C. M. Surko

Time‐resolved reflectivity of ion‐implanted silicon during laser annealing

The time‐resolved reflectivity at 0.63 μm from arsenic‐implanted silicon crystals has been measured during annealing by a 1.06‐μm laser pulse of 50‐ns duration. The reflectivity was observed to change abruptly to the value consistent with liquid silicon and to remain at that value for a period of time which ranged from a few tens of nanoseconds to several hundreds of nanoseconds, depending on the annealing pulse intensity. Concurrently, the transmission of the primary annealing beam dropped abruptly. These observations confirm the formation of a metallic liquid phase at the crystal surface during the annealing process.

Creation and uses of positron plasmas

Advances in positron trapping techniques have led to room‐temperature plasmas of 107 positrons with lifetimes of 103 s. Improvements in plasma manipulation and diagnostic methods make possible a variety of new experiments, including studies just being initiated of electron–positron plasmas. The large numbers of confined positrons have also opened up a new area of positron annihilation research, in which the annihilation cross sections for positrons with a variety of molecules have been measured, as well as the energy spread of the resulting gamma rays. Such measurements are of interest for fundamental physics and for the modeling of astrophysical plasmas.

Use of the positron as a plasma particle

The use of positrons in laboratory plasma physics experiments is considered. Recent progress in this area is discussed, including the creation of a single‐component positron plasma in the laboratory. Specific applications of such antimatter plasmas are also discussed, with emphasis on areas where existing plasma physics technology and that currently under development are likely to produce results in the next few years.

Study of density fluctuations in plasmas by small‐angle CO2 laser scattering

Small‐angle CO2 laser scattering techniques are described which provide detailed information about density fluctuations in plasmas. These techniques are nonperturbative and measure the frequencies, wave vectors, and amplitudes of the density fluctuations. The theories of the scattering and the heterodyne and homodyne detection processes are described in detail for the case of small‐angle scattering. Several models are developed to describe recent experiments involving both coherent and stochastic fluctuations. The laser, detector, and physical arrangement used in the experiments are described in detail. Experimental studies of driven Bernstein waves, ion acoustic turbulence in a positive column discharge, and fluctuations in tokamaks are described to illustrate these techniques. It is demonstrated that CO2 laser scattering is useful in studying fluctuations with wavelengths between 0.01 and 1 cm and frequencies from 1 kHz to tens of giga‐Hertz in plasmas with mean densities ranging from 1010 to 1017 cm−3.

New directions in antimatter chemistry and physics

Preface. I: Positron Sources and Beams. 1. A laser-cooled positron plasma B.M. Jelenkovic, et al. 2. Trap-based positron beams R.G. Greaves, C.M. Surko. 3. Intense radioisotope sources for spin polarized positron beams F. Saito, et al. II: Antihydrogen, Bose-Condensed Positronium, and Positrons in Materials. 4. Collisions of H and H A. Dalgarno, et al. 5. Positron physics in a new perspective: Low-energy antihydrogen scattering by simple atoms and molecules E.A.G. Armoud, C.W. Chamberlain. 6. The Bose-Einstein condensation of positronium in submicron cavities D.B. Cassidy, J.A. Golovchenko. 7. Cooling and quenching of positronium in porous material H. Saito, T. Hyodo. 8. New experiments with bright positron and positronium beams A.P. Mills, Jr., P.M. Platzman. 9. Positron states in materials: density functional and quantum monte carlo studies B. Barbiellini. 10. Depth-profiled positron annihilation spectroscopy of thin insulating films D.W. Gidley, et al. III: Positron and Positronium Interactions with Atoms. 11. The scattering of positrons and positronium by atomic targets H.R.J. Walters, et al. 12. Positronic atoms J. Mitroy, et al. 13. Perspectives on physics with low energy positrons: fundamentals, beams and scattering M. Charlton. 14. Positron chemistry by quantum monte carlo M. Mella, et al. 15. Antimatter compounds D.M. Schrader, J. Moxom. 16. Positronium-atom/molecule interactions: momentum-transfer cross sections and Zeff Y. Nagashima, et al. 17. Correlations between cross sections and threshold energies for positronium formation and direct ionization J.W. Humberston, et al. IV: Positron and Positronium Interactions with Molecules. 18. Scattering of positronium atom off atomic hydrogen and helium targets A.S. Ghosh, P.K. Sinha. 19. Atomic and molecular physics using positron traps and trap-based beams C.M. Surko. 20. Experimental studies of positron scattering using a weak radioactive isotope source O. Sueoka. 21. Future opportunities in positron-atom (molecule) scattering S.J. Buckman. 22. Theory of positron annihilation on molecules G. Gribakin. 23. Bound states of positron with molecules M. Tachikawa, et al. 24. Low-energy positron dynamics in polyatomic gases F.A. Gianturco, et al. 25. A test calculation on SF6 of model potentials for correlation and polarization effects in positron scattering from molecules R.R. Lucchese, et al. 26. On the contribution of polarization-correlation forces to high annihilation rates in positron-molecule collisions M.T. do N. Varella, et al. Author Index. Index.

