Low-pressure Argon Discharge Research Articles

Langmuir probe measurements of the secondary electron population across the cathodic pre-sheath of a DC argon discharge

Cylindrical Langmuir probe measurements in a low-pressure DC argon discharge are used to obtain the spatial evolution of ion, cold and hot electron parameters across the cathodic pre-sheath. The cathodic pre-sheath is formed by a stainless steel plate allowed to float negatively with respect to the plasma. The velocity distribution function of the hot electron population in the pre-sheath is shown to be matched by a drifting Maxwellian that thermalizes across it. The source of the hot electron population is hypothesized to arise from secondary electron emission from the plate. A Bayesian estimation routine is proposed to compare and validate different Langmuir probe models as well as calculating the relative uncertainty between models. The results are analyzed using existing pre-sheath theory for which experimental evidence of the spatial influence of energetic electrons is lacking. The data are shown to follow Riemann's pre-sheath model that the ion-neutral mean free path λ i is proportional to the pre-sheath characteristic length l, and that the potential drop ϕ ( x ) with distance x from the sheath/pre-sheath boundary x0 follows ϕ ( x ) ∝ ( x 0 − x ) / l.

Understanding Surface Modifications Induced via Argon Plasma Treatment through Secondary Electron Hyperspectral Imaging.

Understanding the effects that sterilization methods have on the surface of a biomaterial is a prerequisite for clinical deployment. Sterilization causes alterations in a material's surface chemistry and surface structures that can result in significant changes to its cellular response. Here we compare surfaces resulting from the application of the industry standard autoclave sterilisation to that of surfaces resulting from the use of low‐pressure Argon glow discharge within a novel gas permeable packaging method in order to explore a potential new biomaterial sterilisation method. Material surfaces are assessed by applying secondary electron hyperspectral imaging (SEHI). SEHI is a novel low‐voltage scanning electron microscopy based characterization technique that, in addition to capturing topographical images, also provides nanoscale resolution chemical maps by utilizing the energy distribution of emitted secondary electrons. Here, SEHI maps are exploited to assess the lateral distributions of diverse functional groups that are effected by the sterilization treatments. This information combined with a range of conventional surface analysis techniques and a cellular metabolic activity assay reveals persuasive reasons as to why low‐pressure argon glow discharge should be considered for further optimization as a potential terminal sterilization method for PGS‐M, a functionalized form of poly(glycerol sebacate) (PGS).

Characterization of a pulsed low pressure argon discharge in a cylindrical magnetron reactor by plasma diagnostic and 3D plasma modeling

This work describes a combined experimental and numerical study of a low pressure argon plasma in a cylindrical magnetron reactor. The plasma properties are first investigated experimentally using optical emission spectroscopy as a function of the position in the reactor and the applied power, for two excitation modes (pulsed direct current and pulsed bipolar) at a constant pressure of 10 Pa. Langmuir probe measurements are also performed to complement the plasma diagnostic. The results are compared with three-dimensional particle-in-cell simulations combined with a Monte Carlo Collision scheme. A consistent agreement occurs between the experimentally measured and numerically calculated electron temperatures, enabling one to validate the numerical model and to explain the influence of the magnetic field on the spatial variations of the electron temperature. The aim of this work is to pave the way for more complex simulations, such as the modeling of a plasma enhanced chemical vapor deposition process. In a future article, we will investigate both experimentally and numerically the plasma polymerization of cyclopropylamine in the same reactor.

The electrical asymmetry effect in a multi frequency geometrically asymmetric capacitively coupled plasma: A study by a nonlinear global model

We investigate the electrical asymmetry effect (EAE) and the current dynamics in a geometrically asymmetric capacitively coupled radio frequency plasma driven by multiple consecutive harmonics based on a nonlinear global model. The discharge symmetry is controlled via the EAE, i.e., by varying the total number of harmonics and tuning the phase shifts (θk) between them. Here, we systematically study the EAE in a low pressure (4 Pa) argon discharge with different geometrical asymmetries driven by a multifrequency rf source consisting of 13.56 MHz and its harmonics. We find that the geometrical asymmetry strongly affects the absolute value of the DC self-bias voltage, but its functional dependence on θk is similar at different values of the geometrical asymmetry. Also, the values of the DC self-bias are enhanced by adding more consecutive harmonics. The voltage drop across the sheath at the powered and grounded electrode is found to increase/decrease, respectively, with the increase in the number of harmonics of the fundamental frequency. For the purpose of validating the model, its outputs are compared with the results obtained in a geometrically and electrically asymmetric 2f capacitively coupled plasmas experiment conducted by Schuengel et al. [J. Appl. Phys. 112, 053302 (2012)]. Finally, we study the self-excitation of nonlinear plasma series resonance oscillations and its dependence on the geometrical asymmetry as well as the phase angles between the driving frequencies.

