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  • 11
    Electronic Resource
    Electronic Resource
    Local transport barrier formation and relaxation in reverse-shear plasmas on the Tokamak Fusion Test Reactor : The 38th annual meeting of the Division of Plasma Physics (DPP) of the American Physical Society (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 1736-1744 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The roles of turbulence stabilization by sheared E×B flow and Shafranov shift gradients are examined for Tokamak Fusion Test Reactor [D. J. Grove and D. M. Meade, Nucl. Fusion 25, 1167 (1985)] enhanced reverse-shear (ERS) plasmas. Both effects in combination provide the basis of a positive-feedback model that predicts reinforced turbulence suppression with increasing pressure gradient. Local fluctuation behavior at the onset of ERS confinement is consistent with this framework. The power required for transitions into the ERS regime are lower when high power neutral beams are applied earlier in the current profile evolution, consistent with the suggestion that both effects play a role. Separation of the roles of E×B and Shafranov shift effects was performed by varying the E×B shear through changes in the toroidal velocity with nearly steady-state pressure profiles. Transport and fluctuation levels increase only when E×B shearing rates are driven below a critical value that is comparable to the fastest linear growth rates of the dominant instabilities. While a turbulence suppression criterion that involves the ratio of shearing to linear growth rates is in accord with many of these results, the existence of hidden dependencies of the criterion is suggested in experiments where the toroidal field was varied. The forward transition into the ERS regime has also been examined in strongly rotating plasmas. The power threshold is higher with unidirectional injection than with balance injection. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872275
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  • 12
    Electronic Resource
    Electronic Resource
    Far infrared tangential interferometry/polarimetry on the National Spherical Tokamak Experiment : Proceedings of the 12th topical conference on high temperature plasma diagnostics (1999)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 710-713 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Measurement of the core BT(r,t) value is essential in the National Spherical Tokamak Experiment (NSTX), since the effects of paramagnetism and diamagnetism in the NSTX are expected to be considerably greater than that in higher aspect ratio tokamaks. Therefore, without independent BT(r,t) measurement, plasma parameters dependent upon BT such as the q profile and local β value cannot be evaluated. Tangential interferometer/polarimeter systems (eight channels) [H. Park, L. Guttadora, C. Domier, W. R. Geck, and N. C. Luhman, Jr., First and Second NSTX Research Forums, Princeton, NJ, 1997 (unpublished)] for the NSTX will provide temporally and radially resolved toroidal field profile [BT(r,t)] and two-dimensional electron density profile [ne(r,t)] data. The outcome of the proposed system is extremely important to the study of confinement, heating, and stability of the NSTX plasmas. The research task is largely based on utilizing existing hardware from the TFTR multichannel infrared interferometer system [D. K. Mansfield, H. K. Park, L. C. Johnson, H. Anderson, S. Foote, B. Clifton, and C. H. Ma, Appl. Opt. 26, 4469 (1987) and H. K. Park, D. K. Mansfield, and C. L. Johnson, Proceedings of the 3rd International Symposium on Laser-Aided Plasma Diagnostic, Los Angeles, CA, 28–30 Oct. 1987 (unpublished), pp. 96–104] which will be reconfigured into a tangential system for NSTX, and to develop the additional hardware required to complete the system. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1149364
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  • 13
    Electronic Resource
    Electronic Resource
    Zeff profiles measured by visible and soft x-ray bremsstrahlung arrays in TFTR (abstract)a) : Proceedings of the tenth topical conference on high temperature plasma diagnostics (1995)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 66 (1995), S. 635-635 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The Zeff profiles from the tangential array of visible bremsstrahlung (VB) emission (5235 A(ring)) have shown several interesting features in TFTR. The observed Zeff profile was flat during the ohmic phase and hollow during the neutral-beam heating phase. In order to crosscheck the measured Zeff profiles from the VB array, a poloidal array which measures bremsstrahlung emission in the soft x-ray region has been analyzed. The analysis of array measurements in the two different spectral regions was performed on the same geometry determined from the Abel inversion of the electron density (interferometry). The two independent measurements of Zeff profiles have been consistent with each other for most of the discharges in TFTR. However, in a discharge where the two measurements are inconsistent, the Zeff profile deduced from the VB array indicates some degree of problems. In this paper, the detailed analysis of the bremsstrahlung array (both poloidal and tangential) measurements is presented. Particularly, the effects of geometry in the inversion process, such as a poloidal elongation of flux surfaces, are investigated. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146310
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  • 14
    Electronic Resource
    Electronic Resource
    Identification of Alfvén mode location via multichannel interferometer measurements (abstract) : Proceedings of the eleventh topical conference on high temperature plasma diagnostics (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 698-698 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Quasicoherent magnetohydrodynamics modes with an Alfvén frequency scaling are seen routinely in TFTR neutral beam heated plasmas as well as some Ohmic plasmas. So far, they are only observed in external magnetic fluctuation measurement (Mirnov coils). A close correlation is observed between the changes in the mode frequency and electron density measured by the multichannel infrared interferometer. This correlation allows us to determine the location of the Alfvén modes. The result shows that they are near the plasma edge, r/a〉0.85. This method is also used to identify the location of the toroidicity-induced Alfvén eigenmode driven by fast ions in a radiowave heating experiment. The result is consistent with the location determined by the reflectometer measurement.© 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147925
