210 research outputs found
Design of a Magnetic Field Mapping Rover System for a Neutron Lifetime Experiment
The beta decay lifetime of the free neutron is an important input to the Standard Model of particle physics, but values measured using different methods have exhibited substantial disagreement. The UCNtau experiment in development at Los Alamos National Laboratory (LANL) plans to explore better methods of measuring the neutron lifetime using ultracold neutrons (UCNs). In this experiment, UCNs are confined in a magneto-gravitational trap formed by a curved, asymmetric Halbach array placed inside a vacuum vessel and surrounded by holding field coils. If any defects present in the Halbach array are sufficient to reduce the local field near the surface below that needed to repel the desired energy level UCNs, loss by material interaction can occur at a rate similar to the loss by beta decay. A map of the magnetic field near the surface of the array is necessary to identify any such defects, but the array\u27s curved geometry and placement in a vacuum vessel make conventional field mapping methods difficult. A system consisting of computer vision-based tracking and a rover holding a Hall probe has been designed to map the field near the surface of the array, and construction of an initial prototype has begun at LANL. A description of the design and prototype will be presented
Compatibility of Photomultiplier Tube Operation with SQUIDs for a Neutron EDM Experiment
An experiment at the Spallation Neutron Source at Oak Ridge National Laboratory with the goal of reducing the experimental limit on the electric dipole moment (EDM) of the neutron will measure the precession frequencies of neutrons when a strong electric field is applied parallel and antiparallel to a weak magnetic field. A difference in these frequencies would indicate a nonzero neutron EDM. To correct for drifts of the magnetic field in the measurement volume, polarized Helium-3 will be used as a comagnetometer. In one of the two methods built into the apparatus, superconducting quantum interference devices (SQUIDs) will be used to read out Helium-3 magnetization. Photomultiplier tubes (PMTs) will be used concurrently to measure scintillation light from neutron capture by Helium-3. However, the simultaneous noise-sensitive magnetic field measurement by the SQUIDs makes conventional PMT operation problematic due to the alternating current involved in generating the high voltages needed. Tests were done at Los Alamos National Laboratory to study the compatibility of simultaneous SQUID and PMT operation, using a custom battery-powered high-voltage power supply developed by H.O. Meyer and P.T. Smith to operate the PMT. The results of these tests will be presented
Remnants from Gamma-Ray Bursts
We model the intermediate time evolution of a "jetted" gamma-ray burst by two
blobs of matter colliding with the interstellar medium. We follow the
hydrodynamical evolution of this system numerically and calculate the
bremsstrahlung and synchrotron images of the remnant. We find that for a burst
energy of erg the remnant becomes spherical after years
when it collects of interstellar mass. This result is
independent of the exact details of the GRB, such as the opening angle. After
this time a gamma-ray burst remnant has an expanding sphere morphology. The
similarity to a supernova remnant makes it difficult distinguish between the
two at this stage. The expected number of non-spherical gamma-ray burst
remnants is per galaxy for a beaming factor of 0.01 and a burst
energy of erg. Our results suggest that that the double-shell object
DEM L 316 is not a GRB remnant.Comment: 16 pages, 9 figures, Substantial revisions, Accepted by Ap
Simulating hypervelocity impact effects on structures using the smoothed particle hydrodynamics code MAGI
Analysis of interaction occurring between space debris and orbiting structures is of great interest to the planning and survivability of space assets. Computer simulation of the impact events using hydrodynamic codes can provide some understanding of the processes but the problems involved with this fundamental approach are formidable. First, any realistic simulation is necessarily three-dimensional, e.g., the impact and breakup of a satellite. Second, the thickness of important components such as satellite skins or bumper shields are small with respect to the dimension of the structure as a whole, presenting severe zoning problems for codes. Thirdly, the debris cloud produced by the primary impact will yield many secondary impacts which will contribute to the damage and possible breakup of the structure. The problem was approached by choosing a relatively new computational technique that has virtues peculiar to space impacts. The method is called Smoothed Particle Hydrodynamics
Design of a loop-gap resonator with bimodal uniform fields using finite element analysis
The loop-gap resonator (LGR) was originally developed to provide a uniform microwave magnetic field on a sample for electron spin resonance (ESR) experiments. The LGR is composed of one or more loops and gaps acting as inductances and capacitances respectively. Typical LGR designs produce a uniform field on a sample at a single resonant frequency, but for certain experiments it is necessary to study the response of a material to uniform fields at multiple frequencies applied simultaneously. In this work we develop an empirical design procedure using finite element method calculations to design an asymmetric loop-gap resonator with uniform fields at two frequencies in the same sample volume and analyze the field uniformity, frequency tunability and filling factors, providing comparison to a manufactured device
Theory of magnon-polaritons in quantum Ising materials
We present a theory of magnon-polaritons in quantum Ising materials, and
develop a formalism describing the coupling between light and matter as an
Ising system is tuned through its quantum critical point. The theory is applied
to Ising materials having multilevel single-site Hamiltonians, in which
multiple magnon modes are present, such as the insulating Ising magnet
LiHoF . We find that the magnon-photon coupling strengths may be tuned by
the applied transverse field, with the coupling between the soft mode present
in the quantum Ising material and a photonic resonator mode diverging at the
quantum critical point of the material. A fixed system of spins will not
exhibit the diamagnetic response expected when light is coupled to mobile spins
or atoms. Without the diamagnetic response, one expects a divergent
magnon-photon coupling strength to lead to a superradiant quantum phase
transition. However, this neglects the effects of damping and decoherence
present in any real system. We show that damping and decoherence may block the
superradiant quantum phase transition, and lead to weak coupling between the
soft magnon mode and the resonator mode. The results of the theory are applied
to experimental data on the model system LiHoF in a microwave resonator.Comment: 22 pages, 6 figure
Numerical Simulations of Highly Porous Dust Aggregates in the Low-Velocity Collision Regime
A highly favoured mechanism of planetesimal formation is collisional growth.