Plasma flow around and charge distribution of a dust cluster in a rf discharge

We employ a particle-in-cell Monte Carlo collision/particle-particle particle-mesh simulation to study the plasma flow around and the charge distribution of a three-dimensional dust cluster in the sheath of a low-pressure rf argon discharge. The geometry of the cluster and its position in the sheath are fixed to the experimental values, prohibiting a mechanical response of the cluster. Electrically, however, the cluster and the plasma environment, mimicking also the experimental situation, are coupled self-consistently. We find a broad distribution of the charges collected by the grains. The ion flux shows on the scale of the Debye length strong focusing and shadowing inside and outside the cluster due to the attraction of the ions to the negatively charged grains, whereas the electron flux is characterized on this scale only by a weak spatial modulation of its magnitude depending on the rf phase. On the scale of the individual dust potentials, however, the electron flux deviates in the vicinity of the cluster strongly from the laminar flow associated with the plasma sheath. It develops convection patterns to compensate for the depletion of electrons inside the dust cluster.

DC Discharge Electronic Non-equilibrium Effects Investigations on a M = 2 Rarefied Supersonic Flow Over a Flat Plate

This work aims at studying the effects of a low-pressure Argon discharge (P = 0.5 Torr) on a supersonic Argon flow (M = 2) around a flat plate. The observed phenomena during high speed-flow control with a plasma discharge are exhaustively described. The present investigation is of great interest not only to aviation but also to numerous other areas like the wind power industry. The computations have been carried out using the DC discharge and the High Mach Number Flow Comsol Multiphysics modules. To simulate the DC discharge, chemical reactions near the cathode region along with their corresponding Townsend coefficients need to be defined. The latter are calculated using the Bolsig + computer code. The other reactions cross sections are imported from the LXCAT data base. The imported data are used to calculate the reactions rates. The plasma discharge effects on the rarefied supersonic flow are described using a 2D hydrodynamical model under the Drift–Diffusion approximation. The hydrodynamical model was validated by comparing its results for a supersonic air flow with experiments. The main results on an Argon supersonic flow coupled to an Argon discharge show an increase in the pitot pressure and the shock angle.

2D laser-collision induced fluorescence in low-pressure argon discharges

Development and application of laser-collision induced fluorescence (LCIF) diagnostic technique is presented for the use of interrogating argon plasma discharges. Key atomic states of argon utilized for the LCIF method are identified. A simplified two-state collisional radiative model is then used to establish scaling relations between the LCIF, electron density, and reduced electric fields (E/N). The procedure used to generate, detect and calibrate the LCIF in controlled plasma environments is discussed in detail. LCIF emanating from an argon discharge is then presented for electron densities spanning 109 e cm−3 to 1012 e cm−3 and reduced electric fields spanning 0.1 Td to 40 Td. Finally, application of the LCIF technique for measuring the spatial distribution of both electron densities and reduced electric field is demonstrated.

Collisionless “thermalization” in the sheath of an argon discharge

We performed kinetic Vlasov simulations of the plasma-wall transition for a low-pressure argon discharge without external magnetic fields, using the same plasma parameters as in the experiments of Claire et al. [Phys. Plasmas 13, 062103 (2006)]. Experimentally, it was found that the ion velocity distribution function is highly asymmetric in the presheath, but, surprisingly, becomes again close to Maxwellian inside the sheath. Here, we show that this “thermalization” can be explained by purely collisionless effects that are akin to the velocity bunching phenomenon observed in charged particles beams. Such collisionless thermalization is also observed in the presheath region close to the sheath entrance, although it is much weaker there and in practice probably swamped by collisional processes (standard or enhanced by instabilities).

