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

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 3mm 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 theend 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

Study of atmospheric pressure radiofrequency Ar/02 plasma afterglow used for PTFE surface modification

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Electron impact transfer rates between metastable and resonant states of argon investigated by laser pump-probe technique

\u3cp\u3eThe laser pump-probe technique is used to study the electron impact transfer between the 1s\u3csub\u3e5\u3c/sub\u3e and 1s\u3csub\u3e4\u3c/sub\u3e states of argon (in Paschen's notation) belonging to the \u3csup\u3e2\u3c/sup\u3eP\u3csub\u3e3/2\u3c/sub\u3e ion core for electron temperatures in the range of 1-2 eV. A rate coefficient of m\u3csup\u3e3\u3c/sup\u3es\u3csup\u3e-1\u3c/sup\u3e is determined for the transfer from 1s\u3csub\u3e5\u3c/sub\u3e to 1s\u3csub\u3e4\u3c/sub\u3e state. Different pumping schemes between the 1s and 2p states but also 3p states are used to verify the obtained value but also to probe the transfers with ion-core change toward the \u3csup\u3e2\u3c/sup\u3eP\u3csub\u3e1/2\u3c/sub\u3e ion-core. Our results show the presence of an important transfer channel between 1s\u3csub\u3e2\u3c/sub\u3e and 1s\u3csub\u3e4\u3c/sub\u3e states, and a rate coefficient of m\u3csup\u3e3\u3c/sup\u3e s\u3csup\u3e-1\u3c/sup\u3e is estimated for this transfer. The present results confirm that recent quantum mechanical calculations by Zatsarinny et al [1] underestimate significantly the cross sections for electron impact transfers between 1s states of argon.\u3c/p\u3

Effect of the driving frequency on an atmospheric pressure RF capacitively coupled plasma in Argon

The plasma properties of a capacitively coupled atmospheric pressure plasma in Argon as a function of the driving frequency fd is studied for a number of frequencies in the range 4–40 MHz by means of a 2D fluid model. It is observed that the increase of fd while keeping the applied voltage constant results into amplification of the chemical processes. More neutral active species are produced, which is mainly due to the increase of the electron density ne. However the increase of ne will also facilitate the destruction of excited species Ar* so that the increase of excited species density n(Ar*) will lag behind that of ne

Laser assisted electron gas heating: revision of the criterion for high pressure non-thermal plasmas

Lasers are commonly used nowadays to investigate the plasma properties (laser-aided plasma diagnostics LAPD). However, in using LAPD, one should always be careful in tuning the laser power density to avoid perturbation of the plasma during the diagnostic. A general formula was given by Kunze [1] to determine the heating of electrons during laser diagnostics via electron-ion inverse bremsstrahlung (IB). In this contribution, it is shown that for low ionization degree and high pressure plasmas, electron-ion IB is a negligible heating process and that electron-atom IB becomes dominant. The criterion for non-invasive LAPD needs then to be revised consequently

PTFE treatment by remote atmospheric Ar/O\u3csub\u3e2\u3c/sub\u3e plasmas:a simple reaction scheme model proposal

\u3cp\u3ePolytetrafluoroethylene (PTFE) samples were treated by a remote atmospheric pressure microwave plasma torch and analyzed by water contact angle (WCA) and X-ray photoelectron spectroscopy (XPS). In the case of pure argon plasma a decrease of WCA is observed meanwhile an increase of hydrophobicity was observed when some oxygen was added to the discharge. The WCA results are correlated to XPS of reference samples and the change of WCA are attributed to changes in roughness of the samples. A simple kinetics scheme for the chemistry on the PTFE surface is proposed to explain the results.\u3c/p\u3

A power pulsed low-pressure argon microwave plasma investigated by Thomson scattering : evidence for molecular assisted recombination

A squared-wave power pulsed low-pressure plasma is investigated by means of Thomson scattering. By this method the values of the electron density and temperature are obtained, directly. The plasma is created by a surfatron launcher in pure argon at gas pressures of 8–70 mbar. Features of the pulse rise and decay are studied with microsecond time resolution. During the pulse rise we observe initial high temperature values, while the density is still rising. At power switch-off we find decay times of the electron density that are smaller than what is expected on the basis of diffusion losses. This implies that the dominant decay mechanism in the studied pressure regime is provided by molecular assisted recombination