80 research outputs found
The penetration of plasma clouds across magnetic boundaries : the role of high frequency oscillations
Experiments are reported where a collisionfree plasma cloud penetrates a
magnetic barrier by self-polarization. We here focus on the resulting anomalous
magnetic field diffusion into the plasma cloud, two orders of magnitude faster
than classical, which is one important aspect of the plasma cloud penetration
mechanism. Without such fast magnetic diffusion, clouds with kinetic beta below
unity would not be able to penetrate magnetic barriers at all. Tailor-made
diagnostics has been used for measurements in the parameter range with the
kinetic beta ? 0.5 to 10, and with normalized width w/r(gi) of the order of
unity. Experimental data on hf fluctuations in density and in electric field
has been combined to yield the effective anomalous transverse resistivity
eta(EFF). It is concluded that they are both dominated by highly nonlinear
oscillations in the lower hybrid range, driven by a strong diamagnetic current
loop that is set up in the plasma in the penetration process. The anomalous
magnetic diffusion rate, calculated from the resistivity eta(EFF), is
consistent with single-shot multi-probe array measurements of the diamagnetic
cavity and the associated quasi-dc electric structure. An interpretation of the
instability measurements in terms of the resistive term in the generalized (low
frequency) Ohm's law is given.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
Conditions for plasmoid penetration across magnetic barriers
The penetration of plasma clouds, or plasmoids, across abrupt magnetic
barriers (of the scale less than a few ion gyro radii, using the plasmoid
directed velocity) is studied. The insight gained earlier, from experimental
and computer simulation investigations of a case study, is generalised into
other parameter regimes. It is concluded for what parameters a plasmoid should
be expected to penetrate the magnetic barrier through self-polarization,
penetrate through magnetic expulsion, or be rejected from the barrier. The
scaling parameters are n(e), v(0), B(perp), m(i), T(i), and the width w of the
plasmoid. The scaling is based on a model for strongly driven, nonlinear
magnetic field diffusion into a plasma, which is a generalization of the
laboratory findings. The results are applied to experiments earlier reported in
the literature, and also to the proposed application of impulsive penetration
of plasmoids from the solar wind into the Earth's magnetosphere.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
Nucleation of titanium nanoparticles in an oxygen-starved environment, I: Experiments
A constant supply of oxygen has been assumed to be necessary for the growth
of titanium nanoparticles by sputtering. This oxygen supply can arise from a
high background pressure in the vacuum system or from a purposely supplied gas.
The supply of oxygen makes it difficult to grow metallic nanoparticles of
titanium and can cause process problems by reacting with the target. We here
report that growth of titanium nanoparticles in the metallic hexagonal titanium
({\alpha}Ti) phase is possible using a pulsed hollow cathode sputter plasma and
adding a high partial pressure of helium to the process instead of trace
amounts of oxygen. The helium cools the process gas in which the nanoparticles
nucleate. This is important both for the first dimer formation and the
continued growth to a thermodynamically stable size. The parameter region where
the synthesis of nanoparticles is possible is mapped out experimentally and the
theory of the physical processes behind this process window is outlined. A
pressure limit below which no nanoparticles were produced was found at 200 Pa,
and could be attributed to a low dimer formation rate, mainly caused by a more
rapid dilution of the growth material. Nanoparticle production also disappeared
at argon gas flows above 25 sccm. In this case the main reason was identified
as a gas temperature increase within the nucleation zone, giving a too high
evaporation rate from nanoparticles (clusters) in the stage of growth from
dimers to stable nuclei. These two mechanisms are in depth explored in a
companion paper [1]. A process stability limit was also found at low argon gas
partial pressures, and could be attributed to a transition from a hollow
cathode discharge to a glow discharge.Comment: 22 pages, 11 figure
Сравнительная характеристика методов оценки знаний у студентов I курса фармацевтического факультета
МЕДИЦИНСКИЕ ИНСТИТУТЫОБРАЗОВАНИЕ, СИСТЕМА ОЦЕНОКСТУДЕНТЫфармацевтический факульте
Nucleation of titanium nanoparticles in an oxygen-starved environment, II: Theory
The nucleation and growth of pure titanium nanoparticles in a low-pressure
sputter plasma has been believed to be essentially impossible. The addition of
impurities, such as oxygen or water, facilitates this and allows the growth of
nanoparticles. However, it seems that this route requires so high oxygen
densities that metallic nanoparticles in the hexagonal aTi-phase cannot be
synthesized. Here we present a model which explains results for the nucleation
and growth of titanium nanoparticles in the absent of reactive impurities. In
these experiments, a high partial pressure of helium gas was added which
increased the cooling rate of the process gas in the region where nucleation
occurred. This is important for two reasons. First, a reduced gas temperature
enhances Ti2 dimer formation mainly because a lower gas temperature gives a
higher gas density, which reduces the dilution of the Ti vapor through
diffusion. The same effect can be achieved by increasing the gas pressure.
Second, a reduced gas temperature has a "more than exponential" effect in
lowering the rate of atom evaporation from the nanoparticles during their
growth from a dimer to size where they are thermodynamically stable, r*. We
show that this early stage evaporation is not possible to model as a
thermodynamical equilibrium. Instead, the single-event nature of the
evaporation process has to be considered. This leads, counter intuitively, to
an evaporation probability from nanoparticles that is exactly zero below a
critical nanoparticle temperature that is size-dependent. Together, the
mechanisms described above explain two experimentally found limits for
nucleation in an oxygen-free environment. First, there is a lower limit to the
pressure for dimer formation. Second, there is an upper limit to the gas
temperature above which evaporation makes the further growth to stable nuclei
impossible.Comment: 32 pages, 7 figure
Transition between the discharge regimes of high power impulse magnetron sputtering and conventional direct current magnetron sputtering
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