An extensive study of the threshold law for the cross section of double photoionization in helium is presented. It provides quantitative information about the Wannier exponent α, which agrees with the theoretical prediction. In addition, the hitherto unknown constant of proportionality ${\sigma }_0$ for the cross section ${\sigma }^{++}$ was determined as well as the energy range of validity, which was found to be smaller than expected.

The absolute yield of multiply charged ions of the noble gases argon, krypton, and xenon as a function of laser intensity at λ=586 nm is reported. The measurements were performed with a well characterized picosecond dye laser in the range ${10}^{13}$ to 4×${10}^{14}$ W/${\mathrm{cm}}^2$. Charge states up to ${\mathrm{Ar}}^{+4\mathrm{}}$, ${\mathrm{Kr}}^{+5\mathrm{}}$, and ${\mathrm{Xe}}^{+6\mathrm{}}$ were observed at the highest intensities. An extension of the Keldysh-Reiss-Faisal theory which accounts for the Coulomb field of the residual ion is presented and found to be in good agreement with the experimental results with no adjustable parameters.

Electron cooling of positrons should make possible a large increase in the luminosity of future high-energy linear colliders, leading to greatly enhanced event rates at these machines. An evaluation of the electron-cooling-time requirement indicates that a positron-source repetition rate of 100 Hz is possible. Final positron-beam normalized emittances of ${10}^{\mathrm{-}7}$ m rad should result, implying a tremendous increase in positron-beam density over that currently obtained.

A theoretically predicted instability of the ion-focus–regime guiding of intense relativistic electron beams, known as the ion hose instability, has been observed in an experiment. A 1.5-μs, 1.2-kA, 2.2-MeV electron beam has been successfully injected into and propagated down a 21-m preionized channel. Off-axis beam oscillations have been found to grow and then saturate at large amplitude. Observed growth times, real frequencies, and saturated amplitudes are in agreement with theoretical scaling predictions and with numerical simulation of the ion hose instability.

We report on molecular-dynamics simulations of the low–Reynolds-number flow of Lennard-Jones fluids through a channel. Application of a pressure gradient to a single fluid produces Poiseuille flow with a no-slip boundary condition and Taylor-Aris hydrodynamic dispersion. For an immiscible two-fluid system we find a (predictable) static contact angle and, when accelerated, velocity-dependent advancing and receding contact angles. The approximate local velocity field is obtained, in which the no-slip condition appears to break down near the contact line.

Cross-field particle transport is discussed for the case of a magnetically confined pure electron plasma column. The analysis is based on a 2D guiding-center model and yields a flux which scales with magnetic field strength like 1/B. The magnitude of the flux exceeds that predicted previously provided that two conditions are met. The axial bounce frequency for a typical electron must exceed the E×B drift rotation frequency, and this rotation frequency must be a nonmonotonic function of radius.

Transport of magnetically confined pure electron plasmas to global thermal equilibrium has recently been observed. In equilibrium, the E×B and diamagnetic drifts calculated from the measured temperature and density profiles combine to give rigid rotation (i.e., 〈$v_{\theta }$〉=ωr), as expected. However, the density profile relaxes towards equilibrium up to 5000 times faster than predicted by Boltzmann transport theory: Over a decade range of magnetic field, the density equilibration time is always less than predicted, and scales as $B^1$ rather than as $B^4$.

Lower hybrid waves with a narrow, high-velocity wave spectrum have been used to achieve high central electron temperatures in a tokamak plasma. Waves with a frequency of 2.45 GHz launched by a sixteen-waveguide grill at a power level less than 600 kW were used to increase the central electron temperature of the PLT plasma from 2.2 to 5 keV. Associated with this temperature increase was an apparent reduction in the central electron thermal diffusivity. The magnitude of the temperature increase depends strongly on the phase difference between waveguides and on the direction of the launched wave.

The self-focusing of intense electromagnetic waves in a very underdense plasma is studied in computer simulations. Relativistic self-focusing occurs initially. This is followed by ‘‘ponderomotive blowout’’ and filamentation at the edge of the channel walls. The self-focusing is more intense for resonant double-frequency than for single-frequency illumination.

A solution for the radiation power and frequency spectrum of a relativistic electron beam passing through a strongly turbulent plasma is presented. Regions of intense, localized electrostatic fields in the plasma are characterized as dipolar ‘‘solitons.’’ A new regime of the beam-plasma interaction is identified with the range of beam parameters for which radiation from the beam dominates over the plasma radiation.

Neutral-beam–heated plasmas in TFTR show evidence of substantial non-Ohmically driven toroidal current, even for balanced beam momentum input. The observations are inconsistent with calculations including only Ohmic and beam-driven currents, and presently can only be matched by models including the neoclassical bootstrap current.

