Plasma Front Propagation Research Articles

Numerical analysis of structural change process in millimeter-wave discharge at subcritical intensity

Plasma-front propagation processes of 170 GHz millimeter-wave discharge were investigated under subcritical incident electric field intensity by using a one-dimensional model. The discharge structure was numerically reproduced at more than 0.2 MV/m by introducing the detailed chemical reaction and radiation transport processes into the conventional model. The results revealed that the propagation mechanism of the plasma front in the millimeter-wave discharge changes depending on the incident electric field intensity. At intensities greater than 1.4 MV/m, the plasma front propagated at supersonic speed, while forming a discrete structure, which has intervals of 1/4 wavelength of the millimeter wave. This structure was generated by electron-impact ionization and photoionization processes. At the intermediate intensities, the plasma front propagated continuously rather than discretely because the gas expansion increased the reduced electric field and induced electron-impact ionization. The dominant heating process at the plasma front was fast gas heating. At intensities less than 0.3 MV/m, the plasma front propagated continuously, but the dominant heating process changed to vibrational–translational relaxation. The discharge was maintained by thermal ionization and associative ionization. The simulation results were in good agreement with the past millimeter discharge experiments at this intensity.

Plasma propagation via radiation transfer in millimeter-wave discharge under subcritical condition

An improved model was proposed to reduce a computational cost for subcritical millimeter-wave discharge. The proposed model was able to reproduce the plasma-front propagation via radiation transport as similar to the conventional model, and the plasma-front propagation speed was in agreement with the previous simulation. An electron transport effect by neutral fluid advection, which has been introduced into the conventional model, does not affect the propagation speed. By using the presented model, a computational time was reduced by 35%, which was suitable for a multi-dimensional simulation in the future.

Infrared long nanosecond laser pulse ablation of silicon: Integrated two-dimensional modeling and time-resolved experimental study

Nanosecond (ns) laser ablation may provide good solutions to many industrial silicon micromachining applications. However, most of the previous work is on lasers in ultraviolet (UV) or visible spectral ranges, and ns laser ablation of silicon at the infrared (IR) wavelength has not been well understood, particularly for long ns pulses with durations on the order of ∼100ns. IR ns lasers often have lower costs and less external energy consumption for the same laser energy output than UV or visible lasers, which is desirable for many practical applications. This paper aims to understand the mechanism of IR ns laser ablation of silicon, by combining time-resolved experimental observations with physics-based modeling study. The observation is through a ns-gated intensified charged-coupled devices (ICCD) camera coupled with a microscope tube, while the model is based on two-dimensional (2D) gas dynamic equations for the gaseous phases coupled with the condensed phase heat transfer equation through the Knudsen layer relations. The research shows that the material removal mechanism under the studied laser ablation conditions is surface vaporization in the early stage (yielding a plasma plume above the target), followed by subsequent liquid ejection. The measured plasma front propagation matches reasonably well with the model prediction. The experimentally observed spatial distribution of the plasma radiation intensity is consistent with and has been understood through the model. The study also shows that the observed liquid ejection is induced by the total surface pressure difference between the near-boundary region of the target melt pool and the other remaining region of the pool. The pressure difference is mainly due to the surface vaporization flux drop after laser pulse ends.

Ionization–diffusion plasma front propagation in a microwave field

A 1D model of the expansion of a collisional plasma under the combined effect of diffusion and ionization is presented. It is shown that a simple quasi-neutral model of the plasma using an effective diffusion coefficient can accurately describe the plasma front propagation. The effective diffusion coefficient describes the transition from free electron diffusion in the plasma front to ambipolar diffusion in the bulk. Comparisons with ‘exact’ solutions from a drift-diffusion–Poisson model show excellent agreement not only in the simple case of a constant ionization frequency, but also when the plasma front propagation is due to microwave breakdown. In the latter case the plasma model is solved together with Maxwell's equations and the ionization frequency in the front is modulated in time due to the formation of standing waves in the plasma front region, leading to the formation of plasma patterns. The effect of electron–ion recombination on the observed plasma pattern is discussed.

Time-resolved observation of the plasma induced by laser metal ablation in air at atmospheric pressure

Most of the previous studies on nanosecond (ns) laser-induced plasma typically use relatively short ns laser pulses (pulse duration less than ∼30 to 50 ns). In this paper, relatively long ns laser pulses with 200 ns duration have been used, and the produced plasma during metal ablation in air at atmospheric pressure has been studied through time-resolved observation using an intensified charge-coupled device camera. Due to the much longer ns laser pulse duration, the plasma radiation intensity distribution and the plasma front propagation have different physical features from those produced by much shorter ns laser pulses. In particular, it has been observed that during the laser pulse the plasma has two high-radiation-intensity regions: one is located right above the target surface while the other is behind the expanding plasma front. The former region will disappear once the laser pulse completes. This interesting physical phenomenon has been rarely reported, and requires further experimental and modeling study to completely understand.

