Speaker
Joshua Timothy Moody
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.2016.pdf
Laser Ionized Rubidium Plasma Column Geometric Effects on Wakefields
at AWAKE
J. T. Moody for the AWAKE Collaboration
Max Planck Institute for Physics, Munich, Germany
AWAKE is a proton driven plasma wakefield electron acceleration experiment at CERN [1].
In this experiment a proton bunch much longer than the plasma wavelength, i.e., σz >> λpe = c/fpe ,
undergoes seeded self-modulation through a plasma, resulting in a radially modulated proton
bunch that effectively drives wakefields. The plasma column through which this process occurs
is created by a laser field in the intermediate Keldysh/photoionization regime of rubidium va-
por. The rubidium vapor’s density is controlled within <0.5% over its 10 m length, resulting
in a plasma column of uniform density at its transverse center and sharply falling radial edges
(σedge << λpe ) for the full length of the vapor. Resonances of two neutral rubidium valence elec-
tronic transitions fall within the bandwidth of the ionizing laser pulse. These resonances create
a region of dynamically saturated anomalous dispersion near the radial edge of the plasma col-
umn where the ionizing laser field is strong but significant populations of unionized rubidium
atoms exist. This dynamically dispersive region causes the ionizing laser pulse to propagate in
an unusual way, changing the geometry of the radial plasma boundary. At AWAKE the opera-
tional density range allows plasmas where λpe ∼ Rplasma , resulting in wakefields that can react
with the radial plasma boundary. This effect is more pronounced at the lower limit of the density
range and particularly if there is inexact spatial overlap between the ionizing laser pulse and the
proton bunch through the rubidium vapor. In this presentation, we explore the determination of
the plasma column and the effects that the column wall may have on wakefields driven by a self
modulated proton bunch with and without perfectly symmetric cylindrical overlap.
References
[1] A. Caldwell et al., Phys. Plasmas 18, 103101 (2011)
