Role of low-energy electrons in Ar emission from low-pressure radio frequency discharge plasma

Optical emission spectra from a low-pressure Ar plasma were studied with high spatial resolution. It has been shown that the intensity ratios of Ar lines excited through metastable levels to those excited directly from the ground state are sensitive to the shape of electron energy distribution function. From these measurements, important information on the spatial variation of plasma parameters can be obtained. (C) 1999 American Institute of Physics. [S0003-6951(99)01629-0].

Structure observed in measured electron energy distribution functions in capacitively coupled radio frequency hydrogen plasmas

Electron energy probability functions measured with a passively compensated Langmuir probe in asymmetric capacitively coupled hydrogen and deuterium plasmas exhibit structure. The otherwise relatively continuous distribution appears to have an abrupt peak in electron density near 5 eV. This structure occurs at a higher energy in deuterium than hydrogen and there is a correlation between floating potential and the voltage at which the structure is observed in the second derivative of the I(V) characteristic. While the cause of the structure has yet to be clarified, spectroscopic observations and computer-based hydrogen models indicate that the high energy tail of the distribution is strongly modulated during the radio frequency cycle. The effect of this modulation on plasma properties and probe measurements has yet to be explored. (C) 1999 American Institute of Physics. [S0003-6951(99)00819-0].

TIME-RESOLVED OPTICAL-EMISSION AND ELECTRON-ENERGY DISTRIBUTION FUNCTION MEASUREMENTS IN RF PLASMAS

Comparisons between experimentally measured time-dependent electron energy distribution functions and optical emission intensities are reported for low-frequency (100 and 400 kHz) radio-frequency driven discharges in argon. The electron energy distribution functions were measured with a time-resolved Langmuir probe system. Time-resolved optical emissions of argon resonance lines at 687.1 and 750.4 nm were determined by photon-counting methods. Known ground-state and metastable-state excitation cross sections were used along with the measured electron energy distribution functions to calculate the time dependence of the optical emission intensity. It was found that a calculation using only the ground-state cross sections gave the best agreement with the time dependence of the measured optical emission. Time-dependent electron density, electron temperature, and plasma potential measurements are also reported.

Analysis of Fixed-Speed Wind Farm Low-Frequency Power Pulsations using a Wavelet-Prony Method

The increasing penetration of wind generation on the Island of Ireland has been accompanied by close investigation of low-frequency periodic pulsations contained within the active power flow from different wind farms. A primary concern is excitation of existing low-frequency oscillation modes already present on the system, particularly the 0.75 Hz mode as a consequence of the interconnected Northern and Southern power system networks. Recently grid code requirements on the Northern Ireland power system have been updated stipulating that wind farms connected after 2005 must be able to control the magnitude of oscillations in the range of 0.25 - 1.75 Hz to within 1% of the wind farm's registered output. In order to determine whether wind farm low-frequency oscillations have a negative effect (excite other modes) or possibly a positive impact (damping of existing modes) on the power system, the oscillations at the point of connection must be measured and characterised. Using time - frequency methods, research presented in this paper has been conducted to extract signal features from measured low-frequency active power pulsations produced by wind farms to determine the effective composition of possible oscillatory modes which may have a detrimental effect on system dynamic stability. The paper proposes a combined wavelet-Prony method to extract modal components and determine damping factors. The method is exemplified using real data obtained from wind farm measurements.

Cold atmospheric pressure plasma jets:Interaction with plasmid DNA and tailored electron heating using dual-frequency excitation

Recent progress in plasma science and technology has enabled the development of a new generation of stable cold non-equilibrium plasmas operating at ambient atmospheric pressure. This opens horizons for new plasma technologies, in particular in the emerging field of plasma medicine. These non-equilibrium plasmas are very efficient sources for energy transport through reactive neutral particles (radicals and metastables), charged particles (ions and electrons), UV radiation, and electro-magnetic fields. The effect of a cold radio frequency-driven atmospheric pressure plasma jet on plasmid DNA has been investigated. The formation of double strand breaks correlates well with the atomic oxygen density. Taken with other measurements, this indicates that neutral components in the jet are effective in inducing double strand breaks. Plasma manipulation techniques for controlled energy delivery are highly desirable. Numerical simulations are employed for detailed investigations of the electron dynamics, which determines the generation of reactive species. New concepts based on nonlinear power dissipation promise superior strategies to control energy transport for tailored technological exploitations. © 2012 American Institute of Physics.

Piezoresponse force microscopy of ferroelectric thin films: Frequency dependence of phase imaging

The objective of this work is an evaluation of quantitative measurements of piezoresponse force microscopy for nanoscale characterization of ferroelectric films. To this end, we investigate how the piezoresponse phase difference Delta Phi between c domains depends on the frequency omega of the applied ac field much lower than the cantilever first resonance frequency. The main specimen under study was a 102 nm thick film of Pb(Zr(0.2)Ti(0.8))O(3). For the sake of comparison, a 100 nm thick PbTiO(3) film was also used. From our measurements, we conclude a frequency dependent behavior Delta Phi similar to omega(-1), which can only be partially explained by the presence of adsorbates on the surface. (C) 2008 American Institute of Physics.

