Background Neutral Gas Research Articles

Grand challenges in low-temperature plasma physics

INTRODUCTION A plasma is a hot ionized gas and classification of this fourth state of matter can be initially done using the basic concept of temperature. A prime example is the Sun which exhibits a very hot plasma in its core with temperatures of about 1.5 × 107 K (a result of fusion reactions with a proton density of ∼1026 cm−3) and cooler surface temperatures of about 6000 K [1]: the Solar wind originates from the Solar Corona, expands into the universe and impacts the Earth’ magnetosphere and ionosphere, the two plasma layers surrounding Earth’s gaseous atmosphere. Aurorae in the Northern and Southern skies near the magnetic poles are examples of this interaction. The electron density in the ionosphere is low (104–106 cm−3) and the background neutral gas density is also low (about 108 cm−3 or 3 × 109 Torr at 300 km altitude) approaching the “space-like” environment created in laboratories to develop and test hardware for space use (i.e., satellites and payloads). At the surface of the Earth lightning strikes of a storm are naturally occurring plasmas operating near atmospheric pressure (neutral density of about 2 × 1019cm−3) with large electron densities (about 1015– 1017 cm−3) characteristic of streamers, arcs and filamentary discharges. The temperatures and densities of neutral and charged particles are critical parameters affecting the physical mechanisms within the plasma such as particle transport and collisional processes [2] and their range spans many orders of magnitude, opening doors to an extremely wide range of plasma applications. Plasmas in which fusion reactions take place are often referred to as “hot” plasmas. The high-temperature plasma community has a well-defined aim of triggering and controlling fusion plasmas (as can be found in the Sun’s core) for energy production, with various worldwide large scale programs or experiments in place (i.e., the International Thermonuclear Experimental Reactor ITER, the National Ignition Facility ICF. . .) [3]. Hot plasmas in space also include relativistic plasmas (highest electron temperatures) and quantum plasmas (highest electron densities). All other plasmas are classified as low-temperature or “cold” plasmas: those gaseous plasmas or electrical discharges have been successfully harnessed and studied in the laboratory since the 1920s and in space using satellites since the 1960s. The latter two plasma examples essentially sit at the opposite ends of the “cold” plasma spectrum. The wide range of available plasma parameters has largely contributed to the long and expanding list of plasma applications as a result of both scientific and economic drivers: the temperature range of the neutral and/or ionized species allow heat and particle control to burn, melt, cut, coat, grow materials from the macroscopic to the microand nanoscale via “so called” plasma processes. Amongst thousands of plasma applications processes, a few examples are presented to show the past, present and future key role cold plasma physics must play in addressing the challenges facing the modern world: predicted energy crisis, environmental issues and ecosystems, global climate variation, population growth and biomedical concerns. In addressing those issues we must better understand the physics of the atmosphere, ionosphere and magnetosphere, the physics of the plasma interacting with a boundary and develop new clean sources of energy.

Flute growth rate of plasma jet in mirror machine

The evolution of flute instability in a cold, high-density hydrogen plasma jet, injected into a mirror machine, is studied. The experiment was designed to minimize the interaction of the plasma with the walls, thus bringing it close to the ideal magnetic Rayleigh–Taylor instability conditions. The modal growth rate was measured in various settings to demonstrate the effects of the finite Larmor radius, Bohm diffusion, conductive limiter, biased limiter and neutral background gas. In this paper we will demonstrate that lowering the magnetic field increases stability, as does the insertion of a conducting ring. However, if the ring is biased, the stability is reduced due to inhomogeneous coupling between the plasma and the limiter. It was also found that heavy background gas dramatically reduces the flute instability growth rate.

Pre-sheath density drop induced by ion-neutral friction along plasma blobs and implications for blob velocities

The pre-sheath density drop along the magnetic field in field-aligned, radially propagating plasma blobs is investigated in the TORPEX toroidal experiment [Fasoli et al., Plasma Phys. Controlled Fusion 52, 124020 (2010)]. Using Langmuir probes precisely aligned along the magnetic field, we measure the density nse at a poloidal limiter, where blobs are connected, and the upstream density n0 at a location half way to the other end of the blobs. The pre-sheath density drop nse/n0 is then computed and its dependence upon the neutral background gas pressure is studied. At low neutral gas pressures, the pre-sheath density drop is ≈0.4, close to the value of 0.5 expected in the collisionless case. In qualitative agreement with a simple model, this value decreases with increasing gas pressure. No significant dependence of the density drop upon the radial distance into the limiter shadow is observed. The effect of reduced blob density near the limiter on the blob radial velocity is measured and compared with predictions from a blob speed-versus-size scaling law [Theiler et al., Phys. Rev. Lett. 103, 065001 (2009)].

