the atomic species density in nitrogen post-discharge plasma processes. It is based on the differential recombination coefficients of N-atoms on metallic/textile fibres which are intrinsically bonded together in a fabric matrix which serves as support for a thermochromic ink. The specific heating of the metallic fibres by N-atom recombination heats the whole of the fabric, leading to a visible colour change of the thermochromic ink, and therefore, of the indicator. Through modelling, it was possible to estimate that the inclusion of copper fibres to a pure cotton matrix leads to a 60% increase of the global N-atom recombination coefficient of the fabric, sufficient enough to provide a clearly visible colour change.     

Electron temperature measurement in a surface-wave-produced argon plasma at intermediate pressures

The main objective of this work is to obtain the electron temperature in an argon surface-wave-produced plasma column at intermediate gas pressures. After proving that argon upper excited states remain in Excitation Saturation Balance, the value of electron temperature along the plasma column has been obtained using a modified Saha equation and a corrected Boltzmann-plot. Moreover, the electron energy distribution function has been verified to be nearly Maxwellian in a 0.8-2.8 torr intermediate pressure range. Received 24 July 2000 and Received in final form 19 January 2001     

Magnetogasdynamic radial wall jet in the presence of uniform transerse magnetic field

Summary The effects of compressibility on the flow of a laminar viscous, electrically conducting fluid, in the presence of an uniform transverse magnetic field, in a radial wall jet are studied. A perturbation on the Glauert-Riley model is applied and numerical solutions, for Prandtl number 0.72 and Mach number 2.0, for first- and second-order perturbation terms are obtained. The effects of the magnetic field on the rate at which heat is transferred from the wall, per unit area, when the wall is maintained at a constant temperatureT _∞, and the wall temperature, when the jet is flowing over a thermally insulated wall, have been studied.

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Riassunto Si studiano gli effetti di compressibilità sul flusso di un fluido lminare vischioso e con conducibilità elettrica in presenza di un campo magnetico trasverso uniforme in un getto a parete radiale. Si applica una perturbazione sul modello di Glauert-Riley e si ottengono soluzioni numeriche per il numero di Prandtl 0.72 e quello di Mach 2.0, per termini di perturbazione di primo e second’ ordine. Sono stati studiati gli effetti del campo magnetico sul valore al quale il calore è trasferito dalla parete, per area unitaria, quando la parete è mantenuta a temperatura costanteT _∞, e la temperatura della parete quando il getto passa su una parete isolata termicamente.

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Резюме Исслелуется влияуие сжимаемости на ламинарный поток вязкой злектропроводящей жидкости В присутствии одномерного попереьного магнитного поля В радиальной струе. Рассматривается возмущение модели Глаузрта-Рилея. Получаются численные решения для числа Прандтля 0.72 и числа Мах 2.0 для пертурбационных членов первого и второго порядков. Исследуется влияние магнитного поля на интенсивность площади, когда стенка поддерживается гри постоянной температуреT _∞, и на температуру стенки, когда струя протекает над теплоизолированной стенкой.

     

Determination of the OH radical in atmospheric pressure dielectric barrier discharge plasmas using near infrared cavity ring-down spectroscopy

The hydroxyl radical (OH) plays an important role in combustion systems, atmospheric chemistry and the removal of air pollutants by non-thermal plasmas. The present work reports the determination of the hydroxyl radicals in atmospheric dielectric barrier discharge plasmas via near infrared continuous wave cavity ring-down spectroscopy. The P-branches of OH X²Π_i (ν' = 2 ←ν^′′ = 0) bands were used for its number density measurements. The minimum measurable absorption coefficient is about 3 × 10^-8 cm^-1 in DBD plasmas. At certain experimental conditions (a.c. frequency of 70 kHz, 6700 ppm H₂O in He, 1 atm), when the peak-to-peak discharge voltage varied from 6 kV to 10.4 kV, the determined OH radical concentration increased from (2.1 ± 0.1) × 10¹³ molecules cm^-3 to (3.7 ± 0.1) × 10¹³ molecules cm^-3. The plasma gas temperature, derived from the Boltzmann plots of OH rotational population distributions, ranged from 312 ± 10 K to 363 ± 10 K when the discharge voltage was raised in the above range. The influences of O₂ and N₂ addition on the production of OH radicals have been also investigated.     

