[en] Tunnel ionizing neutral gas with the self-field of a charged particle beam is explored as a possible way of creating plasma sources for a plasma wakefield accelerator [Bruhwiler et al., Phys. Plasmas (to be published)]. The optimal gas density for maximizing the plasma wakefield without preionized plasma is studied using the PIC simulation code OSIRIS [R. Hemker et al., in Proceeding of the Fifth IEEE Particle Accelerator Conference (IEEE, 1999), pp. 3672-3674]. To obtain wakefields comparable to the optimal preionized case, the gas density needs to be seven times higher than the plasma density in a typical preionized case. A physical explanation is given

No abstract available

[en] An experimental technique is presented that permits diagnostics viewing light from the plasma edge to be spatially calibrated relative to one another. By sweeping the plasma edge, each chord of each diagnostic sweeps out a portion of the light emission profile. A nonlinear least-squares fit to such data provides superior cross-calibration of diagnostics located at different toroidal locations compared with simple surveying. Another advantage of the technique is that it can be used to monitor the position of viewing chords during an experimental campaign to ensure that alignment does not change over time. Moverover, should such a change occur, the data can still be cross-calibrated and its usefulness retained

[en] The laser propagation and energy absorption of an argon spark induced by a laser at different pressures is investigated. 8 ns pulses from a frequency-doubled Q-switched Nd:YAG laser are used to create the spark. The pressure of the argon is varied from 1 atm to 10 Torr. Significant energy absorption by the plasma is observed at high pressures (>100 Torr) while there is negligible absorption when the pressure is lower than 50 Torr. The plasma kernel showed distinct behavior with respect to laser energy. At a laser energy well above the breakdown threshold, the spark moved only in the backward direction and the forward component was absent indicating the strong absorption of the laser by the spark front. A spiky behavior is observed in the transmitted temporal profiles of the laser at higher energies and at high pressures and can be due to the formation of a self-regulating regime

[en] It is demonstrated that relaxation of GaAs/In_xGa_1-xAs/GaAs strained-layer heterostructures can be brought about by postfabrication thermal processing. Misfit dislocations are introduced into the structure during thermal processing, even though the thickness of the strained layer is well below the critical value predicted by the Matthews-Blakeslee model. The misfit dislocations are observed to be of both 60 deg. mixed type and 90 deg. pure edge type. As no relaxation occurs at the lower temperatures encountered during fabrication by molecular-beam epitaxy, it can be inferred that the critical condition for the formation of misfit dislocations is not only a function of strained-layer thickness and composition, but also of temperature. This observation cannot be accounted for by differential thermal expansion or diffusion across the strained-layer interfaces, but the temperature-dependent Peierls force may offer an explanation. The high temperature required to produce relaxation of these structures suggests that they are sufficiently thermally stable for most practical applications

[en] Postannealed structure and electron transport properties of the magnetic tunnel junctions with preoxidized CoFe pinned electrode were compared with those of the conventional plasma oxidized junctions. The preoxidized junction exhibited its peak tunneling magnetoresistance ratio at 375 deg. C which is well above the optimal annealing of the normal junction. Using Auger electron spectroscopy and x-ray photoelectron spectroscopy of the thermally annealed junctions, structural and chemical changes after annealing were observed in the tunnel barrier as well as near the interface for both types of the junctions and these changes closely corresponded to the respective postannealed electrical properties. X-ray magnetic circular dichroism analysis indicated that the Co moment in the preoxidized CoFe electrode rose near the tunnel barrier/electrode interface as the optimal annealing temperature was reached. Our results demonstrated that the magnitude of spin polarized tunneling current is very sensitive to the interface structure and that any changes near the barrier interface during thermal annealing can greatly alter the electrical properties of the magnetic tunnel junctions

[en] In a standard low-energy electron generator the beam is formed by particles traveling close to the axis of symmetry. However some electron trajectories are unstable and strongly dependent on the initial conditions. Numerical ray tracing shows that ultimate beam coherence is limited by these trajectories that pass far from the symmetry axis. This contribution can be partly eliminated by modulating the initial conditions and selecting the modulated response. This is exemplified with a low-energy electron beam used for electron diffraction, where the beam current modulation produces a modulated diffraction pattern that displays noteworthy improvement (sevenfold) in wave vector resolution

[en] In the present analysis we study the broad-band triplet interaction in regimes of large amplitudes. Linear response theories associated with nonlinear arguments are used to show that even though coherence of high-frequency modes is lost as one first encounters chaotic regimes, it can be restored as field amplitudes grow further. We discuss implications of the feature for fixed-phase interactions

[en] Sequential structure-factor data have been collected during the transformation from relaxed high-density to low-density amorphous ice at temperatures above 105 K. Trends in the measured structure factor and in particular the height of the first sharp diffraction peak as a function of its position show that, at ambient pressure, this transformation is clearly divided into two distinct structural relaxation trends. The corresponding radial-distribution functions show that the first of these relaxational processes may be described by a continuous depletion of the 'interstitial oxygen' located in the 3.6 A region, i.e., between the first- and second-coordination shells of the low-density amorphous form, and a corresponding sharpening of oxygen-oxygen correlations with characteristic lengths of 2.75 and 4.5 A. At the end of this relaxation process, the basic structural features of the low-density amorphous form have been established and are described by well-defined first- and second-coordination shells. This is followed by a second, distinctly different, relaxation process that is characterized by much smaller changes at 2.75 and 3.6 A and a smooth expansion at longer length scales

[en] The surface electronic structure of the hydrogenated diamond C(100)-(2x1):H surface was studied using scanning tunneling microscopy (STM), valence band and core-level photoemission, and near-edge x-ray-absorption fine structure. The hydrogenated diamond surface was prepared ex situ by hydrogen plasma treatment. The STM topographies of the hydrogenated diamond surface, recorded with an atomic resolution, indicated the atomically smooth diamond surface. The current-voltage (I-V) spectroscopy was used to study the transport of electrons injected from the STM tip into the diamond surface. It has been shown that the electron transport is determined not only by the surface states available but it depends crucially on the amount of subsurface hydrogen and the charge redistribution at the surface. Annealing of the diamond surface induces desorption of the subsurface hydrogen. That affects the electron transport as evidenced from the analysis of the I-V spectroscopy curves. A qualitative model for the electron transport, taking into account the specific electronic structure of the hydrogenated diamond surface, has been proposed