Plasma Sheath Expansion Research Articles

Operation and Qualification of Coaxial Plasma Gun Modes With a Gas Puff Inlet

We describe the design, operation, and qualification of a coaxial plasma gun (CPG) in a low-pressure environment with a gas puff inlet. This infrastructure enables a CPG to operate in the deflagration mode followed by snowplow plasma sheath formation. These modes inject radially expanding and axially propagating plasma sheaths into a low-pressure residual background. This manuscript details the pulse power delivery circuit, including automatic logic control of subcircuits. High-speed camera images (12800 frames/s) of the formed plasma elucidate the ionization of an axially propagating plasma. Long-exposure images (1.3 s) capture the full propagation distance of the plasmoid. Rogowski coil current diagnostics elucidate the timing of a self-crowbar discharge that delineates between deflagration and snowplow modes. An empirically determined 50-ms gas puff duration resulted in a maximum plasma propagation distance of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 23$ </tex-math></inline-formula> cm. Shorter durations did not ionize a sufficient amount of gas for elongated axial propagation, whereas longer durations suppressed axial plasma sheath expansion. This type of gas injection CPG device is now qualified for use in CPG experiments that require the snowplow mode and maximum mean free path of fast ions resulting from various plasma heating mechanisms. These include CPG operation within external magnetic fields in which significant plasma propagation distance is required, including solar physics experiments and space propulsion development.

Enhanced thermionic emission of mayenite electride composites in an Ar glow discharge plasma

Mayenite electride has attracted increasing research interests because of its unique electronic properties. The thermionic emission behavior of the mayenite electride is relatively unknown. Previous studies revealed that mayenite electride exhibited a bare work function ranged from 2.1–2.6 eV when the thermionic emission was tested in vacuum, and enhanced emission currents could be achieved by applying a super-high external electric field. In this paper, the thermionic emission behavior and the corresponding effective work function of two types of mayenite electride based composites, mayenite electride-titanium and mayenite electride-carbon, were investigated in an Ar glow discharge plasma at elevated temperatures (400–1000 K) without applying a high external electric field, which is critical for the application in electric propulsion and other aerospace apparatus, yet never had been done before. During the testing, the thermal equilibrium process and plasma sheath expansion were observed. The effective work function of the two mayenite electride composites were determined as a function of temperature. The Rasor-Warner model was applied to determine the bare work function and adsorption-site density of the mayenite electride based composites. Results suggested that the adsorption of Ar ions led to the enhanced thermionic emission (~30 A/m2 at 985 K) and low effective work functions (0.9–2.2 eV) of mayenite electride based composites, without the need of applying high electric fields. Our findings will pave the ways for the application of mayenite electride and its composites as the thrust cathode materials for electric propulsion where plasma is present.

Spatial inhomogeneity effects on burst temperature estimation using a triple probe configuration in Tokamak Chauffage Alfvén Brésilien tokamak

The effects of coherent structures in plasma edge turbulence are an important topic in the study of plasma cross-field transport in magnetically confined plasmas. To properly characterize these structures, their temperature must be measured, which is often done by using Langmuir probes. In this work, the techniques of Langmuir sweep and triple probe are used, considering the effect of plasma sheath expansion in both methods. It is shown that if the sheath expansion is ignored, the triple probe technique gives overestimated temperature values. In addition, the conditional analysis is applied to both techniques in order to evaluate the mean temperature time trace of the coherent structures. It is shown that these time traces can be distorted in the case of the triple probe technique, due to fact that coherent structures destroy the homogeneity condition on the pins array. Therefore, the sweep technique with a conditional selection of points is more suitable than the triple probe to study coherent structures related to bursts, as it performs a local measurement.

Theoretical investigation of plasma immersion ion implantation of cylindrical bore using hollow cathode plasma discharge

Plasma immersion ion implantation of the internal surface of a cylindrical bore with a small diameter is difficult. The use of radio-frequency hollow cathode discharge as the internal plasma source for ion implantation is proposed. The implantation dynamics and plasma sheath expansion are investigated numerically using the particle-in-cell model. The inner diameter of the tube in our simulation is 20 mm and the external diameter of the hollow cathode is 12 mm. Three electric field zones are observed due to the existence of the electrode with ground potential in the tube. The ions undergo acceleration in the region close to the hollow cathode, maintain their velocity in the zone without the electric field due to overlapping of the plasma sheath, and decelerate in the region near the open end of the tube. Most of the ions are implanted into the surface away from the open plane of the hollow cathode. This is attributed to the special configuration of vertical electric field. The simulation results have demonstrated that plasma immersion ion implantation using a hollow cathode can be effectively applied to the treatment of the inner wall of a cylindrical bore, especially the ones with a small diameter.

