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We investigated chemical sputtering of silicon films by Hy+ ions (with y being 2 and 3) in an asymmetric VHF Plasma Enhanced Chemical Vapor Deposition (PECVD) discharge in detail. In experiments with discharges created with pure H2 inlet flows, we observed that more Si was etched from the powered than from the grounded electrode, and this resulted in a net deposition on the grounded electrode. With experimental input data from a power density series of discharges with pure H2 inlet flows, we were able to model this process with a chemical sputtering mechanism. The obtained chemical sputtering yields were (0.3-0.4) ± 0.1 Si atom per bombarding Hy+ ion at the grounded electrode and at the powered electrode the yield ranged from (0.4 to 0.65) ± 0.1. Subsequently, we investigated the role of chemical sputtering during PECVD deposition with a series of silane fractions SF (SF(%)=[SiH4]/[H2]*100) ranging from SF=0% to 20%. We experimentally observed that the SiHy+ flux is not proportional to SF but decreasing from SF=3.4% to 20%. This counterintuitive SiHy+ flux trend was partly explained by an increasing chemical sputtering rate with decreasing SF and partly by the reaction between H3+ and SiH4 that forms SiH3+. [ABSTRACT FROM AUTHOR]

We present a kinetic simulation of the plasma formed by photoionization in the intense flux of an extreme ultraviolet lithography (EUVL) light source. The model is based on the particle-in-cell plus Monte Carlo approach. The photoelectric effect and ionization by electron collisions are included. The time evolution of the low density argon plasma is simulated during and after the EUV pulse and the ion-induced sputtering of the coating material of a normal incidence collector mirror is computed. The relation between the time and position at which the ions are created and their final energy is studied, revealing how the evolution and the properties of the sheath influence the amount of sputtered material. The influence of the gas pressure and the source intensity is studied, evaluating the behavior of Ar+ and Ar2+ ions. A way to reduce the damage to the collector mirror is presented. [ABSTRACT FROM AUTHOR]

A dusty radio-frequency silane/hydrogen discharge is simulated, with the use of a one-dimensional fluid model. In the model, discharge quantities like the fluxes, densities, and electric field are calculated self consistently. A radius and an initial density profile for the spherical dust particles are given and the charge and the density of the dust are calculated with an iterative method. During the transport of the dust, its charge is kept constant in time. The dust influences the electric field distribution through its charge and the density of the plasma through recombination of positive ions and electrons at its surface. In the model this process gives an extra production of silane radicals, since the growth of dust is not included. Results are presented for situations in which the dust significantly changes the discharge characteristics, both by a strong reduction of the electron density and by altering the electric field by its charge. Simulations for dust with a radius of 2 µm show that the stationary solution of the dust density and the average electric field depend on the total amount of the dust. The presence of dust enhances the deposition rate of amorphous silicon at the electrodes because of the rise in the average electron energy associated with the decrease of the electron density and the constraint of a constant power input. [ABSTRACT FROM AUTHOR]

Presents a study which described a fluid model for an argon radio frequency discharge in a cylindrical discharge chamber. Values of the transport coefficients; Use of the Sharfetter-Gummel exponential scheme for the spatial discretization; Simulation results.

Deals with a study which considered the response of a radio-frequency discharge in an electronegative gas to laser-induced photodetachment. Background to the study; Theoretical model; Numerical technique; Conclusions.

Presents a numerical study on the ion and neutral angular impact energy distribution at the radio frequency-driven electrode of a reactive ion etcher. Contribution of the collision processes to the angular ion impact energy distribution; Derivation of expressions that can be used to predict average impact energies and angles; Calculation of the sputter yield.

Collections of micrometer-sized solid particles immersed in plasma are used to mimic many systems from solid state and fluid physics, due to their strong electrostatic interaction, their large inertia, and the fact that they are large enough to be visualized with ordinary optics. On Earth, gravity restricts the so-called dusty plasma systems to thin, two-dimensional (2D) layers, unless special experimental geometries are used, involving heated or cooled electrons, and/or the use of dielectric materials. In micro-gravity experiments, the formation of a dust-free void breaks the isotropy of 3D dusty plasma systems. In order to do real 3D experiments, this void has somehow to be closed. In this paper, we use a fully self-consistent fluid model to study the closure of a void in a micro-gravity experiment, by lowering the driving potential. The analysis goes beyond the simple description of the 'virtual void', which describes the formation of a void without taking the dust into account. We show that self-organization plays an important role in void formation and void closure, which also allows a reversed scheme, where a discharge is run at low driving potentials and small batches of dust are added. No hysteresis is found this way. Finally, we compare our results with recent experiments and find good agreement, but only when we do not take charge-exchange collisions into account. [ABSTRACT FROM AUTHOR]

