Plasma Density Distribution Research Articles

Propagation characters of Gaussian laser beams in collisionless plasma: Effect of plasma temperature

This paper presents an investigation of the propagation characters of Gaussian laser beam in cold collisionless plasma by considering the plasma temperature variation. The ponderomotive nonlinearity is involved and induces the nonlinear self-focusing against spatial diffraction. The second-order differential equation of dimensionless beam width parameter f(ξ) in terms of plasma temperature is established from Maxwell’s equations with paraxial approximation. The initial (P0,ρ0) plane, behaviors of f(ξ) parameter, and perturbed density n/nn0n0 of different plasma temperatures are plotted and analyzed. The plasma temperature is divided into four regions, which leads to different propagation characters including oscillatory divergence, self-trapping, self-focusing, and steady divergence. The plasma density distribution modified by the electric field intensity and plasma temperature is discussed.

Non-self-sustained glow discharge with electrostatic confinement of electrons sustained by a fast neutral molecule beam

Experimental study of plasma produced at the nitrogen pressure 0.2–1 Pa in the chamber volume V ≈ 0.12 m3 as a result of injection into the chamber of a broad nitrogen molecule beam with 1–4 keV energy and 0.1–1 A equivalent current is carried out, and the study results are presented. Dependences of the plasma density distribution on the beam equivalent current Ib, energy Eb, and gas pressure p indicate a crucial role of fast molecules in gas ionization, and the probe characteristics reveal two groups of plasma electrons with the temperatures Te ∼ 0.4 eV and Te ∼ 16 eV. Immersion in plasma of an electrode isolated from the chamber and application to the electrode of a positive voltage U result in non-self-sustained discharge. When U changes from ∼0.5 to ∼1.5 V, the discharge current I rapidly rises to a certain value I*, and after that the rate of rise dI/dU drops by an order of magnitude. At U ∼ 10 V, the current I rises to I0 ≈ 1.5I*, and dI/dU once again drops by an order of magnitude. Current I0 specifies the number of electrons produced inside the chamber per second, and it grows up with Eb, Ib, and p. At U > 20 V, due to gas ionization by fast electrons emitted by the chamber and accelerated up to the energy ∼eU in the sheath between the plasma and the chamber walls, the current I rises again. When U grows up to ∼50 V, production of fast electrons with energies exceeding the ionization threshold begins inside the sheath, and the ionization intensity rises dramatically. At U > 150 V, contribution of fast electrons to gas ionization already exceeds the contribution of fast molecules, and the plasma density and its distribution homogeneity inside the chamber both grow up substantially. However, even in this case, the discharge is non-self-sustained, and only at U > 300 V it does not expire when the beam source is switched off.

Investigation of the dynamics of the Z-pinch imploding plasma for a laser-assisted discharge-produced Sn plasma EUV source

Dynamics of the imploding plasma and its relations to the 13.5 nm EUV emissions have been experimentally investigated for a laser-assisted Sn based discharge-produced plasma EUV source. The behaviours and two-dimensional electron density distributions of the EUV-emitting plasma were obtained using the time-resolved shadowgraph and Nomarski interferometric techniques. Observation of the plasma piston in the prepinch phase justified the validity of the zero-dimensional thin-shell model, from which the ion charge state of the prepinch plasma in the cathode region was estimated. The sausage (m = 0) instability that usually enhances the EUV emission was observed, with the radial electron density distribution that displays a concave shape at the crest of the plasma and a bell shape at the neck; the maximum of the electron density is located at one peak of the concave distribution at the crest instead of the neck. Intense EUV emission was produced by the Z-pinch plasma with the electron density (2.0–3.0) × 1018 cm−3. Moreover, the shock waves generated in the anode region can also produce in-band EUV emission with the intensity of 30% of that from the Z-pinch plasma.