Pulsed laser deposition of thin carbon films in a neutral gas background

We studied carbon film deposition using a laser-produced plasma, in argon and helium background gas, at pressures between 0.5 and 700 mTorr. A Nd : YAG, 370 mJ, 3.5 ns, at 1.06 µm, operating at 10 Hz, with a fluence of 6.7 J cm−2 was used. The laser plasma was characterized using space resolved OES and a fast response Faraday cup. The resulting carbon films were analysed using AFM, Raman spectroscopy, XPS and SIMS. The structural properties of the carbon films were found to be strongly correlated with the laser carbon plasma composition. Films with a relatively high content of sp3, characteristic of DLC, were obtained at pressures below 200 mTorr. For these conditions the characteristic carbon ion energies in the expanding laser plasma were of the order of 100 eV. At higher pressures sp2 bonds, associated with amorphous carbon, were dominant, which coincides with a high content of C2 molecules in the laser plasma, and a characteristic carbon ion energy around 20 eV.

Dust Streaming in Toroidal Traps

Molecular dynamic simulations were performed to study the characteristics of particle flows in a toroidal trap driven by the ion drag under the influence of gravity, interparticle forces, and friction with a neutral gas background. This paper is focused on the influence of the driving force and the strength of friction on the dynamics of the dust flow and its different regimes. For small values of the ion drag, the particle flow exhibits instabilities with strong velocity fluctuations, while for high values of the ion drag, a quiescent particle flow is obtained. The transition from unstable flows to quiescent flows is studied in detail.

Dust cluster explosion

A model for the dust cluster explosion where micron/sub-micron sized particles are accelerated at the expense of plasma thermal energy, in the afterglow phase of a complex plasma discharge is proposed. The model is tested by molecular dynamics simulations of dust particles in a confining potential. The nature of the explosion (caused by switching off the discharge) and the concomitant dust acceleration is found to depend critically on the pressure of the background neutral gas. At low gas pressure, the explosion is due to unshielded Coulomb repulsion between dust particles and yields maximum acceleration, while in the high pressure regime it is due to shielded Yukawa repulsion and yields much feebler acceleration. These results are in agreement with experimental findings. Our simulations also confirm a recently proposed electrostatic (ES) isothermal scaling relation, PE∝Vd−2 (where PE is the ES pressure of the dust particles and Vd is the confining volume).

Plasma species dynamics in a laser produced carbon plasma expanding in low pressure neutral gas background

We present time and space resolved spectroscopic observations of a laser produced carbon plasma, in an argon background. An Nd:YAG laser pulse, 370 mJ, 3.5 ns, at 1.06 μm, with a fluence of 6.8 J/cm2, is used to produce a plasma from a solid graphite target, at a base pressure of 0.5 mTorr, and with 80 mTorr Argon background. The spectral emission in the visible is recorded with 15 ns time resolution. 20 ns time resolution plasma imagining, filtered at characteristic carbon species emission wavelengths, is used to study the dynamics of the expanding plasma. Two different fronts with ionic or molecular compositions are seen to detach from de laser target plasma.

Pulsed laser deposition of carbon films in low pressure neutral gas background

We have investigated Pulsed Laser Deposition (PLD) of thin carbon films using a graphite target, in a low pressure Argon gas background. Raman spectroscopy based structural analysis of the films shows a correlation between films properties and pressure of the buffer gas background. The morphology of the resulting thin films was characterized with Raman spectroscopy. Time resolved Optical Emission Spectroscopy (OES) observations indicate that, at the substrate position, and for background pressures above 80 mTorr, the dominant species in the expanding carbon plasma are C2 molecules. This is also consistent with Faraday cup observations, which show a strong decrease in the carbon ions content, as the pressure is increased. Our main result is the observation of a sharp transition in the film morphology, around 300 mTorr, from Diamond like Carbon (DLC) at lower pressures, to amorphous Carbon at higher pressures.

