Ising Field Theory on a Pseudosphere

We show how the symmetries of the Ising field theory on a pseudosphere can be exploited to derive the form factors of the spin fields as well as the non-linear differential equations satisfied by the corresponding two-point correlation functions. The latter are studied in detail and, in particular, we present a solution to the so-called connection problem relating two of the singular points of the associated Painleve VI equation. A brief discussion of the thermodynamic properties is also presented.Comment: 39 pages, 6 eps figures, uses harvma

Automatic Estimation of the Seafloor Geomorphology of the Santos Basin, Brazil

The bathymetry and acoustic backscatter of Santos Basin, Brazil were mapped using a SeaBeam 2112 (12 kHz, 151 beam) Multibeam Echosounder (MBES) aboard the R/V Falcon Explorer. This MBES data was acquired from January-November, 2000, during a high-resolution multi-channel 3D seismic survey, resulting in 380 parallel lines of 90 km length, spaced 250 m apart. The final survey mapped an area of 5,000 km${}^2$ in water depths of 900--2000 m. These closely spaced multibeam tracks resulted in an average overlap between swaths of 1000%, thereby ensonifying most areas of the seafloor at least ten times. Traditional (hand) processing of a dataset this dense is time-consuming and tedious, and is prone to subjective decisions and operator fatigue. However, the density of the survey makes it ideal for automatic processing methods. Recently, we have developed an algorithm called CUBE that addresses the twin concerns of robustness and reliability that are often raised about automatic processing methods. Based on a very robust multiple hypothesis Bayesian estimator, CUBE processes MBES bathymetry directly into a set of gridded products representing the best estimate of probable depth, and a measure of the uncertainty associated with this estimate. We apply CUBE to the Santos Basin data, illustrating in terms of processing time and human effort the advantages of processing such data automatically. We compare the automatically generated data with a hand-processed set, showing that the results agree to within the estimated experimental uncertainty. We next illustrate the use of CUBE as a data quality measure, indicating areas of concern in the data. Finally, we utilize the bathymetric grid resulting from CUBE to investigate the seafloor morphology, which includes a set of linear depressions parallel and perpendicular to the Shelf break. These linear depressions are the surface expression of fault planes related to subsurface salt walls. In the shallowest part, the detailed bathymetry also shows various pockmarks (350 m wide) possibly associated with fluid expulsion, while in the deeper portion we observe a small number of larger ones (2500 m wide), which are clearly inactive as they are partially filled with recent sediments. Some pockmarks are aligned with fault planes, suggesting a preferential pathway for fluid expulsion. The acquisition geometry for this survey allowed us to analyze the behavior of the backscatter response as a function of grazing angle for any given piece of seafloor, thus eliminating the need to assume a homogeneous seafloor across the swath. Although the backscatter is not calibrated, the variation in response can be used to investigate the effects of gas in shallow sediments of the survey area

The ion motion in self-modulated plasma wakefield accelerators

The effects of plasma ion motion in self-modulated plasma based accelerators is examined. An analytical model describing ion motion in the narrow beam limit is developed, and confirmed through multi-dimensional particle-in-cell simulations. It is shown that the ion motion can lead to the early saturation of the self-modulation instability, and to the suppression of the accelerating gradients. This can reduce the total energy that can be transformed into kinetic energy of accelerated particles. For the parameters of future proton-driven plasma accelerator experiments, the ion dynamics can have a strong impact. Possible methods to mitigate the effects of the ion motion in future experiments are demonstrated.Comment: 11 pages, 3 figures, accepted for publication in Phys. Rev. Let

Ion motion in the wake driven by long particle bunches in plasmas

We explore the role of the background plasma ion motion in self-modulated plasma wakefield accelerators. We employ J. Dawson's plasma sheet model to derive expressions for the transverse plasma electric field and ponderomotive force in the narrow bunch limit. We use these results to determine the on-set of the ion dynamics, and demonstrate that the ion motion could occur in self-modulated plasma wakefield accelerators. Simulations show the motion of the plasma ions can lead to the early suppression of the self-modulation instability and of the accelerating fields. The background plasma ion motion can nevertheless be fully mitigated by using plasmas with heavier plasmas.Comment: 23 pages, 6 figure

