1,382 research outputs found
Some Characteristics of an Effective Communicator in the Light of the New Testament Parable as a Symbol
Ytterbium-doped Fiber-seeded Thin-disk Master Oscillator Power Amplifier Laser System
Lasers which operate at both high average power and energy are in demand for a wide range of applications such as materials processing, directed energy and EUV generation. Presented in this dissertation is a high-power 1 μm ytterbium-based hybrid laser system with temporally tailored pulse shaping capability and up to 62 mJ pulses, with the expectation the system can scale to higher pulse energies. This hybrid system consists of a low power fiber seed and pre-amplifier, and a solid state thin-disk regenerative amplifier. This system has been designed to generate high power temporally tailored pulses on the nanosecond time scale. Temporal tailoring and spectral control are performed in the low power fiber portion of the system with the high pulse energy being generated in the regenerative amplifier. The seed system consists of a 1030 nm fiber-coupled diode, which is transmitted through a Mach-Zehnder-type modulator in order to temporally vary the pulse shape. Typical pulses are 20-30 ns in duration and have energies of ~0.2 nJ from the modulator. These are amplified in a fiber pre-amplifier stage to ~100 nJ before being used to seed the free-space Yb:YAG thin-disk regenerative amplifier. Output pulses have maximum demonstrated pulse energies of 62 mJ with 20 ns pulse after ~250 passes in the cavity. The effects of thermal distortion in laser and passive optical materials are also. Generally the development of high power and high energy lasers is limited by thermal management strategies, as thermally-induced distortions can degrade laser performance and potentially cause catastrophic damage. Novel materials, such as optical ceramics, can be used to mitigate thermal distortions; however, thorough analysis is required to optimize their fabrication and minimize thermal distortions. iv Using a Shack-Hartmann wavefront sensor (SHWFS), it is possible to analyze the distortion induced in passive and doped optical elements by high power lasers. For example, the thin-disk used in the regenerative amplifier is examined in-situ during CW operation (up to 2 kW CW pump power). Additionally, passive oxide-based optical materials and Yb:YAG optical ceramics are also examined by pumping at 2 and 1 μm respectively to induce thermal distortions which are analyzed with the SHWFS. This method has been developed as a diagnostic for the relative assessment of material quality, and to grade differences in ceramic laser materials associated with differences in manufacturing processes and/or the presence of impurities. In summation, this dissertation presents a high energy 1 μm laser system which is novel in its combination of energy level and temporal tailoring, and an analysis of thermal distortions relevant to the development of high power laser systems
Computational study of the mechanism of Bcl-2 apoptotic switch
Programmed cell death - apoptosis is one of the most studied biological
phenomenon of recent years. Apoptotic regulatory network contains several
significant control points, including probably the most important one - Bcl--2
apoptotic switch. There are two proposed hypotheses regarding its internal
working - the indirect activation and direct activation models. Since these
hypotheses form extreme poles of full continuum of intermediate models, we have
constructed more general model with these two models as extreme cases.
By studying relationship between model parameters and steady-state response
ultrasensitivity we have found optimal interaction pattern which reproduces
behavior of Bcl-2 apoptotic switch. Our results show, that stimulus-response
ultrasensitivity is negatively related to spontaneous activation of Bcl-2
effectors - subgroup of Bcl-2 proteins. We found that ultrasensitivity requires
effector's activation, mediated by another subgroup of Bcl-2 proteins -
activators. We have shown that the auto-activation of effectors forms
ultrasensitivity enhancing feedback loop, only if mediated by monomers, but not
by oligomers. Robustness analysis revealed that interaction pattern proposed by
direct activation hypothesis is able to conserve stimulus-response dependence
and preserve ultrasensitivity despite large changes of its internal parameters.
This ability is strongly reduced as for the intermediate to indirect side of
the models.
Computer simulation of the more general model presented here suggest, that
stimulus-response ultrasensitivity is an emergent property of the direct
activation model, that cannot originate within model of indirect activation.
