Determination of plasma density from data on the ion current to cylindrical and planar probes

To improve probe methods of plasma diagnostics, special probe measurements were performed and numerical models describing ion transport to a probe with allowance for collisions were developed. The current–voltage characteristics of cylindrical and planar probes were measured in an RF capacitive discharge in argon at a frequency of 81 MHz and plasma densities of 10¹⁰–10¹¹ cm^–3, typical of modern RF reactors. 1D and 2D numerical models based on the particle-in-cell method with Monte Carlo collisions for simulating ion motion and the Boltzmann equilibrium for electrons are developed to describe current collection by a probe. The models were used to find the plasma density from the ion part of the current–voltage characteristic, study the effect of ion collisions, and verify simplified approaches to determining the plasma density. A 1D hydrodynamic model of the ion current to a cylindrical probe with allowance for ion collisions is proposed. For a planar probe, a method to determine the plasma density from the averaged numerical results is developed. A comparative analysis of different approaches to calculating the plasma density from the ion current to a probe is performed.     

Correlation between the parameters of ion and X-ray emissions from the plasma of a micropinch discharge

Ion emission from the plasma of a micropinch discharge is studied by analyzing the plasma flow from the discharge region with the help of time-of-flight technique and probe diagnostics. Concurrently, soft Xray emission from the micropinch is recorded. The experimental data are interpreted using the radiative contraction model.     

Study of mechanisms for magnetic field diffusion into an expanding laser plasma

The interaction of plasma clouds generated during laser irradiation of a spherical target in a background gas with a magnetic field was studied on the MKV-4 test bench of the Iskra-5 facility. The dynamics of the plasma cloud expansion in a 300- to 500-Oe magnetic field was investigated using magnetic and probe diagnostics. The results obtained are compared with calculations by different models of laser plasma diffusion in a magnetic field.     

Langmuir probe study of an inductively coupled magnetic-pole-enhanced helium plasma

This study reports the effects of RF power and filling gas pressure variation on the plasma parameters, including the electron number density n _e , electron temperature T _e , plasma potential V _p , skin depth δ, and electron energy probability functions (EEPFs) in a low-pressure inductively coupled helium plasma source with magnetic pole enhancement. An RF compensated Langmuir probe is used to measure these plasma parameters. It is observed that the electron number density increases with both the RF power and the filling gas pressure. Conversely, the electron temperature decreases with increasing RF power and gas pressure. It is also noted that, at low RF powers and gas pressures, the EEPFs are non-Maxwellian, while at RF powers of ≥50 W, they evolve into a Maxwellian distribution. The dependences of the skin depth and plasma potential on the RF power are also studied and show a decreasing trend.     

Specificity of probe measurements in diffuse plasmas of dense gases in strong electric fields

The article is devoted to extending the applicability of the probe diagnostics to the range of higher pressures of the plasma-forming gas by taking into account the effect of the probe shadow on the anode. The probe current–voltage characteristic in the diffuse plasma of a dense gas in a strong electric field was measured, and the influence of the probe potential and probe current on the dimensions of the probe shadow on the anode was studied experimentally. The experiments were carried at different currents of a steady-state glow discharge and different velocities of the gas flow through the discharge. The plasma-forming gas was nitrogen at a pressure of P = 100 Torr.     

Effect of an external RF field on the beam-plasma discharge in a magnetic field

The effect of an RF field on a steady-state beam-plasma discharge with a plane electrode placed parallel to a sheetlike electron beam is studied experimentally. The plasma parameters were measured by a single probe, and the electron distribution function was determined with the use of an electrostatic analyzer. The energy and current of the electron beam were E _B=2.5 keV and J _B=0.05–1.5 A, respectively. The working pressure was p=2×10^?5–10^?3 torr. The frequency of the external RF field was 13.56 MHz. Both the steady-state regimes in which the RF field had no effect on the plasma parameters and regimes with a pronounced effect of the RF field were observed. The experiments show that the regime of the discharge depends strongly on the plasma density and the magnetic field. The parametric instability is studied theoretically in the weak-turbulence approximation. It is shown that, due to the decay nature of the spectrum of plasma oscillations, the onset of instability is accompanied by the transfer of the energy of fluctuations over the spectrum, from the pump frequency toward its harmonics.     