Ion dynamics and acceleration in relativistic shocks

Ab-initio numerical study of collisionless shocks in electron-ion unmagnetized plasmas is performed with fully relativistic particle in cell simulations. The main properties of the shock are shown, focusing on the implications for particle acceleration. Results from previous works with a distinct numerical framework are recovered, including the shock structure and the overall acceleration features. Particle tracking is then used to analyze in detail the particle dynamics and the acceleration process. We observe an energy growth in time that can be reproduced by a Fermi-like mechanism with a reduced number of scatterings, in which the time between collisions increases as the particle gains energy, and the average acceleration efficiency is not ideal. The in depth analysis of the underlying physics is relevant to understand the generation of high energy cosmic rays, the impact on the astrophysical shock dynamics, and the consequent emission of radiation.Comment: 5 pages, 3 figure

Spatial-temporal evolution of the current filamentation instability

The spatial-temporal evolution of the purely transverse current filamentation instability is analyzed by deriving a single partial differential equation for the instability and obtaining the analytical solutions for the spatially and temporally growing current filament mode. When the beam front always encounters fresh plasma, our analysis shows that the instability grows spatially from the beam front to the back up to a certain critical beam length; then the instability acquires a purely temporal growth. This critical beam length increases linearly with time and in the non-relativistic regime it is proportional to the beam velocity. In the relativistic regime the critical length is inversely proportional to the cube of the beam Lorentz factor $\gamma_{0b}$. Thus, in the ultra-relativistic regime the instability immediately acquires a purely temporal growth all over the beam. The analytical results are in good agreement with multidimensional particle-in-cell simulations performed with OSIRIS. Relevance of current study to recent and future experiments on fireball beams is also addressed

Magnetically assisted self-injection and radiation generation for plasma based acceleration

It is shown through analytical modeling and numerical simulations that external magnetic fields can relax the self-trapping thresholds in plasma based accelerators. In addition, the transverse location where self-trapping occurs can be selected by adequate choice of the spatial profile of the external magnetic field. We also find that magnetic-field assisted self-injection can lead to the emission of betatron radiation at well defined frequencies. This controlled injection technique could be explored using state-of-the-art magnetic fields in current/next generation plasma/laser wakefield accelerator experiments.Comment: 7 pages, 4 figures, accepted for publication in Plasma Physics and Controlled Fusio

Transverse self-modulation of ultra-relativistic lepton beams in the plasma wakefield accelerator

The transverse self-modulation of ultra-relativistic, long lepton bunches in high-density plasmas is explored through full-scale particle-in-cell simulations. We demonstrate that long SLAC-type electron and positron bunches can become strongly self-modulated over centimeter distances, leading to wake excitation in the blowout regime with accelerating fields in excess of 20 GV/m. We show that particles energy variations exceeding 10 GeV can occur in meter-long plasmas. We find that the self-modulation of positively and negatively charged bunches differ when the blowout is reached. Seeding the self-modulation instability suppresses the competing hosing instability. This work reveals that a proof-of-principle experiment to test the physics of bunch self-modulation can be performed with available lepton bunches and with existing experimental apparatus and diagnostics.Comment: 8 pages, 8 figures, accepted for publication in Physics of Plasma

Beam loading in the nonlinear regime of plasma-based acceleration

A theory that describes how to load negative charge into a nonlinear, three-dimensional plasma wakefield is presented. In this regime, a laser or an electron beam blows out the plasma electrons and creates a nearly spherical ion channel, which is modified by the presence of the beam load. Analytical solutions for the fields and the shape of the ion channel are derived. It is shown that very high beam-loading efficiency can be achieved, while the energy spread of the bunch is conserved. The theoretical results are verified with the Particle-In-Cell code OSIRIS.Comment: 5 pages, 2 figures, to appear in Physical Review Letter