Hypergravity influence on gliding arc in noble gases

The effects of increased artificial gravity (in the range 1g-18g) on gliding arcs in four noble gases (He, Ne, Ar, Kr) were the subject of this experimental study

Zwaar stof

Stofdeeltjes zijn zeer ongewenst in industriële plasma’s, zoals toegepast in de halfgeleider- en zonnecelindustrie, maar ook in nucleaire fusiereactoren. Echter, voor het verwerven van fundamentele kennis, zoals bijvoorbeeld het bepalen van elektrische velden in plasma’s, zijn deze deeltjes met een afmeting van slechts enkele micrometers uitermate geschikt. Door deze microdeeltjes op te sluiten in een plasma en ze bloot te stellen aan grote versnellingen in een centrifuge, kunnen bovengenoemde elektrische velden, die van groot belang zijn voor veel plasmatoepassingen, nauwkeurig worden gemeten

Oxygen content dependent etch rate of single polymer microparticles confined in the sheath region of a low pressure radiofrequency argon/oxygen plasma

\u3cp\u3eTo study the etching of polymer microparticles confined in low pressure radiofrequency plasmas, the size and refractive index of single 2 μm particles are experimentally obtained as a function of both time and oxygen content (0%-50%) added to the argon background gas. The etch rate was found to depend heavily on the oxygen (O\u3csub\u3e2\u3c/sub\u3e) content, especially for mixtures with low fractions of O\u3csub\u3e2\u3c/sub\u3e. As expected the etch rate was found to be close to zero in absence of O\u3csub\u3e2\u3c/sub\u3e and increases to a value of 2 nm min\u3csup\u3e-1\u3c/sup\u3e for 0.5% O\u3csub\u3e2\u3c/sub\u3e and to roughly 3.5 nm min\u3csup\u3e-1\u3c/sup\u3e for 5% O\u3csub\u3e2\u3c/sub\u3e. Above 5% O\u3csub\u3e2\u3c/sub\u3e the etch rate saturates. It is shown that these results are consistent with a steady state etch model taking the effects of both atomic oxygen and positive ions into account.\u3c/p\u3

Absolute measurement of the total ion-drag force on a single plasma-confined microparticle at the void edge under microgravity conditions

We present an absolute measurement of the total ion-drag force on one single microparticle at the edge of the dust free region in low pressure complex plasmas: the void. In order to do so, the particle confinement position was monitored as a function of the gas pressure for two particle sizes under normal gravity conditions and under microgravity conditions during parabolic flights. At the border of the void, the ion-drag force on a particle with a radius of 4.90 µm appeared to be (3.6±0.3)×10 -12 N

Conclusive evidence of abrupt coagulation inside the void during cyclic nanoparticle formation in reactive plasma

In this letter, we present scanning electron microscopy (SEM) results that confirm in a direct way our earlier explanation of an abrupt coagulation event as the cause for the void hiccup. In a recent paper, we reported on the fast and interrupted expansion of voids in a reactive dusty argon–acetylene plasma. The voids appeared one after the other, each showing a peculiar, though reproducible, behavior of successive periods of fast expansion, abrupt contraction, and continued expansion. The abrupt contraction was termed “hiccup” and was related to collective coagulation of a new generation of nanoparticles growing in the void using relatively indirect methods: electron density measurements and optical emission spectroscopy. In this letter, we present conclusive evidence using SEM of particles collected at different moments in time spanning several growth cycles, which enables us to follow the nanoparticle formation process in great detail

Temperature dependence of dust particle formation in low pressure argon/methane RF discharge

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Anion dynamics in the first 10 milliseconds of an argon–acetylene radio-frequency plasma

The time evolution of the smallest anions (C2H- and H2CC-), just after plasma ignition, is studied by means of microwave cavity resonance spectroscopy (MCRS) in concert with laser-induced photodetachment under varying gas pressure and temperature in an argon–acetylene radio-frequency (13.56 MHz) plasma. These anions act as an initiator for spontaneous dust particle formation in these plasmas. With an intense 355 nm Nd¿:¿YAG laser pulse directed through the discharge, electrons are detached only from these anions present in the laser path. This results in a sudden increase in the electron density in the plasma, which can accurately and with sub-microsecond time resolution be measured with MCRS. By adjusting the time after plasma ignition at which the laser is fired through the discharge, the time evolution of the anion density can be studied. We have operated in the linear regime: the photodetachment signal is proportional to the laser intensity. This allowed us to study the trends of the photodetachment signal as a function of the operational parameters of the plasma. The density of the smallest anions steadily increases in the first few milliseconds after plasma ignition, after which it reaches a steady state. While keeping the gas density constant, increasing the gas temperature in the range 30–120 °C limits the number of smallest anions and saturates at a temperature of about 90 °C. A reaction pathway is proposed to explain the observed trends

End-bridging monte carlo simulation of bulk and grafted amorphous polyethylene above and below the glass transition

The very efficient end-bridging Monte Carlo (EBMC) method has been employed in order to simulate an amorphous, polydisperse 80-chain large C156 polyethylene (PE) system in atomistic detail over a wide range of temperatures (from 600 down to 150 K) and determine its glass transition temperature (Tg). Two sets of simulations have been performed: one with a bulk, isotropic sample and the other with a thin film in which all the 80 PE chains were grafted on a hard substrate on one side (corresponding to a high grafting density equal to = 1.75 nm-2) and exposed to vacuum on the other side. In the simulations, a united-atom model was employed for PE ensuring that only the purely amorphous phase of PE was simulated at all temperatures. In all cases, very long simulations were carried out in order to give enough time for the system to relax at all length scales. For all temperatures studied, the longest relaxation time was found to be present by descriptors associated with the system's long-range conformational characteristics. In contrast, more local, internal structural features were always faster in equilibrating. As a result, the time autocorrelation function for the chain end-to-end unit vector, fu(t), was found to drop to zero and then clearly fluctuate around this value only for temperatures higher than about 220 K for both systems. For lower temperatures, fu(t) did not relax completely, even after 2 × 107 CPU seconds. Additional volumetric simulation data demonstrated a sharp change in the density and potential energy of both systems in the neighborhood of the 230 K, which are considered as features of the glass transition for amorphous PE. The Tg value suggested by the present EBMC simulations for amorphous (bulk or grafted) PE is (230 ± 10) K, which is consistent with the value of 237 K measured experimentally by Wunderlich [J. of Chem. Phys. 1962, 37, 1203] and Loufakis and Wunderlich [J. Phys. Chem. 1988, 92, 4205] for PE in the limit of zero crystallinity. Further, the predicted change in the heat capacity at constant pressure at the glass transition is cp = 1.2 × 10-4 kcal g-1 K-1, which is very close to the value of 1.5 × 10-4 kcal g-1 K-1 measured experimentally by Wunderlich [J. of Chem. Phys. 1962, 37, 1203]. Additional results on the temperature dependence of the conformational and structural properties in the two PE systems are also reported and discussed in detail. -------------------------------------------------------------------------------