Electrode-less Configuration Research Articles

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 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.

Tunable liquid microlens arrays in electrode-less configuration and their accurate characterization by interference microscopy

A special class of tunable liquid microlenses is presented here. The microlenses are generated by an electrowetting effect under an electrode-less configuration and they exhibit two different regimes that are named here as separated lens regime (SLR) and wave-like lens regime (WLR). The lens effect is induced by the pyroelectricity of polar dielectric crystals, as was proved in principle in a previous work by the same authors (S. Grilli et al., Opt. Express 16, 8084, 2008). Compared to that work, the improvements to the experimental set-up and procedure allow to reveal the two lens regimes which exhibit different optical properties. A digital holography technique is used to reconstruct the transmitted wavefront during focusing and a focal length variation in the millimetre range is observed. The tunability of such microlenses could be of great interest to the field of micro-optics thanks to the possibility to achieve focus tuning without moving parts and thus favouring the miniaturization of the optical systems.

Surface-Charge Lithography for Direct PDMS Micro-Patterning

Direct patterning of PDMS films is achieved by modulating the wettability of polar dielectric substrates. Periodic array structures of microbumps can be madeup by functionalizing periodically poled lithium niobate crystals. The modulation of surface wettability is obtained through the spatial distribution of the surface electric charges generated by the pyroelectric effect under electrode-less configuration. An appropriate thermal treatment of the substrates assures both the wettability patterning and the fast cross-linking of the PDMS film.

Dielectrophoretic trapping of suspended particles by selective pyroelectric effect in lithium niobate crystals

We have developed a dielectrophoretic approach for trapping suspended dielectric particles. The electric forces were produced by using the pyroelectric effect in periodically poled lithium niobate substrates. Complex electric field distributions can be built up through an appropriate control of the temperature and the particles can be distributed according to the geometry of the reversed ferroelectric domain structure. The electrode-less configuration makes the technique easier to accomplish compared to conventional dielectrophoretic devices, with interesting possibilities of applications in the field of biology and biomedicine.

Wettability patterning of lithium niobate substrate by modulating pyroelectric effect to form microarray of sessile droplets

The wettability of lithium niobate crystals was patterned by inducing the pyroelectric effect onto domain engineered samples. Compared to conventional electrowetting experiments, this technique allows one to obtain wettability patterning by an electrodeless configuration. In fact, the surface charges inducing the effect were generated pyroelectrically and spatially modulated by the domain grating. A microarray of sessile droplets was obtained on a single chip. Results and physical interpretation of the phenomena are reported and discussed.

The disk generator, its status and its potential

The MHD disk generator has been used as an experimental tool to study plasma properties and to explore new diagnostic techniques, since it provides a near-ideal geometry for plasma studies due to its electrodeless configuration combined with near-perfect insulating walls. Both experimental and theoretical studies have also determined the conditions for favorable performance of the disk, often with inlet swirl, as a configuration for electrical power generation in both open and closed cycle applications. This paper describes the state-of-the-art of the disk geometry for power generation, and it also reviews recent system studies which have integrated the disk generator in electrical power plants using coal as a fuel. These studies indicate that the disk system can achieve overall efficiencies comparable to linear generator systems, but they also show that the disk configuration might provide significant reliability and require lower capital investments. These cost advantages are derived from the simplicity of the superconducting magnet and power management systems.