Channel measurements of device–to–device communications in an urban outdoor environment

In this paper, the results of radio channel measurements between two hypothetical cellular handsets in an outdoor urban environment are reported. The device-to-device channel measurements were made at 868 MHz and investigated a number of different everyday usage scenarios such as the devices being held at the user's heads, placed in a pocket while one of the users rotated or both moved randomly. It was found that shadowing of the main signal path caused by the human body will be an important factor in future device-to-device communications at this frequency. The recently proposed shadowed κ-μ fading model was used to characterize these channels and shown to provide a good description of the measured data.

Starting slow: the effects of response-switching frequency on patterns of cardiovascular reactivity

Research findings suggest that switching between competing response sets can be resource demanding. The current study focused on concurrent health-relevant physiological effects of task switching by assessing cardiovascular response at varying levels of switch frequency. The participants performed a response-switching task at three different levels of response set switching frequency (low, medium and high) while measurements of blood pressure and heart rate were taken. One group was exposed to response-switching frequency conditions in the order low → medium → high, while the other group was exposed to the same task conditions in the reverse order (i.e. high → medium → low). The results showed that the participants in the low → medium → high switch frequency group recovered faster from initially heightened systolic blood pressure when compared with participants in the high → medium → low group. It is concluded that the results point to a physiological "carry over" effect associated with beginning a task at rapid response switching frequency levels, and suggest the importance of habituation to task demands as a means of offsetting potentially unhealthy levels of reactivity. Implications for modern work environments are discussed.

Assessing elements of a family approach to reduce adolescent drinking frequency: parent-adolescent relationship, knowledge management, and keeping secrets

AIMS: To estimate 1) the associations between parent-adolescent relationship, parental knowledge and subsequent adolescent drinking frequency and 2) the influence of alcohol use on parental knowledge.

DESIGN: Path analysis of school based cohort study with annual surveys SETTING: Post primary schools from urban and intermediate/rural areas in Northern Ireland PARTICIPANTS: 4,937 post primary school students aged around 11 years in 2000 followed until around age 16 in 2005.

MEASUREMENTS: Pupil reported measures of: frequency of alcohol use; parental-child relationship quality; sub-dimensions of parental monitoring: parental control, parental solicitation, child disclosure and child secrecy.

FINDINGS: Higher levels of parental control (Ordinal logistic OR 0.86 95% CI 0.78, 0.95) and lower levels of child secrecy (OR 0.83 95% CI 0.75 0.92) were associated with less frequent alcohol use subsequently. Parental solicitation and parent-child relationship quality were not associated with drinking frequency. Weekly alcohol drinking was associated with higher subsequent secrecy (Beta -0.42 95% CI -0.53, -0.32) and lower parental control (Beta -0.15 95% CI -0.26, -0.04). Secrecy was more strongly predictive of alcohol use at younger compared with older ages (P=0.02), and alcohol use was less strongly associated with parental control among families with poorer relationships (P=0.04).

CONCLUSIONS: Adolescent alcohol use appears to increase as parental control decreases and child secrecy increases. Greater parental control is associated with less frequent adolescent drinking subsequently, while parent-child attachment and parental solicitation have little influence on alcohol use. 

Footbridge system identification using wireless inertial measurement units for force and response measurements

With the main focus on safety, design of structures for vibration serviceability is often overlooked or mismanaged, resulting in some high profile structures failing publicly to perform adequately under human dynamic loading due to walking, running or jumping. A standard tool to inform better design, prove fitness for purpose before entering service and design retrofits is modal testing, a procedure that typically involves acceleration measurements using an array of wired sensors and force generation using a mechanical shaker. A critical but often overlooked aspect is using input (force) to output (response) relationships to enable estimation of modal mass, which is a key parameter directly controlling vibration levels in service.

This paper describes the use of wireless inertial measurement units (IMUs), designed for biomechanics motion capture applications, for the modal testing of a 109 m footbridge. IMUs were first used for an output-only vibration survey to identify mode frequencies, shapes and damping ratios, then for simultaneous measurement of body accelerations of a human subject jumping to excite specific vibrations modes and build up bridge deck accelerations at the jumping location. Using the mode shapes and the vertical acceleration data from a suitable body landmark scaled by body mass, thus providing jumping force data, it was possible to create frequency response functions and estimate modal masses.

The modal mass estimates for this bridge were checked against estimates obtained using an instrumented hammer and known mass distributions, showing consistency among the experimental estimates. Finally, the method was used in an applied research application on a short span footbridge where the benefits of logistical and operational simplicity afforded by the highly portable and easy to use IMUs proved extremely useful for an efficient evaluation of vibration serviceability, including estimation of modal masses.