High Power Laser Science and Engineering, (2016), Vol. 4, e39, 8 pages.
© The Author(s) 2016. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/
licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
doi:10.1017/hpl.2016.38
Ultrahigh temporal contrast performance of the
PHELIX petawatt facility
V. Bagnoud
1,2
and F. Wagner
1,2
1
Plasma physics dep., GSI Helmholtzzentrum f
¨
ur Schwerionenforschung GmbH, Planckstrasse 1, 64291 Darmstadt, Germany
2
Helmholtz Institute Jena, Fr
¨
obelstieg 3, 07743 Jena, Germany
(Received 17 June 2016; revised 2 September 2016; accepted 22 September 2016)
Abstract
We report on the temporal contrast performance of the PHELIX facility in view of the requirements imposed by solid-
target interaction experiments. The requirement analysis for the nanosecond and picosecond temporal contrast is derived
from empirical data and simple theoretical modeling, while the realization shows that using an ultrafast optical parametric
amplifier and plasma mirrors enables meeting this specification.
Keywords: amplified spontaneous emission; chirped-pulse amplification; optical parametric amplification; temporal contrast
1. Introduction
The Petawatt High-Energy Laser for heavy Ion Experiment
(PHELIX) facility at GSI is a versatile dual front end glass
laser system offering beam time to the international commu-
nity since 2008
[1]
for experiments in the fields of atomic and
plasma physics, exploiting the world-wide unique capability
of making combined heavy ion and laser beam experiments.
The short-pulse operation mode of PHELIX is based on the
chirped-pulse amplification (CPA) scheme to efficiently use
the full spectral bandwidth of the glass main amplifier and
produce pulses as short as 400 fs on target. One of the
drawbacks of CPA is the generation of pulses that exhibit
a nanosecond pedestal related to amplified spontaneous
emission (ASE) generated by the amplifier. For this reason
and based on the widespread idea that the ASE is favorably
reduced when the seed pulse energy is increased
[2, 3]
, the
short-pulse front end of PHELIX has been recently upgraded
with a high temporal contrast amplifier module
[4]
. This
enables the injection of energetic temporally clean short
pulses in the CPA amplifier, which overcomes the limitation
of standard CPA laser systems.
The generation and amplification of temporally clean short
laser pulses is motivated by the necessity to avoid the gen-
eration and expansion of a pre-plasma in an unwanted and
uncontrolled manner before the peak of the pulse is reached.
For this reason, it is essential to understand the temporal
Correspondence to: V. Bagnoud, GSI Helmholtzzentrum f
¨
ur Schwe-
rionenforschung, Planckstrasse 1, 64291 Darmstadt, Germany. Email:
v.bagnoud@gsi.de
contrast requirements of solid-target laser interaction from
a pre-plasma generation stand point, at first. And second,
a thorough analysis of the temporal degradation sources
in the laser amplifier must be conducted, followed by the
implementation of the solutions that exist to overcome them.
Indeed, the goal is not to avoid target ionization because it
is clear that the ionization of the target will be reached at
some point during the intensity rise of the pulse, but rather
to ensure that this has a limited effect on the interaction
conditions.
In a first part, the paper revisits the theme of quantum
noise in amplifiers to derive a simple description of the ASE
contrast level in CPA lasers, which is put in perspective of the
laser ionization threshold of materials. Then a simple plasma
expansion model is used to quantify some requirements on
the speed of the pulse rising front for various solid materials,
where the pre-plasma is not allowed to flow excessively until
the peak of the intensity is reached. In a second part, the
paper describes the experimental implementation at PHELIX
of two techniques aiming at controlling the ASE level by
use of an ultrafast optical parametric amplifier (uOPA) and
aiming at stiffening the pulse leading edge by use of one
or two plasma mirrors. Altogether, this demonstrates that
PHELIX operates with an ultrahigh temporal contrast level
that fulfils the requirements of most of the targets commonly
used in modern laser–matter interaction experiments.
2. Requirement on the ASE level in petawatt lasers
The ASE level in CPA lasers has been historically problem-
atic because it is naturally above the ionization threshold
1