Numerical Investigation of the Effect of Driving Voltage Pulse Shapes on the Characteristics of Low Pressure Argon Dielectric Barrier Discharge

Numerical Investigation of the Effect of Driving Voltage Pulse Shapes on the Characteristics of Low Pressure Argon Dielectric Barrier Discharge

Two-Dimensional Numerical Study on Characterization of Low-Pressure Capacitively Coupled Argon Discharges

This paper presents an investigation of argon capacitively coupled plasma at low pressure. A two-dimensional, time-dependent fluid model is used to describe the production, transport, and destruction of electrons and positive ions. The model is solved for a GEC(gaseous electronics conference) Cell reactor type (with 4cm diameter and 2.5cm interelectrode distance) operating at frequency 13.56MHz, pressure 1Torr and applied voltage 1000V, in pure argon. Two-dimensional distributions are presented in the stationary state, including electron temperature and species density i.e. electron, ion and metastable atom. The electric field and electric potential at different phases in one RF cycle is also studied.

The effects of neutral gas heating on H mode transition and maintenance currents in a 13.56 MHz planar coil inductively coupled plasma reactor

The H mode transition and maintenance currents in a 13.56 MHz laboratory 6 turn planar coil inductively coupled plasma (ICP) reactor are simulated for low pressure argon discharge range of 0.02–0.3 mbar with neutral gas heating and at ambient temperature. An experimentally fitted 3D power evolution plot for 0.02 mbar argon pressure is also shown to visualize the effects of hysteresis in the system. Comparisons between simulation and experimental measurements show good agreement in the pressure range of 0.02–0.3 mbar for transition currents and 0.02–0.1 mbar for maintenance currents only when neutral gas heating is considered. This suggests that neutral gas heating plays a non-negligible role in determining the mode transition points of a rf ICP system.

Mode transition in magnetic pole enhanced inductively coupled argon plasmas

The electrical probe (Langmuir probe) diagnostics of different plasma parameters and operation regimes (E/H modes) of magnetic pole enhanced, inductively coupled (MaPE-ICP) argon plasmas are investigated. It is shown that uniform, high density (ne ∼ 1012 cm-3) and low electron temperature (Te ∼ 1.5 eV) plasma can be produced in low pressure argon discharges at a low power (100 W). It is found that an MaPE-ICP reactor operates in two different modes; capacitive (E mode) and inductive (H mode). No density jump or hysteresis are reported between these modes. The effect of pressure on transition power, where the mode changes from E to H mode at 20 sccm gas flow rate are studied and it is found that for all pressures tested (∼7.5 mTorr to 75 mTorr) the transition power remains same. In the inductive mode, the above plasma parameters show a smooth variation with increasing filling gas pressure at fixed power. The intensity of the emission line at 750.4 nm due to 2p1 → 1s2 (Paschen’s notation) transition, closely follows the variation of ne with RF power and filling gas pressure. Measured electron energy probability function (EEPF) shows that electron occupation mostly changes in the high-energy tail, which enlightens close similarity of the 750.4 nm argon line to electron number density (ne). The behaviour of the electron energy probability function (EEPF) with regard to pressure and RF power in two operational modes is presented.

On the effect of ion-neutral collisions on dust grain screening in a low-pressure gas discharge plasma

The effect of ion-neutral collisions on charging of micrometer-sized dust grains immersed in a low-pressure argon discharge plasma is studied on the basis of the Vlasov-Bhatnagar-Gross-Krook kinetic equations. The equations are solved numerically using the method described in our previous work [I. L. Semenov et al., Phys. Plasmas 18, 103707 (2011)]. A modified version of the numerical method is proposed to reduce the required computational time. Numerical calculations are carried out for typical plasma parameters used in laboratory investigations of dusty plasma. On the basis of the obtained results, the influence of collisions on the ion flux and grain charge is analyzed. A comparison of our results with those obtained using different analytical models proposed earlier is presented. In addition, applicability of simple kinetic models describing the influence of collisions on the electric potential around a dust grain [S. A. Khrapak et al., Phys. Rev. Lett. 100, 225003 (2008); A. G. Zagorodny et al. Ukr. J. Phys. 54, 1089 (2009)] is examined. The influence of ion-neutral collisions on the distribution of plasma macroparameters near the grain surface is also demonstrated.