The existence of a dust-free void, as often observed in dusty plasmas with small particles, or in dusty plasmas under micro-gravity conditions, requires a maximum of the ionization inside the void. Enhanced optical emission inside the void has indeed been observed. The extra losses of plasma on the dust has to be compensated for by extra ionization, which means that the electron temperature must rise. Inside the void there is no depletion of electrons, so a rise in electron temperature is not immediately obvious. It was therefore proposed that the relatively high electron density in the void with respect to the surrounding dusty region, where the electrons are depleted, causes the higher ionization inside the void. Different observations and models have until now not given a decisive answer, however. Using a global model, we predict that a homogeneous dusty plasma without a void should have an increased electron temperature, but at a reduced electron density. A dusty plasma with a fully formed void should have both an increased electron temperature and density in the void. A fully self-consistent two-dimensional model agrees with these conclusions and also shows that the void is a complex system, which is heated by the dust on the outside, but has most of the ionization on the inside. [ABSTRACT FROM AUTHOR]

Describes the determination of Mach numbers for gases and plasmas in a convergent-divergent cascaded arc. Analysis of a plasma flowing from a subsonic to a supersonic state; Shift in sonic conditions due to the lack of isentropic behavior of the plasma flow; Factors causing the anisotropy.

Examines the isentropic exponent in plasmas. Demonstration that isentropic exponent for atomic plasmas is constant; Maintenance of weak dependence on the electron temperature and the two nonequilibrium parameters.

Recently it was shown that, by using the analysis of electrostatic waves entering the plasma–sheath edge, the direct-current (dc) Bohm criterion also holds for discharges under radio-frequency (rf) conditions. In this paper, the influence of Bohm’s criterion on the sheath characteristics for generator frequencies much higher than the ion plasma frequency has been examined by means of an analysis which is based directly on the numerical solution of the Poisson equation. The present calculations indicate that for high rf voltages the time-dependent potential profile does not always increase monotonically, even if Bohm’s criterion is fulfilled. However, as this nonmonotonic behavior does not lead to a more stringent Bohm criterion, the statement that the original dc Bohm criterion also holds in the high-frequency regime is confirmed. The calculations show further that the time-dependent sheath potential is almost completely modulated in the major part of the sheath and that the modulation drops abruptly to zero close to the plasma–sheath edge. [ABSTRACT FROM AUTHOR]

The supersonic expansion of an underexpanding argon plasma from a high density arc source with small dimensions into a low-pressure vessel with large dimensions is studied by an extended one-dimensional nonlocal thermal equilibrium fluid model, called SPIRIT. In an expanding plasma the velocity increases and the pressure, the density, and the temperatures decrease severely. In this article the virtual source model is discussed first, which is a model describing the expanding plasma as originating from a virtual source. The virtual source model includes some viscosity and heat transport in simplified form, but most of the viscosity and heat transport contributions are neglected. The SPIRIT code includes the full energy and momentum balances. The inclusion of viscosity and heat sources may lead to deviations from an adiabatic and/or isentropic expansion. The SPIRIT code can analyze the deviations. When deviations are small, the isentropic expressions from gas dynamics can be used to model expanding plasma too. Model outcomes are compared with experimental data. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The well-known de Bohm criterion is extended to rf discharges. Both low- (ω[sub rf « ω[sub pi]]) and high- (ω[sub pi] « ω[sub rf]) frequency regimes are considered. For low frequencies, the dc Bohm criterion holds. This criterion states that the initial energy of the ions entering the sheath must exceed a limit in order to obtain a stable sheath. For high frequencies, a modified limit is derived, which is somewhat lower than that of the de Bohm criterion. The resulting ion current density in a high-frequency sheath is only a few percent lower than that for the dc case. [ABSTRACT FROM AUTHOR]

Examines a self-consistent hybrid Monte Carlo-fluid model for a direct current glow discharge. Inclusion of collision rates of the fast electrons; Influence of the negative glow on the calculation in the cathode dark space; Overview of the fluid model for the ions and slow electrons.

