463 research outputs found
Mechanism of degradation and breakdown in PET films under high intensity AC fields
No full textThe mechanism of electrical degradation and breakdown of polymeric
insulating films is examined by this paper as a two-stage process.
Electron injection by Fowler-Nordheim tunnelling, provides the
energy for the creation of the first cavity near to the injection
contact (the beginning of the formation of the so-called
low-density region) where impact ionisation of molecules and hence
electron avalanche, can occur. This stage is assisted by intrinsic
tunnelling of electrons through local potential barriers. The
second stage, the macroion bond scission and the creation of
another macroion and a free radical is a thermofluctuational
process which involves the action of the stretching force by the
local electric field. From the other hand breakdown initiation
starts when an electron following a Poole-Frenkel hopping
mechanism, is accelerated in a hole with sufficiently large
dimensions. The role of the applied electric field Fa and its
relation to the local electric field Fl in both stages is
examined. Experiments were executed for measuring the breakdown AC
voltage Vb and dielectric strength Fb, with voltage
rising rate 3 KV/s, in order to examine their relation with
specimen thickness d and hence to derive a steady state
breakdown criterion. Measurements of PET films life-time were also
taken in order to examine the degradation mechanism and the
polymer's lifetime
The role of interfaces in the dielectric strength of polymeric films under high intensity fields
No full textThis paper examines the role of interfaces in the polymeric
insulating films under the scope of their dielectric strength. PET
(Polyethylene terephthalate) films of different thickness are put
together forming a planar sample of a typical thickness of m
and subjected to AC ramp voltage, until the dielectric
breakdown. The role of interface as a potential barrier for
electrons was examined as well as the influence of interface
number for a standard thickness. Experiments were also executed
for investigating the role of the position of the first, the
second and the last interface in the sample. All the experiments
were executed with the same voltage rising rate, the breakdown AC
voltage Vb was measured and the dielectric strength Fb
was calculated. It was shown that interfaces act as deep trapping
centers for electrons giving rise to the sample dielectric
strength for sort time experiments. Moreover the thinner the first
film, the higher the dielectric strength for the sample. It was
also found that the position of the second interface does not play
an important role in the process
Measurement of the <i>t</i> dependence in exclusive photoproduction of ϒ(1 <i>S</i>) mesons at HERA
No full textThe exclusive photoproduction reaction γp→ϒ(1 <i>S</i>)<i>p</i> has been studied with the ZEUS detector in <i>ep</i> collisions at HERA using an integrated luminosity of 468 pb<sup>−1</sup>. The measurement covers the kinematic range60<W<220 GeV and Q<sup>2</sup><1 GeV<sup>2</sup>, where <i>W</i> is the photon–proton centre-of-mass energy and Q<sup>2</sup> is the photon virtuality. The exponential slope, <i>b</i>, of the <i>t</i> dependence of the cross section, where <i>t</i> is the squared four-momentum transfer at the proton vertex, has been measured, yielding b = 4.3{+2.0 =1.3}(stat.){+0.5 -0.6}(syst.) GeV<sup>-2</sup>. This constitutes the first measurement of the <i>t</i> dependence of the γp→ϒ(1 <i>S</i>)<i>p</i> cross section
Search for single-top production in <i>ep</i> collisions at HERA
No full textA search for single-top production,<i>ep</i>→ <i>et X</i>, has been performed with the ZEUS detector at HERA using data corresponding to an integrated luminosity of 0.37 fb<sup>−1</sup>. No evidence for top production was found, consistent with the expectation from the Standard Model. Limits were computed for single-top production via flavour changing neutral current transitions involving a neutral electroweak vector boson, γ or Z. The result was combined with a previous ZEUS result yielding a total luminosity of 0.50 fb<sup>−1</sup>. A 95% credibility level upper limit of 0.13 pb was obtained for the cross section at the centre-of-mass energy of √s = 315 GeV
Measurement of Inelastic and Photoproduction at HERA
No full textThe cross sections for inelastic photoproduction of J/ψ and ψ′ mesons have been measured in epcollisions with the ZEUS detector at HERA, using an integrated luminosity of 468 pb<sup>−1</sup> collected in the period 1996–2007. The ψ′ to J/ψ cross section ratio was measured in the range 0.55 < z< 0.9 and 60 < W < 190 GeV as a function of <i>W, z</i> and <i>p T</i> . Here W denotes the photon-proton centre-of-mass energy, <i>z</i> is the fraction of the incident photon energy carried by the meson and <i>p T</i> is the transverse momentum of the meson with respect to the beam axis. The J/ψ cross sections were measured for 0.1 < <i>z</i> < 0.9, 60 < W < 240 GeV and <i>p T</i> > 1 GeV. Theoretical predictions within the non-relativistic QCD framework including NLO colour-singlet and colour-octet contributions were compared to the data, as were predictions based on the <i>k T -</i> factorisation approach
Measurement of high-Q (2) charged current deep inelastic scattering cross sections with a longitudinally polarised positron beam at HERA
No full textMeasurements of the cross sections for charged current deep inelastic scattering in e+p collisions with a longitudinally polarised positron beam are presented. The measurements are based on a data sample with an integrated luminosity of 132 pb-1 collected with the ZEUS detector at HERA at a centre-of-mass energy of 318 GeV. The total cross section is presented at positive and negative values of the longitudinal polarisation of the positron beams. The single-differential cross-sections dσ/dQ2, dσ/dx and dσ/dy are presented for Q2>200 GeV2. The reduced cross-section σ̃ is presented in the kinematic range 200<Q2<60000 GeV2 and 0.006<x<0.562. The measurements agree well with the predictions of the Standard Model. The results are used to determine a lower limit on the mass of a hypothetical right-handed W boson. © 2010 The Author(s)
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