Attainment of high confinement in neutral beam heated divertor discharges in the PDX tokamak

Abstract The PDX divertor configuration has recently been converted from an open to a closed geometry to inhibit the return of neutral gas from the divertor region to the main chamber. Since then, operation in a regime with high energy confinement in neutral beam heated discharges (ASDEX H-mode) has been routine over a wide range of operating conditions. These H-mode discharges are characterized by a sudden drop in divertor density and H α emission and a spontaneous rise in main chamber plasma density during neutral beam injection. The confinement time is found to scale nearly linearly with plasma current, but can be degraded due either to the presence of edge instabilities or heavy gas puffing. Detailed Thomson scattering temperature profiles show high values of T c near the plasma edge (∼ 450 eV) with sharp radial gradients (∼ 400 eV/cm) near the separatrix. Density profiles are broad and also exhibit steep gradients close to the separatrix.

Low-energy positron interactions with atoms and molecules

This paper is a review of low-energy positron interactions with atoms and molecules. Processes of interest include elastic scattering, electronic and vibrational excitation, ionization, positronium formation and annihilation. An overview is presented of the currently available theoretical and experimental techniques to study these phenomena, including the use of trap-based positron beam sources to study collision processes with improved energy resolution. State-resolved measurements of electronic and vibrational excitation cross sections and measurement of annihilation rates in atoms and molecules as a function of incident positron energy are discussed. Where data are available, comparisons are made with analogous electron scattering cross sections. Resonance phenomena, common in electron scattering, appear to be less common in positron scattering. Possible exceptions include the sharp onsets of positron-impact electronic and vibrational excitation of selected molecules. Recent energy-resolved studies of positron annihilation in hydrocarbons containing more than a few carbon atoms provide direct evidence that vibrational Feshbach resonances underpin the anomalously large annihilation rates observed for many polyatomic species. We discuss open questions regarding this process in larger molecules, as well as positron annihilation in smaller molecules where the theoretical picture is less clear.

Emerging science and technology of antimatter plasmas and trap-based beams

Progress in the ability to accumulate and cool positrons and antiprotons is enabling new scientific and technological opportunities. The driver for this work is plasma physics research—developing new ways to create and manipulate antimatter plasmas. An overview is presented of recent results and near-term goals and challenges. In atomic physics, new experiments on the resonant capture of positrons by molecules provide the first direct evidence that positrons bind to “ordinary” matter (i.e., atoms and molecules). The formation of low-energy antihydrogen was observed recently by injecting low-energy antiprotons into a cold positron plasma. This opens up a range of new scientific opportunities, including precision tests of fundamental symmetries such as invariance under charge conjugation, parity, and time reversal, and study of the chemistry of matter and antimatter. The first laboratory study of electron-positron plasmas has been conducted by passing an electron beam through a positron plasma. The next maj...

Use of positrons to study transport in tokamak plasmas (invited)

It now appears feasible to deposit positrons (e+) in a tokamak plasma by injecting bursts of neutral positronium atoms (e+e−), which are then ionized by the plasma. The annihilation time of these positrons in the plasma is long compared with typical particle containment times. Thus the subsequent transport of the positrons can be studied by monitoring the time dependence of the annihilation, gamma radiation produced when the positrons strike a limiter. This paper discusses the design of such an experiment, the kinds of data which can be obtained, and the physics questions which this experiment might address. This diagnostic technique could also be useful in studying transport in other magnetic confinement devices such as reversed‐field pinches and magnetic mirrors.

Calculation of the dynamics of surface melting during laser annealing

We present a thermal transport model to describe the melting and resolidification of semiconductors which is observed to occur during annealing with a pulsed laser. The temperature‐dependent properties of both the solid and liquid are included. We compare this calculation with experimental results for the time duration of the melted surface for crystalline Si and Ge. The temperature of the liquid surface as a function of time is calculated and effects associated with the hot liquid and the vapor are also discussed.