Production of a Periodic Plasma Structure for Basic Experiments on Plasma Photonic Crystals in Microwave Range

A 2-D periodic plasma structure is produced by a low-pressure direct-current argon discharge for basic experiments on plasma photonic crystals in the microwave range of a few gigahertz. A cathode consisting of 7 × 11 stainless steel rods located at 3-cm intervals, which are covered by an insulator, and a grounded anode are set in a vacuum chamber. An axial profile of a plasma density measured by a Langmuir probe shows the production of the periodic plasma structure with the same intervals as the electrode. The highest and lowest plasma densities are about 1.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> and 0.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , respectively. The image of the discharge suggests the production of a "2-D" periodic structure composed of plasma columns.

Analytical Evaluation of a Reduced-Pressure Microwave-Induced Plasma Studied by Optical Emission Spectrometry Method

ABSTRACT The authors describe a system that utilizes a reduced-pressure (RP) air-cooled microwave-induced plasma (MIP) torch to interface an ultrasonic nebulizer (USN) with an optical emission spectrometer (OES). Argon was investigated as plasma gas. The analytical potential of such techniques was illustrated for the determination of elements. A univariate approach and simplex optimization procedure was used to achieve optimized conditions and derive analytical figures of merit. Analytical performance of the RP-MIP was characterized by determination of the limits of detection (LODs) and precision (RSDs) with the RP-MIP-OES observed at flow rate of 10 µL min−1 without removal of any matrix. The experimental concentration detection limits for simultaneous determination, calculated as the concentration giving a signal equal to three times the standard deviation of the blank (LOD, 3σblank criterion, peak height), were 15, 4.5, 6.2, 2.9, 31, 6.3, 3.1, 13, 5.4, and 33 n g mL−1 for Ba, Ca, Cd, Cu, Fe, Mg, Mn, Ni, Sr, and Zn, respectively. Absolute limits of detection were 167, 50, 68, 32, 350, 69, 34, 143, 59, and 363 pg for Ba, Ca, Cd, Cu, Fe, Mg, Mn, Ni, Sr, and Zn, respectively. The method offers relatively good precision (RSD ranged from 7 to 12%) for liquid analysis and microsampling capability. The accuracy of the method was verified by the use of digested certified reference materials (SRM 1648 (Urban Particulate Matter), IAEA 336 (Lichen), SRM 2710 (Montand Soil), INCT-SBF-4 (Soya Bean Flour)) and by aqueous standard calibration technique. The analyte concentrations in reference materials were in satisfactory agreement with the certified values. The method requires small amounts of reagents and reduces contamination and losses. In general, low-pressure argon discharges proved to be superior, in terms of detection limits (DLs), to atmospheric pressure MIPs for the excitation of the analyte atomic or ionic emission.

Measurements of the total energy lost per electron–ion pair lost in low-pressure inductive argon, helium, oxygen and nitrogen discharge

Experimental measurements of the total energy lost per electron–ion pair lost, εT, were performed in a low-pressure inductive atomic gases (Ar, He) and molecular gases (O2, N2) discharge. The value of εT was determined from a power balance based on the electropositive global (volume-averaged) model. A floating harmonic method was employed to measure ion fluxes and electron temperatures at the discharge wall. In the pressure range 5–50 mTorr, it was found that the measured εT ranged from about 70 to 150 V for atomic gases, but from about 180 to 1300 V for molecular gases. This difference between atomic and molecular discharge is caused by additional collisional energy losses of molecular gases. For argon discharge, the stepwise ionization effect on εT was observed at relatively high pressures. For different gases, the measured εT was evaluated with respect to the electron temperature, and then compared with the calculation results, which were derived from collisional and kinetic energy loss. The measured εT and their calculations showed reasonable agreement.

