ε (eV)

several samples were kept more than 80 hours at a pressure lower than 10 −4 Pa.
However, after one day of incubation the whole spore population was recovered.
Also, a rapid recovery was observed when oxygen was flowing in the experimental
chamber at the working pressure.
The set-up was cleaned after every experiment to avoid cross-contamination.
The above series of tests showed that the low pressure oxygen environment does
not plays any role in spore inactivation, if plasma is not present.
2.4 Plasma diagnostics
2.4.1 Measurement of electrical properties
Plasma electrical properties in the vicinity of the bacteria sample were evalu-
ated from the I(V ) characteristics of a movable Pt cylindrical electrostatic probe,
0.4 mm in diameter and 6.25 mm in length (LP in Fig. 2.1) insertedinoxygen
plasma. During the experiments, the collecting surface of the probe was placed
horizontally, at 14 mm above the contaminated sample, on the symmetry axis of
both the ring antenna and the bacteria support. The Langmuir probing circuit is
presented in Fig. 2.6.
The compensation of the probe was realized to prevent the RF power influence
on the probe circuit. This was done by inserting an RF signal blocking filter in
the probe circuit, containing three LC circuits in series with the probe tip, having
the resonance frequencies the RF power supply frequency (i.e., f =13.56 MHz)
37