Electrical characterization of radio frequency discharges

Two electrical techniques that are frequently used to characterize radio frequency plasmas are described: current-voltage probes for plasma power input and compensated Langmuir probes for electron energy probability functions and other parameters. The following examples of the use of these techniques, sometimes in conjunction with other diagnostic methods, are presented: plasma source standardization, plasma system comparison, power efficiency, plasma modelling and complex processing plasma mechanisms.

Abnormal integrity of association fiber tracts in amnestic mild cognitive impairment.

The association fiber tracts integrity of the inter-hemispheric and within-hemispheric communication was poor understood in amnestic type mild cognitive impairment (aMCI) patients by diffusion tensor imaging (DTI). A region of interest-based DTI approach was applied to explore fiber tract differences between 22 aMCI patients and 22 well-matched normal aging. Correlations were also sought between fractional anisotropy (FA) values and the cognitive performance scores in the aMCI patients. Extensive impairment of association fiber tracts integrity was observed in aMCI patients, including bilateral inferior fronto-occipital fascicles, the genu of corpus callosum, bilateral cingulate bundles and bilateral superior longitudinal fascicles II (SLE II) subcomponent. In addition, the FA value of right SLE II was significantly negatively correlated to the performance of Trail Making Test A and B, whilst the values of right posterior cingulate bundle was significantly positive correlation with MMSE score. As aMCI is a putative prodromal syndrome to Alzheimer's disease (AD), this study suggested that investigation of association fiber tracts between remote cortexes may yield important new data to predict whether a patient will eventually develop AD.

Absent gender differences of hippocampal atrophy in amnestic type mild cognitive impairment.

Hippocampus displayed progressively gender-associated damage in Alzheimer's disease. However, gender effects have been largely neglected in studies of amnestic type mild cognitive impairment (aMCI) patients who were believed to represent an early stage of this disease. The goal of this study was to use in vivo neuroimaging techniques to determine whether there were any evidences of gender differences in hippocampal atrophy in aMCI. A region of interest-based magnetic resonance imaging approach was used to compare hippocampal volume between aMCI patients (22 male, 17 female) and normal aging controls (12 male, 11 female). Independent of group, male hippocampal volumes were larger than female volumes and right hippocampal volumes were typically smaller than left volumes. Hippocampal volumes were significantly reduced in the clinical group but no gender differences were noted in terms of degree of atrophy present. However, female patients showed more impaired cognitive function than male patients despite this apparent equivalence in atrophy. The absence of a gender difference suggested that early neuropathological progression might be independent of gender. However, the data also suggested female aMCI patients had an increased vulnerability to cognitive impairment earlier in the illness course.

Abnormal Functional Connectivity of Hippocampus During Episodic Memory Retrieval Processing Network in Amnestic Mild Cognitive Impairment.

BACKGROUND: Functional connectivity magnetic resonance imaging technique has revealed the importance of distributed network structures in higher cognitive processes in the human brain. The hippocampus has a key role in a distributed network supporting memory encoding and retrieval. Hippocampal dysfunction is a recurrent finding in memory disorders of aging such as amnestic mild cognitive impairment (aMCI) in which learning- and memory-related cognitive abilities are the predominant impairment. The functional connectivity method provides a novel approach in our attempts to better understand the changes occurring in this structure in aMCI patients. METHODS: Functional connectivity analysis was used to examine episodic memory retrieval networks in vivo in twenty 28 aMCI patients and 23 well-matched control subjects, specifically between the hippocampal structures and other brain regions. RESULTS: Compared with control subjects, aMCI patients showed significantly lower hippocampus functional connectivity in a network involving prefrontal lobe, temporal lobe, parietal lobe, and cerebellum, and higher functional connectivity to more diffuse areas of the brain than normal aging control subjects. In addition, those regions associated with increased functional connectivity with the hippocampus demonstrated a significantly negative correlation to episodic memory performance. CONCLUSIONS: aMCI patients displayed altered patterns of functional connectivity during memory retrieval. The degree of this disturbance appears to be related to level of impairment of processes involved in memory function. Because aMCI is a putative prodromal syndrome to Alzheimer's disease (AD), these early changes in functional connectivity involving the hippocampus may yield important new data to predict whether a patient will eventually develop AD.

Helicon Mode Formation and Radio Frequency Power Deposition in a Helicon Produced Plasma

Time- and space-resolved magnetic (B-dot) probe measurements in combination with measurements of the plasma parameters were carried out to investigate the relationship between the formation and propagation of helicon modes and the radio frequency (rf) power deposition in the core of a helicon plasma. The Poynting flux and the absorbed power density are deduced from the measured rf magnetic field distribution in amplitude and phase. Special attention is devoted to the helicon absorption under linear and nonlinear conditions. The present investigations are attached to recent observations in which the nonlinear nature of the helicon wave absorption has been demonstrated by showing that the strong absorption of helicon waves is correlated with parametric excitation of electrostatic fluctuations.

The dynamics of radio-frequency driven atmospheric pressure plasma jets

The complex dynamics of radio-frequency driven atmospheric pressure plasma jets is investigated using various optical diagnostic techniques and numerical simulations. Absolute number densities of ground state atomic oxygen radicals in the plasma effluent are measured by two-photon absorption laser induced fluorescence spectroscopy (TALIF). Spatial profiles are compared with (vacuum) ultra-violet radiation from excited states of atomic oxygen and molecular oxygen, respectively. The excitation and ionization dynamics in the plasma core are dominated by electron impact and observed by space and phase resolved optical emission spectroscopy (PROES). The electron dynamics is governed through the motion of the plasma boundary sheaths in front of the electrodes as illustrated in numerical simulations using a hybrid code based on fluid equations and kinetic treatment of electrons.

Space and phase resolved optical emission in mode transitions of radio-frequency inductively coupled plasmas

Inductively coupled radio-frequency plasmas can be operated in two distinct modes. At low power and comparatively low plasma densities the plasma is sustained in capacitive mode (E-mode). As the plasma density increases a transition to inductive mode (H-mode) is observed. This transition region is of particular interest and governed by non-linear dynamics, which under certain conditions results in structure formation with strong spatial gradients in light emission. These modes show pronounced differences is various measureable quantities e.g. electron densities, electron energy distribution functions, ion energy distribution functions, dynamics of optical light emission. Here the transition from E- to H- mode in an oxygen containing inductively coupled plasma (ICP) is investigated using space and phase resolved optical emission spectroscopy (PROES). The emission, measured phase resolved, allows investigation of the electron dynamics within the rf cycle, important for understanding the power coupling and ionization mechanisms in the discharge. The temporal variation of the emission reflects the dynamics of relatively high-energy electrons. It is possible to distinguish between E- and H-mode from the intensity and temporal behaviour of the emission.