Introduction of indirect-model-specific interactions does not provide better
explanation of Bcl-2 functioning compared to direct model
Low-temperature properties of the heavy-fermion system U Cd
We present electrical-resistivity, magnetic-susceptibility, specific-heat, and thermal-expansion data for UCd11. The low-temperature specific heat indicates that the electronic subsystem has a highly enhanced specific heat which is partially removed by a phase transition at 5.0 K. © 1984 The American Physical Society
3D chaotic model for sub-grid turbulent dispersion in Large Eddy Simulations
We introduce a 3D multiscale kinematic velocity field as a model to simulate
Lagrangian turbulent dispersion. The incompressible velocity field is a
nonlinear deterministic function, periodic in space and time, that generates
chaotic mixing of Lagrangian trajectories. Relative dispersion properties, e.g.
the Richardson's law, are correctly reproduced under two basic conditions: 1)
the velocity amplitudes of the spatial modes must be related to the
corresponding wavelengths through the Kolmogorov scaling; 2) the problem of the
lack of "sweeping effect" of the small eddies by the large eddies, common to
kinematic simulations, has to be taken into account. We show that, as far as
Lagrangian dispersion is concerned, our model can be successfully applied as
additional sub-grid contribution for Large Eddy Simulations of the planetary
boundary layer flow
How does flow in a pipe become turbulent?
The transition to turbulence in pipe flow does not follow the scenario
familiar from Rayleigh-Benard or Taylor-Couette flow since the laminar profile
is stable against infinitesimal perturbations for all Reynolds numbers.
Moreover, even when the flow speed is high enough and the perturbation
sufficiently strong such that turbulent flow is established, it can return to
the laminar state without any indication of the imminent decay. In this
parameter range, the lifetimes of perturbations show a sensitive dependence on
initial conditions and an exponential distribution. The turbulence seems to be
supported by three-dimensional travelling waves which appear transiently in the
flow field. The boundary between laminar and turbulent dynamics is formed by
the stable manifold of an invariant chaotic state. We will also discuss the
relation between observations in short, periodically continued domains, and the
dynamics in fully extended puffs.Comment: for the proceedings of statphys 2
Magnetic Field and Pressure Phase Diagrams of Uranium Heavy-Fermion Compound UZn
We have performed magnetization measurements at high magnetic fields of up to
53 T on single crystals of a uranium heavy-fermion compound UZn
grown by the Bridgman method. In the antiferromagnetic state below the N\'{e}el
temperature = 9.7 K, a metamagnetic transition is found at
32 T for the field along the [110] direction (-axis). The
magnetic phase diagram for the field along the [110] direction is
given. The magnetization curve shows a nonlinear increase at 35
T in the paramagnetic state above up to a characteristic
temperature where the magnetic susceptibility or
electrical resistivity shows a maximum value. This metamagnetic behavior of the
magnetization at is discussed in comparison with the metamagnetic
magnetism of the heavy-fermion superconductors UPt, URuSi, and
UPdAl. We have also carried out high-pressure resistivity measurement
on UZn using a diamond anvil cell up to 8.7 GPa. Noble gas argon was
used as a pressure-transmitting medium to ensure a good hydrostatic
environment. The N\'{e}el temperature is almost
pressure-independent up to 4.7 GPa and starts to increase in the
higher-pressure region. The pressure dependences of the coefficient of the
term in the electrical resistivity , the antiferromagnetic gap
, and the characteristic temperature are
discussed. It is found that the effect of pressure on the electronic states in
UZn is weak compared with those in the other heavy fermion
compounds
The underscreened Kondo effect: a two S=1 impurity model
The underscreened Kondo effect is studied within a model of two impurities
S=1 interacting with the conduction band and via an interimpurity coupling
. Using a mean-field treatment of the bosonized
Hamiltonian, we show that there is no phase transition, but a continuous
cross-over versus K from a non Kondo behaviour to an underscreened Kondo one.
For a small antiferromagnetic coupling (K>0), a completely asymmetric situation
is obtained with one s= component strongly screened by the Kondo effect
and the other one almost free to yield indirect magnetism, which shows finally
a possible coexistence between a RKKY interaction and a local Kondo effect, as
observed in Uranium compounds such as .Comment: 27 pages, RevTeX, to be published in PR
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