Publications (3)0 Total impact
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ABSTRACT: Although vacuum arcs were first identified over 110 years ago, they are not
yet well understood. We have since developed a model of breakdown and gradient
limits that tries to explain, in a self-consistent way: arc triggering, plasma
initiation, plasma evolution, surface damage and gra- dient limits. We use
simple PIC codes for modeling plasmas, molecular dynamics for modeling surface
breakdown, and surface damage, and mesoscale surface thermodynamics and finite
element electrostatic codes for to evaluate surface properties. Since any given
experiment seems to have more variables than data points, we have tried to
consider a wide variety of arcing (rf structures, e beam welding, laser
ablation, etc.) to help constrain the problem, and concentrate on common
mechanisms. While the mechanisms can be comparatively simple, modeling can be
challenging.
08/2011;
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ABSTRACT: Continuing the description of rf vacuum arcs from an earlier paper, we
describe some aspects of the interaction of vacuum arcs that involve the
surface. This paper describes aspects of plasma materials interactions that
affect the arc and models measurement of the surface field using the
Tonks-Frenkel and the spinodal electrohydrodynamic instabilities, a realistic
model for the generation and evaluation of high field enhancements, unipolar
arcs, creep and other effects.
06/2010;
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ABSTRACT: We describe breakdown in 805 MHz rf accelerator cavities in terms of a number
of self consistent mechanisms. We divide the breakdown process into three
stages: 1) we model surface failure using molecular dynamics of fracture caused
by electrostatic tensile stress, 2) we model the ionization of neutrals
responsible for plasma initiation and plasma growth using a particle in cell
code, and 3) we model surface damage by assuming a process similar to unipolar
arcing. We find that the cold, dense plasma in contact with the surface
produces very small Debye lengths and very high electric fields over a large
area, consistent with unipolar arc behavior, although unipolar arcs are
strictly defined with equipotential boundaries. These high fields produce
strong erosion mechanisms, primarily self sputtering, compatible with the
crater formation that we see. We use OOPIC modeling to estimate very high
surface electric fields in the dense plasma and measure these field using
electrohydrodynamic arguments to relate the dimensions of surface damage with
the applied electric field. We also present a geometrical explanation of the
large enhancement factors of field emitters.This is consistent with the
apparent absence of whiskers on surfaces exposed to high fields. The
enhancement factors we derive, when combined with the Fowler-Nordheim analysis
produce a consistent picture of breakdown and field emission from surfaces at
local fields of 7 - 10 GV/m. We show that the plasma growth rates we obtain
from OOPIC are consistent with growth rates of the cavity shorting currents
using x ray measurements. We believe the general picture presented here for rf
breakdown arcs should be directly applicable to a larger class of vacuum arcs.
Results from the plasma simulation are included as a guide to experimental
verification of this model.
03/2010;
