Millimeter and submillimeter wave technology developments for the next generation of fusion devices

There is increasing demand for compact watt-level coherent sources in the millimeter and submillimeter wave region. The approach that we have taken to satisfy this need is to fabricate two-dimensional grids loaded with oscillators, electronic beam steerers, and frequency multipliers for quasioptical coherent spatial combining of the outputs of a large number of low-power devices

Optical properties of NaxV2O5

The optical properties of sodium-deficient NaxV2O5 (0.85 < x <1) single crystals are analyzed in the wide energy range, from 0.012 to 4.5 eV, using ellipsometry, infrared reflectivity, and Raman scattering techniques. The material remains insulating up to the maximal achieved hole concentration of about 15%. In sodium deficient samples the optical absorption peak associated to the fundamental electronic gap develops at about 0.44 eV. It corresponds to the transition between vanadium dxy and the impurity band, which forms in the middle of the pure NaV2O5 gap. Raman spectra measured with incident photon energy larger then 2 eV show strong resonant behavior, due to the presence of the hole-doping activated optical transitions, peaked at 2.8 eV.Comment: 7 pages, 4 fugures, to be published in PR

Shock Structure Analysis and Aerodynamics in a Weakly Ionized Gas Flow

The structure of a shock wave propagating through a weakly ionized gas is analyzed using an electrofluid dynamics model composed of classical conservation laws and Gauss Law. A viscosity model is included to correctly model the spatial scale of the shock structure, and quasi-neutrality is not assumed. A detailed analysis of the structure of a shock wave propagating in a weakly ionized gas is presented, together with a discussion of the physics underlying the key features of the shock structure. A model for the flow behind a shock wave propagating through a weakly ionized gas is developed and used to analyze the effect of the ionization on the aerodynamics and performance of a two-dimensional hypersonic lifting body

Electronic Structure of the Perovskite Oxides: La1-xCaxMnO3

URL:http://link.aps.org/doi/10.1103/PhysRevLett.76.960 DOI:10.1103/PhysRevLett.76.960The electronic structures of the perovskite oxides, LaMnO3 and CaMnO3, are studied using density-functional methods. Antiferromagnetic insulating (AFI) solutions are obtained for both compounds within the local-density approximation (LDA). For LaMnO3 the Jahn-Teller distortion, found necessary for the AFI solution, produces occupied Mn(z2-1) orbitals pointed along the long, basal-plane Mn-O bonds. The large on-site Coulomb U and exchange J, obtained from “constrained” LDA calculations, U≈8-10eV and J≈0.9eV, indicate important correlation effects and yield large redistribution of the spectral weight within the LDA+U approach.We thank O. Gunnarsson for stimulating discussions. This work was supported in part by the Office of Naval Research under Contract No. ONR N00014-95-1-0439

Evidence for multiple impurity bands in sodium-doped silicon MOSFETs

We report measurements of the temperature-dependent conductivity in a silicon metal-oxide-semiconductor field-effect transistor that contains sodium impurities in the oxide layer. We explain the variation of conductivity in terms of Coulomb interactions that are partially screened by the proximity of the metal gate. The study of the conductivity exponential prefactor and the localization length as a function of gate voltage have allowed us to determine the electronic density of states and has provided arguments for the presence of two distinct bands and a soft gap at low temperature.Comment: 4 pages; 5 figures; Published in PRB Rapid-Communication

Magnetohydrodynamic Power Generation in the Laboratory Simulated Martian Entry Plasma

This paper addresses the magnetohydrodynamic (MHD) conversion of the energy released during the planetary entry phase of an interplanetary vehicle trajectory. The effect of MHD conversion is multi-fold. It reduces and redirects heat transferred to the vehicle, and regenerates the dissipated energy in reusable and transportable form. A vehicle on an interplanetary mission carries about 10,000 kWh of kinetic energy per ton of its mass. This energy is dissipated into heat during the planetary atmospheric entry phase. For instance, the kinetic energy of Mars Pathfinder was about 4220 kWh. Based on the loss in velocity, Mars Pathfinder lost about 92.5% of that energy during the plasma-sustaining entry phase that is approximately 3900 kWh. An ideal MHD generator, distributed over the probe surface of Mars Pathfinder could convert more than 2000 kWh of this energy loss into electrical energy, which correspond to more than 50% of the kinetic energy loss. That means that the heat transferred to the probe surface can be reduced by at least 50% if the converted energy is adequately stored, or re-radiated, or directly used. Therefore, MHD conversion could act not only as the power generating, but also as the cooling process. In this paper we describe results of preliminary experiments with light and microwave emitters powered by model magnetohydrodynamic generators and discuss method for direct use of converted energy

Bipolar-Driven Large Magnetoresistance in Silicon

Large linear magnetoresistance (MR) in electron-injected p-type silicon at very low magnetic field is observed experimentally at room temperature. The large linear MR is induced in electron-dominated space-charge transport regime, where the magnetic field modulation of electron-to-hole density ratio controls the MR, as indicated by the magnetic field dependence of Hall coefficient in the silicon device. Contrary to the space-charge-induced MR effect in unipolar silicon device, where the large linear MR is inhomogeneity-induced, our results provide a different insight into the mechanism of large linear MR in non-magnetic semiconductors that is not based on the inhomogeneity model. This approach enables homogeneous semiconductors to exhibit large linear MR at low magnetic fields that until now has only been appearing in semiconductors with strong inhomogeneities.Comment: 23 pages, 4 figures (main text), 6 figures (supplemental material