Single dust grains, which follow gas flows in the protoplanetary disc, hit each
other, stick due to van der Waals forces and form fluffy aggregates up to
centimetre size. The mechanism of further growth is unclear since the outcome
of aggregate collisions in the relevant velocity and size regime cannot be
investigated in the laboratory under protoplanetary disc conditions. Realistic
statistics of the result of dust aggregate collisions beyond decimetre size is
missing for a deeper understanding of planetary growth. Joining experimental
and numerical efforts we want to calibrate and validate a computer program that
is capable of a correct simulation of the macroscopic behaviour of highly
porous dust aggregates. After testing its numerical limitations thoroughly we
will check the program especially for a realistic reproduction of various
benchmark experiments. We adopt the smooth particle hydrodynamics (SPH)
numerical scheme with extensions for the simulation of solid bodies and a
modified version of the Sirono porosity model. Experimentally measured
macroscopic material properties of silica dust are implemented. We calibrate
and test for the compressive strength relation and the bulk modulus. SPH has
already proven to be a suitable tool to simulate collisions at rather high
velocities. In this work we demonstrate that its area of application can not
only be extended to low-velocity experiments and collisions. It can also be
used to simulate the behaviour of highly porous objects in this velocity regime
to a very high accuracy.The result of the calibration process in this work is
an SPH code that can be utilised to investigate the collisional outcome of
porous dust in the low-velocity regime.Comment: accepted by Astronomy & Astrophysic
Collisions between equal sized ice grain agglomerates
Following the recent insight in the material structure of comets,
protoplanetesimals are assumed to have low densities and to be highly porous
agglomerates. It is still unclear if planetesimals can be formed from these
objects by collisional growth. Therefore, it is important to study numerically
the collisional outcome from low velocity impacts of equal sized porous
agglomerates which are too large to be examined in a laboratory experiment. We
use the Lagrangian particle method Smooth Particle Hydrodynamics to solve the
equations that describe the dynamics of elastic and plastic bodies.
Additionally, to account for the influence of porosity, we follow a previous
developed equation of state and certain relations between the material strength
and the relative density. Collisional growth seems possible for rather low
collision velocities and particular material strengths. The remnants of
collisions with impact parameters that are larger than 50% of the radius of the
colliding objects tend to rotate. For small impact parameters, the colliding
objects are effectively slowed down without a prominent compaction of the
porous structure, which probably increases the possibility for growth. The
protoplanetesimals, however, do not stick together for the most part of the
employed material strengths. An important issue in subsequent studies has to be
the influence of rotation to collisional growth. Moreover, for realistic
simulations of protoplanetesimals it is crucial to know the correct material
parameters in more detail.Comment: 7 pages, 11 figures, accepted by A&
Updated Smoothed Particle Hydrodynamics for Simulating Bending and Compression Failure Progress of Ice
In this paper, an updated Smoothed Particle Hydrodynamics (SPH) method based on the Simplified Finite Difference Interpolation scheme (SPH_SFDI) is presented to simulate the failure process of ice. The Drucker–Prager model is embedded into the SPH code to simulate the four point bending and uniaxial compression failure of ice. The cohesion softening elastic–plastic model is also used in the SPH_SFDI framework. To validate the proposed modeling approach, the numerical results of SPH_SFDI are compared with the standard SPH and the experimental data. The good agreement demonstrated that the proposed SPH_SFDI method including the elastic–plastic cohesion softening Drucker–Prager failure model can provide a useful numerical tool for simulating failure progress of the ice in practical field. It is also shown that the SPH_SFDI can significantly improve the capability and accuracy for simulating ice bending and compression failures as compared with the original SPH scheme
Collisions of inhomogeneous pre-planetesimals
In the framework of the coagulation scenario, kilometre-sized planetesimals
form by subsequent collisions of pre-planetesimals of sizes from centimetre to
hundreds of metres. Pre-planetesimals are fluffy, porous dust aggregates, which
are inhomogeneous owing to their collisional history. Planetesimal growth can
be prevented by catastrophic disruption in pre-planetesimal collisions above
the destruction velocity threshold. We develop an inhomogeneity model based on
the density distribution of dust aggregates, which is assumed to be a Gaussian
distribution with a well-defined standard deviation. As a second input
parameter, we consider the typical size of an inhomogeneous clump. These input
parameters are easily accessible by laboratory experiments. For the simulation
of the dust aggregates, we utilise a smoothed particle hydrodynamics (SPH) code
with extensions for modelling porous solid bodies. The porosity model was
previously calibrated for the simulation of silica dust, which commonly serves
as an analogue for pre-planetesimal material. The inhomogeneity is imposed as
an initial condition on the SPH particle distribution. We carry out collisions
of centimetre-sized dust aggregates of intermediate porosity. We vary the
standard deviation of the inhomogeneous distribution at fixed typical clump
size. The collision outcome is categorised according to the four-population
model. We show that inhomogeneous pre-planetesimals are more prone to
destruction than homogeneous aggregates. Even slight inhomogeneities can lower
the threshold for catastrophic disruption. For a fixed collision velocity, the
sizes of the fragments decrease with increasing inhomogeneity.
Pre-planetesimals with an active collisional history tend to be weaker. This is
a possible obstacle to collisional growth and needs to be taken into account in
future studies of the coagulation scenario.Comment: 12 pages, 9 figures, 4 table
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