Measurements of electron density and Stark width of neutral helium lines in a helium arc plasma

Electron densities in an atmospheric helium arc plasma have been measured with the Stark broadening parameters of helium spectral lines. The spatially distributed radiation intensities are converted to plasma emission coefficients at every wavelength by means of Abel inversion. From the inverted profiles of He I lines of 4713 ?, 5016 ?, and 6678 ? electron density has been calculated, which ranges from 0.5 ×10¹⁶ to 4 ×10¹⁶ cm^-3 for a helium arc with current 200 A. Stark widths of He I lines of 3889 ? and 7065 ? are determined based on the measurements and compared with existing data.     

A study of OH radicals in an atmospheric AC discharge plasma using near infrared diode laser cavity ringdown spectroscopy combined with optical emission spectroscopy

Simultaneous measurements of absolute concentrations of H₂O and OH radicals in an atmospheric AC discharge using continuous wave cavity ringdown spectroscopy (cw-CRDS) are reported. Formation of OH radicals and plasma temperatures are characterized by optical emission spectroscopy. The concentration of OH radical at the edge of the discharge plume at 380 K is measured by the cw-CRDS technique to be 1.1 ×10¹⁵ molecule cm^-3. Ringdown measurements of the H₂O (120-000) band and the OH first overtone around 1515 nm enable us to determine an OH generation yield, , to be 4.8 ×10^-3, where N_OH and are the number densities of OH and H₂O, respectively. The minimum detectable absorption coefficient of the cw-CRDS system is 8.9 ×10^-9  cm^-1, which corresponds to a 1σ detection limit of OH number density of 1.2 ×10¹³ molecule cm^-3 in the discharge. This experimental approach is demonstrated for the first time ever in an AC discharge, and can be applied in general to a variety of atmospheric plasmas to help study OH formation mechanisms and OH-related plasma applications.     

An analytic colllslonai-radlatlve model Incorporating non-LTE and optical depth effects

In this paper we analyze the variations in line intensities ratios due to a non-equilibrium situation and to optical depth effects. A four level model is proposed and the two particular situations for the possible transitions are considered. Electron density and temperature as well as the source thickness are used as independent parameters to find out in which way and extent they modify the ratios of levels populations compared with the ideal case of an equilibrium state and optically thin source. Accordingly with the ion of interest, electron temperatures ranging from I/20 to I/7 eV (I being the ionization energy), whereas electron densities in the interval from 10¹⁴ to 10²⁰ cm^-3 will be considered. These ranges are of special interest for diverse applications such as LIBS and measurement of transition probabilities. Some results are presented for real ions and a new expression for the escape factor is also proposed for general plasma conditions. Received 12 June 2001 and Received in final form 24 October 2001     

Hall effects on thermosolutal instability of a rotating plasma

Summary The thermosolutal instability of a rotating plasma in the presence of a uniform vertical magnetic field is studied to include the effects of Hall current. When the instability sets in as stationary convection for the case of no rotation, the Hall effects are found to be destabilizing. The stable solute gradient and rotation are found to have stabilizing effects. In the presence of rotation the Hall currents are found to be stabilizing forT ₁>M(1+x)². the case of overstability is also considered and it is shown that such solutions exist. The variation of the frequency with respect to the wave number at the neutral state is graphically shown. The authors of this paper have agreed to not receive the proofs for correction.     

Similarity solutions for two-dimensional magnetogasdynamic boundary layer flow in a transverse magnetic field

Summary The conditions for the existence of the similarity solution of a two-dimensional boundary layer flow past a plane wall of an electrically conducting gas, in the presence of a transverse magnetic field at small magnetic Reynolds number, are studied. It is found that, in general, the similarity requirements involve as many as eleven parameters out of which eight are the same as these of nonmagnetic gasdynamic boundary layer flow. The solution is obtained for power law velocity distribution, in Illingworth variables, at the outer edge of the boundary layer and for the corresponding variable magnetic field. The Prandtl number of the fluid is taken as unity and the magnetic-field effects are confined to the boundary layer only.

