Abstracts Brochure - CERN

THPCH001
THPCH002
THPCH003
29-Jun-06 16:00 - 18:00 THPCH — Poster Session
THPCH — Poster Session
Nonlinear Stability in the Transport of Mismatched Beams in a Uniform Focusing Field
A nonlinear stability analysis of mismatched
R. Pakter, F.B. Rizzato, W. Simeoni (IF-UFRGS)
breathing beams considering nonaxysymmetric
perturbations is performed. It is
shown that breathing oscillations of an initially quasi-round beam may nonlinearly induce quadrupole-like oscillations,
with a possible increase of the beam size along one direction. A simple model for the nonlinear coupling is
developed to clarify the instability mechanism. Growth rates are determined and compared to that of other instabilities
that affect mismatched beams, such as halo formation. Self-consistent simulations with different beam loading
distributions are used to verify the findings.
Combined Centroid-envelope Dynamics of Intense, Magnetically Focused Charged Beams
Surrounded by Conducting Walls
This paper analyses the combined envelope-
F.B. Rizzato, K. Fiuza, R. Pakter (IF-UFRGS)
centroid dynamics of magnetically focused
high-intensity charged beams surrounded by
conducting walls. Similarly to the case were conducting walls are absent*, we show that the envelope and centroid
dynamics decouples from each other. Mismatched envelopes still decay into equilibrium with simultaneous emittance
growth, but the centroid keeps oscillating with no appreciable energy loss. Some estimates are performed to
analytically obtain some characteristics of halo formation seen in the full simulations.
*Moraes et al. Phys. Rev. Lett. 93, 244801 (2004).
Influence of Beam-Breakup Instabilities on Electron Focusing
M. Caron, F. Cartier, D.C. Collignon, L.H. Hourdin, M. Mouillet,
D.P. Paradis, N. Pichoff (CEA)
386
High intensity electron beam focusing is a
key issue for the successful development
of flash radiography at hydro test facilities.
AIRIX is a 2 kA, 19 MeV, 60 ns, single shot
linear accelerator that produces X-rays from the interaction between relativistic electrons and a Tantalum solid target
(Ta). In that emittance dominated process, different physical mechanisms can limit the end focusing of the intense
electron beam on Ta. In the present paper, the role of the beam break-up (BBU) instabilities is pointed out. The theory
of BBU for induction linacs has been developed for quite some time. For an accelerating beam with steady state radial
oscillations at the accelerator entrance, the theoretical prediction regarding the amplitude growth after n successive
cavities agrees with the experimental observations. Therefore, as a result of the subsequent emittance growth a spot
size enlargement would be expected. As a matter of fact, we found out experimentally that no direct correlation can
be drawn up between BBU growth and focal spot dimensions over the range of transverse beam motion investigated.
This finding gives now new orientations on the way to reduce the spot size.