Self-organized filaments, striations and other nonuniformities in nonthermal atmospheric microwave excited microdischarges

Self-organized filaments, stationary striations, and spherical nonuniformities have been observed in atmospheric argon microdischarges sustained within a 120-µm gap between two coplanar electrodes. The microdischarges are driven by opposite ends of a half-wave split-ring resonator constructed using microstrip transmission lines. The microdischarge generator operates at 900 MHz using 0.5–2 W of power

Low-power microwave plasma source based on a microstrip split-ring resonator

Microplasma sources can be integrated into portable devices for applications such as bio-microelectromechanical system sterilization, small-scale materials processing, and microchemical analysis systems. Portable operation, however, limits the amount of power and vacuum levels that can be employed in the plasma source. This paper describes the design and initial characterization of a low-power microwave plasma source based on a microstrip split-ring resonator that is capable of operating at pressures from 0.05 torr (6.7 Pa) up to one atmosphere. The plasma source’s microstrip resonator operates at 900 MHz and presents a quality factor of Q = 335. Argon and air discharges can be self-started with less than 3Win a relatively wide pressure range. An ion density of 1.3 X 10(11) cm-3 in argon at 400 mtorr (53.3 Pa) can be created using only 0.5W. Atmospheric discharges can be sustained with 0.5 W in argon. This low power allows for portable air-cooled operation. Continuous operation at atmospheric pressure for 24 h in argon at 1 W shows no measurable damage to the source

Rotational, vibrational, and excitation temperatures of a microwave-frequency microplasma

Integration of microplasma sources in portable systems sets constraints in the amount of power and vacuum levels employed in these plasma sources. Moreover, in order to achieve good power efficiency and prevent physical deterioration of the source, it is desirable to keep the discharge temperature low. In this paper, the thermal characteristics of an atmospheric argon discharge generated with a low-power microwave plasma source are investigated to determine its possible integration in portable systems. The source is based on a microstrip split-ring resonator and is similar to the one reported by Iza and Hopwood, 2003. Rotational, vibrational, and excitation temperatures are measured by means of optical emission spectroscopy. It is found that the discharge at atmospheric pressure presents a rotational temperature of ~300 K, while the excitation temperature is ~0.3 eV (~3500 K). Therefore, the discharge is clearly not in thermal equilibrium. The lowrotational temperature allows for efficient air-cooled operation and makes this device suitable for portable applications including those with tight thermal specifications such as treatment of biological materials

Ultrahigh frequency microplasmas from 1 pascal to 1 atmosphere

The generation of plasma-on-a-chip is examined for two extremes in gas pressure. The application of microplasmas as sensors of industrial vacuum processes requires stable operation at gas pressures of less than 1 Pa. In this low-pressure regime, the addition of a static magnetic field that causes electron cyclotron resonance is shown to increase the emission intensity of the microplasma by 50%. Using atomic emission spectrometry, the detection of helium in air is found to have a detection limit of 1000 ppm, which is three orders of magnitude worse than the DL of SO2 in argon. The loss of sensitivity is traced to the high excitation energy threshold of He, and to the poor ionization efficiency inherent in an air plasma. At atmospheric pressure (105 Pa), a microdischarge is described that operates in a 25 mm-wide gap in a microstrip transmission line resonator operating at 900 MHz. The volume of the discharge is ~10-7 cm3, and this allows an atmospheric air discharge to be initiated and sustained using less than 3 W of power

Split-ring resonator microplasma : microwave model, plasma impedance and power efficiency

The microstrip split-ring resonator (MSRR) microplasma source is analysed and characterized using a microwave model of the device. Throughout the discussion, experimental data for three MSRR designs are also presented. The model identifies the key parameters that control the performance of the device and results in the formulation of closed-form expressions useful for designing, analysing and comparing MSRR designs. Matching the microstrip characteristic impedance to the microplasma impedance is found to be a key factor in the performance of these devices and it can be even more critical than the quality factor of the ring resonator. Based on the model, average rf electric fields of up to 4MVm−1 at 1Wof input power are estimated to be generated in a 45μm gap device. Furthermore, the model is used to determine the plasma impedance and thereby obtain information on physical properties of the microdischarge. Electron densities of the order of 10(14) cm−3 are estimated in a 1W argon discharge at atmospheric pressure. Based on the values of the plasma impedance, it is also determined that up to 70% of the power input to the MSRR is coupled to the electrons in the microdischarge

Influence of operating frequency and coupling coefficient on the efficiency of microfabricated inductively coupled plasma sources

Microfabricated inductively coupled plasma (mICP) generators, operating at 690 and 818 MHz, have been constructed and characterized. The mICP consists of a single-turn coil that is 5mm in diameter and a microfabricated matching network. Ion densities of ~9 × 10(10) cm−3 in argon at 400mTorr consuming only 1W were obtained. This ion density is three times larger than previous mICP sources under the same conditions. The influence of the frequency of operation and the coupling coefficient on the power efficiency has also been studied. Contrary to what was observed in former generations of mICP sources operating at lower frequencies, the efficiency of the new mICP sources decreases as the frequency increases. A model that incorporates the electron inertia, the power dependence of the plasma resistance and the frequency dependence of the coil resistance agrees with the new experimental results as well as with the results of previous mICP sources. It was also observed that bringing the coil closer to the plasma increases the coupling coefficient of the ICP sources and thereby improves the efficiency of the device. The improvement in efficiency, however, is limited by the non-scalable plasma sheath width near the coil

A microfabricated atmospheric-pressure microplasma source operating in air

An atmospheric-pressure air microplasma is ignited and sustained in a 25μm wide discharge gap formed between two co-planar gold electrodes. These electrodes are the two ends of a microstrip transmission line that is microfabricated on an Al2O3 substrate in the shape of a split-ring resonator operating with a resonant frequency of 895 MHz. At resonance, the device creates a peak gap voltage of ~390V with an input power of 3W, which is sufficient to initiate a plasma in atmospheric pressure air. Optical emission from the discharge is primarily in the ultraviolet region. In spite of an arc-like appearance, the discharge is not in thermal equilibrium as the N2 rotational temperature is 500–700 K. The intrinsic heating of the Al2O3 substrate (to 100°C) causes a downward shift in the resonant frequency of the device due to thermal expansion. The temperature rise also results in a slight decrease in the quality factor (142 > Q > 134) of the resonator. By decreasing the power supply frequency or using a heat sink, the microplasma is sustained in air. Microscopic inspection of the discharge gap shows no plasma-induced erosion after 50 h of use