104 research outputs found
Special Issue on Plenary and Invited Papers From ICOPS 2012
The 39th International Conference on Plasma Science (ICOPS) was held in Edinburgh, Scotland in July 2012. This was the third time that the conference was organized outside North America, the previous ones being in Karlsruhe (Germany) in 2008 and Jeju (S. Korea) in 2003. The technical programme combined seven technical areas of plasma science and technology covering a wide range of topics. The conference featured a wide range of advances in innovative plasma and beam science and applications, and served as a venue for an international community to meet and discuss their ideas and research results. More than 800 abstracts were received in 35 different topical areas, with more than half the papers originating outside the United State. The conference was attended by over 600 delegates and enjoyed the participation of over 200 registered students
Electron heating in radio-frequency capacitively coupled atmospheric-pressure plasmas
In atmospheric-pressure plasmas the main electron heating mechanism is Ohmic heating, which has
distinct spatial and temporal evolutions in the α and γ modes. In γ discharges, ionizing avalanches
in the sheaths are initiated not only by secondary electrons but also by metastable pooling reactions.
In α discharges, heating takes place at the sheath edges and in contrast with low-pressure plasmas,
close to 50% of the power absorbed by the electrons is absorbed at the edge of the retreating sheaths.
This heating is due to a field enhancement caused by the large collisionality in atmospheric-pressure
discharges
Evolution of the light emission profile in radio-frequency atmospheric pressure glow discharges
Time-resolved images of the light emission profiles of RF atmospheric pressure plasmas are presented. As the current increases, the emission profile evolves from bell shaped to double humped. At moderate currents, bright layers above each of the electrodes are observed, and it is found that both layers light up and fade simultaneously during the RF cycle
Reactive oxygen species production in atmospheric-pressure low-temperature He+O2+H2O plasmas
Low-temperature atmospheric pressure plasmas have received growing interest in recent
years, due to their increasing popularity in technological and biological applications. There
are many advantages to using these plasmas, for example, they are relatively cheap to run as
they do not require expensive vacuum equipment, they are portable, they can be run at near
room temperature and they can create complex reactive chemistries inside and outside the
discharge region
Electron avalanches and diffused γ-mode in radio-frequency capacitively coupled atmospheric-pressure microplasmas
Space-, time- and wavelength-resolved optical emission profiles suggest that the helium emission at 706 nm can be used to indicate the presence of high energy electrons and estimate the sheath in helium rf discharges containing small concentration of air impurities. Furthermore, the experimental data supports the theoretical predictions of energetic electron avalanches transiting across the discharge gap in rf microdischarges and the absence of an α-mode. Nonetheless, microdischarges sustained between bare metal electrodes and operating in the γ-mode can produce diffuse glowlike discharges rather than the typical radially constricted plasmas observed in millimeter-size rf atmospheric-pressure γ discharges
Atmospheric glow discharges from the high-frequency to very high-frequency bands
This letter reports an experimental investigation of an atmospheric glow discharge in both the high-frequency (HF) band of 3–30 MHz and the very high frequency band of 30–300 MHz. At constant input power, increased frequency is found to change little the electron density and to reduce slightly the electron excitation temperature. Significantly, an eightfold frequency increase from 20 to 80 MHz leads to a 20-fold increase in the maximum plasma power without plasma constriction. The maximum power density of 355 W/cm3 achieved at 80 MHz is far greater than those reported in the HF band
From submicrosecond-to nanosecond-pulsed atmospheric-pressure plasmas
We have developed a time-hybrid computational
model to study pulsed atmospheric-pressure discharges and compared
simulation results with experimental data. Experimental
and computational results indicate that increasing the applied
voltage results in faster ignition of the discharge and an increase
in the mean electron energy, opening the door to tunable plasma
chemistry by means of pulse shaping. Above a critical electric field
of ~2 kV/mmfor ~1-mm discharges, pulsed plasmas ignite right
after the application of an externally applied voltage pulse. Despite
the large pd value (30–300 torr · cm) and the high applied electric
field, the discharges are found to be streamer free in a desirable
glowlike mode. The comparison of the time evolution of the mean
electron kinetic energy as a function of the pulse rise time suggests
that a fast rise time is not necessarily the best way of achieving
high mean electron energy
Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC
The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes
(LEDs) is limited due to their poor light extraction efficiency (LEE). To
improve the LEE of AlGaN LEDs, we developed a fabrication technology to process
AlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with high
LEE. This process transfers the AlGaN LED epi onto a new substrate by
wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a
highly selective SF6 plasma etch that stops at the AlN buffer layer. We
optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a
substrate-removal process with high reliability and precise epitaxial control,
without creating micromasking defects or degrading the health of the plasma
etching system. The SiC etch rate by SF6 plasma was ~46 \mu m/hr at a high RF
bias (400 W), and ~7 \mu m/hr at a low RF bias (49 W) with very high etch
selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and
AlN was essential for removing the SiC substrate and exposing a pristine,
smooth AlN surface. We demonstrated the epi-transfer process by fabricating
high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further
enhance the light extraction, the exposed N-face AlN was anisotropically etched
in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at
room temperature. This AlGaN TFFC LED process establishes a viable path to high
external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C
LEDs.Comment: 22 pages, 6 figures. (accepted in Semiconductor Science and
Technology, SST-105156.R1 2018
Chaos in atmospheric-pressure plasma jets
We report detailed characterization of a low-temperature atmospheric-pressure plasma jet that exhibits regimes of periodic, quasi-periodic and chaotic behaviors. Power spectra, phase portraits, stroboscopic section and bifurcation diagram of the discharge current combine to comprehensively demonstrate the existence of chaos, and this evidence is strengthened with a nonlinear dynamics analysis using two control parameters that maps out periodic, period-multiplication, and chaotic regimes over a wide range of the input voltage and gas flow rate. In addition, optical emission signatures of excited plasma species are used as the second and independent observable to demonstrate the presence of chaos and period-doubling in both the concentrations and composition of plasma species, suggesting a similar array of periodic, quasi-periodic and chaotic regimes in plasma chemistry. The presence of quasi-periodic and chaotic regimes in structurally unbounded low-temperature atmospheric plasmas not only is important as a fundamental scientific topic but also has interesting implications for their numerous applications. Chaos may be undesirable for industrial applications where cycle-to-cycle reproducibility is important, yet for treatment of cell-containing materials including living tissues it may offer a novel route to combat some of the major challenges in medicine such as drug resistance. Chaos in low-temperature atmospheric plasmas and its effective control are likely to open up new vistas for medical technologies
Atmospheric-pressure gas breakdown from 2 to 100 MHz
We report a detailed study of breakdown voltage of atmospheric-pressure helium gas between two
parallel-plate electrodes from 2 to 100 MHz. Experimental data show that the breakdown voltage
reduces initially with increasing frequency due to a diminishing contribution of drift-dominated
electron wall loss and then begins to increase with increasing frequency. The latter is contrary to the
current understanding that relies largely on the electron wall loss mechanism. Particle-in-cell
simulation suggests that rapid oscillation of the applied voltage prevents electrons from reaching
their maximum achievable kinetic energy, thus compromising the ionization efficiency and
increasing the breakdown voltage
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