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Plasma Sources Science and Technology , 2014 , Volume 23 Number 1.
Emile Carbone , Sander Nijdam.
Department of Applied Physics, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands.
Abstract
In this communication, we investigate the ignition of pulsed microwave plasmas in a narrow dielectric tube with an electrodeless configuration. The plasma is generated using a surfatron cavity. The power is modulated as a square wave with a rise-time of 30 ns at variable frequencies from 100 Hz up to 5 MHz. The ignition and plasma propagation inside the 3 mm radius quartz tube are imaged spatially and resolved with nanosecond time resolution using an iCCD camera.
The plasma is found to propagate in the form of a front moving from the launcher to the end of the plasma column with the microwave power being gradually absorbed behind it. The velocity of the plasma front decreases while the plasma goes towards a steady state.
The ionization front is found to be strongly non-uniform and various structures as a function of the pulse repetition frequency (i.e. power-off time) are shown in the axial and radial directions. At low frequencies, finger-like structures are found. The plasma becomes more hollow at smaller power-off times. At higher repetition frequencies (kHz regime), a critical repetition frequency is found for which the plasma light intensity sharply increases at the head of the propagation front, taking a shape resembling a plasma bullet. This critical frequency depends on the pressure and power. For even higher frequencies, the bullet shape disappears and plasma volume ignition from the launcher to the end of the plasma column is observed.
These results bring a new insight into the ignition mechanisms of pulsed microwave plasmas inside dielectric tubes. A wide variety of effects are found which seem to mostly depend on the background ionization degree. Moreover, the results show that only a 3D time-dependent model can, in general, correctly describe the ignition of a pulsed microwave discharge.
Additional Information
Plasma assisted technologies have nowadays an undisputed impact for many industrial relevant applications. They traditionally played a key role in analytical spectrometry since the beginning of the twentieth century but also in various fields such as welding, the microelectronic industry, nanotechnologies, air purification, lighting, satellite thrusters and more recently in medicine as well. For all these applications, various plasma sources were developed over the years which range from vacuum pressure up to several hundreds of atmospheres. The power is usually coupled inside the discharge volume via a static or oscillating electromagnetic field. More recently, pulsed modulation of the power input has been found to be an extra method to efficiently tune plasma properties. In this paper, the effect of ultra fast power modulation of a microwave surface wave discharge was analyzed by mean of nanosecond imaging. Such discharges are used for instance in the field of optical fibers and solar cells production. Various ignition modes of the plasma were found as a function of the frequency of the repetitively pulsed microwave square power input. These new findings bring a better understanding of pulsed microwave plasmas but also give some limitations to what can be achieved with ultra-fast plasma power pulsing.