On plasma ion beam formation in the Advanced Plasma Source

The Advanced Plasma Source (APS) is employed for plasma ion-assisted deposition (PIAD) of optical coatings. The APS is a hot cathode dc glow discharge which emits a plasma ion beam to the deposition chamber at high vacuum (p ≲ 2 × 10−4 mbar). It is established as an industrial tool but to date no detailed information is available on plasma parameters in the process chamber. As a consequence, the details of the generation of the plasma ion beam and the reasons for variations of the properties of the deposited films are barely understood. In this paper the results obtained from Langmuir probe and retarding field energy analyzer diagnostics operated in the plasma plume of the APS are presented, where the source was operated with argon. With increasing distance to the source exit the electron density (ne) is found to drop by two orders of magnitude and the effective electron temperature (Te,eff) drops by a factor of five. The parameters close to the source region read ne ≳ 1011 cm−3 and Te,eff ≳ 10 eV. The electron distribution function exhibits a concave shape and can be described in the framework of the non-local approximation. It is revealed that an energetic ion population leaves the source region and a cold ion population in the plume is build up by charge exchange collisions with the background neutral gas. Based on the experimental data a scaling law for ion beam power is deduced, which links the control parameters of the source to the plasma parameters in the process chamber.

Biomedical applications and diagnostics of atmospheric pressure plasma

Numerous applications of non-equilibrium (cold, low temperature) plasmas require those plasmas to operate at atmospheric pressure. Achieving non-equilibrium at atmospheric pressure is difficult since the ionization growth is very fast at such a high pressure. High degree of ionization on the other hand enables transfer of energy between electrons and ions and further heating of the background neutral gas through collisions between ions and neutrals. Thus, all schemes to produce non-equilibrium plasmas revolve around some form of control of ionization growth. Diagnostics of atmospheric pressure plasmas is difficult and some of the techniques cannot be employed at all. The difficulties stem mostly from the small size. Optical emission spectroscopy and laser absorption spectroscopy require very high resolution in order to resolve the anatomy of the discharges. Mass analysis is not normally applicable for atmospheric pressure plasmas, but recently systems with triple differential pumping have been developed that allow analysis of plasma chemistry at atmospheric pressures which is essential for numerous applications. Application of such systems is, however, not free from problems. Applications in biomedicine require minimum heating of the ambient air. The gas temperature should not exceed 40 °C to avoid thermal damage to the living tissues. Thus, plasmas should operate at very low powers and power control is essential. We developed unique derivative probes that allow control of power well below 1 W and studied four different sources, including dielectric barrier discharges, plasma needle, atmospheric pressure jet and micro atmospheric pressure jet. The jet operates in plasma bullet regime if proper conditions are met. Finally, we cover results on treatment of bacteria and human cells as well as treatment of plants by plasmas. Localized delivery of active species by plasmas may lead to a number of medical procedures that may also involve removal of bacteria, fungi and spores.

A particle-in-cell Monte Carlo code for electron beam ion source simulation

FAR-TECH, Inc., has developed a particle-in-cell Monte Carlo code (EBIS-PIC) to model ion motions in an electron beam ion source (EBIS). First, a steady state electron beam is simulated by the PBGUNS code (see http://far-tech.com/pbguns/index.html). Then, the injected primary ions and the ions from the background neutral gas are tracked in the trapping region using Monte Carlo method. Atomic collisions and Coulomb collisions are included in the EBIS-PIC model. The space charge potential is updated by solving the Poisson equation each time step. The preliminary simulation results are presented and compared with BNL electron beam test stand (EBTS) fast trapping experiments.

A study on backstreaming positive ions on a high power negative ion source for fusion

Future magnetic-confinement nuclear fusion experiments will be using neutral beam injection (NBI) systems for plasma heating that are based on sources for negative hydrogen ions as opposed to the positive ions mostly used to date. An unavoidable effect in negative NBI (NNBI) systems is the creation of positive ions in the acceleration region due to collisions between the fast negative ions and the neutral background gas. These positive ions are accelerated back into the ion source. At the high extracted ion current densities from NNBI sources and the high acceleration voltages—1 MeV in the case of ITER—the resulting heat load on the backplate of the source and the sputtering rates of the backplate material can be substantial. In this work, sputtering probes and a simple 1D calculation were employed to estimate the flux density of the backstreaming ions in the rear part of the ion source at the long pulse NNBI testbed MANITU (20 kV extraction voltage). It was found that the flux of backstreaming ions is approximately between 0.8 and 2.5% of the flux of negative ions extracted from the source. Experiment and theory are in fair agreement.

