Charge Exchange Collisions Of Ions Research Articles

An assessment of the first wall and limiter erosion during limiter discharge stages in DEMO

The model elaborated on previously (Tokar 2018 Nucl. Fusion 58 016016) for the transport of neutral and charged plasma components in the DEMO scrape-off layer (SOL) with a divertor is reworked to describe limiter discharge phases. It provides radial profiles of the plasma density and temperatures of electrons and ions, averaged over flux surfaces, which are calculated self-consistently at the edge of the confined region and in the SOL, without prescribing any decay lengths at the last closed flux surface. It is assumed that on the limiter discharge phase in question the plasma will be in the L-mode of the energy confinement and the radial transport in the SOL is modeled with coefficients of the Bohm type. Computations give the plasma particle flows both to the limiters and to the first wall (FW). This allows us to assess the erosion rates by plasma ions of these reactor components. In the case of the FW physical sputtering with hot atoms generated in charge-exchange collisions of ions with neutral species, recycling from the wall and limiters is also taken into account.

Influence of Ar gas pressure on ion energy and charge state distributions in pulsed cathodic arc plasmas from Nb–Al cathodes studied with high time resolution

For cathodic arcs, the cathode material is one of the most important determinants of plasma properties. Consequently, the cathode material—plasma relationship is of special interest in related fundamental research as well as in applications like the synthesis of thin films and coatings. In the latter, the use of multi-element cathodes in inert as well as reactive gas atmospheres is common practice. To further improve the physical understanding of cathodic arcs in such settings, we analyze ions in pulsed cathodic arc plasmas from Nb, Al and two composite Nb–Al cathodes with high time-resolution using a mass-energy-analyzer. The experiments were conducted in Ar atmosphere at total pressures of 0.04, 0.20 and 0.40 Pa, and are compared to earlier results in high vacuum at 10−4. In addition to examining the influence of Ar on ion properties and their cathode material dependence, the results are used to discuss physical concepts in cathodic arcs, like the gas-dynamic expansion of the cathode spot plasma, or the influence of charge exchange collisions of ions with neutrals. While such inelastic collisions e.g. with Ar atoms cause a reduction of charge states to mainly Al+ and Nb2+ at the highest pressure, Ar atoms also seem to take part in near-cathode processes. Ar ions in different time and energy regimes up to 150 eV were observed and compared to Nb and Al ions, showing overlapping velocity distributions for Nb, Al and Ar+ ions, but also Ar2+ ions faster than other ion species.

Equipment and Technology for Combined Ion–Plasma Strengthening of Cutting Tools

A combined strengthening of cutting tools for finishing has been carried out in glow discharge plasma filling a process vacuum chamber. At the first stage, reamers rotating around the axis distanced from the magnetron targets at 8 cm were bombarded by fast argon atoms produced due to charge exchange collisions of ions accelerated in space charge sheathes between the plasma and a negatively biased to 3 kV grid with a 25 cm radius of its concave surface curvature. The reamer bombardment by fast neutral atoms led to a reduction of its cutting-edge radius from ~7 μm to ~2 μm. At the second stage, the reamer surface was nitrided within 1 h at a temperature of 500 °C stabilized by regulation of the negative bias voltage accelerating the nitrogen ions. At the third stage, a 3 μm thick TiN coating has been synthesized on the reamer bombarded by pulsed beams of 3 keV neutral atoms at a 50 Hz repetition rate of 50 μs wide pulses. After the combined strengthening, the cutting edge radius of the coated reamer amounted to ~5 μm and the roughness of the area machined by the reamer holes in blanks made of structural steel reduced by about 1.5 times.

Coating synthesis on dielectric substrates assisted by pulsed beams of high-energy gas atoms

Titanium nitride and aluminum nitride coatings have been deposited on glass and aluminum oxide substrates in a flow of metal atoms accompanied by high-energy gas atoms. The metal atoms are produced due to sputtering of a flat rectangular magnetron target. The gas atoms with energy up to 25 keV are produced due to charge exchange collisions of ions extracted from the magnetron discharge plasma and accelerated by high-voltage pulses applied to a flat grid parallel to the target. The metal atoms pass through the grid and deposit on the substrate. Conjunction of their trajectories with those of gas atoms bombarding the growing coating enables the coating synthesis on complex-shape dielectric products planetary rotating inside the vacuum chamber. Mixing high-energy gas atoms of the coating and substrate atoms substantially improves the coating adhesion.

