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[en] The low pressure gas breakdown described by Paschen's law in Townsend theory, i.e. the breakdown voltage as a function of gas pressure p and the electrode distance d, provides an accurate description of breakdown in DC discharges when the ratio between inter-electrode gap distance d and electrode radii R tends to zero. On increasing of the ratio d/R, the Paschen's curves are shifted to the region of higher breakdown voltage and higher pd values. A modified Paschen's law recently proposed is well satisfied in our measurements. However, the value of constant b changes not only due to gas type but also according to electrode gap distance; furthermore, gas breakdown voltages are considerably modified by plasma-wall interactions due to glass tube proximity in the discharge.
[en] Because of their capabilities to generate plasmas that are not confined between electrodes, low-temperature plasma jets offer unique opportunities for applications such as material processing and biomedicine. The need to generate multiple jets in order to cover larger treatment areas has recently become desirable. However, the interaction between neighbouring jets is unavoidable. It is therefore crucial to elucidate the physical processes that occur between jets. In this paper we present the case of two counter-propagating jets generated by two DBD-based devices. We show that the plasma bullets emitted by the two jets interact with each other as soon as they leave their respective DBD device, resulting in a decrease in their velocities. The bullets do not actually meet but rather approach each other at a minimum approach distance. The location of the region of minimum approach is not midway between the nozzles of the jet devices but rather depends on the operating conditions. In addition, we discovered the emergence of a ‘secondary’ discharge exactly in the region of minimum approach. This discharge exhibits a pink glow, reminiscent of the pink afterglow occurring in some nitrogen discharges. Time-resolved spectroscopic measurements and current measurement analysis showed that the pink glow is a transient negative glow discharge that cannot be attributed to kinetic processes associated with re-excitation of nitrogen molecules. It is rather ignited by electrons accelerated from both jets towards the region of minimum approach. This process is found to be exactly timed with the measured current reversal. (paper)
[en] Electrostatic particle-in-cell simulations of a Penning discharge are performed in order to investigate azimuthally asymmetric, spoke-like structures previously observed in experiments. Two-dimensional simulations show that for Penning-discharge conditions, a persistent nonlinear spoke-like structure forms readily and rotates in the direction of E × B and electron diamagnetic drifts. The azimuthal velocity is within about a factor of 2 of the ion acoustic speed. The spoke frequency follows the experimentally observed scaling with ion mass, which indicates the importance of ion inertia in spoke formation. The spoke provides enhanced (anomalous) radial electron transport, and the effective cross-field conductivity is several times larger than the classical (collisional) value. The level of anomalous current obtained in the simulations is in good agreement with the experimental data. Furthermore, the rotating spoke channels most of the radial current, observable by an edge probe as short pulses.
[en] An in-line sensor has been constructed with 50 Ω characteristic impedance to accurately measure rf power dissipated in a matched or unmatched load with a view to being implemented as a rf discharge diagnostic. The physical construction and calibration technique are presented. The design is a wide band, hybrid directional coupler/current-voltage sensor suitable for fundamental and harmonic power measurements. A comparison with a standard wattmeter using dummy load impedances shows that this in-line sensor is significantly more accurate in mismatched conditions
[en] By employing a self-consistent kinetic approach, an analytical expression is derived for the potential of a test charge in a weakly ionized plasma with ion drift. The drift is assumed to be due to an external electric field, with the velocity being mobility-limited and much larger than the thermal velocity of neutrals. The derived expression is proven to be in excellent agreement with the measurements by Konopka et al. [Phys. Rev. Lett. 84, 891 (2000)] performed in the sheath region of a rf discharge
[en] The microscopic channeling dynamics of projectiles in subexcitable chain bundle dusty plasma liquids consisting of long chains of negatively charged dusts suspended in low pressure glow discharges is investigated experimentally using fast video-microscopy. The long distance channeling of the projectile in the channel formed by the surrounding dust chain bundles and the excitation of a narrow wake associated with the elliptical motions of the background dusts are demonstrated. In the high projectile speed regime, the drag force due to wake wave excitation increases with the decreasing projectile speed. The excited wave then leads the slowed down projectile after the projectile speed is decreased below the resonant speed of wave excitation. The wave-projectile interaction causes the increasing projectile drag below the resonant speed and the subsequent oscillation around a descending average level, until the projectile settles down to the equilibrium point. Long distance projectile surfing through the resonant crest trapping by the externally excited large amplitude solitary wave is also demonstrated.
[en] Full text: Until recently in the rapidly expanding area of discharge processing the emphasis has been on developing improved or new processes with less attention to the characteristics of the plasma itself. In part this was caused by the difficulty of understanding rf plasmas in contrast to the extensively studied low pressure DC discharges. Another aspect different classical low pressure discharges is the use of strongly attaching gases in processing plasmas. The need to better characterize the processing plasma and to acquire a predictive capability through modeling based on measurements reproducible at different laboratories was recently recognized. A standardized discharge configuration and electrical measurement methods which properly consider the influence of stray capacitances and inductances were developed. Based on these methods, we measured discharges characteristics in argon, helium and nitrogen. In addition, using a microwave interferometer, the line integrated electron densities were measured for these discharges as a function of power and pressure. Information on plasma loss mechanisms and ion species can be deduced from these measurements. The discharge also was operated impulsed mode. The DC bias was found to reach final values at times much later than would be expected from circuit time constants. These time constants are important if negative ions and dust precursors are to be eliminated by pulsed operation. To obtain reduced DC bias, increase plasma density and operation at lower pressure, one can apply a magnetic field of a few hundred Gauss parallel to the electrodes. Measurements in argon, C F4 and S F6 in a small parallel plate discharge showed that the effect to the Β-field depend on the attachment coefficient of the gas which determines the width of the electrode sheaths. In S F6 for example, the sheath width a higher pressure is of the order of the gyro radius and the Β-field has little influence. To separate the plasma generation from the substrate treatment area and to operate at lower pressure and with controlled ion energy, different discharge configurations using excitation methods, such as E C R, helicon waves, inductive or helical discharges have been used. Studies in our laboratory using inductive discharges and an electron energy analyzer showed that at the lower pressure with possible these discharge configurations, the percentage of high energy electrons increases considerably. This may have implications for generating defects in the substrate. Investing discharges with helical resonators, we measured electron densities comparable or exceeding those in parallel plate discharges at similar average power densities. (author)