Instabilities in high-power impulse magnetron plasmas: from stochasticity to periodicity

We observed that high-power pulsed magnetron plasmas show instabilities in the form of rotating emission structures. Fast CCD camera measurements show motion into the E x B direction. Their characteristic frequencies are in the 100 kHz range. They are correlated with a modulation of the floating potential and changes in the ion saturation current of a Langmuir probe. Rotation velocities are of the order of 10 km s(-1). The azimuthal mode number, i.e. the number of emission nodes depends on current. The fast Fourier transform (FFT) analysis of the frequency spectra indicates that the structures condense out of broad band fluctuations suggesting transition from stochastic to periodic behaviour. FFT of the Langmuir probe signals also identifies other types of instabilities at around 2.4 MHz, which is corresponding to the frequency of a modified two-stream instability.