  Two different oscillation modes in microdischarge with parallel-plate
geometry has been observed: relaxation oscillations with frequency range
between 1.23 and 2.1 kHz and free-running oscillations with 7 kHz frequency.
The oscillation modes are induced by increasing power supply voltage or
discharge current. For a given power supply voltage, there is a spontaneous
transition from one to other oscillation mode and vice versa. Before the
transition from relaxation to free-running oscillations, the spontaneous
increase of oscillation frequency of relaxation oscillations form 1.3 kHz to
2.1 kHz is measured. Fourier Transform Spectra of relaxation oscillations
reveal chaotic behaviour of microdischarge. Volt-Ampere characteristics
associated with relaxation oscillations describes periodical transition between
low current, diffuse discharge and normal glow. However, free-running
oscillations appear in subnormal glow only.
