Harmonic Balance and Averaging Techniques for Stick-Slip Limit-Cycle Determination in Mode-Coupling Friction Self-Excited Systems

A minimal model for mode-coupling friction induced instability with Coulomb-type frictional nonlinearity is set up to investigate the applicability and quality of approximative methods to determine the limit cycles of unstable system configurations. It turns out that - due to the multi-degree-of-freedom nature of the mode-coupling instability - harmonic balance approaches yield reasonable results only if applied carefully, i.e. with respect to the special effects of the nonlinearities under consideration. The Krylov-Bogoliubov-Mitropolsky approach yields good results in a straightforward manner, the technique is however formally much more cumbersome

Femtosecond Pump-Probe Diagnostics Of Preformed Plasma Channels

We report on recent ultrafast pump-probe experiments 28 in He plasma waveguides using 800 nm, 80 fs pump pulses of 0.2 x 1018 W/cm2 peak guided intensity, and single orthogonally-polarized 800 nm probe pulses with similar to0.1% of pump intensity. The main results are: (1) We observe frequency-domain interference between the probe and a weak, depolarized component of the pump that differs substantially in mode shape from the injected pump pulse; (2) we observe spectral blue-shifts in the transmitted probe that are not evident in the transmitted pump. The evidence indicates that pump depolarization and probe blue-shifts both originate near the channel entrance.Physic

Dynamics and stability of Bose-Einstein solitons in tilted optical lattices

Bloch oscillations of Bose-Einstein condensates realize sensitive matter-wave interferometers. We investigate the dynamics and stability of bright-soliton wave packets in one-dimensional tilted optical lattices with a modulated mean-field interaction $g(t)$. By means of a time-reversal argument, we prove the stability of Bloch oscillations of breathing solitons that would be quasistatically unstable. Floquet theory shows that these breathing solitons can be more stable against certain experimental perturbations than rigid solitons or even non-interacting wave packets.Comment: final, published versio

Stability and decay of Bloch oscillations in presence of time-dependent nonlinearity

We consider Bloch oscillations of Bose-Einstein condensates in presence of a time-modulated s-wave scattering length. Generically, interaction leads to dephasing and decay of the wave packet. Based on a cyclic-time argument, we find---additionally to the linear Bloch oscillation and a rigid soliton solution---an infinite family of modulations that lead to a periodic time evolution of the wave packet. In order to quantitatively describe the dynamics of Bloch oscillations in presence of time-modulated interactions, we employ two complementary methods: collective-coordinates and the linear stability analysis of an extended wave packet. We provide instructive examples and address the question of robustness against external perturbations.Comment: 15 pages, 8 figures. Slightly amended final versio

A Higgs Mechanism for Gravity

In this paper we elaborate on the idea of an emergent spacetime which arises due to the dynamical breaking of diffeomorphism invariance in the early universe. In preparation for an explicit symmetry breaking scenario, we consider nonlinear realizations of the group of analytical diffeomorphisms which provide a unified description of spacetime structures. We find that gravitational fields, such as the affine connection, metric and coordinates, can all be interpreted as Goldstone fields of the diffeomorphism group. We then construct a Higgs mechanism for gravity in which an affine spacetime evolves into a Riemannian one by the condensation of a metric. The symmetry breaking potential is identical to that of hybrid inflation but with the non-inflaton scalar extended to a symmetric second rank tensor. This tensor is required for the realization of the metric as a Higgs field. We finally comment on the role of Goldstone coordinates as a dynamical fluid of reference.Comment: 15 pages, 2 figures, 3 tables, appendix C on on-shell d.o.f. added, references adde

Outcomes of Salvage Arthrodesis and Arthroplasty for Failed Osteochondral Allograft Transplantation of the Ankle

