  The performance and long term stability of an optically readout Time
Projection Chamber with an electron amplification structure based on three Gas
Electron Multipliers was studied. He/CF$_4$ based gas mixtures were used in two
different proportions (60/40 and 70/30) in a CYGNO prototype with 7 litres
sensitive volume. With electrical configurations providing very similar
electron gains, an almost full detection efficiency in the whole detector
volume was found with both mixtures, while a light yield about 20\% larger for
the 60/40 was found. The electrostatic stability was tested by monitoring
voltages and currents during 25 days. The detector worked in very stable and
safe condition for the whole period. In the presence of less CF$_4$, a larger
probability of unstable events was clearly detected.