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  • 15
    Electronic Resource
    Electronic Resource
    Hydrogenation and annealing effects in p-type ZnSe thin films grown on GaAs (100) substrates (1998)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 84 (1998), S. 3125-3128 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Photoluminescence (PL) and Raman scattering measurements have been carried out to investigate the hydrogenation and annealing effects in p-type ZnSe epilayers grown on n-type GaAs (100) substrates by molecular beam epitaxy. After hydrogenation, the PL spectra showed that the deep acceptor-bound exciton peak disappeared and that the peak position of the donor–acceptor pair peak shifted to a higher energy. When the hydrogenated ZnSe/GaAs heterostructure was annealed at 300 °, a bound exciton due to neutral acceptors appeared. The Raman intensity of the plasma longitudinal optical-phonon-coupling mode increased after hydrogenation. These results indicate that the crystallinity of the p-type ZnSe epilayers grown on n-type GaAs substrates is improved by hydrogenation and that hydrogenated and annealed ZnSe films grown on GaAs substrates hold promise for potential application as buffer layers for the growth of Zn1−xMgxSySe1−y active layers. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.368510
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  • 16
    Electronic Resource
    Electronic Resource
    Optical and structural properties of ZnCdSe/ZnSSe/ZnMgSSe separate confinement heterostructures (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 5951-5954 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have grown ZnCdSe/ZnSSe/ZnMgSSe separate confinement heterostructures by molecular-beam epitaxy. Strain on the ZnSSe layer is calculated from x-ray and photoluminescence data. The temperature dependence of band-gap energy and the photoluminescence intensity in the Cl-doped ZnCdSe active layers is compared with that of undoped ones. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363590
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  • 17
    Electronic Resource
    Electronic Resource
    Thermal stability of sulfur-treated InP investigated by photoluminescence (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 4052-4057 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of sulfur (S) treatments on InP is investigated by low-temperature photoluminescence (PL) measurements. For both n- and p-InP, the PL intensity is observed to increase about four times in magnitude if the scattering by the S overlayer is relatively small. Some PL bands are observed to disappear after S treatments and then reappear if the S-treated surface is heat treated at 220 °C in a vacuum of 10−3 Torr. By observing their dependence on the excitation power density, the doping level of the samples, and measurement temperature, these PL bands are ascribed to the optical transitions via surface states. Our results thus indicate that the S-treated InP surface may not be stable at a subsequent processing temperature of about 250 °C. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363366
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  • 18
    Electronic Resource
    Electronic Resource
    The hydridation and nitridation of GeSi oxide annealed in ammonia (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 2631-2634 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A 110-nm-thick Ge0.38Si0.62O2 film on Ge0.38Si0.62 was annealed in NH3 at 700 °C for 4 h and analyzed by Auger electron spectroscopy, backscattering spectrometry, x-ray photoelectron spectroscopy, and secondary-ion mass spectrometry. In the surface region of the oxide film, this annealing results in an incorporation of nitrogen bonded to germanium by the nitridation of GeO2. In the bottom region of the oxide film near the GeSi/oxide interface, elemental Ge appears. We attribute this process to the hydridation of GeO2 with hydrogen that comes from dissociated ammonia. Results obtained at 800 °C for a 380-nm-thick oxide film are similar. A model is proposed to explain the observed changes of the oxide after the ammonia annealing. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.360123
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  • 19
    Electronic Resource
    Electronic Resource
    Laser beam interference effects on the photovoltage of a p-n junction diode (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 5459-5463 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The photovoltage developed by a p-n junction diode illuminated by an interference pattern through a slit depends both on the distance between fringes and the phase of the interference pattern relative to the position of the slit. For a slit width which accommodates an integer number N of fringes, the voltage is independent of the phase of the pattern but this is no longer the case for a noninteger number of fringes. The maximum dependence is observed for N+1/2 fringes within the slit but the effect decreases as N increases. When the fringe distance is no longer negligible compared to the minority carrier diffusion length the dependence of the photovoltage on the number of fringes becomes more pronounced. A theory is presented which shows how the diffusion length can be obtained from the experimental data. Experiments on GaAs and Si diodes are reported. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.362735
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  • 20
    Electronic Resource
    Electronic Resource
    High transconductance heterostructure field-effect transistors based on AlGaN/GaN (1996)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 69 (1996), S. 794-796 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report on the fabrication and characterization of Al0.15Ga0.85N/GaN heterostructure field-effect transistors (HFETs) with transconductance as high as 120 mS/mm and saturated current density of 0.35 A/mm for a device with a gate length and width of 1 and 100 μm. This represents one of the best results for such device. A comparison of the maximum transconductance of devices on wafers with different channel conductance is presented to analyze the factors limiting the performance. Our data indicates the series resistance between the source and drain to be the limiting factor for the maximum dc transconductance. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.117894
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