Expanding sheath in a bounded plasma in the context of the post-arc phase of a vacuum arc

A numerical model of sheath expansion and plasma decay in a bounded plasma subjected to a linearly increasing voltage has been developed. Numerical results obtained with a hybrid-MB model (Maxwell–Boltzmann electrons, particle ions and Poisson's equations) are compared with analytical theory and results from particle-in-cell (PIC) simulations. The hybrid-MB model is similar to models used for plasma immersion ion implantation except that plasma decay due to particle losses to the electrodes is taken into account. The comparisons with more accurate and much more time consuming PIC models show that the hybrid-MB model provides a very satisfactory description of the sheath expansion and plasma decay even for conditions where the grid spacing is much larger than the Debye length. The model is used for high plasma density conditions, corresponding to the post-arc phase of a vacuum arc circuit breaker where a vacuum gap is subject to a transient recovery voltage (TRV) after it has ceased to sustain a vacuum arc. The results show that the plasma sheath expansion is subsonic under these conditions, and that the plasma starts to decay exponentially after two rarefaction waves from the cathode and anode merge in the centre of the gap. A parametric study also shows the strong influence of the TRV rise rate and initial plasma density on the plasma decay time and on the ion current collected by each electrode. The effect of collisions between charged particles and metal atoms resulting for the electrode evaporation is also discussed.

Two dimensional computer simulation of plasma immersion ion implantation

Department of Physics and Chemistry, Engineering Faculty of Guaratinguet´a FEG, UNESPAv. Ariberto Perreira da Cunha 333, Guaratinguet´a, SP, BrasilReceived on 30 January, 2004; revised version received on 6 May, 2004The biggest advantage of plasma immersion ion implantation (PIII) is the capability of treating objects withirregular geometry without complex manipulation of the target holder. The effectiveness of this approach relieson the uniformity of the incident ion dose. Unfortunately, perfect dose uniformity is usually difficult to achievewhen treating samples of complex shape. The problems arise from the non-uniform plasma density and ex-pansion of plasma sheath. A particle-in-cell computer simulation is used to study the time-dependent evolutionof the plasma sheath surrounding two-dimensional objects during process of plasma immersion ion implanta-tion. Before starting the implantation phase, steady-state nitrogen plasma is established inside the simulationvolume by using ionization of gas precursor with primary electrons. The plasma self-consistently evolves toa non-uniform density distribution, which is used as initial density distribution for the implantation phase. Asa result, we can obtain a more realistic description of the plasma sheath expansion and dynamics. Ion currentdensity on the target, average impact energy, and trajectories of the implanted ions were calculated for threegeometrical shapes. Large deviations from the uniform dose distribution have been observed for targets withirregular shapes. In addition, effect of secondary electron emission has been included in our simulation and noqualitative modifications to the sheath dynamics have been noticed. However, the energetic secondary electronschange drastically the plasma net balance and also pose significant X-ray hazard. Finally, an axial magneticfield has been added to the calculations and the possibility for magnetic insulation of secondary electrons hasbeen proven.

Modelling and experiment of plasma-based ion-implanted two-dimensional target

A prismoidal-shaped target with trapezoidal section containing four different convex corners was implanted with nitrogen using plasma-based ion implantation (PBII) in order to study the effect of target shape on the retained dose and its distribution with depth. Nitrogen was implanted into a silicon wafer clamped on the side wall of the sample holder, and Auger electron spectroscopy was employed to obtain the nitrogen depth distribution and the retained dose. Both a former simulation and the present experimental analysis exhibit dependence of the dose on the target shape but with a reversed trend. A method that combines a fluid dynamic model to simulate plasma sheath expansion during a high voltage pulse and the Monte Carlo method of the TRIM code to simulate the incident ion distribution in the solid was presented to predict the concentration depth profile after PBII. When establishing the model, the mechanism of the resulting lower retained dose near the corner with the higher density of ion impact flux was discussed. It was found that the oblique impact of the ion flux reduces the retained dose of the modified layer in three ways and changes the form of the profile remarkably. The continuous distribution of ion impact energy and the low N+/N2 ratio in the plasma shift the N depth profile nearer to the surface, which reduces the implantation depth significantly. In addition, the oblique impact near the edge of the convex corner decreases the reduction in ion range and retained dose and should account for the gradient in the retained dose distribution on the target surface. The model presented can give a good prediction and explanation for the experimental results.

