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  • 1
    Digitale Medien
    Digitale Medien
    Magnetic susceptibility of RCo3B2 (R=Y, Sm, Gd, and Dy) : 38th Annual Conference on Magnetism and Magnetic Materials (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 7140-7142 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The temperature dependence of magnetic susceptibility has been measured in a temperature region from about 100 to 500 K for the compounds RCo3B2 with R=Y, Sm, Gd, and Dy. Their magnetization curves at T=5 K have also been measured in a pulsed high field up to 170 kOe. YCo3B2 seems to have a Pauli paramagnetic nature down to T=5 K. In the paramagnetic temperature region, SmCo3B2 exhibits Van Vleck paramagnetism; however, GdCo3B2 and DyCo3B2 show Curie–Weiss character. The paramagnetic susceptibility for all compounds mentioned above is found to include a Pauli paramagnetic contribution from the Co sublattice. In the case of GdCo3B2 at T=5 K, the magnetic moment of 0.2μB/Co, which is antiparallel to the Gd moment, is considered to be induced by the exchange field from the Gd spin. A similar induced moment of 0.14μB/Co is also estimated in DyCo3B2. The Sm- and Dy-sublattice moments at T=5 K are smaller than the free-ion values, which can be explained by the crystal-field theory. A relatively large value of B02=7.0 K for Dy3+ in DyCo3B2 can explain the experimental Dy3+ spontaneous moment and its increase by external field. The Sm moment in SmCo3B2 at T=5 K may be antiparallel to the induced Co-sublattice moment.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.356704
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  • 2
    Digitale Medien
    Digitale Medien
    Diagnostics for investigation of magnetic reconnection in sawtooth crash on TFTR : Proceedings of the 9th topical conference on high temperature plasma diagnostics (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 4623-4626 
    Details
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: Magnetic reconnection phenomena are documented by a set of noninvasive fast diagnostics during the crash phase of sawtooth oscillations. The electron cyclotron emission diagnostic system provides the highest resolution for measuring time evolution of electron temperature profile during a typical Tokamak Fusion Test Reactor fast sawtooth crash. The x-ray tomography also contains fast time scale information of the electron temperature profile and additionally the impurity concentration. Just before the crash, a shrinking circular hot peak and growing crescent-shaped flat island appear in the inside of the inversion radius on a bird's-eye view of the electron temperature profile. The electron temperature gradient inside the inversion radius diminishes to nearly zero after the crash. Concomitantly, q(r) profile [q(r)=local safety factor] is measured by the motional stark effect (MSE) diagnostics to verify a magnetic field line reconnection during the sawtooth oscillation. Initial MSE data indicate that central q values increases by 5%–10% during the sawtooth crash phase even when the pressure gradient diminishes inside the q=1 region.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1143637
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  • 3
    Digitale Medien
    Digitale Medien
    Investigation of magnetic reconnection during a sawtooth crash in a high-temperature tokamak plasma (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 3269-3276 
    Details
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: In this paper a laboratory investigation is made on magnetic reconnection in high-temperature Tokamak Fusion Test Reactor (TFTR) plasmas [Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 51]. The motional Stark effect (MSE) diagnostic is employed to measure the pitch angle profile of magnetic field lines, and hence the q profile. An analytical expression that relates pitch angle to q profile is presented for a toroidal plasma with circular cross section. During the crash phase of sawtooth oscillations in plasma discharges, the ECE (electron cyclotron emission) diagnostic measures a fast flattening of the two-dimensional (2-D) electron temperature profile in a poloidal plane, an observation consistent with the Kadomtsev reconnection theory. On the other hand, the MSE measurements indicate that central q values do not relax to unity after the crash, but increase only by 5%–15%, typically from 0.7 to 0.8. The latter result is in contradiction with the 2-D models of Kadomtsev and/or Wesson. In the present study this puzzle is addressed by a simultaneous analysis of electron temperature and q profile evolutions. Based on a heuristic model for magnetic reconnection during the sawtooth crash, the small change of q, i.e., partial reconnection, is attributed to the precipitous drop of pressure gradients that drive the instability and the reconnection process, as well as flux conserving plasma dynamics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.870479
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  • 4
    Digitale Medien
    Digitale Medien
    Experimental investigation of three-dimensional magnetic reconnection by use of two colliding spheromaks (1993)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 5 (1993), S. 3691-3701 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Experimental investigation of three-dimensional (3-D) effects of magnetic reconnection dynamics has been extended by use of axially colliding spheromaks [M. Yamada et al., Phys. Fluids B 3, 2379 (1991)]. The two toroidal shape spheromak plasmas with major radii of 15–20 cm and with parallel toroidal currents of up to 30 kA collide to merge in an external equilibrium field. It is important to note that the present experimental setup allows one to investigate magnetic reconnection comprehensively from both local and global points of view. Reconnection angle θ between the merging field lines is varied by changing the polarity of the internal toroidal field and the magnitude of an external toroidal field. It is observed that the speed of counterhelicity merging with θ∼180° is about three times faster than that of cohelicity merging with θ∼90°. This suggests the significance of a 3-D effect on the reconnection process. This difference is attributed to the property of the neutral current sheets with and without the magnetic field component parallel to the reconnection (X) line. In the counterhelicity merging, the neutral current sheet is compressed in much shorter time than in the cohelicity merging, resulting in much higher current density and subsequent faster decay of the current sheet. This induces a faster magnetic reconnection. The reconnection speed increases proportionally with the initial approaching speed of the spheromaks, suggesting that a compressible driven reconnection model is consistent with the present reconnection experimental results.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.860840
