40 research outputs found
Sheath parameters for non-Debye plasmas: simulations and arc damage
This paper describes the surface environment of the dense plasma arcs that
damage rf accelerators, tokamaks and other high gradient structures. We
simulate the dense, non-ideal plasma sheath near a metallic surface using
Molecular Dynamics (MD) to evaluate sheaths in the non-Debye region for high
density, low temperature plasmas. We use direct two-component MD simulations
where the interactions between all electrons and ions are computed explicitly.
We find that the non-Debye sheath can be extrapolated from the Debye sheath
parameters with small corrections. We find that these parameters are roughly
consistent with previous PIC code estimates, pointing to densities in the range
. The high surface fields implied by these
results could produce field emission that would short the sheath and cause an
instability in the time evolution of the arc, and this mechanism could limit
the maximum density and surface field in the arc. These results also provide a
way of understanding how the "burn voltage" of an arc is generated, and the
relation between self sputtering and the burn voltage, while not well
understood, seems to be closely correlated. Using these results, and equating
surface tension and plasma pressure, it is possible to infer a range of plasma
densities and sheath potentials from SEM images of arc damage. We find that the
high density plasma these results imply and the level of plasma pressure they
would produce is consistent with arc damage on a scale 100 nm or less, in
examples where the liquid metal would cool before this structure would be lost.
We find that the sub-micron component of arc damage, the burn voltage, and
fluctuations in the visible light production of arcs may be the most direct
indicators of the parameters of the dense plasma arc, and the most useful
diagnostics of the mechanisms limiting gradients in accelerators.Comment: 8 pages, 16 figure
Hydrostatic Level Sensors as High Precision Ground Motion Instrumentation for Tevatron and Other Energy Frontier Accelerators
Particle accelerators pushed the limits of our knowledge in search of the
answers to most fundamental questions about micro-world and our Universe. In
these pursuits, accelerators progressed to higher and higher energies and
particle beam intensities as well as increasingly smaller and smaller beam
sizes. As the result, modern existing and planned energy frontier accelerators
demand very tight tolerances on alignment and stability of their elements:
magnets, accelerating cavities, vacuum chambers, etc. In this article we
describe the instruments developed for and used in such accelerators as
Fermilab's Tevatron (FNAL, Batavia, IL USA) and for the studies toward an
International Linear Collider (ILC). The instrumentation includes Hydrostatic
Level Sensors (HLS) for very low frequency measurements. We present design
features of the sensors, outline their technical parameters, describe test and
calibration procedures and discuss different regimes of operation. Experimental
results of the ground motion measurements with these detectors will be
presented in subsequent paper
NERAZLIKOVNOST RAZLIKA
Analiziraju se leksikološko-leksikografska polazišta i metodološki postupci u izradi Razlikovnog rječnika srpskog i hrvatskog jezika Vladimira Brodnjaka