25,796 research outputs found
Interaction of a Modulated Electron Beam with a Plasma
The results of a theoretical and experimental investigation of the high-frequency interaction of an electron beam with a plasma are reported. An electron beam, modulated at a microwave frequency, passes through a uniform region of a mercury arc discharge after which it is demodulated. Exponentially growing wave amplification along the electron beam was experimentally observed for the first time at a microwave frequency equal to the plasma frequency. Approximate theories of the effects of 1) plasma-electron collision frequencies, 2) plasma-electron thermal velocities and 3) finite beam diameter, are given. In a second experiment the interaction between a modulated electron beam and a slow electrostatic wave on a plasma column has been studied. A strong interaction occurs when the velocity of the electron beam is approximately equal to the velocity of the wave and the interaction is essentially the same as that which occurs in traveling-wave amplifiers, except that here the plasma colum replaces the usual helical slow-wave circuit. The theory predicting rates of growth is presented and compared with the experimental results
Post Big Bang Processing of the Primordial Elements
We explore the Gnedin-Ostriker suggestion that a post-Big-Bang
photodissociation process may modify the primordial abundances of the light
elements. We consider several specific models and discuss the general features
that are necessary (but not necessarily sufficient) to make the model work. We
find that with any significant processing, the final D and He abundances,
which are independent of their initial standard big bang nucleosynthesis (SBBN)
values, rise quickly to a level several orders of magnitude above the
observationally inferred primordial values. Solutions for specific models show
that the only initial abundances that can be photoprocessed into agreement with
observations are those that undergo virtually no processing and are already in
agreement with observation. Thus it is unlikely that this model can work for
any non-trivial case unless an artificial density and/or photon distribution is
invoked.Comment: 12 page Latex file (AASTEX style). Tarred, gzipped, and uuencoded
postscript files of seven figures. Also available (with ps file of paper) at
ftp://www-physics.mps.ohio-state.edu/pub/nucex/phot
On causality, apparent 'superluminality' and reshaping in barrier penetration
We consider tunnelling of a non-relativistic particle across a potential
barrier. It is shown that the barrier acts as an effective beam splitter which
builds up the transmitted pulse from the copies of the initial envelope shifted
in the coordinate space backwards relative to the free propagation. Although
along each pathway causality is explicitly obeyed, in special cases reshaping
can result an overall reduction of the initial envelope, accompanied by an
arbitrary coordinate shift. In the case of a high barrier the delay amplitude
distribution (DAD) mimics a Dirac -function, the transmission amplitude
is superoscillatory for finite momenta and tunnelling leads to an accurate
advancement of the (reduced) initial envelope by the barrier width. In the case
of a wide barrier, initial envelope is accurately translated into the complex
coordinate plane. The complex shift, given by the first moment of the DAD,
accounts for both the displacement of the maximum of the transmitted
probability density and the increase in its velocity. It is argued that
analysing apparent 'superluminality' in terms of spacial displacements helps
avoid contradiction associated with time parameters such as the phase time
The effect of composition on the mechanism of stress-corrosion cracking of titanium alloys in nitrogen tetroxide, and aqueous and hot- salt environments Annual summary report, 1 May 1967 - 30 Apr. 1968
Stress corrosion data for titanium alloys in aqueous, hot salt, and nitrogen dioxide environment
A summary of the active flexible wing program
A summary of the NASA/Rockwell Active Flexible Wing Program is presented. Major elements of the program are presented. Key program accomplishments included single- and multiple-mode flutter suppression, load alleviation and load control during rapid roll maneuvers, and multi-input/multi-output multiple-function active controls tests above the open-loop flutter boundary
Study of tooling concepts for manufacturing operations in space Final report
Mechanical linkage device for manufacturing operations with orbital workshop
Search for the second forbidden beta decay of 8B to the ground state of 8Be
A significant decay branch of 8B to the ground state of 8Be would extend the
solar neutrino spectrum to higher energies than anticipated in the standard
solar models. These high-energy neutrinos would affect current neutrino
oscillation results and also would be a background to measurements of the hep
process. We have measured the delayed alpha particles from the decay of 8B,
with the goal of observing the two 46-keV alpha particles arising from the
ground-state decay. The 8B was produced using an in-flight radioactive beam
technique. It was implanted in a silicon PIN-diode detector that was capable of
identifying the alpha-particles from the 8Be ground state. From this
measurement we find an upper limit (at 90% confidence level) of 7.3 x 10^{-5}
for the branching ratio to the ground state. In addition to describing this
measurement, we present a theoretical calculation for this branching ratio.Comment: One reference corrected. Minor edits in tex
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