לוח שנה משולב

A gas-puff z-pinch begins with the injection of a column of gas between the electrodes of a pulsed-power driver using a fast valve. This gas is preionized and then cylindrically compressed by discharge of a high current through the column. Using Argon gas, these current-driven implosions are a well-developed, high-intensity source of ~3keV, radiation [1]. Optimization of gas-puff sources depends on manipulation of the radial mass-density and gas-species profiles injected by the puff valve, achieved using multiple concentric nozzles backed by independently pressurized gas plena [2]. In particular, the Rayleigh-Taylor unstable boundary between the lower density current sheath and higher density shell of entrained gas, must be carefully controlled. We present experimental results of the response of this instability to radiative losses during the implosion phase using both Argon (Ar) and Krypton (Kr) gas puffs. Experiments are performed using a 7cm outer diameter, triple-annular nozzle designed, developed and characterized at the Weizmann Institute and fielded on the 200 ns, 1 MA COBRA generator at Cornell University. Time-gated density measurements are recorded by 532 nm small-shift shearing interferometers, acceleration of the boundary by streaked and time-gated x-ray cameras, and plasma temperatures are inferred from 527 nm Thomson scattering across the implosion region. RT amplitude e-folding times of 20 ns are recorded in Kr, 20% faster than in Ar under otherwise identical conditions. Ion temperatures of 100 eV are recorded in Kr shells, ~40% lower than in Ar, and it appears this cooling is responsible for the observed increase in RT growth rate.

* This work was supported by students and staff of the COBRA facility at Cornell University, and sponsored by the NNSA Stewardship Sciences Academic Programs.
[1] B. Jones et al. IEEE Trans. Plasma Sci. 42 1145-1152 (2014)
[2] A. Velikovich et al. Phys. Rev. Lett. 77 853-856 (1996)
[3] C. Jennings et al. Phys. Plasmas accepted (2015)

Rydberg states are electronically excited states, the spectral position of which can be described by Rydberg’s well-known formula. The physical properties of these states become very unusual at high values of the principal quantum number n: Their polarizability scales as n7, the van der Waals interaction between two Rydberg atoms or molecules as n11, the maximal electric dipole that can be induced by an electric field as n2, the threshold field for field ionization as n-4, the spacing between adjacent states of a given Rydberg series as n-3, the absorption cross section from the ground state as n-3, the transition moment between neighboring Rydberg states as n2, their lifetime as n3, etc. These physical properties and their rapid variation with n form the basis of a growing number of applications of Rydberg states in chemistry and physics.

The talk will begin with an overview of the properties of Rydberg states and their applications in physics and chemistry. I shall then present specific applications of Rydberg states my group is interested in. In the first, we use Rydberg states to carry out precision measurements of dissociation and ionization energies of molecules to test quantum electrodynamics calculations in small molecular systems such as H2 and He2. In the second, we exploit the large dipole moments of Rydberg states to control the translational motion of atoms and molecules, and develop methods to accelerate, focus, deviate and reflect beams of cold atoms and molecules, the latest experiments being carried out near the surface of chips. In the third, we study ion-molecule reactions at low temperatures within the Rydberg-electron orbit.

In this talk I will briefly review the peer review system used by most publishing houses and briefly review some alternative models. I will then move on to discuss some more advanced elements of peer review including open access, copyright and ethical issues.

We consider a simple supersymmetric hidden sector: pure SU(N) gauge theory. Dark matter is made up of hidden glueballinos and hidden glueballs with mass near the confinement scale. For glueballino mass ~TeV and glueball mass ~100 MeV, the glueballinos freeze out with the correct relic density and self-interact through glueball exchange to resolve small-scale structure puzzles. An immediate consequence is that the glueballino spectrum has a hyperfine splitting. We show that the radiative decays of the excited state can explain the observed 3.5 keV X-ray line signal from clusters of galaxies, Andromeda, and the Milky Way.

The LAO/STO interface hosts a two-dimensional electron system that is unusually sensitive to the application of an in-plane magnetic field. Low-temperature experiments have revealed a giant negative magnetoresistance (dropping by 70\%), attributed to a magnetic-field induced transition between interacting phases of conduction electrons with Kondo-screened magnetic impurities. Here we report on experiments over a broad temperature range, showing the persistence of the magnetoresistance up to the 20~K range --- indicative of a single-particle mechanism. Motivated by a striking correspondence between the temperature and carrier density dependence of our magnetoresistance measurements we propose an alternative explanation. Working in the framework of semiclassical Boltzmann transport theory we demonstrate that the combination of spin-orbit coupling and scattering from finite-range impurities can explain the observed magnitude of the negative magnetoresistance, as well as the temperature and electron density dependence. I will present both experimental results and our theoretical transport model.