Electron Density Oscillations in CC-RF Oxygen Plasma Investigated by Gaussian Beam Microwave Interferometry
Gaussian beam microwave interferometry (160 GHz, beam waist of 5 mm) is applied to study the electron density in capacitively coupled radio-frequency (RF) oxygen plasma. The microwave interferometry provides immediately the line-integrated electron density without model assumptions. In the considered range of RF power and total pressure, the investigations have shown that the oxygen plasma is dominated by electron density oscillations due to the attachment-induced ionization instability. The frequency of the mostly nonharmonic oscillations ranges between 0.3 and 3 kHz with peak-to-peak line-integrated electron density variations up to 3.5 × 1015 m-2.
Attachment-induced ionization instability of a radio frequency excited discharge in oxygen
The plasma of a 13.56 MHz capacitively coupled oxygen discharge of the Gaseous Electronics Conference Reference Cell type exhibits fluctuations of the plasma potential and of light emission in the kilohertz range. This behavior can be explained in terms of an attachment-induced ionization instability in connection with a sufficiently high fraction of negative ions. A necessary condition for the appearance of this type of instability is a strong positive dependence on electron temperature of the electron attachment coefficient describing the formation of negative oxygen ions. A strong volume loss of negative ions by neutral particles—metastable molecules in the present case—is an additional condition. Calculations of the temperature dependent coefficients for ionization and attachment by solving the time-independent Boltzmann equation in the two-term approximation in combination with the fluid equations of the charge carriers show that the criteria for the development of an attachment-induced ionization instability are fulfilled in the pressure domain around 30 Pa and for medium radio frequency voltages around 300 V.
The Limits of Vulnerability in Explaining Fear of Crime: A Comparative Neighborhood Analysis
Recent research on the fear of crime in the United States has focused on the sources of fear. Many of these studies have identified neighborhood-level variables that are related to fear, but most have examined the differences in fear among individual residents of one or more cities. There has been substantially less research on the differences in the sources of fear among residents of different areas within a single city. This article examines the causes of fear through a comparative analysis of three different neighborhoods in San Francisco. An analysis of covariance model is used to assess the importance of differences in the sources of fear that may be associated with an individual's neighborhood of residence. The findings demonstrate that age, as a measure of physical vulnerability previously associated with fear among individuals, is not related to fear in neighborhoods where residents express the greatest general concern about crime. It is concluded that in those urban areas where crime problems are regular features of the neighborhood environment, measures of physical vulnerability are less important in predicting differences in fear among individuals.