Diffusion positive column of electric discharge in transverse magnetic field

The effect of a transverse magnetic field on the characteristics of planar diffusion positive column of electric discharge has been studied. It is shown that, as the magnetic induction increases, the distributions of plasma density and particle fluxes to walls become asymmetric; the density maximum shifts in the direction of Ampere’s force action, and the ion flux in this force direction can significantly exceed the reverse flux. It is established that there is a maximum value of magnetic induction, which bounds from above the region of magnetic fields in which a stationary state of the positive column is possible. In the region where a stationary state of the positive column is possible, each value of the magnetic induction corresponds to two positive-column regimes with different values of the electron energy, drift velocity, and electric field strength.

Comparison of calorimetric plasma diagnostics in a plasma downstream reactor

The energy influx in a non-equilibrium plasma in the afterglow of a plasma downstream reactor has been measured by two different calorimetric probe types. The radio frequency discharge is investigated for oxygen (10–100%)–argon (90–0%) mixtures at relatively high gas flow rates (750–3000 sccm) and pressures (100–350 Pa). The main process parameters influencing the energy influx are the plasma power, the system pressure and the total gas flow rate. Even though the size, geometry and material composition of the used probe types are different, comparable energy flux values are achieved. Depending on the process parameters, energy fluxes between around 100 and 3500 W m−2 are found and the variation of the pressure and total gas flow rate suggests a highly flow-dependent plasma density distribution in the reactor.

Density measurement and MHD simulation ofgas-filled capillary discharge waveguide

To accelerate the electrons efficiently in the laser wake-field accelerator, it is necessary to suppress the instability induced by the diffraction and the defocusing of the laser pulse. The gas-filled capillary discharge waveguide can generate an approximately parabolic density distribution, which can guide the laser pulse efficiently and suppress the instability. Using the Stark effect, this plasma density distribution is measured in this paper, and the relationship between plasma density and filled pressure is presented. By using the MHD code CRMHA, the formation of the capillary waveguide is simulated and researched in detail.

On the Possibility of ECR-Discharge with Overcritical Plasma Density in Axisymmetrical Magnetic Trap

New approach to realizing high plasma parameters in electron cyclotron resonance discharge supported by microwave radiation in axisymmetric magnetic trap has been proposed. This approach is based on using the inverted radial distribution of plasma density (with minimum at the trap axis). Such plasma density distributions allow efficient cyclotron heating and sustaining the steady-state discharge with high plasma density including densities above the critical value (corresponding to the density for which plasma frequency equals to the frequency of heating radiation). Simulations of power deposition for EC heating of plasma with overcritical density in axisymmetric mirror magnetic trap are presented, in which the effect of increasing the heating efficiency for inverted radial plasma density profiles has been demonstrated.

Effect of Excitation Frequency on the Spatial Distributions of a Surface Wave Plasma

Using different frequencies of 2.45 GHz and 915 MHz, the effect of excitation frequency on the spatial distributions of a surface wave plasma for 450 mm wafer processing was experimentally investigated at a medium pressure of 1 Torr. As a result, it was found that the mode number of standing waves which the surface waves form in the radial direction has great influence on the radial distribution of plasma density. Consequently, the plasma uniformity in the downstream region at 915 MHz was better than that at 2.45 GHz. On the other hand, the electron temperature was roughly constant and below 1.5 eV except in the vicinity of the quartz window at both frequencies.

Characteristics of field line resonance type geomagnetic pulsations and variations of plasmaspheric plasma density distribution

The period of field line resonance (FLR) type geomagnetic pulsations depends on the length of the field line and on the plasma density in the inner magnetosphere (plasmasphere), where field lines are closed. Here as FLR period, the period belonging to the maximum occurrence frequency of the occurrence frequency spectrum (equivalent resonance curve) of pulsations has been considered. The resonance system may be replaced by an equivalent resonant circuit. The plasma density would correspond to the ohmic load. The plasma in the plasmasphere originates from the ionosphere, thus FLR period, occurrence frequency are also affected by the maximum electron density in the ionosphere. The FLR period has shown an enhancement with increasing F region electron density, while the occurrence frequency indicated diminishing trend (possible damping effect). Thus, the increased plasma density may be the cause of the decreased occurrence of FLR type pulsations in the winter months of solar activity maximum years (winter anomaly).

