Evidence for a propagating density structure in a pulsed DC magnetron discharge

Using a Langmuir probe the temporal and spatial evolution of the plasma density along the centreline of a pulsed DC magnetron has been studied. We restrict the study to one set of discharge parameters, namely a 2 kHz pulse frequency, a 50% duty cycle, the use of argon gas of pressure 0.53 Pa and a titanium target. The results show that during the transition from duty on to off phases a ‘density-wave’ propagates from the cathode downstream into the plasma bulk at a speed of about 1100 ms−1. This velocity is close to the mean ion-acoustic speed in the decaying plasma. A discussion of the plasma density variation throughout the pulse cycle is given.