Nuclear Inst. and Methods in Physics Research, A 884 (2018) 25–30
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Nuclear Inst. and Methods in Physics Research, A
journal homepage: www.elsevier.com/locate/nima
Real-time interferometric diagnostics of rubidium plasma
G.P. Djotyan, J.S. Bakos, M.Á. Kedves *, B. Ráczkevi, D. Dzsotjan, K. Varga-Umbrich, Zs. Sörlei,
J. Szigeti, P. Ignácz, P. Lévai, A. Czitrovszky, A. Nagy, P. Dombi, P. Rácz
Wigner Research Centre for Physics of the Hungarian Academy of Sciences, 29-33 Konkoly-Thege Street, H-1121 Budapest, Hungary
a r t i c l e i n f o
Keywords:
CERN AWAKE project
Laser plasma
Interferometry
Plasma diagnostics
a b s t r a c t
A method of interferometric real-time diagnostics is developed and applied to rubidium plasma created by
strong laser pulses in the femtosecond duration range at different initial rubidium vapor densities using a
Michelson-type interferometer. A cosine fit with an exponentially decaying relative phase is applied to the
obtained time-dependent interferometry signals to measure the density–length product of the created plasma
and its recombination time constant. The presented technique may be applicable for real-time measurements of
rubidium plasma dynamics in the AWAKE experiment at CERN, as well as for real-time diagnostics of plasmas
created in different gaseous media and on surfaces of solid targets.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
Plasma generation in gases and on surfaces of solid targets by
intense ultrashort laser pulses has found an extremely broad spectrum
of applications in different fields of science and technology. These
include high order harmonics generation widely used for ultrashort laser
pulse generation, (see review paper [1] and references therein), plasma
wake-field based particle acceleration in novel compact accelerators for
charged particles, [2–6] and many others.
In this communication, we report results of our study of plasma
generation in rubidium (Rb) vapors by intense laser pulses of femtosec-
ond duration using the interferometry method. This study is directly
related to the Advanced Wake-Field Experiment (AWAKE) project at
CERN, [7,8], which will be the first ever in the world proton-driven
plasma wake-field experiment. The aim of this project is the construction
of a relatively compact (and cheaper) accelerator of electrons (positrons)
to TeV energies in a single acceleration stage utilizing the proton bunch
available at the Super Proton Synchrotron (SPS) at CERN. The length
of the SPS proton bunch is in the range of 10–30 cm. The estimations
and simulations have shown that the maximum wake-field acceleration
in plasma is achieved when the length of the proton bunch is equal to
the length of the plasma wave, which is in the range of 100 μm [7,9].
Since short (∼ 100 μm) proton bunches are not available at SPS up to
now, transverse modulation of the current proton bunch was proposed
by means of the self-modulation-instability in plasmas. Thus, it would
be possible to obtain micro-bunches with length equal to the plasma
wavelength, [10].
*
Corresponding author.
E-mail address: kedves.miklos@wigner.mta.hu (M.
˙
Kedves).
As earlier estimations have shown, strict conditions on the plasma
density uniformity must be satisfied for an efficient self-modulation
instability effect [11], but according to recent numerical simulations,
some longitudinal plasma density gradient could be useful [12]. In any
case, the plasma density and its spatial distribution is a critical issue
for the successful transverse modulation of the proton bunch and the
efficient electron acceleration in the plasma wake field driven by the
produced proton micro-bunches. To assure the desired plasma density
it is necessary to ionize the single outer electron in all of the atoms
in the Rb vapor along the pass of the proton bunch. In this case, the
plasma density will be the same as the neutral Rb vapor initial density,
which can be controlled by the temperature of the Rb vapor. The strict
requirement on the plasma density uniformity can be fulfilled by confin-
ing Rb vapor in a steel tube with extremely homogeneous distribution
of the vapor temperature provided by external oil heating [11]. The
relatively low ionization potential of Rb, (4.18 eV) [13,14], makes it
relatively easy to ionize the single outer electron of Rb atoms. Thus,
one could fully ionize Rb vapor in the extended volume of the tube
by sufficiently intense (energetic) ultra-short laser pulses. It is worth
noting that the laser plasma source is an essential part of the setup of
the AWAKE experiment.
Diagnostics of the plasma generated by the ionizing laser pulses
in such a system is crucial for having an insight into the ionization
processes of the Rb atoms, and for detecting the density of the created
plasma as well as its time–space evolution. In Ref. [15], a method for
accurate measurement of the density of neutral Rb atoms was developed
https://doi.org/10.1016/j.nima.2017.12.004
Received 27 July 2017; Received in revised form 30 November 2017; Accepted 4 December 2017
Available online 7 December 2017
0168-9002/© 2017 Elsevier B.V. All rights reserved.