Background: Osteochondral allograft (OCA) transplantation is a useful treatment for posttraumatic ankle arthritis in young patients, but failure rates are high and reoperations are not uncommon. The aim of this study was to evaluate the outcomes of failed ankle OCA transplantation converted to ankle arthrodesis (AA) or total ankle arthroplasty (TAA). Methods: We evaluated 24 patients who underwent salvage procedures (13 AA and 11 TAA) after primary failed ankle OCA transplantation. Reoperations were assessed. Failure of the salvage procedure was defined as an additional surgery that required a revision AA/TAA or amputation. Evaluation among nonfailing ankles included the American Academy of Orthopaedic Surgeons Foot and Ankle Module (AAOS-FAM), pain, and satisfaction. Results: In the salvage AA cohort, 3 patients were classified as failures (2 revision AA and 1 amputation). The 10 nonfailing patients had a mean follow-up of 7.4 years. Eighty-eight percent were satisfied with the procedure, but 63% reported continued problems with their ankle (eg, pain, swelling, stiffness). Mean pain level was 1.9 and AAOS-FAM core score was 83±13. In the salvage TAA cohort, 2 patients were classified as failures (both revision TAA). The 9 nonfailing patients had a mean follow-up of 3.8 years. Fifty percent were satisfied with the procedure, but 40% reported continued problems with their ankle. The mean pain level was 1.3, and the median AAOS-FAM core score was 82±26. Conclusion: Revision and reoperation rates for salvage procedures following failed OCA transplantation of the ankle are higher compared to published data for primary AA and TAA procedures. However, we believe OCA transplantation can serve as an interim procedure for younger patients with advanced ankle joint disease who may not be ideal candidates for primary AA or TAA at the time of initial presentation. Level of Evidence: Level IV, case series

Bogoliubov Excitations of Disordered Bose-Einstein Condensates

We describe repulsively interacting Bose-Einstein condensates in spatially correlated disorder potentials of arbitrary dimension. The first effect of disorder is to deform the mean-field condensate. Secondly, the quantum excitation spectrum and condensate population are affected. By a saddle-point expansion of the many-body Hamiltonian around the deformed mean-field ground state, we derive the fundamental quadratic Hamiltonian of quantum fluctuations. Importantly, a basis is used such that excitations are orthogonal to the deformed condensate. Via Bogoliubov-Nambu perturbation theory, we compute the effective excitation dispersion, including mean free paths and localization lengths. Corrections to the speed of sound and average density of states are calculated, due to correlated disorder in arbitrary dimensions, extending to the case of weak lattice potentials.Comment: 23 pages, 11 figure

Multi-GeV Electron Generation Using Texas Petawatt Laser

We present simulation results and experimental setup for multi-GeV electron generation by a laser plasma wake field accelerator (LWFA) driven by the Texas Petawatt (TPW) laser. Simulations show that, in plasma of density n(e) = 2 - 4 x cm(-3), the TPW laser pulse (1.1 PW, 170 fs) can self-guide over 5 Rayleigh ranges, while electrons self-injected into the LWFA can accelerate up to 7 GeV. Optical diagnostic methods employed to observe the laser beam self-guiding, electron trapping and plasma bubble formation and evolution are discussed. Electron beam diagnostics, including optical transition radiation (OTR) and electron gamma ray shower (EGS) generation, are discussed as well.Physic

Stability analysis of rotating beams rubbing on an elastic circular structure

This paper presents the stability analysis of a system composed of rotating beams on a flexible, circular fixed ring, using the Routh-Hurwitz criterion. The model displayed has been fully developed within the rotating frame by use of an energy approach. The beams considered possess two degrees of freedom (dofs), a flexural motion as well as a traction/compression motion. In-plane deformations of the ring will be considered. Divergences and mode couplings have thus been underscored within the rotating frame and in order to simplify understanding of all these phenomena, the dofs of the beams will first be treated separately and then together. The dynamics of radial rotating loads on an elastic ring can create divergence instabilities as well as post-critical mode couplings. Moreover, the flexural motion of beam rubbing on the ring can also lead to mode couplings and to the locus-veering phenomenon. The presence of rubbing seems to make the system unstable as soon as the rotational speed of the beams is greater than zero. Lastly, the influence of an angle between the beams and the normal to the ring's inner surface will be studied with respect to system stability, thus highlighting a shift frequency phenomenon