4,658 research outputs found
He-3 cooling systems for space
The development of a space-compatible He(3) refrigerator would provide a significant improvement in several areas of research in the 0.3 to 1 K temperature range. There are several methods of achieving these temperatures on Earth: He(3) refrigeration, dilution refrigeration, and adiabatic demagnetization refrigeration. The progress of adapting He(3) refrigeration for use in space is described. Various cycles and possible embodiments of He(3) refrigerators are described. Also included is an analysis of the liquid confinement and liquid-vapor phase-separation system. A possible configuration is then analyzed. Finally, the results of ground-based experiments will be discussed
Losses in fountain-effect pumps
Three loss mechanisms in fountain-effect pumps are identified and analyzed. Two of these mechanisms reduce the mechano-caloric effect, by reducing the mass transferred per unit heat input. The first loss mechanism of these losses is the result of normal fluid leakage through the pump. The second loss mechanism is the result of the finite separation between the porous plug and the heater. The third loss mechanism reduces the thermomechanical effect. It results from the Gorter-Mellink interaction within the porous plug. All three of these loss mechanisms are shown to reduce pump efficiency. They are then applied to an example to illustrate the relative significance of the three mechanisms
Passive storage technologies
Advances in storage technology and how passive techniques could be applied to the storage of propellants at the space station are described. The devices considered are passive orbital disconnect struts, cooled shield optimization, liftweight shields and catalytic converters
Design considerations for a HE-3 refrigerator for space applications
The low temperature provided by He-3 refrigerators (0.3 to 3 K) have useful space applications. However, the low temperatures and the low surface tension of He-3 require special design considerations. The considerations include the need for small pores to contain the liquid in a matrix; the effects of bubble nucleation and growth; and the effects of the thermal conductivity within the matrix. These design considerations are discussed and a possible confinement system is analyzed
Superfluid Helium On-Orbit Transfer (SHOOT) operatons
The in-flight tests and the operational sequences of the Superfluid Helium On-Orbit Transfer (SHOOT) experiment are outlined. These tests include the transfer of superfluid helium at a variety of rates, the transfer into cold and warm receivers, the operation of an extravehicular activity coupling, and tests of a liquid acquisition device. A variety of different types of instrumentation will be required for these tests. These include pressure sensors and liquid flow meters that must operate in liquid helium, accurate thermometry, two types of quantity gauges, and liquid-vapor sensors
Cold atom dynamics in a quantum optical lattice potential
We study a generalized cold atom Bose Hubbard model, where the periodic
optical potential is formed by a cavity field with quantum properties. On the
one hand the common coupling of all atoms to the same mode introduces cavity
mediated long range atom-atom interactions and on the other hand atomic
backaction on the field introduces atom-field entanglement. This modifies the
properties of the associated quantum phase transitions and allows for new
correlated atom-field states including superposition of different atomic
quantum phases. After deriving an approximative Hamiltonian including the new
long range interaction terms we exhibit central physical phenomena at generic
configurations of few atoms in few wells. We find strong modifications of
population fluctuations and next-nearest neighbor correlations near the phase
transition point.Comment: 4 pages, 5 figures, corrected typo
Dynamical instability and loss of p-band bosons in optical lattices
We study how the bosonic atoms on the excited p-band of an optical lattice
are coupled to the lowest s-band and the 2nd excited d-band. We find that in
some parameter regimes the atom-atom interactions can cause a dynamical
instability of the p-band atoms towards decay to the s- and d-bands.
Furthermore, even when dynamical instability is not expected s- and d-bands can
become substantially populated.Comment: 7 figures, minor changes to the earlier versio
Formation and Stability of Cellular Carbon Foam Structures:An {\em Ab Initio} Study
We use ab initio density functional calculations to study the formation and
structural as well as thermal stability of cellular foam-like carbon
nanostructures. These systems with a mixed bonding character may be
viewed as bundles of carbon nanotubes fused to a rigid contiguous 3D honeycomb
structure that can be compressed more easily by reducing the symmetry of the
honeycombs. The foam may accommodate the same type of defects as graphene, and
its surface may be be stabilized by terminating caps. We postulate that the
foam may form under non-equilibrium conditions near grain boundaries of a
carbon-saturated metal surface
Dirac electrons in a Kronig-Penney potential: dispersion relation and transmission periodic in the strength of the barriers
The transmission T and conductance G through one or multiple one-dimensional,
delta-function barriers of two-dimensional fermions with a linear energy
spectrum are studied. T and G are periodic functions of the strength P of the
delta-function barrier V(x,y) / hbar v_F = P delta(x). The dispersion relation
of a Kronig-Penney (KP) model of a superlattice is also a periodic function of
P and causes collimation of an incident electron beam for P = 2 pi n and n
integer. For a KP superlattice with alternating sign of the height of the
barriers the Dirac point becomes a Dirac line for P = (n + 1/2) pi.Comment: 5 pages, 6 figure
Number statistics of molecules formed from ultra-cold atoms
We calculate the number statistics of a single-mode molecular field excited
by photoassociation or via a Feshbach resonance from an atomic Bose-Einstein
condensate (BEC), a normal atomic Fermi gas and a Fermi system with pair
correlations (BCS state). We find that the molecule formation from a BEC is a
collective process that leads for short times to a coherent molecular state in
the quantum optical sense. Atoms in a normal Fermi gas, on the other hand, are
converted into molecules independently of each other and result for short times
in a molecular state analogous to that of a classical chaotic light source. The
BCS situation is intermediate between the two and goes from producing an
incoherent to a coherent molecular field with increasing gap parameter.Comment: 5 pages, 4 figure
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