Toroidal and poloidal asymmetry in carbon deposition on gaps of plasma facing tiles

In order to characterize and mitigate the carbon deposition in gaps between plasma facing tiles, a simulation calculation is performed for transport of hydrocarbons in the plasma density and potential distribution of the gap calculated using 2D gyro-kinetic model. The plasma distribution is strongly asymmetric between toroidal and poloidal gaps, depending the magnetic angle and the gap width. Ionized hydrocarbons are deposited in the gap edge where the toroidal and poloidal asymmetry is found, whereas neutral species are reflected repeatedly at the side surface so that they are deposited deeply in the gap and are found at the bottom. For low temperatures (<10eV) the asymmetry is small due to dominant contribution of the neutral species to the deposition. Small tilt of the tile surface hides the poloidal gap from direct access of both plasma hydrogen ions and returning hydrocarbons, so that the deposition on the plasma-open side is suppressed.

Compression and strong rarefaction in high power impulse magnetron sputtering discharges

Gas compression and strong rarefaction have been observed for high power impulse magnetron sputtering (HIPIMS) discharges using a copper target in argon. Time-resolved ion saturation currents of 35 probes were simultaneously recorded for HIPIMS discharges operating far above the self-sputtering runaway threshold. The argon background pressure was a parameter for the evaluation of the spatial and temporal development of the plasma density distribution. The data can be interpreted by a massive onset of the sputtering flux (sputter wind) that causes a transient densification of the gas, followed by rarefaction and the replacement of gas plasma by the metal plasma of sustained self-sputtering. The plasma density pulse follows closely the power pulse at low pressure. At high pressure, the relatively remote probes recorded a density peak only after the discharge pulse, indicative for slow, diffusive ion transport.

300mm Cu BEOL Top Trench Etch Process Development for 65nm Logic Technology

The challenges for the top trench etch of small size technology node logic products, include how to get minimal etch depth loading and superior CD (critical dimension) uniformity, free faceting or fencing and free striations. Enabler dielectric etch chamber is designed to provide excellent etch performance and productivity for 65nm technology and beyond. Enabler chamber is equipped with an NSTU (neutral species tuning unit), a CSTU (charged species tuning unit), and 2 MHz and 13.56MHz dual frequency bias power, as well as 162MHz high frequency source power, which is used to re-shape the plasma density distribution. In this work, a 5-step in-situ process (including two ashing steps) was developed using an Enabler chamber. Overall, mass production demonstrated excellent inline and electrical performance with high reliability and MTBC of as high as 460 RF hours was achieved.

Interaction of high contrast laser pulse with foam-attached target

Interactions of high contrast laser pulses with foam-attached targets are investigated via particle-in-cell simulations in order to enhance the energy coupling from laser to plasmas. A foam layer whose mass density is much lower than that of the solid state is used for controlling the plasma density distribution of the laser irradiation region with the aid of the high contrast laser pulses, which are by contrast with the expanding profile of preformed plasmas which are inevitably generated by the irradiation of the low contrast laser pulses. The ionization process plays a role in the laser and foam interaction, which results in the formation of periodic structure of ion charge density. The bulk electrons inside the foam layer are heated by the laser pulse, which results in the generation of abundant MeV electrons and higher energy coupling from laser to plasma. These features are utilized for laser ion acceleration by using a foam-attached thin foil target. It is shown that the laser accelerated ion energy is enhanced by properly choosing the foam parameters.