Ion velocity distribution functions in argon and helium discharges: detailed comparison of numerical simulation results and experimental data

Using the Monte Carlo collision method, we have performed simulations of ion velocity distribution functions (IVDF) taking into account both elastic collisions and charge exchange collisions of ions with atoms in uniform electric fields for argon and helium background gases. The simulation results are verified by comparison with the experiment data of the ion mobilities and the ion transverse diffusion coefficients in argon and helium. The recently published experimental data for the first seven coefficients of the Legendre polynomial expansion of the ion energy and angular distribution functions are used to validate simulation results for IVDF. Good agreement between measured and simulated IVDFs shows that the developed simulation model can be used for accurate calculations of IVDFs.

Deposition of wear-resistant coatings using a combined source of metal atoms and fast gas molecules

A source of metal atoms accompanied by fast gas atoms has been developed and studied. The fast particles are produced due to charge exchange collisions of ions accelerated by potential difference between a plasma emitter inside the source and an emissive grid. The metal atoms are produced due to sputtering a target placed at the hollow cathode bottom by ions with energy up to 3 keV from the plasma emitter. Sputtered atoms cross the emitter and the grid, together with accelerated ions enter the chamber and deposit on the products. Two different discharge modes can be used to produce the plasma emitter and bombard the growing nitride coating interruptedly by atoms and molecules with energy variable from 10 to 300 eV.

Gas discharge source of metal vapor and fast gas atoms

A source of metal atom flow coinciding in time and space with a flow of fast gas atoms has been studied and the study results are presented. The fast particles are produced due to charge exchange collisions of ions accelerated by potential difference between a plasma emitter inside the source and secondary plasma inside a process vacuum chamber. The emitter is the glow discharge plasma, whose electrons are confined in an electrostatic trap formed by a cold hollow cathode and an emissive grid the latter being negative both to the cathode and the chamber. The metal atoms are produced due to sputtering a target placed at the hollow cathode bottom by ions from the plasma emitter with energy up to 3 keV. Sputtered atoms cross the emitter, together with accelerated ions enter the chamber through the emissive grid and deposit on pieces placed therein. When a mixture of argon and nitrogen is used, the metal nitride coatings are being synthesized and interruptedly bombarded during the synthesis by atoms and molecules with energy variable from ∼10 to ∼300 eV.

Numerical studies of a plasma diode with external forcing

With reference to laboratory Q-machine studies we analyze the dynamics of a plasma diode under external forcing. Assuming a strong axial magnetic field, the problem is analyzed in one spatial dimension by a particle-in-cell code. The cathode is assumed to be operated in electron rich conditions, supplying an abundance of electrons. We compare different forcing schemes with the results obtained by solving the van der Pol equation. In one method of forcing we apply an oscillation in addition to the DC end plate bias and consider both amplitude and frequency variations. An alternative method of perturbation consists of modelling an absorbing grid at some internal position. Also in this case we can have a constant frequency with varying amplitude or alternatively an oscillation with chirped frequency but constant amplitude. We find that the overall features of the forced van der Pol equation are recovered, but the details in the plasma response need more attention to the harmonic responses, requiring extensions of the model equation. The analysis is extended by introducing collisional effects, where we emphasize charge exchange collisions of ions, since these processes usually have the largest cross sections and give significant modifications of the diode performance. In particular we find a reduction in oscillator frequency, although a linear scaling of the oscillation time with the system length remains also in this case.

Fluid Model of Plasma Sheath Involving Ion Energy Spectrum

A common fluid model of ion drift in a strong electric field, appearing, e.g., in the sheath, describes ions by the Newtonian equation of motion, with ion-neutral collisions involved by an effective friction force. Such an approach implicitly anticipates a monoenergetic ion beam. We demonstrate that, due to nonlinear effects, the concept based on “average” ions is too simplified. In the suggested model, the conventional equation of ion motion is replaced by a formula considering the energy spectrum of ion flux. The formula is based on the 1-D description of charge-exchange collisions of ions with neutrals. The Newtonian equation of motion is then interpreted from the suggested collision model. It is shown that the ion fluid velocity should be identified with the rms velocity in the ion beam and that the effective ion mean free path appearing in the friction term should be chosen as a double of the ion mean free path for the charge-exchange collisions.

Broad beam source of fast atoms produced as a result of charge exchange collisions of ions accelerated between two plasmas

Experimental study of a fast argon atom beam source is carried out and the study results are presented. The source comprises a 90-mm deep and 210-mm in diameter hollow cathode as well as a flat emission grid, both electrodes made of titanium. The study revealed main factors, which influence the zone diameter of homogeneous substrate etching by a broad beam of fast argon atoms, produced as a result of charge exchange collisions of ions, accelerated between a plasma emitter inside the hollow cathode and a secondary plasma in the working vacuum chamber, the plasmas being separated from each other with the grid. It is shown that at a distance from the grid, exceeding the resonant charge exchange length up to 4 times, elastic collisions have no appreciable impact on the spatial distribution of the etching rate in the vacuum chamber. The homogeneous etching zone diameter is mainly influenced by angular characteristics of accelerated particles in the grid plane. At a constant beam power up to 3–5 kW the diameter is rising with a decrease of their energy and with a corresponding increase of the beam current.

