44,370 research outputs found
Instability of fixed, low-thrust drag compensation
FORCED drag compensation using continuous low-thrustpropulsion has been considered for satellites in low Earth orbit. This simple, but nonoptimal, scheme merely requires that the thrust vector is directed opposite to the drag vector and that the magnitude of the two are equal. In principle, the drag force acting on the spacecraft could be determined onboard using accurate accelerometers. However, for small, low-cost spacecraft such sensors may beunavailable. An alternative strategy would be to Ĺ˝ x the thrust magnitude equal to the expected air drag that would be experienced by the spacecraft. The thrust levelwould be periodically updated based on ground-based orbit determination. In this Engineering Note, it is shown that such a forced circular orbit with a Ĺ˝ fixed thrust levelis exponentially unstable for all physically reasonable atmosphere models
Interesting thermomagnetic history effects in the antiferromagnetic state of SmMn_2Ge_2
We present results of magnetization measurements showing that the magnetic
response of the antiferromagnetic state of SmMn_2Ge_2 depends on the path used
in the field(H)-temperature(T) phase space to reach this state. Distinct
signature of metastablity is observed in this antiferromagnetic state when
obtained via field-cooling/field-warming paths. The isothermal M-H loops show
lack of end-point memory, reminiscent of that seen in metastable vortex states
near the field-induced first order phase transition in various type-II
superconductors.Comment: 11 pages of text and 3 figure
Diffuse-interface model for nanopatterning induced by self-sustained ion etch masking
We construct a simple phenomenological diffuse-interface model for
composition-induced nanopatterning during ion sputtering of alloys. In
simulations, this model reproduces without difficulties the high-aspect ratio
structures and tilted pillars observed in experiments. We investigate the time
evolution of the pillar height, both by simulations and by {\it in situ}
ellipsometry. The analysis of the simulation results yields a good
understanding of the transitions between different growth regimes and supports
the role of segregation in the pattern-formation process.Comment: 10 pages, 3 figures; minor revisions with respect to first version;
figures nicened; journal ref. adde
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