S. Khorram

Adaptation of Sing Lee's model to the Filippov type plasma focus geometry

A new model for plasma behaviour in Filippov type plasma focus (PF) systems has been described and used. This model is based on the so-called slug model and Sing Lees model for Mather type PF devices. Using the model, the discharge current and its derivative as a function of time, and the pinch time and the maximum discharge current as a function of pressure, have been predicted. At the end, the predicted data are compared with the experimental data obtained through a Filippov type PF facility with a nominal maximum energy of 90 kJ.

HIERARCHY OF CHAOTIC MAPS WITH AN INVARIANT MEASURE

We give hierarchy of one-parameter family Φ(α,xa0x) of maps at the interval [0,xa01] with an invariant measure. Using the measure, we calculate Kolmogorov-Sinai entropy, or equivalently Lyapunov characteristic exponent of these maps analytically, where the results thus obtained have been approved with the numerical simulation. In contrary to the usual one-parameter family of maps such as logistic and tent maps, these maps do not possess period doubling or period-n-tupling cascade bifurcation to chaos, but they have single fixed point attractor for certain values of the parameter, where they bifurcate directly to chaos without having period-n-tupling scenario exactly at those values of the parameter whose Lyapunov characteristic exponent begins to be positive.We generate new hierarchy of many-parameter family of maps of the interval [0,1] with an invariant measure, by composition of the chaotic maps of reference [1]. Using the measure, we calculate Kolmogorov-Sinai entropy, or equivalently Lyapunov characteristic exponent, of these maps analytically, where the results thus obtained have been approved with numerical simulation. In contrary to the usual one-dimensional maps and similar to the maps of reference [1], these maps do not possess period doubling or period-n-tupling cascade bifurcation to chaos, but they have single fixed point attractor at certain region of parameters values, where they bifurcate directly to chaos without having period-n-tupling scenario exactly at these values of parameter whose Lyapunov characteristic exponent begins to be positive.

Level crossing analysis of growing surfaces

We investigate the average frequency of positive slope ν + ,c rossing th eh eight α = h − ¯ in the surface growing processes. The exact level crossing analysis of the random deposition model and the Kardar–Parisi–Zhang equation in the strong coupling limit before creation of singularities is given.

Fractal analysis of discharge current fluctuations

We use multifractal detrended fluctuation analysis (MF-DFA) to study the electrical discharge current fluctuations in plasma and show that it has multifractal properties and behaves as a weak anti-correlated process. Comparison of the MF-DFA results for the original series with those for the shuffled and surrogate series shows that correlation of the fluctuations is responsible for the multifractal nature of the electrical discharge current.

Measured density of copper atoms in the ground and metastable states in argon magnetron discharge correlated with the deposition rate

In a dc-magnetron discharge with argon feed gas, densities of copper atoms in the ground state Cu(2S1/2) and metastable state Cu*(2D5/2) were measured by the resonance absorption technique, using a commercial hollow cathode lamp as light source. The operating conditions were 0.3?14??bar argon pressure and 10?200?W magnetron discharge power. The deposition rate of copper in a substrate positioned at 18?cm from the target was also measured with a quartz microbalance. The gas temperature, in the range 300?380?K, was deduced from the emission spectral profile of N2(C?3?u ? B?3?g) 0?0 band at 337?nm when trace of nitrogen was added to the argon feed gas. The isotope-shifts and hyperfine structures of electronic states of Cu have been taken into account to deduce the emission and absorption line profiles, and hence for the determination of atoms densities from the measured absorption rates. To prevent error in the evaluation of Cu density, attributed to the line profile distortion by auto-absorption inside the lamp, the lamp current was limited to 5?mA. Density of Cu(2S1/2) atoms and deposition rate both increased with the enhanced magnetron discharge power. But at fixed power, the copper density augmented with argon pressure whereas the deposition rate followed the opposite trend. Whatever the gas pressure, the density of Cu*(2D5/2) metastable atoms remained below the detection limit of 1 ? 1010?cm?3 for magnetron discharge powers below 50?W and hence increased much more rapidly than the density of Cu(2S1/2) atoms, over passing this later at some discharge power, whose value decreases with increasing argon pressure. This behaviour is believed to result from the enhancement of plasma density with increasing discharge power and argon pressure, which would increase the excitation rate of copper into metastable states. At fixed pressure, the deposition rate followed the same trend as the total density of copper atoms in the ground and metastable states. Two important conclusions of this work are (i) copper atoms sputtered from the target under ion bombardment are almost all in the ground state Cu(2S1/2) and hence in the plasma volume they can be excited into the metastable states; (ii) all atoms in the long-lived ground and metastable states contribute to the deposition of copper layer on the substrate.