Analysis of the electron and ion fluxes to the wall of a hot-filament discharge device

The current densities of ions and electrons to the wall of a hot-filament discharge device are examined both experimentally and theoretically. The ion current to the wall as a function of neutral gas pressure is found theoretically from a model of the sheath and presheath that includes charge-exchange collisions of the ions with neutrals. The electron current is found from a model based upon the energy distributions of secondary electrons from ionization of the neutral gas and of secondary electrons from the wall. In a hot-filament discharge device with argon plasma (density (0.2–4.5)×109cm−3, electron temperature 0.14–0.21eV, and pressure 0.3–12mTorr), a gridded energy analyzer is placed behind a slit in the wall and the current collected is recorded as a function of the retarding potential. The dependence of the collector current on the grid bias potential identifies the electrons in the 10–65eV range as being mostly secondaries from ionization and those in the 0–10eV range as being mostly secondaries from the wall. Ions are collected at the most negative grid bias voltages. The measured ion currents are within about 40% of values calculated from a model with charge exchange collisions of ions, and at the highest pressure differ by about a factor of 3 from the collisionless value, indicating that the Bohm ion current should be corrected for collisions when the charge exchange mean free path is less than about 0.2 of the plasma radius.

Time-dependence of ion charge State distributions of vacuum arcs: an interpretation involving atoms and charge exchange collisions

Experimentally observed charge state distributions are known to be higher at the beginning of each arc discharge. Until know, this has been attributed to cathode surface effects in terms of changes in temperature, chemical composition, and spot mode. Here it is shown that the initial decay of charge states of cathodic arc plasmas may be at least in part due to charge exchange collisions of ions with neutrals. Neutrals gradually fill the discharge volume, and therefore, the effect of charge exchange shows delayed onset after arc initiation. Besides desorbed gases, sources of neutrals may include evaporated atoms from macroparticles and still-hot craters of previously active arc spots. More importantly, atoms are also produced by energetic condensation of the cathodic arc plasma. Self-sputtering is significant, and additionally, ions have a low sticking probability when impacting at oblique angle of incidence. Estimates show that the characteristic time for filling the discharge volume agrees well with the charge state decay time, and the likelihood of charge exchange is reasonably large to be taken into account.

Numerical solutions to a kinetic model for the plasma-sheath problem with charge exchange collisions of ions.

A kinetic model of the plasma-sheath problem is presented that includes the effects of charge-exchange collisions of the ion. The collisions are modeled as a sink for accelerated ions and as a source of cold ions. Solutions are obtained by numerical integration of Poisson's equation from a point near the plasma midplane to the wall. In the quasineutral region, these solutions agree with earlier analytic work. As the mean free path is decreased, the current density at the wall decreases and the potential profile in the quasineutral region shows a smooth transition from a parabolic profile to a nearly cubic profile determined by the ion mobility. An approximate expression is found for the ion flux to the wall in the collisional limit.

Stable Sheath Formation in Expanding Magnetic Field to Divertor Plate

The stable sheath formation and the effects of charge exchange collisions of ions with cold neutrals for the stable presheath formation in an expanding magnetic field towards a divertor plate is studied by a one-dimensional analysis. The requirement for flow velocity of ions at a plasma-sheath boundary is more restricted than that of the uniform magnetic field, which should be greater than the ion sound speed. The difference, however, between both cases is an order of the Debye length to plasma radius, which is negligibly small. The requirement for ion flow velocity inside a quasi-neutral plasma is investigated by taking into account the effects of the charge exchange collisions. Without neutrals in the quasi-neutral plasma, the ion flow velocity at an injection point should be much greater than the ion sound speed. The unisotropic velocity distribution of injected ions with coupling the expanding magnetic field and the charge exchange collisions might mitigate this requirement.

On the turbulent heating and the threshold condition in the critical ionization velocity interaction

On the basis of the nonlinear treatment of the ion beam instability and the modified two-stream instability, threshold conditions for the critical ionization velocity interaction are derived. There are three different regimes of interaction: (1) additional ionization for relative velocities smaller than the critical velocity, (2) the self-sustained discharge for velocities greater than the critical velocity which indeed turns out as a sharp threshold, and (3) an explosive growth regime for velocities exceeding 1.5 times the critical velocity. Additional charge exchange collisions of ions and energy loss of electrons due to excitation do not change the basic threshold behaviour but modify the value of the critical velocity.