The cascaded arc hydrogen plasma of Pilot-PSI is studied in a non-LTE model. We demonstrate that the effect of vibrationally excited molecules on the heavy-particle-assisted dissociation is crucial for obtaining thermal constriction. To the best of our knowledge, thermal constriction has not been obtained before in a non-LTE simulation. Probably, realistic numerical studies of this type of plasma were hindered by numerical problems, preventing the non-LTE simulations to show characteristic physical mechanisms such as thermal constriction. In this paper we show that with the help of appropriate numerical strategies thermal constriction can be obtained in a non-LTE simulation. To this end, a new source term linearization technique is developed, which ensures physical solutions even near chemical equilibrium where the composition is dominated by chemical source terms. Results of the model are compared with experiments on Pilot-PSI and show good agreement with pressure and voltage measurements in the source. [ABSTRACT FROM AUTHOR]

The effects of magnetization on an expanding high-enthalpy plasma jet in argon are studied with a non-LTE model. The magnetic field is taken into account with a complete approach where the calculation of diffusion velocities, the electromagnetic field and the flow field are self-consistently coupled via the current density. Results of the simulations show that inside the plasma source there are no significant changes due to the magnetic field. However, in the expansion the magnetic field completely changes the appearance of the jet. Due to the magnetic field, shockwaves disappear and power dissipation not only takes place inside the source but also in the expansion region. [ABSTRACT FROM AUTHOR]

The operation of a cascaded arc hydrogen plasma source was experimentally investigated to provide an empirical basis for the scaling of this source to higher plasma fluxes and efficiencies. The flux and efficiency were determined as a function of the input power, discharge channel diameter, and hydrogen gas flow rate. Measurements of the pressure in the arc channel show that the flow is well described by Poiseuille flow and that the effective heavy particle temperature is approximately 0.8 eV. Interpretation of the measured I-V data in terms of a one-parameter model shows that the plasma production is proportional to the input power, to the square root of the hydrogen flow rate, and is independent of the channel diameter. The observed scaling shows that the dominant power loss mechanism inside the arc channel is one that scales with the effective volume of the plasma in the discharge channel. Measurements on the plasma output with Thomson scattering confirm the linear dependence of the plasma production on the input power. Extrapolation of these results shows that (without a magnetic field) an improvement in the plasma production by a factor of 10 over where it was in van Rooij et al. [Appl. Phys. Lett. 90, 121501 (2007)] should be possible. [ABSTRACT FROM AUTHOR]

A magnetized hydrogen plasma beam was generated with a cascaded arc, expanding in a vacuum vessel at an axial magnetic field of up to 1.6 T. Its characteristics were measured at a distance of 4 cm from the nozzle: up to a 2 cm beam diameter, 7.5×1020 m-3 electron density, ∼2 eV electron and ion temperatures, and 3.5 km/s axial plasma velocity. This gives a 2.6×1024 H+ m-2 s-1 peak ion flux density, which is unprecedented in linear plasma generators. The high efficiency of the source is obtained by the combined action of the magnetic field and an optimized nozzle geometry. This is interpreted as a cross-field return current that leads to power dissipation in the beam just outside the source. [ABSTRACT FROM AUTHOR]

The direct simulation Monte Carlo (DSMC) method was used to investigate the efficiency of differential pumping in linear plasma generators operating at high gas flows. Skimmers are used to separate the neutrals from the plasma beam, which is guided from the source to the target by a strong axial magnetic field. In this way, the neutrals are prevented to reach the target region. The neutral flux to the target must be lower than the plasma flux to enable ITER relevant plasma-surface interaction (PSI) studies. It is therefore essential to control the neutral gas dynamics. The DSMC method was used to model the expansion of a hot gas in a low pressure vessel where a small discrepancy in shock position was found between the simulations and a well-established empirical formula. Two stage differential pumping was modeled and applied in the linear plasma devices Pilot-PSI and PLEXIS. In Pilot-PSI a factor of 4.5 pressure reduction for H2 has been demonstrated. Both simulations and experiments showed that the optimum skimmer position depends on the position of the shock and therefore shifts for different gas parameters. The shape of the skimmer has to be designed such that it has a minimum impact on the shock structure. A too large angle between the skimmer and the forward direction of the gas flow leads to an influence on the expansion structure. A pressure increase in front of the skimmer is formed and the flow of the plasma beam becomes obstructed. It has been shown that a skimmer with an angle around 53° gives the best performance. The use of skimmers is implemented in the design of the large linear plasma generator Magnum-PSI. Here, a three stage differentially pumped vacuum system is used to reach low enough neutral pressures near the target, opening a door to PSI research in the ITER relevant regime. [ABSTRACT FROM AUTHOR]

