6,823 research outputs found
Analytical treatment of the wakefields driven by transversely shaped beams in a planar slow-wave structure
The suppression of transverse wakefield effects using transversely elliptical
drive beams in a planar structure is studied with a simple analytical model
that unveils the geometric nature of this phenomenon. By analyzing the
suggested model we derive scaling laws for the amplitude of the longitudinal
and transverse wake potentials as a function of the Gaussian beam ellipticity -
. We explicitly show that in a wakefield accelerator application it
is beneficial to use highly elliptical beams for mitigating transverse forces
while maintaining the accelerating field. We consider two scaling strategies:
1) aperture scaling, where we keep a constant charge to have the same
accelerating gradient as in a cylindrical structure and 2) charge scaling,
where aperture is the same as in the cylindrical structure and charge is
increased to match the gradient.Comment: 10 pages, 6 figure
Topological String Defect Formation During the Chiral Phase Transition
We extend and generalize the seminal work of Brandenberger, Huang and Zhang
on the formation of strings during chiral phase transitions(berger) and discuss
the formation of abelian and non-abelian topological strings during such
transitions in the early Universe and in the high energy heavy-ion collisions.
Chiral symmetry as well as deconfinement are restored in the core of these
defects. Formation of a dense network of string defects is likely to play an
important role in the dynamics following the chiral phase transition. We
speculate that such a network can give rise to non-azimuthal distribution of
transverse energy in heavy-ion collisions.Comment: 10 pages, 4 figures, minor correction
Depletion of New Neurons by Image Guided Irradiation
Ionizing radiation continues to be a relevant tool in both imaging and the treatment of cancer. Experimental uses of focal irradiation have recently been expanded to studies of new neurons in the adult brain. Such studies have shown cognitive deficits following radiation treatment and raised caution as to possible unintentional effects that may occur in humans. Conflicting outcomes of the effects of irradiation on adult neurogenesis suggest that the effects are either transient or permanent. In this study, we used an irradiation apparatus employed in the treatment of human tumors to assess radiation effects on rat neurogenesis. For subjects we used adult male rats (Sprague-Dawley) under anesthesia. The irradiation beam was directed at the hippocampus, a center for learning and memory, and the site of neurogenic activity in adult brain. The irradiation was applied at a dose-rate 0.6 Gy/min for total single-fraction, doses ranging from 0.5 to 10.0 Gy. The animals were returned to home cages and recovered with no sign of any side effects. The neurogenesis was measured either 1 week or 6 weeks after the irradiation. At 1 week, the number of neuronal progenitors was reduced in a dose-dependent manner with the 50% reduction at 0.78 Gy. The dose–response curve was well fitted by a double exponential suggesting two processes. Examination of the tissue with quantitative immunohistochemistry revealed a dominant low-dose effect on neuronal progenitors resulting in 80% suppression of neurogenesis. This effect was partially reversible, possibly due to compensatory proliferation of the remaining precursors. At higher doses (>5 Gy) there was additional, nearly complete block of neurogenesis without compensatory proliferation. We conclude that notwithstanding the usefulness of irradiation for experimental purposes, the exposure of human subjects to doses often used in radiotherapy treatment could be damaging and cause cognitive impairments
Stability of the Black Hole Horizon and the Landau Ghost
The stability of the black hole horizon is demanded by both cosmic censorship
and the generalized second law of thermodynamics. We test the consistency of
these principles by attempting to exceed the black hole extremality condition
in various process in which a U(1) charge is added to a nearly extreme
Reissner--Nordstr\"om black hole charged with a {\it different\/} type of U(1)
charge. For an infalling spherical charged shell the attempt is foiled by the
self--Coulomb repulsion of the shell. For an infalling classical charge it
fails because the required classical charge radius exceeds the size of the
black hole. For a quantum charge the horizon is saved because in order to avoid
the Landau ghost, the effective coupling constant cannot be large enough to
accomplish the removal.Comment: 12 pages, RevTe
Semiclassical wave equation and exactness of the WKB method
The exactness of the semiclassical method for three-dimensional problems in
quantum mechanics is analyzed. The wave equation appropriate in the
quasiclassical region is derived. It is shown that application of the standard
leading-order WKB quantization condition to this equation reproduces exact
energy eigenvalues for all solvable spherically symmetric potentials.Comment: 13 page
Progress on the hybrid gun project at UCLA
UCLA/INFN-LNF/Univ. Rome has been developing the hybrid gun which has an RF gun and a short linac for velocity bunching in one structure. After the cavity was manufactured at INFN-LNF in 2012, tests of the gun was carried out at UCLA. The field in the standing wave part was 20 % smaller than the simulation but the phase advance was fine. The cavity was commissioned successfully up to 13 MW. The beam test was performed at 11.5 MW and demonstrated the bunch compression
Observation of Plasma Focusing of a 28.5 GeV Positron Beam
The observation of plasma focusing of a 28.5 GeV positron beam is reported.
The plasma was formed by ionizing a nitrogen jet only 3 mm thick. Simultaneous
focusing in both transverse dimensions was observed with effective focusing
strengths of order Tesla per micron. The minimum area of the beam spot was
reduced by a factor of 2.0 +/- 0.3 by the plasma. The longitudinal beam
envelope was measured and compared with numerical calculations
Semiclassical Decay of Excited String States on Leading Regge Trajectories
We study the decay of hadrons based on a semiclassical string model. By
including quark mass effects we find that the width to mass ratio \G/m is an
increasing function of , which increases most rapidly for massive quarks.
This is consistent with the available data. The decay probability of hadrons on
the leading Regge trajectories is computed taking the effect of the string
rotation into account. The resulting decay probability is no longer uniform
along the length of the string but varies in a manner that is in qualitative
agreement with the available data. We argue in favour of possible experiments
that would test our predictions more accurately and help open a window to the
nonperturbative aspects of QCD.Comment: 15 PAGES, UR-1326, ER-40685-776, SU-4240-55
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