Nonlinear enhancement of plasma density in linear combination of multiple collisional internal inductively coupled plasma sources

The nonlinear enhancement of plasma density in a combination of two and four collisional internal inductively coupled plasma sources has been experimentally investigated at different mediate gas pressures and radio-frequency powers, where each source can be considered as independent and the combination is linear. In the nonlinear enhancement phenomenon, the plasma density distribution in combination is not only larger than that of every individual source, but also larger than the linear summation of them in most region of the midplane. The nonlinear enhancement effect in the center of the midplane has stronger functional relation to the plasma density and is sensitive to the arrangement method, while it is weakly affected by the neutral particle density. Meanwhile, the electron temperature approximately keeps constant during the antenna’s individual to combinational discharge. Furthermore, a nonlinear diffusion model including the multistep ionization is applied to describe the nonlinear enhancement phenomenon in linear combination, of which the solutions are good consistent with the experimental results.

Two dimensional density and its fluctuation measurements by using phase imaging method in GAMMA 10

Two dimensional (2D) plasma image analysis is useful to study the improvement of plasma confinement in magnetically confined fusion plasmas. We have constructed a 2D interferometer system with phase imaging method for studying 2D plasma density distribution and its fluctuation measurement in the tandem mirror GAMMA 10. 2D profiles of electron density and its fluctuation have been successfully obtained by using this 2D phase imaging system. We show that 2D plasma density and fluctuation profiles clearly depends on the axial confining potential formation with application of plug electron cyclotron heating in GAMMA 10.

Perturbation of the ECR plasma ion energy distribution function during the interaction with a laser-generated plasma

The electron cyclotron resonance coupled to a laser ion source for charge state enhancement method, based on the coupling of a laser ion source with an electron cyclotron resonance (ECR) ion source in order to produce highly charged ions of metallic elements, demonstrated its validity some years ago, although some aspects remained unclear, in particular the optimization of the ion capture probability through ion–ion scattering. Another crucial item that determines the efficiency of the coupling is the variation of the energy content of the electron cyclotron resonance ion source (ECRIS) plasma: when the ions produced by means of the laser ablation are captured, they deposit a great amount of energy into the magnetically confined plasma. The released energy perturbs the ECR plasma, thus increasing the losses onto the chamber walls and the emittance of the extracted beam. Only a rough estimation of the energy content perturbations has been available up to now, so Monte Carlo simulations have been used to improve the description of the ion capture mechanism and the energy perturbation. The real magnetic field structure of ECRIS and a realistic plasma density distribution have been taken into account, as the code considers the ECR plasma properties emerging from particle in cell (PIC) simulations and from experimental measurements that permit one to determine locally the energy perturbation. The effects of electric fields on the ion stream coming from the laser-ablated target have been also considered, resulting in better coupling after the ion energy decrease.

Gas Temperature Effect on Discharge-Mode Characteristics of Atmospheric-Pressure Dielectric Barrier Discharge in a Helium–Oxygen Mixture

For a better understanding of gas temperature effects on plasma characteristics, a numerical study has been carried out for a dielectric barrier discharge (DBD) with a helium-oxygen mixture at atmospheric pressure. A one-dimensional time-dependent simulation code has been developed to solve continuity equations for plasma species and Poisson's equation for electric field calculation for a parallel-plate DBD reactor. To include temperature effects, gas heating by enthalpy change and Joule heating with ionic current movement are considered in the helium-oxygen plasma including 13 species reacting with one another according to 34 reactions depending on the gas temperature. Varying the ambient temperature from 300 K to 500 K, the plasma characteristics are calculated for the temporal variations and spatial distributions of electric field and species densities in the DBD region, and the different features of discharge modes are described by the voltage-current characteristic curves. A glowlike mode, which typically shows the formation of cathode fall, Faraday dark space, negative glow, and positive column in the spatial distributions of electric field and plasma density, is found in the discharge at a low ambient temperature, while a Townsend discharge mode with moderate electric field intensity and lower electron density is characterized at higher ambient temperatures. The temperature-dependent reactions strongly influence the generation and loss of species in the DBD plasma, and the decomposition of O3 into O or O2 and the quenching of metastable helium by the resultant O or O2 play an important role in determining the distinct discharge mode in the DBD of a He-O2 mixture. Furthermore, it is understood that the discharge-mode transition is controllable by the coupled effects of oxygen additive concentration, frequency, and gas temperature. A small amount of O2 additive or a high-frequency operation exhibits a glow mode in a specific range of ambient temperature, of which reason can be explained by density variation and quenching of helium metastable species caused by the produced oxygen-related species.

