  This work shows for the first time experimentally the close relation between
return currents from relativistic laser-driven target polarization and the
quality of the relativistic laser plasma interaction for laser driven secondary
sources. Such currents rise in all interaction schemes where targets of any
kind are charged by escaping laser-accelerated relativistic electrons.
Therefore, return currents can be used as a metrological online tool in the
optimization of many laser-driven secondary sources and for diagnosing their
stability. We demonstrate the destruction free measurement of return currents
at the example of a tape target system irradiated by the 1 PW VEGA3 laser at
CLPU at its maximum capabilities for laser-driven ion acceleration. Such
endeavour paves the ground for feedback systems that operate at the
high-repetition-rate of PW-class laser systems.