Effect of plasma density on the distribution of incident ions and depth profile in plasma-based ion implanted layers

The retained dose and compositional depth profile were studied in the context of cylindrical target with different plasma density treated by plasma-based ion implantation (PBII). Nitrogen was implanted into silicon wafer clamped on the samples in order to acquire high quality profiles. Auger electron spectroscopy (AES) was used to acquire the nitrogen depth profile at the middle of Si wafer. A method, that combined fluid dynamic model to simulate plasma sheath expansion during high voltage pulse and TRIM code to simulate incident ion distribution in the solid was presented to simulate the experimental results. Both retained dose and N depth profile were compared with the results of theoretical simulation. The agreement between them for all three cases is good; that is, the model can give a good prediction and explanation to the experimental results. The retained dose for cylinder increases with increasing plasma density. The continuously distributed energy of incident ions and low N +/N 2 ratio in the plasma shift the N depth profile nearer to the surface and reduce the range significantly.

Ion depletion effects in sheath dynamics during plasma immersion ion implantation—models and data

In plasma immersion ion implantation, the wafer is negatively pulsed while immersed in a dc ambient plasma. During this high voltage pulse, the sheath expands, and plasma ions are accelerated to the wafer. The essential character of this plasma sheath expansion can be described by a simple mathematical model, first proposed by Lieberman. In this article, we build on Lieberman’s model, extending it to describe the ion current before and after the pulse. We find that a dip in ion current is predicted immediately after the pulse, due to the depletion of ions within the sheath. This simple model is tested using Faraday cup data, and is also compared to a particle-in-cell simulation.

Heat balance during plasma immersion ion implantation

The substrate temperature was measured during plasma immersion ionimplantation (PIII) of nitrogen as a function of the pulse bias voltage andthe repetition frequency. The variation of the equilibrium frequency fortemperatures between 150 and 500 °C and pulse voltages between -5and -30 kV was investigated. Using these data, the relative dose perpulse for different voltages was obtained and its voltage dependence comparedwith different models for the plasma sheath expansion during PIII. A higherplasma density than measured for a static plasma without pulses, due to theinteraction of secondary electrons with the plasma, must be assumed. Goodagreement with dose measurements of N implanted in Si was also observed. Forhigh pulse frequencies above 1 kHz a deviation was observed, clearly showingthat depletion of the ions from the plasma during the pulses leads to reducedaverage plasma density at high repetition rates.

Influence of electrode-size effects on plasma sheath expansion

Influence of the electrode-size effects on plasma sheath expansion is investigated for a negative voltage at the cathode with maximum amplitude of V0, assuming that the cathode is a thin, flat, conducting, circular disk of radius R. It is shown that propagation of the sheath front is proportional to the one-third power of the combination, 2V0R/π. Experimental measurements have been carried out to verify theoretical predictions. The experimental data agree remarkably well with theoretical results in every aspect.

Effect of pulse waveform on plasma sheath expansion in plasma-based ion implantation

In this paper, the effect of the pulse waveform on plasma sheath evolution around a diamond-shaped target has been simulated using fluid dynamic model in the context of plasma-based ion implantation (PBII). The implanted parameters of ions such as ion-implanted energy, impact angle and impact current have also been studied under different pulse shapes. Understandably, the longer pulse rise time would result in the lower ion impact energy, and less sheath expanding extent. By comparing the sheath structure under different pulse rise time, we found that long pulse rise time would enhance the conformation of the sheath to the target at the earlier stage of the pulse and would reduce the tendency of the ion depletion in the plasma sheath. Accordingly increase the ion impact current at the later stage of the pulse, which is quite important for the PBII process, when the ions have been accelerated adequately.

Utilization of plasma source ion implantation for tribological applications

Plasma source ion implantation (PSII) has been developed as an alternative technique to circumvent the limitations of conventional ion implantation, such as the requirements of complicated target handling and beam raster systems for a uniform ion implantation of three-dimensional samples. By applying negative high voltage pulses, positively charged ions are accelerated to and implanted into the target. The plasma sheath expansion and ion doses, calculated with a theoretical model, are in good agreement with the experimentally obtained data. As the analytical model describes the experimental data sufficiently, it is used to discuss design specifications for PSII systems. Furthermore, the possibility of implanting ions into large, three-dimensional samples is discussed on the basis of predictions of the analytical model. The results of nitrogen PSII treatment of planar samples are shown. Wear and hardness of the implanted and unimplanted flat steel samples with a high chromium content were measured. An increased hardness as well as a decreased wear was obtained.

Numerical simulation of plasma sheath expansion, with applications to plasma-source ion implantation

In plasma-source ion implantation a target is pulse biased to a high negative voltage, forming an expanding plasma sheath. A numerical simulation model for the evolution of the sheath has been developed and compared successfully with experimental results. The model is one dimensional (planar, cylindrical, or spherical). The time-dependent, self-consistent potential profile is calculated from Poisson’s equation coupled with collisionless fluid equations for the ions and a Boltzmann assumption for the electrons. In addition to the density and potential profile, the simulation yields the ion current to the surface and the energy spectrum of the ions hitting the surface.