Modelling and Simulation of the Advanced Plasma Source

Plasma ion assisted-deposition (PIAD) is a combination of conventional thermal evaporation deposition and plasma-beam surface modification; it serves as a well-established technology for the creation of high quality coatings on mirrors, lenses, and other optical devices. It is closely related to ion-assisted deposition to the extent that electrons preserve quasineutrality of the ion beam. This paper investigates the Advanced Plasma Source (APS), a plasma beam source employed for PIAD. A field enhanced glow discharge generates a radially expanding plasma flow with an ion energy of about 80-120 eV. Charge exchange collisions with the neutral background gas (pressure 0.1 Pa and below) produce a cold secondary plasma, which expands as well. A model is developed which describes the primary ions by a simplified Boltzmann equation, the secondary ions by the equations of continuity and momentum balance, and the electrons by the condition of Boltzmann equilibrium. Additionally, quasineutrality is assumed. The model can be reduced to a single nonlinear differential equation for the velocity of the secondary ions, which has several removable singularities and one essential singularity, identified as the Bohm singularity. Solving the model yields macroscopic plasma features, such as fluxes, densities, and the electrical field. An add-on Monte-Carlo simulation is employed to calculate the ion energy distribution function at the substrate. All results compare well to experiments conducted at a commercial APS system.

Interaction energy and closest approach of moving charged particles on a plasma and neutral gas background

AbstractElectric interaction between two negatively charged particles of different sizes on a mixed background of positive, negative, and neutral particles is complex and has relevance both to dusty plasmas and to transports in ionized fluids in general. We consider particularly effects during interaction that particle velocity and neutrals in the background may have on the well-known “dressing” and electric shielding that is due to the charged part of the background and how the interaction energy is modified because of this. Without such effects earlier works show the interaction becomes attractive when the distance between the two particles is a bit larger than the Debye length. We use a model where one of the two interacting particles has a radius much larger than the Debye length and the other a radius shorter than the Debye length. Then, the complex interaction may be more easily determined for particle separation up to a few Debye lengths. We consider the larger particle as stationary while the smaller may move. We find quite simple analytic expressions for the dressed particle interaction energy over the whole range of speed of the incoming smaller particle, assumed coming head on the larger particle, and the whole range of neutral particle densities. We also derive a distance of closest approach of small and large particles for all such parameter values. This distance is important for excluded volume estimations for moving small charged particles in media populated by large charged particles on a background as described above, and hence, important for determining the speed of flow of the smaller particles through such media.

Plasma Catalysis for Enhanced-Thrust Single Dielectric Barrier Discharge Plasma Actuators

S INGLE dielectric barrier discharge (SDBD) plasma actuators have been used to manipulate airflows on a variety of lifting and nonlifting bodies (see, for example, the excellent review article by Corke et al. [1]). These actuators, which consist of a pair of offset electrodes separated by a dielectric material, generate a force on the neutral background gas that results in a paraelectric gas flow. Applications include, for example, active delay of separation near the leading edge of airfoils [2,3], delay of airfoil dynamic stall [4,5], reduction of bluff-body drag by delay of separation [6,7], control of separation on turbomachinery blades [8,9], flow control on wind turbine blades [10], and other similar applications. While plasma actuators have been used with some success, as detailed in the literature cited above, their utility is limited by the small force that is generated by the actuators. The thrust generated by state-of-the-art actuators is limited to about 0.10 to 0.20 N per meter of actuator, inducing a velocity that peaks at about 3.0 to about 6:0 m=s [1]. Although their attributes (particularly the ability to be flush-mounted with almost no parasitic drag, very simple implementation, and nomoving parts) make them very attractive for active flow control, it is generally recognized that a significant increase in control authority is needed in order for plasma actuators to be effective for most practical applications. One approach to increasing actuator thrust may be to apply certain heterogeneous catalysts on the surface of the dielectric exposed to the plasma. In this Note, we present the results of recent experiments using titania (TiO2) as a plasma catalyst in which the actuator thrust was seen to increase by as much as 120% relative to the catalyst-free actuator. We discuss possible mechanisms responsible for the enhanced thrust and suggest further experiments which will improve our understanding of the phenomenon.

Measurement of the force on microparticles in a beam of energetic ions and neutral atoms

The force on microparticles in an energetic ion beam is investigated experimentally. Hollow glass microspheres are injected into the vertically upward directed beam and their trajectories are recorded with a charge-coupled device camera. The net force on the particles is determined by means of the measured vertical acceleration. The resulting beam pressures are compared with Faraday cup measurements of the ion current density and calorimetric measurements of the beam power density. Due to the neutral gas background, the beam consists, besides the ions, of energetic neutral atoms produced by charge-exchange collisions. It is found that the measured composition of the drag force by an ion and a neutral atom component agrees with a beam model that takes charge-exchange collisions into account. Special attention is paid to the momentum contribution from sputtered atoms, which is shown to be negligible in this experiment, but should become measurable in case of materials with high sputtering yields.

