  The central tracking system of the HADES detector, installed at the SIS-18
synchrotron at GSI/Darmstadt (Germany), employs large-area, low-mass drift
chambers, featuring Aluminum potential wires and small cell sizes. The chambers
in front of the magnetic field, closest to the interaction point, have
developed significant self-sustained currents and discharges during operation,
most probably triggered by isobutane-based gas mixtures. Only both, (i)
replacing isobutane by CO2 and (ii) adding 1000 to 3500 ppmv of water into the
Ar/CO2 counting gas mixture, individually optimized for a given chamber,
allowed to recover the chambers, enabling stable operation in several
production runs since then, e.g. with high-intensity heavy-ion induced
reactions. The origin of the instability was found to be deposits on the
cathode wires, provoking the Malter-like effects, by visual inspection and
energy-dispersive X-ray spectroscopy. The charge on the wires accumulated
during their lifetime does not point to so-called classical aging, but
presumably the interaction of isobutane with materials in the gas flow,
residual impurities, and reaction products formed in plasma, e.g., built by
discharges.