Formation of Field-Reversed-Configuration Plasma with Punctuated-Betatron-Orbit Electrons

We describe ab initio, self-consistent, 3D, fully electromagnetic numerical simulations of current drive and field-reversed-configuration plasma formation by odd-parity rotating magnetic fields (RMF{o}). Magnetic-separatrix formation and field reversal are attained from an initial mirror configuration. A population of punctuated-betatron-orbit electrons, generated by the RMF{o}, carries the majority of the field-normal azimuthal electrical current responsible for field reversal. Appreciable current and plasma pressure exist outside the magnetic separatrix whose shape is modulated by the RMF{o} phase. The predicted plasma density and electron energy distribution compare favorably with RMF{o} experiments.

Jovian plasma torus interaction with Europa: 3D hybrid kinetic simulation. First results

The hybrid kinetic model supports comprehensive simulation of the interaction between different spatial and energetic elements of the Europa moon-magnetosphere system with respect to variable upstream magnetic field and flux or density distributions of plasma and energetic ions, electrons, and neutral atoms. This capability is critical for improving the interpretation of the existing Europa flyby measurements from the Galileo orbiter mission, and for planning flyby and orbital measurements (including the surface and atmospheric compositions) for future missions. The simulations are based on recent models of the atmosphere of Europa ( Cassidy et al., 2007; Shematovich et al., 2005). In contrast to previous approaches with MHD simulations, the hybrid model allows us to fully take into account the finite gyroradius effect and electron pressure, and to correctly estimate the ion velocity distribution and the fluxes along the magnetic field (assuming an initial Maxwellian velocity distribution for upstream background ions). Non-thermal distributions of upstream plasma will be addressed in future work. Photoionization, electron-impact ionization, charge exchange and collisions between the ions and neutrals are also included in our model. We consider two models for background plasma: (a) with O ++ ions; (b) with O ++ and S ++ ions. The majority of O 2 atmosphere is thermal with an extended cold population ( Cassidy et al., 2007). A few first simulations already include an induced magnetic dipole; however, several important effects of induced magnetic fields arising from oceanic shell conductivity will be addressed in later work.

Simulation of Toroidal Rotation Effect on Heat Flux Transport in the Edge Plasma of Small Size Divertor Tokamak

The effect of toroidal rotation on heat flux transport in the edge plasma of small size divertor was simulated by B2SOLP0.5.2D transport code. The main results of simulation shows that, the following: (1) the radial heat flux is strongly influenced by toroidal rotation. (2) The amplification of conduction part of radial heat flux imposes nonresilient profile of ion temperature, under which the effect of toroidal rotation on ion temperature profile is strong. (3) The ion distribution and its gradients are lower for counter-injection neutral beam than for co-injection neutral beam. (4) Reversal of toroidal rotation during using neutral beam injection result in reverses of radial electric field and E × B drift velocity. (5) The toroidal rotation strong influence on the ion temperature scale length of the ion temperature gradient (ITG). (6) Switch on and off all drifts leads to higher change in the ion density distribution in edge plasma of small size divertor tokamak when the unbalance neutral beam injection are considered (7) the comparison between radial heat flux at different momentum input shows that, the radial ion heat flux with larger ion temperature scale length in the case of co-injection neutral beam is larger than the ion heat flux with smaller ion temperature scale length in the case of counter-injection neutral beam.