Properties of a low-pressure inductive RF discharge I: Experiment

Results are presented from experimental studies of low-pressure inductive RF discharges (including those with a capacitive component) employed in plasma technology. It is shown that both the RF power absorbed in the plasma and the electron density depend nonmonotonically on the external magnetic field. Discharge disruptions occurring at critical values of the magnetic field and the spatial redistribution and hysteresis of the plasma parameters were observed when varying the magnetic field and RF generator power. The parameters of the plasma of low-pressure (0.5–5 mTorr) inductive RF discharges were investigated, and the discharge properties related to the redistribution of the RF generator power between the plasma and the discharge external circuit were revealed. The experiments were performed with both conventional unmagnetized inductive plasma sources and plasma sources with a magnetic field.     

Radio-frequency stabilization of a nonequilibrium plasma accelerator

Results of experimental studies of the effect of an external RF field on the excitation of oscillations in a magnetoplasmadynamic plasma accelerator are presented. It is found that applying an RF field can suppress the drift component of low-frequency oscillations in the ejected plasma flow. The experimental data agree with the concept of stabilization of the plasma accelerator by the magnetic component of the field generated by the RF current loop. The conditions under which the RF field stabilizes the generation of the plasma flow are determined, and the factors limiting the stabilization efficiency are revealed.     

Properties of a low-pressure inductive RF discharge II: Mathematical simulations

Self-consistent numerical simulations of a low-pressure inductive RF discharge have been carried out. It is shown that, on the one hand, the plasma parameters are determined by the RF power absorbed in the plasma and, on the other, they themselves govern the power absorption. This results in a nonmonotonic dependence of the plasma parameters on the magnetic field, as well as in discharge disruptions, similar to those observed experimentally in such discharges. An inductive RF discharge with a capacitive component is simulated. The experimentally observed characteristic properties of the discharges are explained based on the regular features of the absorption of RF power in the plasma. Traditional inductive plasma sources (both without and with a magnetic field) are considered.     

Metabolomic Analysis in Severe Childhood Pneumonia in The Gambia,West Africa: Findings from a Pilot Study

Background

Pneumonia remains the leading cause of death in young children globally and improved diagnostics are needed to better identify cases and reduce case fatality. Metabolomics, a rapidly evolving field aimed at characterizing metabolites in biofluids, has the potential to improve diagnostics in a range of diseases. The objective of this pilot study is to apply metabolomic analysis to childhood pneumonia to explore its potential to improve pneumonia diagnosis in a high-burden setting.

Methodology/Principal Findings

Eleven children with World Health Organization (WHO)-defined severe pneumonia of non-homogeneous aetiology were selected in The Gambia, West Africa, along with community controls. Metabolomic analysis of matched plasma and urine samples was undertaken using Ultra Performance Liquid Chromatography (UPLC) coupled to Time-of-Flight Mass Spectrometry (TOFMS). Biomarker extraction was done using SIMCA-P⁺ and Random Forests (RF). ‘Unsupervised’ (blinded) data were analyzed by Principal Component Analysis (PCA), while ‘supervised’ (unblinded) analysis was by Partial Least Squares-Discriminant Analysis (PLS-DA) and Orthogonal Projection to Latent Structures (OPLS). Potential markers were extracted from S-plots constructed following analysis with OPLS, and markers were chosen based on their contribution to the variation and correlation within the data set. The dataset was additionally analyzed with the machine-learning algorithm RF in order to address issues of model overfitting and markers were selected based on their variable importance ranking. Unsupervised PCA analysis revealed good separation of pneumonia and control groups, with even clearer separation of the groups with PLS-DA and OPLS analysis. Statistically significant differences (p<0.05) between groups were seen with the following metabolites: uric acid, hypoxanthine and glutamic acid were higher in plasma from cases, while L-tryptophan and adenosine-5′-diphosphate (ADP) were lower; uric acid and L-histidine were lower in urine from cases. The key limitation of this study is its small size.

Conclusions/Significance

Metabolomic analysis clearly distinguished severe pneumonia patients from community controls. The metabolites identified are important for the host response to infection through antioxidant, inflammatory and antimicrobial pathways, and energy metabolism. Larger studies are needed to determine whether these findings are pneumonia-specific and to distinguish organism-specific responses. Metabolomics has considerable potential to improve diagnostics for childhood pneumonia.     

Construction of repeat-free fluorescence in situ hybridization probes

FISH probes are generally made out of BAC clones with genomic DNA containing a variable amount of repetitive DNA that will need to be removed or blocked for FISH analysis. To generate repeat free (RF) Probes without loss in genomic coverage, a random library is made from BAC clones by whole-genome amplification (WGA). Libraries are denatured in the presence of excess C(0)t-1 DNA and allowed to re-anneal followed by digestion of all double-stranded elements by duplex-specific nuclease (DSN). Selective amplification of all elements not containing repetitive sequences is realized by a sequential amplification. The final RF products can be re-amplified and used as a stock for future probe production. The RF probes have a lower background, the signal intensity build up is faster and there is no need for blocking DNA. The signal to background ratio of the RF was higher as compared to repeat containing probes.     