A horizontal cylindrical cavity bounded by isothermal walls was partially filled with liquid tritium which was then frozen by reduction of the temperature to 1.0 K below the triple point. Visual observations revealed that the solid subsequently redistributed itself into a layer of uniform thickness covering the complete interior of the cavity. The time constant for this effect depends on the age (or ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ content) of the tritium and not on the initial filling fraction. Time constants of 14.9, 92, 219, and 234 min were measured for tritium 0.04, 7, 16, and 17 days old, respectively.

Adopting the empirical silicon interatomic potential of Stillinger and Weber, we investigate the effect of the tip profile on the atomic-force microscope images for a prototype system, Si(001)-(2×1), and conclude that the tip profile has a profound effect on the observations. We also study relaxation of the surface under the influence of the tip using a many-body energy minimization procedure and find that the force exerted by the tip should be less than ≃${10}^{\mathrm{-}9}$ N for the atomic-force microscope to be a nondestructive tool.

The temperature dependence of the inelastic neutron-scattering intensity from vitreous silica has been studied between 50 and 300 K down to frequencies of 150 GHz. Above 500 GHz one finds essentially harmonic behavior. Low-frequency anharmonic behavior can be described by a relaxational model using parameters determined from ultrasonic measurements. The dynamic structure factor shows that relaxation involves coupled rotational jumps of ${\mathrm{SiO}}_4$ tetrahedra, with a jump width of the atoms smaller than 0.8 Å. These results support the hypothesis of a common origin of low-temperature glass anomalies.

The segregation and trapping of Au at the moving crystal-amorphous Si interface has been measured. Epitaxial crystallization was induced by 2.5-MeV Ar-ion irradiation. The Au segregation is analogous to behavior at liquid-solid interfaces except that the interfacial segregation coefficient of 0.007 at 320 °C is independent of velocity between 0.6 and 6 Å/sec. The Au is trapped in crystalline Si at concentrations some 10 orders of magnitude in excess of equilibrium concentration.

The stability ranges of monatomic and diatomic steps on the Si(001) surface are determined by high-resolution spot-profile analysis of low-energy diffracted electrons. Diatomic steps are predominantly observed at temperatures below 1200 K. With increasing annealing temperature the steps coagulate and we identify (113) facets stabilized by a (3×1) reconstruction. These facets are stable above 1200 K, and up to 1450 K. Annealing to higher temperatures results in the formation of monatomic steps with a terrace width above the experimental coherence length.

We discuss the temperature-concentration phase diagram of doped ${\mathrm{La}}_2$${\mathrm{CuO}}_4$. The addition of holes introduces a local ferromagnetic exchange coupling between Cu spins. The resulting frustration destroys the 3D Néel state characterizing pure ${\mathrm{La}}_2$${\mathrm{CuO}}_4$, and generates a new 3D spin-glass phase. In the paramagnetic phase, strongly correlated Cu spins in the planes are canted by the holes, yielding an oscillating dipole-dipole attraction between holes. The possible relevance to superconductivity is discussed.

Localization in one dimension in the presence of a pseudorandom potential is investigated. The localization length of the tight-binding model ${\mathrm{V}}_{\mathrm{n}}$${\mathrm{u}}_{\mathrm{n}}$+${\mathrm{u}}_{\mathrm{n}+1\mathrm{}}$+${\mathrm{u}}_{\mathrm{n}\mathrm{-}1}$=${\mathrm{Eu}}_{\mathrm{n}}$ with ${\mathrm{V}}_{\mathrm{n}}$=λ cosπα‖n${\mathrm{\Vert }}^{\nu }$ is calculated numerically and in perturbation theory for λ≪1, for generic values of α and ν. The similarity between the potential ${\mathrm{V}}_{\mathrm{n}}$ and random potentials increases with ν. It is found that for ν≥2 all the states are localized and the localization length is equal to that of the corresponding random model while for 0<ν≤1 there are extended states. The intermediate regime 1<ν<2 is discussed as well.

We demonstrate that a GaAs-AlAs superlattice can be switched from indirect to direct, in both real and reciprocal spaces, by application of a modest axial electric field. The crossover region is found to be an anticrossing, manifesting the presence of Γ-X mixing by a potential measured to be of the order of 1 meV. This value is corroborated by time-decay measurements performed on different superlattices.

A new class of uncertainty relations is derived for pairs of observables in a finite-dimensional Hilbert space which do not have any common eigenvector. This class contains an ‘‘entropic’’ uncertainty relation which improves a previous result of Deutsch and confirms a recent conjecture by Kraus. Some comments are made on the extension of these relations to the case where the Hilbert space is infinite dimensional.