44,142 research outputs found
Electron cloud dynamics in the Cornell Electron Storage Ring Test Accelerator wiggler
The interference of stray electrons (also called âelectron cloudsâ) with accelerator beams is important in modern intense-beam accelerators, especially those with beams of positive charge. In magnetic wigglers, used, for instance, for transverse emittance damping, the intense synchrotron radiation produced by the beam can generate an electron cloud of relatively high density. In this paper the complicated dynamics of electron clouds in wigglers is examined using the example of a wiggler in the Cornell Electron Storage Ring Test Accelerator experiment at the Cornell Electron Storage Ring. Three-dimensional particle-in-cell simulations with the WARP-POSINST computer code show different density and dynamics for the electron cloud at locations near the maxima of the vertical wiggler field when compared to locations near the minima. Dynamics in these regions, the electron cloud distribution vs longitudinal position, and the beam coherent tune shift caused by the wiggler electron cloud will be discussed
Computationally efficient methods for modelling laser wakefield acceleration in the blowout regime
Electron self-injection and acceleration until dephasing in the blowout
regime is studied for a set of initial conditions typical of recent experiments
with 100 terawatt-class lasers. Two different approaches to computationally
efficient, fully explicit, three-dimensional particle-in-cell modelling are
examined. First, the Cartesian code VORPAL using a perfect-dispersion
electromagnetic solver precisely describes the laser pulse and bubble dynamics,
taking advantage of coarser resolution in the propagation direction, with a
proportionally larger time step. Using third-order splines for macroparticles
helps suppress the sampling noise while keeping the usage of computational
resources modest. The second way to reduce the simulation load is using
reduced-geometry codes. In our case, the quasi-cylindrical code CALDER-CIRC
uses decomposition of fields and currents into a set of poloidal modes, while
the macroparticles move in the Cartesian 3D space. Cylindrical symmetry of the
interaction allows using just two modes, reducing the computational load to
roughly that of a planar Cartesian simulation while preserving the 3D nature of
the interaction. This significant economy of resources allows using fine
resolution in the direction of propagation and a small time step, making
numerical dispersion vanishingly small, together with a large number of
particles per cell, enabling good particle statistics. Quantitative agreement
of the two simulations indicates that they are free of numerical artefacts.
Both approaches thus retrieve physically correct evolution of the plasma
bubble, recovering the intrinsic connection of electron self-injection to the
nonlinear optical evolution of the driver
Space-charge compensation experiments at IOTA ring
Space-charge effects belong to the category of the most long-standing issues
in beam physics, and even today, after several decades of very active
exploration and development of counter-measures, they still pose the most
profound limitations on performance of high intensity proton accelerators. We
briefly consider past experience in active compensation of these effects and
present in detail the progress towards experimental studies of novel schemes of
space-charge compensation at the Fermilab's IOTA ring.Comment: 5 p
Magnetic Field Effects on the Structure and Evolution of Overdense Radiatively Cooling Jets
We investigate the effect of magnetic fields on the propagation dynamics and
morphology of overdense, radiatively cooling, supermagnetosonic jets, with the
help of fully three-dimensional SPMHD simulations. Evaluated for a set of
parameters which are mainly suitable for protostellar jets (with density ratios
between the jet and the ambient medium 3-10, and ambient Mach number ~ 24),
these simulations are also compared with baseline non-magnetic and adiabatic
calculations. We find that, after amplification by compression and
re-orientation in nonparallel shocks at the working surface, the magnetic field
that is carried backward with the shocked gas into the cocoon improves the jet
collimation relative to the purely hydrodynamic (HD) systems. Low-amplitude,
approximately equally spaced internal shocks (which are absent in the HD
systems) are produced by MHD K-H reflection pinch modes. The longitudinal field
geometry also excites non-axisymmetric helical modes which cause some beam
wiggling. The strength and amount of these modes are, however, reduced (by ~
twice) in the presence of radiative cooling relative to the adiabatic cases.
Besides, a large density ratio between the jet and the ambient medium also
reduces, in general, the number of the internal shocks. As a consequence, the
weakness of the induced internal shocks makes it doubtful that the magnetic
pinches could produce by themselves the bright knots observed in the overdense,
radiatively cooling protostellar jets.Comment: To appear in ApJ; 36 pages + 16 (gif) figures. PostScript files of
figures are available at http://www.iagusp.usp.br/preprints/preprint.htm
Particle transport and heating in the microturbulent precursor of relativistic shocks
Collisionless relativistic shocks have been the focus of intense theoretical
and numerical investigations in recent years. The acceleration of particles,
the generation of electromagnetic microturbulence and the building up of a
shock front are three interrelated essential ingredients of a relativistic
collisionless shock wave. In this paper we investigate two issues of importance
in this context: (1) the transport of suprathermal particles in the excited
microturbulence upstream of the shock and its consequences regarding particle
acceleration; (2) the preheating of incoming background electrons as they cross
the shock precursor and experience relativistic oscillations in the
microturbulent electric fields. We place emphasis on the importance of the
motion of the electromagnetic disturbances relatively to the background plasma
and to the shock front. This investigation is carried out for the two major
instabilities involved in the precursor of relativistic shocks, the
filamentation instability and the oblique two stream instability. Finally, we
use our results to discuss the maximal acceleration at the external shock of a
gamma-ray burst; we find in particular a maximal synchrotron photon energy of
the order of a few GeV.Comment: 14 pages, 6 figures. Revised versio
Chaotic dynamics in a storage-ring Free Electron Laser
The temporal dynamics of a storage-ring Free Electron Laser is here
investigated with particular attention to the case in which an external
modulation is applied to the laser-electron beam detuning. The system is shown
to produce bifurcations, multi-furcations as well as chaotic regimes. The
peculiarities of this phenomenon with respect to the analogous behavior
displayed by conventional laser sources are pointed out. Theoretical results,
obtained by means of a phenomenological model reproducing the evolution of the
main statistical parameters of the system, are shown to be in a good agreement
with experiments carried out on the Super-ACO Free Electron Laser.Comment: submitted to Europ Phys. Journ.
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