Self-consistent modeling of radio-frequency plasma generation in stellarators

A self-consistent model of radio-frequency (RF) plasma generation in stellarators in the ion cyclotron frequency range is described. The model includes equations for the particle and energy balance and boundary conditions for Maxwell’s equations. The equation of charged particle balance takes into account the influx of particles due to ionization and their loss via diffusion and convection. The equation of electron energy balance takes into account the RF heating power source, as well as energy losses due to the excitation and electron-impact ionization of gas atoms, energy exchange via Coulomb collisions, and plasma heat conduction. The deposited RF power is calculated by solving the boundary problem for Maxwell’s equations. When describing the dissipation of the energy of the RF field, collisional absorption and Landau damping are taken into account. At each time step, Maxwell’s equations are solved for the current profiles of the plasma density and plasma temperature. The calculations are performed for a cylindrical plasma. The plasma is assumed to be axisymmetric and homogeneous along the plasma column. The system of balance equations is solved using the Crank-Nicholson scheme. Maxwell’s equations are solved in a one-dimensional approximation by using the Fourier transformation along the azimuthal and longitudinal coordinates. Results of simulations of RF plasma generation in the Uragan-2M stellarator by using a frame antenna operating at frequencies lower than the ion cyclotron frequency are presented. The calculations show that the slow wave generated by the antenna is efficiently absorbed at the periphery of the plasma column, due to which only a small fraction of the input power reaches the confinement region. As a result, the temperature on the axis of the plasma column remains low, whereas at the periphery it is substantially higher. This leads to strong absorption of the RF field at the periphery via the Landau mechanism.     

小鼠角膜发育期间胎球蛋白受体的定位

用辣根过氧化物酶标记的胎球蛋白(Fet-HRP)为探针小鼠角膜发育期间胎球蛋白受体(RF)在光镜水平的定位和变化。结果表明:角膜上皮于胎龄11天出现RF,主要分布在细胞表面;角膜基质自胎龄13天出现RF,15天时最多,出生后减少;角膜内皮在胚胎期未发现RF。文中讨论了RF与角膜基质组建间的关系。     

Functional and morphological cardiac magnetic resonance imaging of mice using a cryogenic quadrature radiofrequency coil

Cardiac morphology and function assessment by magnetic resonance imaging is of increasing interest for a variety of mouse models in pre-clinical cardiac research, such as myocardial infarction models or myocardial injury/remodeling in genetically or pharmacologically induced hypertension. Signal-to-noise ratio (SNR) constraints, however, limit image quality and blood myocardium delineation, which crucially depend on high spatial resolution. Significant gains in SNR with a cryogenically cooled RF probe have been shown for mouse brain MRI, yet the potential of applying cryogenic RF coils for cardiac MR (CMR) in mice is, as of yet, untapped. This study examines the feasibility and potential benefits of CMR in mice employing a 400 MHz cryogenic RF surface coil, compared with a conventional mouse heart coil array operating at room temperature. The cryogenic RF coil affords SNR gains of 3.0 to 5.0 versus the conventional approach and hence enables an enhanced spatial resolution. This markedly improved image quality - by better deliniation of myocardial borders and enhanced depiction of papillary muscles and trabeculae - and facilitated a more accurate cardiac chamber quantification, due to reduced intraobserver variability. In summary the use of a cryogenically cooled RF probe represents a valuable means of enhancing the capabilities of CMR of mice.     

Focused hyperthermia with a magnetic resonance imaging (MRI) unit and an interstitial grounded probe

The feasibility of using a commercial magnetic resonance imaging (MRI) scanner to do either imaging or hyperthermic treatment was demonstrated. Radiofrequency (RF) induced focal heating of phantoms and animal tissues was performed using a MRI scanner as the RF power source and a grounded interstitial probe as a device to produce hyperthermia via eddy current convergence. In the therapeutic mode, a pulse width of 900 microseconds and interval of 50 ms were used to give 2% duty cycle (closest simulation to continuous wave (CW) mode without bypassing imaging filters). Temperature in the vicinity of the grounded probe was measured with a field nonperturbing fluoroptic probe. Temperatures increased 4.5 degrees C in 5 minutes in a dielectrically uniform phantom, 3.1 degrees C in 6.7 minutes in rats' leg muscles, and 5.0 degrees C in 6.0 minutes in rats' peritoneum. The MRI of the phantom with the grounded probe and the fluoroptic probe was obtained using spin echo sequences. The potential advantage of this approach is visualization of deep-seated tumors and hyperthermic treatment with minimal modification of the MRI scanner.     

