High Power Laser Science and Engineering, (2017), Vol. 5, e4, 5 pages.
© The Author(s) 2017. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/
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doi:10.1017/hpl.2016.47
Proton probing of laser-driven EM pulses travelling in
helical coils
H. Ahmed
1
, S. Kar
1
, A.L. Giesecke
2
, D. Doria
1
, G. Nersisyan
1
, O. Willi
2
, C.L.S. Lewis
1
, and M. Borghesi
1
1
Centre for Plasma Physics, School of Mathematics and Physics, Queen’s University of Belfast, BT7 1NN, UK
2
Institut f
¨
ur Laser-und Plasmaphysik, Heinrich-Heine-Universit
¨
at, D
¨
usseldorf, Germany
(Received 1 August 2016; revised 4 November 2016; accepted 22 November 2016)
Abstract
The ultrafast charge dynamics following the interaction of an ultra-intense laser pulse with a foil target leads to the launch
of an ultra-short, intense electromagnetic (EM) pulse along a wire connected to the target. Due to the strong electric field
(of the order of GV m
−1
) associated to such laser-driven EM pulses, these can be exploited in a travelling-wave helical
geometry for controlling and optimizing the parameters of laser accelerated proton beams. The propagation of the EM
pulse along a helical path was studied by employing a proton probing technique. The pulse-carrying coil was probed
along two orthogonal directions, transverse and parallel to the coil axis. The temporal profile of the pulse obtained from
the transverse probing of the coil is in agreement with the previous measurements obtained in a planar geometry. The
data obtained from the longitudinal probing of the coil shows a clear evidence of an energy dependent reduction of the
proton beam divergence, which underpins the mechanism behind selective guiding of laser-driven ions by the helical coil
targets.
Keywords: acceleration; EM pulse; ion; laser; proton; proton probing
1. Introduction
Ion beams generated via the target normal sheath accel-
eration (TNSA) mechanism posses remarkable character-
istics such as high particle flux, ultra-low emittance and
short pulse duration, but also exhibit large envelope diver-
gence and broad energy distribution
[1]
. Although the two
latter properties are advantageous in plasma radiography
applications
[2–6]
, these are generally undesirable in view of
many other potential applications
[7, 8]
. Therefore controlling
and optimising the laser driven ion beam parameters has
been one of the intensively studied research topic over the
past decade
[8–13]
.
TNSA-driven proton beams have been extensively used as
a radiographic tool to study the dynamics of electric and
magnetic fields generated by intense laser interactions
[2–5]
.
The emission of an ultra-short burst of protons with a
broad energy spectrum from a point-like source allows the
implementation of point-projection probing schemes, while
providing multi-frame snapshots of the probed object. The
ultra-short burst duration enables a high temporal resolution
Correspondence to: S. Kar (Invited Speaker at HPLSE 2016), School of
Mathematics and Physics, Queen’s University of Belfast, BT7 1NN, UK.
Email: s.kar@qub.ac.uk
(typically of a few ps), while the beam laminarity and small
source size ensures a high spatial resolution
[14, 15]
.
The propagation of an electromagnetic (EM) pulse
generated by intense laser interaction with a solid tar-
get was recently studied by employing a self-probing
arrangement
[16–18]
. The ultra-short EM pulses with peak
electric field of the order of 10
9
V m
−1
, are generated
following the rapid charging of the laser-irradiated target
to MV potential, due to the prompt escape of the high energy
(MeV) electrons produced during the interaction
[2, 13, 19]
.
The EM pulse was observed propagating along a thin
metallic wire attached to the main foil target. In this case the
pulse-carrying wire was shaped into a square wave pattern
contained in a plane perpendicular to the axis of the probe
proton beam.
It has been shown recently
[16]
that, by directing such a
high amplitude EM pulse in a helical path around the proton
beam, the spectral and angular properties of the beam can
be controlled and optimized. This motivates the study of the
propagation of the EM pulse in a helical geometry, which
is presented in this paper. By following the spatial and
temporal evolution of the electric field across the helical coil,
probed transversely by the probe protons, the pulse profile
was reconstructed with the help of particle tracing simu-
lation. The characteristic parameters of the pulse, such as
1
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