Structure of the ion wakefield in dusty plasmas.

The magnitude and structure of the ion wakefield potential below a single negatively charged dust particle levitated in the plasma sheath region were measured using a test particle. Attractive and repulsive components of the interaction force were extracted from a trajectory analysis of low-energy collisions between different mass particles in a well-defined electrostatic potential that constrained the dynamics of the collisions to one dimension. As the vertical spacing between the particles increased, the peak attractive force decreased and the width of the potential increased. For the largest vertical separations measured in this study, the lower particle does not form a vertical pair with the upper particle but rather has an equilibrium position offset from the bottom of the parabolic potential confining well.

Measurement of attractive interactions produced by the ion wakefield in dusty plasmas using a constrained collision geometry.

Plasma dust particle interactions, charges, and screening lengths are derived from measurements of time-dependent particle positions in a simplified geometry. The magnitude and structure of the ion wakefield potential below a negatively charged dust particle levitated in the plasma sheath region were measured as functions of the pressure and interparticle spacing. Attractive and repulsive components of the interaction force were extracted from a trajectory analysis of low-energy dust collisions between different mass particles in a well-defined electrostatic potential that constrained the dynamics of the collisions to be one-dimensional. Typical peak attractions varied between 60 and 230 fN while the peak particle-particle repulsion was on the order of 60 fN. Random thermal motion of the particles contributed to observable rates for transitions between different equilibrium configurations of vertically separated particles. The influence of nearest- and non-nearest-neighbor interactions on calculated particle parameters is examined using several methods.

Dynamic probe of dust wakefield interactions using constrained collisions.

The magnitude and the structure of the ion-wakefield potential below a negatively charged dust particle levitated in the plasma-sheath region have been determined. Attractive and repulsive components of the interaction force were extracted from a trajectory analysis of low-energy dust collisions in a well-defined electrostatic potential, which constrained the dynamics of the collisions to be one dimensional. The peak attraction was on the order of 100 fN. The structure of the ion-wakefield-induced attractive potential was significantly different from a screened-Coulomb repulsive potential.

Analysis of the particle interactions in a two-dimensional-plasma dust crystal and the use of dust as a probe of the time-averaged presheath electric field.

A method for measuring the interactions of dust particles within a two-dimensional (2D) layer in a plasma is presented, along with the use of dust as a probe for determining plasma presheath electric fields. Particle interactions were directly determined from the lateral compression of two-dimensional plasma dust crystals confined in a parabolic potential well. The shape of the parabolic potential well was confirmed by observing trajectories of single particles falling within the well. Particle trajectories were in good agreement with values calculated using gas temperatures derived from laser-induced-fluorescence measurements of the argon metastable velocity distributions and assuming diffusive gas scattering. Measured particle separations combined with an equation of state for the crystal were used to derive values for the plasma screening length and the charge on the particles. Screening lengths and charges were measured for a range of plasma power, pressure, and particle diameter (mass). Analysis of the particle heights and charge were used to derive the time-averaged electric fields at the edge of the rf driven plasma sheath. Measured electric fields were between 5 and 22 V/cm. For smaller particle diameters, the ion wind force was comparable to the force of gravity. These measurements showed that the particles are confined to the bulk-plasma side of the classical Bohm point.

Direct determination of particle-particle interactions in a 2D plasma dust crystal.

Particle interactions were directly determined from the lateral compression of two-dimensional plasma dust crystals confined in a parabolic potential well. Measured particle separations combined with an equation of state for the crystal were used to derive values for the particle screening length and the charge on the particles. The shape of the parabolic potential well was confirmed by observing trajectories of single particles falling within the well. No evidence of a lateral particle-particle binding was observed.

Eating difficulties in patients with probable dementia of the Alzheimer type.

Seventy-three institutionalized patients suffering from probable dementia of the Alzheimer type (DAT) were surveyed for the presence of eating difficulties. Among 71 patients fed by natural means four different groups emerged: (1) patients who fed themselves (n = 17, 23.9%), (2) patients who had to be fed but posed no other eating problem (n = 13, 18.3%), (3) patients who refused food although they were able to swallow it (n = 18, 25.4%), and (4) patients who choked on liquid and/or solid food, some of whom also refused food (n = 23, 32.4%). Patients who fed themselves were in a less advanced stage of the disease than those who did not, and their average body weight was equal to the ideal weight. The remaining three groups, ie, those with different eating problems, did not differ in mean severity of DAT, and their body weights were significantly lower. The mortality rate during 2 years following the survey was similar in all four groups of patients, although tube feeding was used in only one case. The mortality rate was also similar in patients whose body weights were 20% or more below the median weight for their age, and in patients whose relative body weight was higher. The results of this study suggest that eating difficulties occur in a majority of institutionalized DAT patients, but can be managed without resorting to tube feeding.

Laser beam divergence utilizing a lateral shearing interferometer.

An analysis of the lateral shearing interferometer is given for laser beams. The results enable a simple determination of the local radius of curvature of the wavefront. In addition, the presence of phase distortion in the beam may be ascertained.