123,813 research outputs found
Laser anemometry techniques for turbine applications
Laser anemometry offers a nonintrusive means for obtaining flow field information. Current research at NASA Lewis Research Center is focused on instrumenting a warm turbine facility with a laser anemometer system. In an effort to determine the laser anemometer system best qualified for the warm turbine environment, the performance of a conventional laser fringe anemometer and a two spot time of flight system were compared with a new, modified time of flight system, called a Four Spot laser anemometer. The comparison measurements were made in highly turbulent flows near walls. The Four Spot anemometer uses elliptical spots to increase the flow acceptance angle to be comparable to that of a Laser Fringe Anemometer. Also, the Four Spot uses an optical code that vastly simplifies the pulse detection processor. The results of the comparison measurements will exemplify which laser anemometer system is best suited to the hostile environment typically encountered in warm rotating turbomachinery
A Remote Laser Mass Spectrometer for Lunar Resource Assessment
The use of lasers as a source of excitation for surface mass spectroscopy has been investigated for some time. Since the laser can be focused to a small spot with intensity, it can vaporize and accelerate atoms of material. Using this phenomenon with a time-of-flight mass spectrometer allows a surface elemental mass analysis of a small region with each laser pulse. While the technique has been well developed for Earth applications, space applications are less developed. NASA Langley recently began a research program to investigate the use of a laser to create ions from the lunar surface and to analyze the ions at an orbiting spacecraft. A multijoule, Q-switched Nd:YAG laser would be focused to a small spot on the lunar surface, creating a dense plasma. This plasma would eject high-energy ions, as well as neutrals, electrons, and photons. An experiment is being set up to determine the characteristics of such a laser mass spectrometer at long flight distances. This experiment will determine the character of a future flight instrument for lunar resource assessment
Single-Mode Fiber Microlens with Controllable Spot Size
A novel method for fabricating microlenses on tapered single-mode fibers is shown to be able to control the lens spot size. The fiber cladding is first symmetrically tapered by etching it with an evaporating ammonium bifluoride solution. A hemispheric lens is then melted on the taper tip with a CO2 laser. The lens can reduce the fiber mode radius to 40% of its original value. A theoretical calculation of the focused spot size agrees well with experimental results
Enhancement of surface activity in CO oxidation on Pt(110) through spatiotemporal laser actuation
We explore the effect of spatiotemporally varying substrate temperature
profiles on the dynamics and resulting reaction rate enhancement for the
catalytic oxidation of CO on Pt(110). The catalytic surface is "addressed" by a
focused laser beam whose motion is computer-controlled. The averaged reaction
rate is observed to undergo a characteristic maximum as a function of the speed
of this moving laser spot. Experiments as well as modelling are used to explore
and rationalize the existence of such an optimal laser speed.Comment: 9 pages, 12 figures, submitted to Phys. Rev.
Spontaneous Expulsion of Giant Lipid Vesicles Induced by Laser Tweezers
Irradiation of a giant unilamellar lipid bilayer vesicle with a focused laser
spot leads to a tense pressurized state which persists indefinitely after laser
shutoff. If the vesicle contains another object it can then be gently and
continuously expelled from the tense outer vesicle. Remarkably, the inner
object can be almost as large as the parent vesicle; its volume is replaced
during the exit process. We offer a qualitative theoretical model to explain
these and related phenomena. The main hypothesis is that the laser trap pulls
in lipid and ejects it in the form of submicron objects, whose osmotic activity
then drives the expulsion.Comment: Plain TeX file; uses harvmac and epsf; .ps available at
http://dept.physics.upenn.edu/~nelson/expulsion.p
Radiation Pressure Acceleration: the factors limiting maximum attainable ion energy
Radiation pressure acceleration (RPA) is a highly efficient mechanism of
laser-driven ion acceleration, with with near complete transfer of the laser
energy to the ions in the relativistic regime. However, there is a fundamental
limit on the maximum attainable ion energy, which is determined by the group
velocity of the laser. The tightly focused laser pulses have group velocities
smaller than the vacuum light speed, and, since they offer the high intensity
needed for the RPA regime, it is plausible that group velocity effects would
manifest themselves in the experiments involving tightly focused pulses and
thin foils. However, in this case, finite spot size effects are important, and
another limiting factor, the transverse expansion of the target, may dominate
over the group velocity effect. As the laser pulse diffracts after passing the
focus, the target expands accordingly due to the transverse intensity profile
of the laser. Due to this expansion, the areal density of the target decreases,
making it transparent for radiation and effectively terminating the
acceleration. The off-normal incidence of the laser on the target, due either
to the experimental setup, or to the deformation of the target, will also lead
to establishing a limit on maximum ion energy.Comment: 17 pages, 6 figure
Multiple colliding electromagnetic pulses: a way to lower the threshold of pair production from vacuum
The scheme of simultaneous multiple pulse focusing on one spot naturally
arises from the structural features of projected new laser systems, such as ELI
and HiPER. It is shown that the multiple pulse configuration is beneficial for
observing pair production from vacuum under the action of sufficiently
strong electromagnetic fields. The field of the focused pulses is described
using a realistic three-dimensional model based on an exact solution of the
Maxwell equations. The pair production threshold in terms of
electromagnetic field energy can be substantially lowered if, instead of one or
even two colliding pulses, multiple pulses focused on one spot are used. The
multiple pulse interaction geometry gives rise to subwavelength field features
in the focal region. These features result in the production of extremely short
bunches.Comment: 10 pages, 4 figure
Intense high-quality medical proton beams via laser fields
During the past decade, the interaction of high-intensity lasers with solid
targets has attracted much interest, regarding its potential in accelerating
charged particles. In spite of tremendous progress in laser-plasma based
acceleration, it is still not clear which particle beam quality will be
accessible within the upcoming multi petawatt (1 PW = 10 W) laser
generation. Here, we show with simulations based on the coupled relativistic
equations of motion that protons stemming from laser-plasma processes can be
efficiently post-accelerated using crossed laser beams focused to spot radii of
a few laser wavelengths. We demonstrate that the crossed beams produce
monoenergetic accelerated protons with kinetic energies MeV, small
energy spreads ( 1) and high densities as required for hadron
cancer therapy. To our knowledge, this is the first scheme allowing for this
important application based on an all-optical set-up.Comment: 14 pages, 3 figures, 1 tabl
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