Characteristic properties of the frame-antenna-produced RF discharge evolution in the Uragan-3M torsatron

In the ? = 3 Uragan-3M torsatron, hydrogen plasma is produced and heated by RF fields in the Alfvén range of frequencies (ω ? ω _ci ). To this end, a frame antenna with a broad spectrum of generated parallel wavenumbers is used. The RF discharge evolution is studied experimentally at different values of the RF power fed to the antenna (the anode voltage of the oscillator and the antenna current) and the initial pressure of the fueling gas. It is shown that, depending on the antenna current and hydrogen pressure, the discharge can operate in two regimes differing in the plasma density, temperature, and particle loss. The change in the discharge regime with increasing anode voltage is steplike in character. The particular values of the anode voltage and pressure at which the change occurs are affected by RF preionization or breakdown stabilization by a microwave discharge. The obtained results will be used in future experiments to choose the optimal regimes of the frame-antenna-produced RF discharge as a target for the production and heating of a denser plasma by another, shorter wavelength three-half-turn antenna.     

Sterilization effect of atmospheric plasma on Escherichia coli and Bacillus subtilis endospores

Aims:  Escherichia coli and Bacillus subtilis spores were treated with an atmospheric plasma mixture created by the ionization of helium and oxygen to investigate the inactivation efficiency of a low-temperature plasma below 70°C.
Methods and results:  An electrical discharge plasma was produced at a radio frequency (RF) of 13·56 MHz, connected to a perforated circular electrode with a discharge spacing of 1–15 mm. The discharge gas was helium with 0–2% oxygen. For the plasma treatment, a dried E. coli cell or B. subtilis endospore suspension on a cover-glass was exposed to oxygen downstream of the plasma from holes in an RF-powered electrode. The sterilization effect of the RF plasma was highest with 0·2% oxygen, corresponding to the maximum production of oxygen radicals.
Conclusions:  Oxygen radicals generated by RF plasma are effective for the destruction of bacterial cells and endospores.
Significance and Impact of the study:  Low-temperature atmospheric plasma can be used for the disinfection of diverse objects, especially for the inactivation of bacterial endospores.     

Measurements of the microwave power absorbed by a plasma during second harmonic ECR heating in the L-2M stellarator

Comparative measurements of the absorbed microwave power are performed using the diamagnetic diagnostics and a multichannel diagnostics receiving the second harmonic electron cyclotron emission from the plasma. The specific features of the experiments and the results obtained are discussed.     

Perspectives with cryogenic RF probes in biomedical MRI

Since discovery of high-temperature superconductive (HTS) ceramics by Bednorz and Muller in 1986, there has been an accelerated development of cold technologies in industry, including the domain of NMR detection. The purpose of this paper is to fix ideas about the stage that cryogenic radio frequency (RF) probe techniques have reached in biomedical magnetic resonance imaging (MRI). Readers confronted to the literature about this emerging topic have to understand a large range of motivations with somewhat unclearly defined technical limitations and actual outlets. An overview of sensitivity issues in the general context of biomedical MRI is provided here and the contribution of RF coil techniques to recent advances is identified. The domains where cooled coil materials such as copper, low- or high-temperature superconductors, could actually increase the RF coil sensitivity are delimited by a quantitative analysis of noise mechanisms. Technical keys, cryogenic means and cold RF coil technologies are considered, and first achievements in different fields of biomedical MRI are reviewed. This survey provides a basis for discussing about the future impact of cryogenic probes for MRI investigations.     

Scanning calorimetry measurements of the gas temperature in an RF-discharge fluorine-containing plasma

The neutral-gas temperature in a low-pressure (50 Pa) capacitive RF discharge in a CF₄+O₂ mixture is determined from the heating kinetics of a gallium arsenide single crystal, which is chemically inert to any radicals in a fluorine-containing plasma. Experimental methods are discussed that make it possible to confirm the absence of heat sources capable of additional heating of the calorimeter in the discharge. The features and applicability limits of the method of non-steady-state gas thermometry in a weakly ionized nonequilibrium plasma are discussed. The method proposed is compared with conventional steady-state methods based on measurements of the established temperature of a thermal probe in the discharge. Temperature scanning makes it possible to study dependences that cannot be investigated by steady-state methods, in particular, the temperature dependence of the calorimeter heating power, which is very important for diagnosing the processes of plasma-surface heat transfer.