Presents a study which used the high-power density of a frequency quadrupled pulsed neodymium-yttrium aluminum garnet laser to photodetach electrons from negative ions in radio frequency plasmas generated within a microwave cavity. Determination of negative ion densities; Measurement of the shape of the resonance curve; Pressure and power dependence of the electron density and of the negative ion density.

In partially ionized, magnetically confined plasmas, the diffusive fluxes of different species are coupled. Additionally, the fluxes are directionally coupled due to the Lorentz force. The challenge in the modelling of multi-component, magnetized plasmas is to take care of this coupling in the numerical method. In this paper, a complex form of the Stefan-Maxwell equations is used to account for the coupling between the flow directions. To handle the coupling between the species fluxes in the finite volume method, a generalized, coupled form of the exponential scheme is used. The presented numerical method is applied to a magnetically confined hydrogen jet. The results show that the numerical method is capable of describing typical characteristics of magnetized plasmas, such as anisotropic diffusion and the presence of a pressure gradient sustained by the Lorentz force. [ABSTRACT FROM AUTHOR]

The Eunomia code is used to study the neutral species in and near a hydrogen plasma beam. Eunomia is a non-linear Monte Carlo transport code that solves the neutral equilibrium, given a fixed background plasma. The code is developed to study the neutral inventory of Pilot-PSI and Magnum-PSI, linear devices developed to study plasma surface interactions in similar conditions as expected in the ITER divertor. Results show the influence of elastic collisions and the outer vessel wall on the neutral species. In the center of the 2 cm diameter Pilot-PSI beam the results show a strong coupling to the plasma. Only millimeters away from the center, the neutral flow, temperature and density are strongly influenced by recombination processes at the vessel wall (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Ion and neutral parameters are determined in the high electron density, magnetized, hydrogen plasma beam of an ITER divertor relevant plasma via measurements of the n = 2 excited neutrals. Ion rotation velocity (up to 7 km/s) and temperature (2-3 eV ∼ Te) are obtained from analysis of Hα spectra measured close to the plasma source. The methodology for neutral density determination is explained whereby measurements in the linear plasma beam of Pilot-PSI are compared to modeling. Ground-state atomic densities are obtained via the production rate of n = 2 and the optical thickness of the Lyman-α transition (escape factor ∼0.6) and yield an ionization degree >85% and dissociation degree in the residual gas of ∼4%. A 30% proportion of molecules with a rovibrational excitation of more than 2 eV is deduced from the production rate of n = 2 atoms. This proportion increases by more than a factor of 4 for a doubling of the electron density in the transition to ITER divertor relevant electron densities, probably because of a large increase in the production and confinement of ground-state neutrals. Measurements are made using laser-induced fluorescence (LIF) and absorption, the suitability of which are evaluated as diagnostics for this plasma regime. Absorption is found to have a much better sensitivity than LW, mainly owing to competition with background emission. [ABSTRACT FROM AUTHOR]

A highly sensitive imaging Thomson scattering system was developed for low temperature (0.1–10 eV) plasma applications at the Pilot-PSI linear plasma generator. The essential parts of the diagnostic are a neodymium doped yttrium aluminum garnet laser operating at the second harmonic (532 nm), a laser beam line with a unique stray light suppression system and a detection branch consisting of a Littrow spectrometer equipped with an efficient detector based on a “Generation III” image intensifier combined with an intensified charged coupled device camera. The system is capable of measuring electron density and temperature profiles of a plasma column of 30 mm in diameter with a spatial resolution of 0.6 mm and an observational error of 3% in the electron density (ne) and 6% in the electron temperature (Te) at ne=4×1019 m-3. This is achievable at an accumulated laser input energy of 11 J (from 30 laser pulses at 10 Hz repetition frequency). The stray light contribution is below 9×1017 m-3 in electron density equivalents by the application of a unique stray light suppression system. The amount of laser energy that is required for a ne and Te measurement is 7×1020/ne J, which means that single shot measurements are possible for ne>2×1021 m-3. [ABSTRACT FROM AUTHOR]

