  The properties of laser-accelerated ion bunches are demanding and require
development of suitable beam diagnostics. In particular, the short and intense
particle bunches with a broad energy spectrum emitted in conjunction with a
strong electromagnetic pulse (EMP) are challenging for conventional and well
established monitoring systems. An approach based on measuring the acoustic
signals of particles depositing their energy in water, referred to as
ionoacoustics [https://doi.org/10.1118/1.4905047] was recently developed into
Ion-Bunch Energy Acoustic Tracing (I-BEAT)\cite{haffa_i-beat:_2019}. I-BEAT
allows online detection of single proton bunches while being cost effective and
EMP resistant. A simple water phantom equipped with only one ultrasound
transducer positioned on the ion axis allows for reconstructing a rather
complex energy spectrum that is typical for (manipulated) laser-accelerated ion
bunches. To deduce the lateral bunch properties, additional transducers can be
added, for example perpendicular to the ion beam axis. This established setup
has been adapted for use closely behind the laser target and tested at the
PHELIX laser at GSI. The capability of the system to retrieve information about
the broad proton spectrum close to the source despite the harsh conditions has
been demonstrated. Future improvements are required, most importantly the
increase of dynamic range. Nevertheless, I-BEAT holds promise to evolve into an
online diagnostic tool particularly suited for laser-driven source development
and optimization at high repetition rates.