Interaction of laser plasmas upon optical breakdown in the normal atmosphere

The plasma front propagation regimes and the spectral characteristics of plasma emission upon laser breakdown in the normal atmosphere are experimentally investigated. The molecular emission of atmospheric gases is recorded at the initial instants of the development of the laser plasma. The behaviour of the intensities of continuous and line emission spectra is investigated upon interaction of counterpropagating plasma fronts; in this case, an increase in the integrated intensity of the plasma emission and a rise in the contrast of emission lines against the background of continuous plasma emission was recorded.

Converting spatial to pseudotemporal resolution in laser plasma analysis by simultaneous multifiber spectroscopy

Traditional chemical analysis based on laser plasma spectroscopy (LPS) requires time-gated detectors, to avoid the initial signal from the hot plasma. These detectors are expensive and often need to be cooled and protected against vapor condensation. We suggest a low-cost setup that may replace these gated detectors, while maintaining acceptable analytical performance. The proposed setup is a result of investigation of plasma-front propagation in LPS analysis. It is known that the LPS plasma propagation is similar to the shock wave propagation after a strong explosion in the atmosphere. We found that the propagation of the plasma fits well the Sedov blast wave theory, providing a good agreement between the theoretical and experimental figures. A proper observation geometry, which is perpendicular to the plasma expansion vector, enables converting spatial to temporal resolution. We take advantage of the fact that the plasma reaches a given distance above the analyzed surface at a certain time delay. Therefore, a single optical fiber, positioned at a well-defined geometry, can provide spectral information corresponding to a certain time delay. A multifiber imaging spectrometer provides information corresponding to a series of delay times, which is adequate for analysis of a variety of matrixes. It was found that the performance of the nongated detector observing a narrow solid angle is similar to that of a gated one observing the whole plasma. For one particular example, observing the plasma from a distance of 4.5 mm is equivalent to a delay of 4 micros and integration time of 2 mircos. The ratio of spectral lines of two elements was investigated using the spatially resolved (nongated) setup, and it was found that this mode is advantageous when internal calibration is applied. It was concluded that sensitive LPS analyses can be carried out by less expensive (nongated) detectors.

Arc discharge sliding over a conducting surface

Results of experimental and theoretical studies of the arc discharge which slides over the surface of a conductor are reported. Experiments were performed in air and argon ambients at various pressures. It is found that the velocity of the discharge plasma front depends linearly on the strength of the average electric field ahead of the front. A physical model of this phenomenon for the range of electric fields near the discharge threshold was developed. It is demonstrated that the mechanism related to thermal conductivity is responsible for the plasma front propagation.

Laser-induced breakdown in large transparent water droplets

Recent experiments on the laser-induced breakdown (LIB) of large transparent liquid droplets are reviewed. A physical model of LIB processes is presented with the aim of integrating the following recent results: (1) the internal and near-field distributions for large transparent spheres; (2) the location of LIB initiation based on spatially resolved plasma emission spectroscopic techniques; (3) spatially resolved but time-averaged density of the plasma plumes and temperature of the atomic species within the plasma; (4) the plasma front propagation velocities inside and outside the droplet; and (5) the fate of the remaining superheated droplet and the expelled material.

Propagation of a laser-supported detonation wave

Experiments were performed in order to determine the aerodynamic values of the gas velocity, pressure and temperature which are produced by a radiation driven wave in air. The wave is initiated on a plane target surface area of 2 cm 2 by the focussed 10.6 μ laser light energy (maximum of 280 joules in 3 μ sec). The resulting flux intensity which was approximately constant during 1 μ sec of the pulse, varied between 10 7 and 1.5 × 10 8 W/cm 2 as related to the emitted laser energy. Comparison of the incident and transmitted pulses permitted the determination of the deposited light energy on the target surface and of the characteristic time for plasma formation. The plasma front propagation velocity was measured by a streak camera. A piezoelectric gage of a relatively small diameter compared to the impact cross-section measured the pressure applied to the target during the interaction. The temperature was deduced from an analysis of the plasma spectra emission which was measured at different distances from the target. From measurements of laser line absorption at 488 nm and 633 nm the plasma density was estimated. The time between the beginning of the laser pulse and the beginning of the front propagation in air was found to be about 100 ns. This time did not seem to depend upon the target material. Considering that the cross-section was sufficiently large, these measurements give the necessary information to illustrate that, for the range of laser flux considered, a one-dimensional model of a detonation wave correctly describes the aerodynamic flow during the time of laser-light energy deposition.