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Riassunto Si studiano le condizioni per l’esistenza della soluzione di somiglianza di un flusso nello stato di confine bidimensionale oltre una parete piana di un gas che conduce electtricità, in presenza di un campo magnetico trasverso con piccolo numero magnetico di Reynolds. Si trova che, in generale, le esigenze di somiglianza coinvolgono undici parametri, otto dei quali sono identici a quelli del flusso della strato di confine non magnetico e gas dinamico. La soluzione è ottenuta per la distribuzione di velocità della legge di potenza, in variabili di Illingworth, nel bordo esterno dello strato limite e per il corrispondente campo magnetico variabile. Il numero di Prandtl del fluido è preso uguale a uno e gli effetti del campo magnetico sono confinati solo allo strato limite.

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Резюме Исследуются условия существования подобного решения для двумерного течения в пограничиом слое вдоль плосокой стенки газа, проводящего электричество, в присутствии поперечного магнитного поля при малых магнитных числах Рейнольдса. Получено, что в общем случае требования подобия включают одиннадцать параметров, восемь из которых являются соответствующими параметрами для немагнитного газодинамического течения в пограничном слое. Ппределено решение в случае степенного закона распределения по скоростям на внешнем краю пограничного слоя и для соотвтствуюего переменного магнитного поля. Число Прандтля для жидкости считается равным единице и эффекты магнитного поля ограничиваются только пограничным слоем.

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Observations of extreme ultraviolet emission from plasma produced by capillary discharges

This study reports the results of a pilot experiment concerning observations of extreme ultraviolet emission from plasma produced by the capillary discharges. A few kA current was applied across the gas-filled alumina capillary (1 mm diameter and 8 mm long) to generate radiation in the EUV region (12–63 nm). Spectroscopic studies were carried out by means of a XEUV spectrometer which was upgraded for special lithography purposes. The results obtained from the EUV spectroscopic measurements provided information about the radiation processes from xenon and argon plasma and testifies that given capillary is an effective source of EUV emission. Additionally we showed a simulation which describes plasma dynamics parameters and dynamics of various ionization stages in capillary discharge. Our computer simulation confirmed the presence of ions, which spectra was registered in the experiment.     

Diagnosis of negative hydrogen ions and rovibrational distribution of H<Subscript>2</Subscript> molecule in non-thermal plasmas

Rovibrational excited hydrogen molecule plays an important role for the production of H^- ions. The correlation between H^- ion density and rovibrational distribution of H₂ molecules has been investigated in dielectric barrier discharge hydrogen plasmas via optical emission spectrometry and molecular beam mass spectrometry. The relative vibrational distribution of molecular hydrogen in the electronic ground state has been determined by the best fitting to the Fulcher-α band emission lines. It is shown that the ratio of the Q_(0-0)(1) to Q_(1-1)(1) line is very suitable and simple for the diagnosis of vibrational temperature in the range of 1500 to 7500 K. At certain discharge conditions (ac 40 kHz, 14 kV), the vibrational temperature decreases from 3600 to 2400 K as the pressure increases from 100 to 200 Pa and the negative ions density near the ground electrode also decreases as the pressure increases. Both the hydrogen ions density and the vibrational temperature increase with the increasing of discharge voltage. It is found that the evolution of negative atomic hydrogen ions density greatly depends on the vibrational temperature.     

Measurement of atomic hydrogen density in non-thermal H<Subscript>2</Subscript> plasmas via threshold ionisation-molecular beam mass spectrometry

Determinations of radical density are essential to investigate the physical-chemical processes in plasmas and setup the related theoretical models. This paper presents the experimental measurement of atomic hydrogen near grounded electrode in dielectric barrier discharge medium-pressure hydrogen plasma via threshold ionisation-molecular beam mass spectrometry. After investigating the possible influences from parent molecules in excited states, background component and space-charge, evolution of atomic hydrogen density as functions of discharge parameters are investigated utilising the signal of H₂ molecule beam as the reference. At fixed gas pressure of 6.0 torr and a discharge voltage of 24 kV, atomic hydrogen density increases monotonously from 1.1×10¹⁴ to 2.0×10¹⁵ cm^-3 as the discharge frequency increases from 9 to 26 kHz. Similarly the rising discharge voltage also lead to enhancement of atomic hydrogen density.