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  • 5
    Digitale Medien
    Digitale Medien
    Effect of neutral particles on the energy confinement of spheromaks (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 115-122 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Neutral density profiles are calculated for plasma and edge conditions consistent with those of the S-1 spheromak [Plasma Physics and Controlled Nuclear Fusion Research, 1984 (IAEA, Vienna, 1985), Vol. 2, p. 535]. Multiple generation charge exchange is the dominant transport mechanism for edge neutrals to the plasma center. Central neutral densities may be as high as 1%–5% of the electron density. The high-edge neutral population makes it possible to explain the value and scaling of resistivity in decaying spheromaks. Large neutral components also provide an ion energy-loss mechanism through charge exchange.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.859583
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  • 6
    Digitale Medien
    Digitale Medien
    Experimental control of the spheromak tilting instability (1985)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 28 (1985), S. 1525-1527 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The magnetohydrodynamic (MHD) tilting instability of the spheromak configuration has been experimentally investigated by recording the time evolution of the magnetic field with small magnetic probes. Passive coil systems such as the figure-eight and saddle coils have been proven experimentally effective against this instability.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.864986
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  • 7
    Digitale Medien
    Digitale Medien
    Experimental study of a transformer-driven spheromak plasma (1991)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 2591-2600 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: An axial transformer was installed in the Proto S-1/C spheromak device [Yamada et al., Phys. Rev. Lett. 46, 188 (1981)] in order to study physics issues pertaining to sustained spheromak operation. An increase in both the toroidal plasma current and the toroidal flux was observed when the transformer current was pulsed. The ratio of these two quantities remained constant (within experimental error) indicating that the equilibrium spheromak magnetic field was maintained through a relaxation mechanism. At transformer currents of 10 kA, the toroidal current increased from 37 to 51 kA and the toroidal flux increased from 3.7×105 to 4.4×105 G cm2.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.859971
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  • 8
    Digitale Medien
    Digitale Medien
    Experimental investigation of magnetic compression of a spheromak plasma (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 3074-3080 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Magnetic compression of S-1 spheromak plasmas [Nucl. Fusion 22, 1407 (1982)] was performed to extend the investigation of confinement properties of the spheromak to plasmas with larger current density and higher plasma pressure than obtained before. Evolution of the magnetic configuration was mapped using internal-probe-based flux plots, and it was found that the plasma compression was approximately self-similar. The Taylor equilibrium state was maintained without flux conversion or relaxation phenomena. With a compression factor of up to 1.6, a significant increase of plasma pressure was observed. The peak electron temperature rose from approximately 40 eV before compression to about 100 eV, and the peak density increased somewhat less than the adiabaticity based scaling, which suggests the presence of an enhanced particle loss mechanism. In general, the electron's beta value was approximately constant during the compression; that is, ne0Te0∝B20. This result is consistent with the S-1 scaling obtained earlier without compression. The ion temperature measured by Doppler broadening of low Z impurity line radiation was observed to rise as high as 0.5 keV.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.859218
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  • 9
    Digitale Medien
    Digitale Medien
    Verification of the Taylor (minimum energy) state in a spheromak (1986)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 1994-1997 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Experimental measurements of the equilibrium in the S-1 spheromak [M. Yamada, J. Sinnis, H. P. Furth, M. Okabayashi, G. Sheffield, T. H. Stix, and A. M. M. Todd, in Proceedings of the US-Japan Symposium on Compact Toruses and Energetic Particle Injection (Princeton Plasma Physics Laboratory, Princeton, NJ, 1979), p. 171] by use of magnetic probes inside the plasma show that the final magnetic equilibrium is one that has relaxed close to the Taylor (minimum-energy) state, even though the plasma is far from that state during formation. The comparison is made by calculating the two-dimensional μ profile of the plasma from the probe data, where μ is defined as μ0 j(parallel)/B. Measurements using a triple Langmuir probe proved evidence to support the conclusion that the pressure gradients in the relaxed state are confined to the edge region of the plasma.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.865627
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  • 10
    Digitale Medien
    Digitale Medien
    Global magnetic fluctuations in spheromak plasmas and relaxation toward a minimum-energy state (1985)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 28 (1985), S. 3667-3675 
    Details
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Globally coherent modes have been observed during formation in the S-1 Spheromak plasma [Plasma Physics and Controlled Nuclear Fusion 1984 (IAEA, Vienna, Austria, 1985), Vol. 2, p. 535] by analysis of magnetic field fluctuations measured from outside the plasma. The modes are of low n number (2≤n≤5), where n is defined by the functional dependence einφ of the fluctuation on toroidal angle φ. These modes are shown to be related to flux conversion and plasma relaxation toward a minimum-energy state during the spheromak formation. The modes are active while the q profile is rapidly changing, with q on axis, q0, rising to 0.7. A significant finding is the temporal progression through the n=5, 4, 3, 2; m=1 mode sequence as q rises through rational fractions m/n. During formation, peak amplitudes of the n=2, 3, 4 modes relative to the unperturbed field have been observed as high as 20%, while more typical amplitudes are below 5%.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.865321
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