The FOM-Institute for Plasma Physics Rijnhuizen is preparing the construction of Magnum-psi, a magnetized (3 T), steady-state, large area (80 cm²) high-flux (up to 1024 H+ions m-2s-1) plasma generator. The aim of the linear plasma device Magnum-psi is to provide a controlled, highly accessible laboratory experiment in which the interaction of a magnetized plasma with different surfaces can be studied in detail. Plasma parameters can be varied over a wide range, in particular covering the high-density, low-temperature conditions expected for the detached divertor plasma of ITER. A vital part of the Magnum-psi experiment is the superconducting magnet system, which generates a magnetic field of 3 T while good diagnostic access to the experiment is guaranteed. In this contribution, we will explain the requirements on the magnet system, which is now in the pre-design phase. The present design consists of 3 cylindrical NbTi coils which generate a plateau shaped field of 3 T in a 1.3 meter room temperature bore. The discrete coils are supported by a 2.4 meter long single cylinder in a shared cryostat with 16 room temperature view-ports of 200 mm diameter. The field will most probably be passively shielded by an iron wall surrounding the experimental area. As background, some elements of the pre-design of the Magnum-psi experiment; i.e. vacuum system, plasma source and diagnostics are presented. [ABSTRACT FROM AUTHOR]

The FOM-Institute for Plasma Physics is preparing the construction of Magnum-psi, a magnetized (3 T), steady-state, large area (100 cm²), high-flux (up to 1024 H+ ions m-2s-1) plasma generator. Magnum-psi will be used to study plasma-surface interaction in conditions similar to those in the divertor of ITER and fusion reactors beyond ITER. The active magnetic field region is required to be 4 meter long, 1 meter diameter and steady state. This, together with the need for minimization of the running costs, makes the application of superconducting coils imperative. The magnet system will be unique because of its maximum transparency to provide optimal radial access to the experimental region inside the magnet bore. In this contribution we present a magnet configuration that consists of 5 cylindrical, conduction cooled NbTi coils. These generate an axial field of 3 T with a maximum field on the coils below 6 T. Two cryogenic structures are proposed: the discrete coils are either placed within separate cryostats or are supported by a single cylinder in a shared cryostat with 32 room temperature view ports. Room temperature iron rings close to the outer coils reduce the axial forces that would otherwise put severe constraints on the mechanical structure. The field will most probably be passively shielded by an iron dome at 2 meters from the cryostat. [ABSTRACT FROM AUTHOR]

Transport of hydrocarbon impurities in a high-density (>1020 m−3), low-temperature (<2 eV) plasma beam was studied with the ERO code. The high ion density and low temperature cause strong Coulomb collisionality between plasma ions and impurity ions. The collisionality is so strong that ions typically do not complete their Larmor orbits. The high collisionality causes impurity entrainment: impurity ions quickly acquire a velocity close to the plasma flow velocity. This causes a relatively high surface impact energy: the calculated mean impact energy of CHx was 8.1 eV in a plasma with Te = 0.7 eV. Simulation results were compared to an a-C : H erosion experiment in the linear plasma generator Pilot-PSI. The large uncertainties in literature values for the sticking probability of hydrocarbon radicals are shown to cause a serious uncertainty in the calculated re-deposition pattern. In contrast, the radial electric field component perpendicular to the axial magnetic field lines did not have a major effect on the redeposition profile. [ABSTRACT FROM AUTHOR]

Plasma-enhanced chemical-vapor deposition of amorphous silicon by a square-wave amplitude-modulated radio-frequency excitation has been studied by optical emission spectroscopy and plasma modeling. By the modulation, the deposition rate is increased or reduced, depending on the plasma parameters. The increase in the deposition rate in powder-free (α-regime) plasmas is explained by the behavior of the electrons. High-energy electrons cause a large production of radicals at the onset of the plasma, as evidenced by an overshoot in optical emission. This is confirmed by a one-dimensional fluid model. An optimum in the deposition rate at a modulation frequency of about 100 kHz is determined by the decay time of the electron density. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]