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[en] Laboratory experiments with thermophoretic levitation of dust are described that aim at the closure of a central dust-free void region. A careful study of the void structure as a function of the discharge and levitation parameters leads to the discovery of an extended parameter region where stable void-free equilibria are found. The void closure is effected by a novel mechanism that involves a self-organized change in the discharge topology, in which the dust cloud becomes surrounded by a toroidal region of plasma production. In this geometry ions are found to stream radially inwards instead of outwards as in clouds with a central void. This change in ion flow is proved by a reversal of the propagation direction of dust-density waves.
[en] A device generating microwave discharges in nitrogen or air is depicted. A spectroscopic study of the N2 discharge has been made for a range of pressures from 10 to 650 torr and UHF power from 130 to 420 watts. The vibrational excitation as measured from the N2 positive second system is high corresponding to a vibrational temperature Tsub(v,x) from 6 to 10x103 K. The comparison with the ionic state as measured from the N2+ negative first system shows that the ion N2+ excitation is predominant for high pressures and high power levels
[fr]Un dispositif permettant de realiser des decharges de microondes (2,450 MHz) dans l'azote et dans l'air a pression elevee est presente. L'etude spectroscopique de la decharge dans l'azote est faite entre 10 et 650 torr et pour des puissances UHF allant de 130 a 420 watts. L'excitation vibrationnelle, determinee a partir du second systeme positif de N2, est forte correspondant a des temperatures vibrationnelles Tsub(v,x) de 6 a 10x103 K. La comparaison avec l'etat ionique, determinee a partir du premier systeme negatif de N2+ montre que l'excitation de l'ion N2+ est predominante aux fortes pressions et fortes puissances
[en] The symmetry of capacitively coupled radio frequency (CCRF) discharges can be controlled electrically by applying a fundamental frequency and its second harmonic with fixed but adjustable phase shift θ between the driving voltages to one electrode. In such a discharge a variable dc self-bias η is generated as an almost linear function of θ for 00 ≤ θ ≤ 900 via the Electrical Asymmetry Effect. The control parameter for η and the discharge symmetry is θ. Here electron dynamics in electrically asymmetric geometrically symmetric dual frequency discharges operated in argon at 13.56 and 27.12 MHz is investigated experimentally by a particle-in-cell simulation and by an analytical model. The electron dynamics is probed by the electron impact excitation rate of energetic electrons from the ground state into highly excited levels. At high pressures (collisional sheaths) the excitation dynamics is found to work differently compared with conventional CCRF discharges. Unlike in classical discharges the maxima of the time modulated excitation at the powered and grounded electrode within one low frequency period will be similar (symmetric excitation), if η is strong at θ ∼ 00, 900, and significantly different (asymmetric excitation), if η ∼ 0 V at θ ∼ 450. At low pressures (collisionless sheaths) the excitation dynamics works similar to classical discharges, i.e. the excitation will be asymmetric, if η is strong, and symmetric, if η ∼ 0 V. This dynamics is understood in the frame of an analytical model, which provides a more detailed insight into electron heating in CCRF discharges and could be applied to other types of capacitive RF discharges as well.
[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)
[en] The destruction of CH+ ions in collisions with H atoms has been studied in a temperature-variable 22 pole ion trap (22PT) combined with a cold effusive H-atom beam. The stored ions are relaxed to temperatures of T22PT ≥ 12 K. The hydrogen atoms, produced in a radio frequency discharge, are slowed down to various temperatures of TACC ≥ 7 K. They are formed into an effusive beam. The effective density of the hydrogen atoms in the trap as well as the H2 background are determined in situ using chemical probing with CO2+. The experimental arrangement allows us not only to measure thermal rate coefficients (T22PT = TACC), but also to extract state-specific rate coefficients k(J,Tt) at selected translational temperatures Tt and for the CH+ rotational states J = 0, 1, and 2. The measured thermal rate coefficients have a maximum at 60 K, k = (1.2 ± 0.5)x10-9 cm3 s-1. Toward higher temperatures, they fall off in accordance with previous measurements and the trend predicted by phase space theory. Toward lower temperatures, the rate coefficients decrease significantly, especially if the rotation of the ions is cooled. At the coldest conditions achieved (beam: 7.3 K; trap: 12.2 K), a value as low as (5 ± 4) x 10-11 cm3 s-1 has been measured. This leads to the conclusion that non-rotating CH+ is protected against attacks of H atoms. This surprising result is not yet understood. It is most probably due to quantum-dynamical effects already occurring at large distances.
[en] The hydrogen atomic beam for a maser was realized with a U.H.F. discharge produced between the plates of the capacitor of a capacitively loaded coaxial cavity at 400 MHz. The degree of dissociation of hydrogen molecules in the discharge and the atomic flux through the collimator of the source were estimated