Plasma Initiation by Neutral Beam Injection

A unique and reliable method of plasma initiation has been established in the Large Helical Device (LHD) by using neutral beam (NB) injection into vacuum. Since LHD is a superconducting machine, the confining magnetic field exists unrelated to plasma. Under these circumstances it is demonstrated that the NB can initiate plasma by itself. A small fraction of injected NB is ionized by collision with the background neutral gas and is confined by the magnetic field. Although these high-energy ions are lost quickly by charge exchange, they work as the energy source for ionizing the background neutral particles and heating the produced plasma. As a result, very thin but hot “seed” plasma is generated, which ionizes puffed gas and makes dense target plasma that is sufficient for NB absorption. This process is simulated numerically and the results agree well with the experimental observations for both absolute values and temporal behavior of plasma parameters. The method does not depend on magnetic field strength strongly, and plasma can be initiated at the magnetic field strength as low as 0.4 T, although standard field strength of LHD is 2.75 T. The progress of high-beta studies in LHD owes this plasma production method much.

Suprathermal electrons in a stationary magnetic arc discharge

Stochastic short voltage spikes occur in the stationary arc discharge of the linear plasma generator PSI-2. Similar spikes are also found when detecting the infrared bremsstrahlung emission at various plasma positions. They are related to suprathermal electrons which have energies up to 150 eV, i.e. 1.5 times the average discharge voltage and 15 times kBTe, the thermal energy of the bulk electrons. These electrons are examined by different diagnostic methods, in particular a newly constructed segmented neutralizer plate was used as a diagnostic tool. The suprathermal particles are found to exist in a thin circular ring of the plasma column which is the region of field lines connected directly to the cathode. For low neutral gas pressure the suprathermal electrons can be treated as collisionless, but when increasing the neutral gas background, scattering with the molecules must be taken into account. In some cases the peak in the potential of the collecting neutralizer plate exceeds the peak of the accelerating voltage. This is explained as a transient event occurring when a bunch of electrons is approaching the collecting surface. The maximum current associated with these electrons is found to compensate the ion saturation current. Their peak density is thus estimated to be in the range of 10−3 of the thermal electrons; on temporal average the ratio ⟨nst⟩/ne is of order 10−8.To the best of our knowledge this is the first extensive study on suprathermal electrons in magnetized arcs. They provide an interesting physical phenomenon but are unlikely to affect the interpretation of electrical probes or optical diagnostic measurements.

Fluid model of inductively coupled plasma etcher based on COMSOL

Fluid dynamic models are generally appropriate for the investigation of inductively coupled plasmas. A commercial ICP etcher filled with argon plasma is simulated in this study. The simulation is based on a multiphysical software, COMSOL™, which is a partial differential equation solver. Just as with other plasma fluid models, there are drift–diffusion approximations for ions, the quasi-neutrality assumption for electrons movements, reduced Maxwell equations for electromagnetic fields, electron energy equations for electron temperatures and the Navier–Stokes equation for neutral background gas. The two-dimensional distribution of plasma parameters are shown at 200 W of power and 1.33 Pa (10 mTorr) of pressure. Then the profile comparison of the electron number density and temperature with respect to power is illustrated. Finally we believe that there might be some disagreement between the predicted values and the real ones, and the reasons for this difference would be the Maxwellian eedf assumption and the lack of the cross sections of collisions and the reaction rates.

Dust-acoustic waves driven by an ion-dust streaming instability in laboratory discharge dusty plasma experiments

Dust acoustic waves (DAWs) are spontaneously excited in dusty plasmas produced in dc and rf discharge plasmas over a wide range of plasma and dust conditions. A common feature of these plasmas is the presence of an ion drift relative to the dust, which is driven by an electric field, Eo in the discharge. Using a three fluid model of the DAWs, including the zero order electric field and collisions of all species with the background neutral gas (pressure Po), DAW stability curves were obtained in the Eo-Po plane, for various dust and wave parameters. The (Eo,Po) data points from several experiments in which DAWs have been observed are also shown in comparison with the theoretical stability boundaries. This analysis supports the conclusion that the DAWs are excited by an ion-dust streaming instability.