On the inertia term in the momentum equation in the free-fall regime of discharge maintenance

AbstractThe study, being on two-dimensional modelling of low pressure discharges, suggests an approach to the nonlinear inertia term in the momentum equation of the positive ions needed to be accounted for in the free-fall regime of the discharge maintenance. On the basis of conclusions that the inertia term acts in the wall sheath, where the ions fly perpendicularly to the walls, it is shown that (i) the parallel – to the walls – velocity component can be neglected, and (ii) the rest of the convective derivative can be determined by using the energy conservation law in the collisionless case. In a way, the inertia term acting as a retarding force is joined to the momentum loss term by introducing effective collision frequencies. The validity of the procedure is proved in a model of a low pressure argon discharge by comparison with the exact solutions for the two-dimensional spatial distribution of the discharge characteristics (ion velocity, electron density and temperature and DC electric field and its potential). The conclusion is that (i) ignoring the velocity component that is parallel to the walls does not cause deviation from the exact solution, and (ii) the approximation of using the energy conservation law in the collisionless case is good enough.

Plasma diagnostics by optical emission spectroscopy on argon and comparison with Thomson scattering

A novel optical emission spectroscopy (OES) technique for the determination of electron temperatures and densities in low-pressure argon discharges is compared with Thomson scattering (TS). The emission spectroscopy technique is based on the measurement of certain line ratios in argon and a collisional–radiative model (CRM) including metastable transport. The investigations are carried out in a planar inductively coupled neutral loop discharge (NLD) over a wide range of pressures, p = 0.05 Pa–5 Pa. This discharge is a weakly magnetized novel radio-frequency (rf) plasma source, proposed for plasma etching. The NLD is operated in pure argon at a frequency of f = 13.56 MHz and powers varied between P = 1 kW and 2 kW. Both diagnostics, OES and TS, are applied in parallel. The electron energy distribution functions obtained by TS are clearly Maxwellian at low pressures but exhibit a certain enhancement of the energetic tail at higher pressures. Electron densities and temperatures obtained by both diagnostic techniques are compared. Further, absolute numbers of the metastable densities derived from the measurement by the CRM are compared with earlier measurements under similar conditions. Excellent agreement is found throughout if depletion of the neutral gas density by increasing gas temperature and electron pressure is included in the CRM. Electron pressure is the dominant depletion mechanism at gas pressures p ⩽ 0.1 Pa and rf powers P > 1 kW. There, the electron pressure exceeds more than 3 times the neutral pressure and the ionization degree approaches 7% while at pressures p > 1 Pa the degree of ionization is relatively low (<10−3) and neutral gas depletion is dominated by gas heating.

Electrical and spectroscopic characterizations of a low pressure argon discharge created by a broad-band helical coupling device

This paper presents the spatial characterization of Ar plasma induced by a broad-band helical coupling device at low pressure (15 Pa). The electric field analysis outside the helix but within the coupling device is carried out by electrostatic probe for several excitation frequencies ranging from 13.56 to 433 MHz. The results prove that this helical cavity behaves as a slow-wave plasma source. At 13.56 MHz, the spatial investigation of electric field with and without plasma led to show a flat profile along the helix structure. At this frequency, the plasma is studied by spatially resolved emission spectroscopy and by means of a double Langmuir probe in order to evaluate the densities of some characteristic species (electron and Ar emitter) and the electron and gas temperatures. These temperatures are invariants along the discharge with average values about 400 ± 50 K and 2.3 ± 0.6 eV for gas and electron, respectively. Consequently, the electron energy distribution function and argon ground state density are also constant along the discharge. No variation of the argon metastable density, estimated by means of a simplified kinetic model, could be shown and an average value equal to 9 × 1017 m−3 is obtained. In contrast, the density profiles of electron and Ar emitter show a minimum situated in the middle of the helical structure and two maxima located at the positions where the RF power is transmitted to the gas and at the end of the helix. The electron density values vary from 3 × 1016 to 1017 m−3. According to the low pressure conditions and the flat evolution of the electric field, these hollow shaped density profiles can be explained as a consequence of a non-linear phenomenon.

Using OES to determine electron temperature and density in low-pressure nitrogen and argon plasmas

A summary of the methods of electron temperature and electron density measurement in low-pressure nitrogen and argon discharges using optical emission spectroscopy (OES) is given. This paper describes recently developed simple kinetic models for nitrogen and argon discharges and their applications in establishing the relationship between the electron parameters (temperature and density) and the OES line ratios from the discharges. In particular, the assumptions and applicability of these models in different kinds of discharges (capacitively coupled and inductively coupled discharges, discharges with different gas mixtures, etc) are discussed in detail.