197 research outputs found
Elastic orifice automatically regulates gas bearings
Elastic, pressure-sensitive orifice is used to automatically regulate the rate of gas flow into bearings under varying loads. Formed of a molded elastomer, theses orifices increase the stability of gas bearings
An accurate Rb density measurement method for a plasma wakefield accelerator experiment using a novel Rb reservoir
A method to accurately measure the density of Rb vapor is described. We plan
on using this method for the Advanced Wakefield (AWAKE)~\cite{bib:awake}
project at CERN , which will be the world's first proton driven plasma
wakefield experiment. The method is similar to the hook~\cite{bib:Hook} method
and has been described in great detail in the work by W. Tendell Hill et.
al.~\cite{bib:densitymeter}. In this method a cosine fit is applied to the
interferogram to obtain a relative accuracy on the order of for the vapor
density-length product. A single-mode, fiber-based, Mach-Zenhder interferometer
will be built and used near the ends of the 10 meter-long AWAKE plasma source
to be able to make accurate relative density measurement between these two
locations. This can then be used to infer the vapor density gradient along the
AWAKE plasma source and also change it to the value desired for the plasma
wakefield experiment. Here we describe the plan in detail and show preliminary
results obtained using a prototype 8 cm long novel Rb vapor cell.Comment: 5 pages 9 figure
Spin and orbital effects in a 2D electron gas in a random magnetic field
Using the method of superbosonization we consider a model of a random
magnetic field (RMF) acting on both orbital motion and spin of electrons in two
dimensions. The method is based on exact integration over one particle degrees
of freedom and reduction of the problem to a functional integral over
supermatrices . We consider a general case when
both the direction of the RMF and the g-factor of the Zeeman splitting are
arbitrary. Integrating out fast variations of we come to a standard
collisional unitary non-linear -model. The collision term consists of
orbital, spin and effective spin-orbital parts. For a particular problem of a
fixed direction of RMF, we show that additional soft excitations identified
with spin modes should appear. Considering % -correlated weak RMF and
putting g=2 we find the transport time . This time is 2 times
smaller than that for spinless particles.Comment: 9 pages, no figure
Anderson transition of three dimensional phonon modes
Anderson transition of the phonon modes is studied numerically. The critical
exponent for the divergence of the localization length is estimated using the
transfer matrix method, and the statistics of the modes is analyzed. The latter
is shown to be in excellent agreement with the energy level statistics of the
disrodered electron system belonging to the orthogonal universality class.Comment: 2 pages and another page for 3 figures, J. Phys. Soc. Japa
Comment on ``Critical Behavior in Disordered Quantum Systems Modified by Broken Time--Reversal Symmetry''
In a recent Letter [Phys. Rev. Lett. 80, 1003 (1998)] Hussein and Pato
employed the maximum entropy principle (MEP) in order to derive interpolating
ensembles between any pair of universality classes in random matrix theory.
They apply their formalism also to the transition from random matrix to Poisson
statistics of spectra that is observed for the case of the Anderson-type
metal-insulator transition. We point out the problems with the latter
procedure.Comment: 1 page in PS, to appear in PRL Sept. 2
Generation of 10-m-lengthscale plasma columns by resonant and off-resonant laser pulses
Creating extended, highly homogeneous plasma columns like that required by
plasma wakefield accelerators can be a challenge. We study the propagation of
ultra-short, TW power ionizing laser pulses in a 10-meter-long rubidium vapor
and the plasma columns they create. We perform experiments and numerical
simulations for pulses with 780 nm central wavelength, which is resonant with
the D transition from the ground state of rubidium atoms, as well as for
pulses with 810 nm central wavelength, some distance from resonances. We
measure transmitted energy and transverse width of the pulse and use schlieren
imaging to probe the plasma column in the vapor close to the end of the vapor
source. We find, that resonant pulses are more confined in a transverse
direction by the interaction than off-resonant pulses are and that the plasma
channels they create are more sharply bounded. Off-resonant pulses leave a
wider layer of partially ionized atoms and thus lose more energy per unit
propagation distance. Using experimental data, we estimate the energy required
to generate a 20-meter-long plasma column and conclude that resonant pulses are
much more suitable for creating a long, homogeneous plasma.Comment: 12 pages, 14 figure
Magnetic Field Effect for Two Electrons in a Two Dimensional Random Potential
We study the problem of two particles with Coulomb repulsion in a
two-dimensional disordered potential in the presence of a magnetic field. For
the regime, when without interaction all states are well localized, it is shown
that above a critical excitation energy electron pairs become delocalized by
interaction. The transition between the localized and delocalized regimes goes
in the same way as the metal-insulator transition at the mobility edge in the
three dimensional Anderson model with broken time reversal symmetry.Comment: revtex, 7 pages, 6 figure
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