• R.P. Nunes, F.B. Rizzato
  IF-UFRGS, Porto Alegre

Non-homogeneity is a characteristic naturally present in non-neutral beams. Recently, a set of works has been developed by us for the case of beams initially homogeneous, making possible that relevant macroscopic quantities such as the RMS radius and emittance could be determined at equilibrium as functions of characteristic parameters of beam phase-space and of initial conditions. The present work intends to investigate the influences of the initial inhomogeneity in the beam route to equilibrium. Through the same methodology introduced in the studies for the homogeneous beams, both emittance and beam envelope have been obtained as functions of the magnitude of the inhomogeneity and some additional parameters associated with geometry of beam phase-space. The results obtained with this investigation have proven to be useful not only to better understand the effects of inhomogeneity over beam dynamics but also to provide physical background to the investigations previously carried out for homogeneous beams.

• R.P. Nunes, F.B. Rizzato
  IF-UFRGS, Porto Alegre

Experiments and numerical simulations show that high-intensity beams composed by charged particles usually reach their final stationary state with a progressive populating of a spatial region external to its original border. This populating process occurs in such terms that beam spatial limits at equilibrium increase by an amount of two or three times its initial nominal size. This is known as halo in Beam Physics. In this way, this work intends to better understand the time scale of halo formation. The carried out investigation has shown that the time scale of halo formation in fact can be segmented in two different quantities, each one associated to distinct physical mechanisms. One is related with the initial non-homogeneity naturally present in such systems, and the other is a result of the initial beam envelope mismatch. This investigation seems to be useful to design more efficient collimation systems and/or non-linear control systems for the next generation high-power accelerators.

• R. Pakter, L.C. Martins, F.B. Rizzato
  IF-UFRGS, Porto Alegre

Funding: Work supported by CNPq, Brazil and the US-AFOSR under Grant No. FA9550-06-1-0345.

In this paper, we investigate the combined envelope-centroid dynamics of relativistic continuous charged beams transported through a uniform focusing field and surrounded by a conducting wall. For such beams, the conducting wall screens the electric field but allows magnetic field penetration, enhancing the induced charges effect on the beam transport. As a consequence, in contrast to the case of nonrelativistic beams where the walls are shown to have little effect*, relativistic beams may have their centroid motion severely affected, leading to limitations in the total current and area occupied by the beam inside the conductor. Self-consisted simulation are used to verify the findings.

*K. Fiuza, F. B. Rizzato, and R. Pakter, Phys. Plasmas, 13, 023101 (2006).

• F.B. Rizzato, A. Endler, R.P. Nunes, R. Pakter, E.G. Souza
  IF-UFRGS, Porto Alegre

Funding: CNPq, Brazil; AFOSR FA9550-06-1-0345, USA

In this work we analyze the dynamics of mismatched inhomogeneous beams of charged particles. Initial inhomogeneities lead to propagating density waves across the beam core, and the presence of density waves eventually results in density build up and particle scattering. Particle scattering off waves in the beam core and the presence of resonances due to envelope mismatches ultimately generate a halo of particles with concomitant emittance growth. Emittance growth directly indicates when the beam relaxes to its final stationary state, and the purpose of the present paper is to describe halo and emittance in terms of test particles moving under the action of the mismatched inhomogeneous beam. To this end we develop an average Lagrangian approach for the beam where both density and envelope mismatches are incorporated. Test particle results compare well with full simulations.

• A. Bonatto, R. Pakter, F.B. Rizzato
  IF-UFRGS, Porto Alegre

Funding: This work has received financial support from AFOSR, Arlington, VA (under Grant FA9550-06-1-0345) and from CNPq, Brazil.

In the present analysis we study the self consistent propagation of intense laser pulses in a cold relativistic ideal-fluid underdense plasma, with particular interest in how the envelope dynamics is affected by the plasma frequency. Analysis of the linear system associated with the chosen model shows the existence of thresholds that can led propagating pulses to distinct modulational instabilities, according to the relation between its transversal wave vector and the plasma frequency.