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  • 1
    Electronic Resource
    Electronic Resource
    Free-carrier absorption of Hg1−xCdxTe epitaxial films (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 7738-7742 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Free-carrier absorption (FCA) of Hg1−xCdxTe epitaxial films is analyzed by considering the composition-in-depth nonuniformity of epilayers. The results show that epilayers exhibit different FCA behavior from bulk materials. Based on the analyses, the carrier concentration, the density and size distribution of Te precipitates, as well as the inclusion in Hg1−xCdxTe epilayers are derived from fitting the measured FCA spectra. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.362342
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  • 2
    Electronic Resource
    Electronic Resource
    Paramagnetism and scaling behavior of volume flux pinning force density in a GdBa2Cu3O6+x thin film (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 3287-3292 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnetic measurements have been performed on a GdBa2Cu3O6+x superconducting thin film. The paramagnetism carried by rare earth Gd3+ ions in the film tilts the magnetic hysteresis loop and broadens the width of the magnetic hysteresis ΔM, then the magnetization critical current density and the volume flux pinning force density based on the Bean critical state model deviate from intrinsic values. Therefore, in order to get useful information on the pinning mechanism, correction for the paramagnetism is essential. And after correction for the paramagnetism, a scaling law of the volume flux pinning force density is obtained as f(b)=(b/4)0.5(1−b/4)1.5, based on which the possible pinning mechanism in the film is discussed. In the end, deviation from the scaling law at high fields is interpreted by the collective pinning theory. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.360019
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  • 3
    Electronic Resource
    Electronic Resource
    Study of magnetization relaxation on zone-melted YBa2Cu3O6+x superconductors (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 77 (1995), S. 2618-2624 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The analytical magnetization versus time expressions for several flux-pinning models for high-Tc superconductors are deduced. Magnetic relaxation and hysteresis studies have been performed on a zone-melted YBa2Cu3O6+x sample by SQUID. Nonlogarithmic magnetization decay is observed at relatively high temperatures; we attribute this to the nonlinear U(J) relationship which may arise from the local pinning potential Up(x), rather than collective pinning. At 70 K, the experimental data fits extremely well to the expression M(t)=M0+a(T)ln ln(t/τ) which is the result of the exponential U(J) relationship. Based on this U(J) law, the local pinning potential is determined to be Up(x)=U0(x/x0)[1−ln (x/x0)]. The voltage–current E(J) resulting from the observed U(J) relationship, as well as the role of the Y2BaCuO5 (211) phase in zone-melted YBCO is discussed. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.358726
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  • 4
    Electronic Resource
    Electronic Resource
    Magnetization relaxation in YBa2Cu3O7−δ (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 1671-1675 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The time and field sweep rate dependencies of magnetization in high temperature superconductors are derived from a combined view based on the thermally activated flux creep. The τ0 in the magnetization relaxation expression M=M0+M1 ln(1+t/τ0) is a macroscopic quantity related primarily to the sample geometry and other experimental conditions via the effective activation energy U(J0). This activation energy provides an alternative interpretation to the nearly inverse proportionality of τ0 to the field sweep rate. The values of τ0 in the magnetization relaxation of single crystal YBa2Cu3O7−δ are consistent with the calculated ones. As the nonlinear U(J) is taken into account, the curvature in the ||M||-ln t plot may change from negative to positive over a sufficiently long time window, and the magnetization hysteresis M(H) depends on the field sweep rate H(overdot) by const+A ln H(overdot)+B ln2 H(overdot), where A is slightly larger than M1 and B(approximately-greater-than)0. A discussion of the temperature dependency of the normalized flux creep rate is at the conclusion of the article.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.356352
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  • 5
    Electronic Resource
    Electronic Resource
    Magnetic fluctuation induced transport in MST (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. 812-812 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We made local measurements of the magnetic fluctuation induced transport of particles and energy in the outer region (r/a(approximately-greater-than)0.75) of the MST reversed field pinch plasma. These measurements allow comparison with the Rochester–Rosenbluth stochastic diffusion coefficients. An electrostatic electron energy analyzer and fast pyrobolometer were employed in combination with magnetic pickup coils to directly measure the correlated products 〈J˜θB˜r〉 and 〈q˜(parallel)B˜r〉, respectively. Magnetic fluctuation induced transport of both particles and energy is negligible at the extreme edge of MST (r/a(approximately-greater-than)0.9), but increases to a level consistent with estimates of the entire particle and energy flux as the core is approached (5×1021 m−2 s−1 and 105 W m−2). Stochastic particle transport is limited by an apparent ambipolar electric field to the ion stochastic diffusion rate (D=50 m2 s−1), the electron transport being dominated by the energetic component of the distribution (E(approximately-greater-than)50 eV). The measured heat transport is consistent with the convective energy transport carried by the fast electron component (Teffective=100 eV). © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146229
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  • 6
    Electronic Resource
    Electronic Resource
    Studies of global stability of field-reversed configuration plasmas using a rigid body model (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 3685-3693 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Global stability of field-reversed configuration (FRC) plasmas has been studied using a simple rigid body model in the parameter space of s (the ratio of the separatrix radius to the average ion gyro-radius) and plasma elongation E (the ratio of the separatrix length to the separatrix diameter). Tilt stability is predicted, independent of s, for FRC's with low E (oblate), while the tilt stability of FRC's with large E (prolate) depends on s/E. It is found that plasma rotation due to ion diamagnetic drift can stabilize the tilt mode when s/E(approximately-less-than)1.7. The so-called collisionless ion gyro-viscosity also is identified to stabilize tilt when s/E(approximately-less-than)2.2. Combining these two effects, the stability regime broadens to s/E(approximately-less-than)2.8, consistent with previously developed theories. A small additional rotation (e.g., a Mach number of 0.2) can improve tilt stability significantly at large E. A similar approach is taken to study the physics of the shift stability. It is found that radial shift is unstable when E〈1 while axial shift is unstable when E〉1. However, unlike tilt stability, gyro-viscosity has little effect on shift stability. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872978