Markov Properties of Electrical Discharge Current Fluctuations in Plasma

Using the Markovian method, we study the stochastic nature of electrical discharge current fluctuations in the Helium plasma. Sinusoidal trends are extracted from the data set by the Fourier-Detrended Fluctuation analysis and consequently cleaned data is retrieved. We determine the Markov time scale of the detrended data set by using likelihood analysis. We also estimate the Kramers-Moyal’s coefficients of the discharge current fluctuations and derive the corresponding Fokker-Planck equation. In addition, the obtained Langevin equation enables us to reconstruct discharge time series with similar statistical properties compared with the observed in the experiment. We also provide an exact decomposition of temporal correlation function by using Kramers-Moyal’s coefficients. We show that for the stationary time series, the two point temporal correlation function has an exponential decaying behavior with a characteristic correlation time scale. Our results confirm that, there is no definite relation between correlation and Markov time scales. However both of them behave as monotonic increasing function of discharge current intensity. Finally to complete our analysis, the multifractal behavior of reconstructed time series using its Keramers-Moyal’s coefficients and original data set are investigated. Extended self similarity analysis demonstrates that fluctuations in our experimental setup deviates from Kolmogorov (K41) theory for fully developed turbulence regime.

Determination of sputtered atom densities and velocities via simulation

Transport of sputtered atoms in a plasma sputtering unit is simulated using two known elastic collision models. The methods of studying the collision of two atoms are isotropic random scattering angle in the center of mass frame and the so-called center to center collision model in which the impact parameter is ignored and the two particles collide such as two material points. The obtained results for the density and velocity of sputtered atoms in both models are compared with some experimental reports. This investigation shows that the copper and argon atoms present pointlike behavior in the collision process.

Simulation study of spheroidal dust gains charging: Applicable to dust grain alignment

The charging process of nonspherical dust grains in an unmagnetized plasma as well as in the presence of a magnetic field is studied. It is shown that unlike the spherical dust grain, due to nonhomogeneity of charge distribution on the spheroidal dust surface, the resultant electric forces on electrons and ions are different. This process produces some surface charge density gradient on the nonspherical grain surface. Effects of a magnetic field and other plasma parameters on the properties of the dust particulate are studied. It has been shown that the alignment direction could be changed or even reversed with the magnetic field and plasma parameters. Finally, the charge distribution on the spheroidal grain surface is studied for different ambient parameters including plasma temperature, neutral collision frequency, and the magnitude of the magnetic field.

The peculiarities of mechanical bending of silicon wafers after diverse manufacturing operations

In this work, peculiarities of mechanical bending and deformation rate of silicon wafers under the influence of applied external forces have been studied. It has been shown that, for all investigated samples, there are three characteristic sections with various slopes irrespective of the types of operation, orientation and thickness of the wafers. By increasing the applied force, the rate of deformation rises, reaches a maximum, and then starts to decrease not monotonically, but spasmodically by further increase of the force. Experimental results are discussed on the base of the creation of dislocations and it is shown that, there is excellent correlation between the density of dislocations and sposmodic decrease of deformation rate. The analysis of the obtained results confirms the availability of plastic deformation and its inclination to localization during deformation process at room temperature. It should be added that, the sposmodic decrease of deformation rate, can be viewed as a self-organized deformable medium.

Erosion of the surface of electrodes in pulsed plasma evaporator

AbstractIn preparing thin films by concentration of a high speed plasma jet onto different substrates and also surface coating technology, many interesting physical problems may arise. We discussed the mechanism by which some evaporated dielectric or semiconductor material fall upon a substrate and estimate the rate of erosion of different evaporating electrode materials (Cu, Mo, C, ...). Dusty plasma jet of relatively high speed (105 ∨ 106 cm/s) is produced by a pulsed evaporating system. The measured erosion rate is compared with e phenomenologically calculated mass deposition on the substrate. In such plasmas with electron currents (10∨20kA), the following empirical relation has been found between mass erosion and discharge current:n