  Micro hollow cathode discharges (MHCD) were produced using 250 m thick
dielectric layer of alumina sandwiched between two nickel electrodes of 8 m
thickness. A through cavity at the center of the chip was formed by laser
drilling technique. MHCD with a diameter of few hundreds of micrometers allowed
us to generate direct current discharges in helium at up to atmospheric
pressure. A slowly varying ramped voltage generator was used to study the
ignition and the extinction periods of the microdischarges. The analysis was
performed by using electrical characterisation of the V-I behaviour and the
measurement of He*(3S1) metastable atoms density by tunable diode laser
spectroscopy. At the ignition of the microdischarges, 2 s long current peak as
high as 24 mA was observed, sometimes followed by low amplitude damped
oscillations. At helium pressure above 400 Torr, an oscillatory behaviour of
the discharge current was observed just before the extinction of the
microdischarges. The same type of instability in the extinction period at high
pressure also appeared on the density of He*(3S1) metastable atoms, but delayed
by a few s relative to the current oscillations. Metastable atoms thus cannot
be at the origin of the generation of the observed instabilities.
