Ultra-fast pulsed microwave plasma breakdown: evidence of various ignition modes

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.

Streamer knotwilg branching: sudden transition in morphology of positive streamers in high-purity nitrogen

“…An alternative explanation is that the streamers follow the ionisation region at a specific critical electron density. Such an electron density has recently been found in pulsed microwave discharges [22]. No indications have been found in literature for the existence of a similar critical density in streamer discharges.…”

“…The authors could however not find any indications of a critical electron density in streamer discharges in literature, either experimentally or theoretically. A critical leftover electron density has recently been found by Carbone and Nijdam [22] in a different plasma discharge (a pulsed microwave discharge in argon). This caused the discharge morphology to change completely on a small change in repetition frequency.…”

Self-consistent fluid modeling and simulation on a pulsed microwave atmospheric-pressure argon plasma jet

“…35,36 As we known, these fluid models cannot simulate the microscopic features of charged particles. Thence, Kwon et al 17 have used an one-dimensional EM particle-in-cell plus Monte Carlo collision (PIC/MCC) method to study the microscopic features of charged particles and the electromagnetic characteristics in its transient states, for a pulsed atmospheric plasmas. The steady-state plasma parameters of surface wave sustained plasmas in a metal bound plasma slab have been simulated by Cooperberg and Birdsall, 37,38 adopting a 2D3V PIC/MCC model.…”

“…25 and 40) and at atmospheric pressure 20 have been simulated in our recent reports by using a 2D3V PIC/MCC model, respectively. In short summary, these fundamental results 17,25,[37][38][39][40] are meaningful for discharges operating over a range of frequencies, gas pressures, device designs, and efficient mechanism of power deposition.…”

“…16 Another important advantage, the temperature of plasma jet plume can be decreased, due to lower average power absorbed in the pulsed microwave discharge. 17 To pay attention on microwave-driven plasma devices, many researches have focused on optimizing the device structure, in order to acquire special plasma composition, plasma size, and more efficient energy absorbed by plasmas. [18][19][20][21][22] Our previous works have proposed some types of microwave-driven plasma devices excited by surface waves of surface plasmon polaritons (SPPs).…”