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  • 7
    Electronic Resource
    Electronic Resource
    Measurement of the dynamo effect in a plasma (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1935-1942 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A series of detailed experiments has been conducted in three laboratory plasma devices to measure the dynamo electric field along the equilibrium field line (the α effect) arising from the correlation between the fluctuating flow velocity and magnetic field. The fluctuating flow velocity is obtained from probe measurement of the fluctuating E×B drift and electron diamagnetic drift. The three major findings are the following: (1) The α effect accounts for the dynamo current generation, even in the time dependence through a "sawtooth'' cycle; (2) at low collisionality the dynamo is explained primarily by the widely studied pressureless magnetohydrodynamic (MHD) model, i.e., the fluctuating velocity is dominated by the E×B drift; (3) at high collisionality, a new "diamagnetic dynamo'' is observed, in which the fluctuating velocity is dominated by the electron diamagnetic drift. In addition, direct measurements of the helicity flux indicate that the dynamo activity transports magnetic helicity from one part of the plasma to another, but the total helicity is roughly conserved, verifying Taylor's [Phys. Rev. Lett. 33, 1139 (1974); Rev. Mod. Phys. 58, 741 (1986)] conjecture. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871989
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  • 8
    Electronic Resource
    Electronic Resource
    Experimental study of lower-hybrid drift turbulence in a reconnecting current sheet (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 3272-3288 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The role of turbulence in the process of magnetic reconnection has been the subject of a great deal of study and debate in the theoretical literature. At issue in this debate is whether turbulence is essential for fast magnetic reconnection to occur in collisionless current sheets. Some theories claim it is necessary in order to provide anomalous resistivity, while others present a laminar fast reconnection mechanism based on the Hall term in the generalized Ohm's law. In this work, a thorough study of electrostatic potential fluctuations in the current sheet of the magnetic reconnection experiment (MRX) [Yamada et al., Phys. Plasmas 4, 1936 (1997)] was performed in order to ascertain the importance of turbulence in a laboratory reconnection experiment. Using amplified floating Langmuir probes, broadband fluctuations in the lower hybrid frequency range (fLH∼5–15 MHz) were measured which arise with the formation of the current sheet in MRX. The frequency spectrum, spatial amplitude profile, and spatial correlation characteristics of the measured turbulence were examined carefully, finding consistency with theories of the lower-hybrid drift instability (LHDI). The LHDI and its role in magnetic reconnection has been studied theoretically for decades, but this work represents the first detection and detailed study of the LHDI in a laboratory current sheet. The observation of the LHDI in MRX has provided the unique opportunity to uncover the role of this instability in collisionless reconnection. It was found that: (1) the LHDI fluctuations are confined to the low-beta edge of current sheets in MRX; (2) the LHDI amplitude does not correlate well in time or space with the reconnection electric field, which is directly related to the rate of reconnection; and (3) significant LHDI amplitude persists in high-collisionality current sheets where the reconnection rate is classical. These findings suggest that the measured LHDI fluctuations do not play an essential role in determining the reconnection rate in MRX. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1494433
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  • 9
    Electronic Resource
    Electronic Resource
    Numerical study of global stability of oblate field-reversed configurations (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 1267-1277 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Global stability of the oblate (small elongation, E〈1) Field-Reversed Configuration (FRC) has been investigated numerically using both three-dimensional magnetohydrodynamic (MHD) and hybrid (fluid electrons and kinetic ions) simulations. For every nonzero value of the toroidal mode number n, there are three MHD modes that must be stabilized. For n=1, these are the interchange, the tilt and the radial shift; while for n〉1 these are the interchange and two co-interchange modes with different polarization. It is shown that the n=1 tilt mode becomes an external mode when E〈1, and it can be effectively stabilized by close-fitting conducting shells, even in the small Larmor radii (MHD) regime. The tilt mode stability improves with increasing oblateness, however at sufficiently small elongations the radial shift mode becomes more unstable than the tilt mode. The interchange mode stability is strongly profile dependent, and all n≥1 interchange modes can be stabilized for a class of pressure profile with separatrix beta larger than 0.035. Our results show that all three n=1 modes can be stabilized in the MHD regime, but the stabilization of the n〉1 co-interchange modes still remains an open question. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1355027
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  • 10
    Electronic Resource
    Electronic Resource
    Experimental study of ion heating and acceleration during magnetic reconnection : The 42nd annual meeting of the division of plasma physics of the American Physical Society and the 10th international congress on plasma physics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 1916-1928 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Ion heating and acceleration has been studied in the well-characterized reconnection layer of the Magnetic Reconnection Experiment [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)]. Ion temperature in the layer rises substantially during null-helicity reconnection in which reconnecting field lines are anti-parallel. The plasma outflow is sub-Alfvénic due to a downstream back pressure. An ion energy balance calculation based on the data and including classical viscous heating indicates that ions are heated largely via nonclassical mechanisms. The Ti rise is much smaller during co-helicity reconnection in which field lines reconnect obliquely. This is consistent with a slower reconnection rate and a smaller resistivity enhancement over the Spitzer value. These observations show that nonclassical dissipation mechanisms can play an important role both in heating the ions and in facilitating the reconnection process. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1356737
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