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  • 1
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: Heating and current drive with ICRF is one of the major missions of TPX and is crucial to its success. In contrast to the integrated nature of core diagnostic programs, diagnostics that measure ICRF propagation and edge interactions are often assigned lower priority, have not been included in the base diagnositics set, or were included as upgrades. This can result in an incomplete and unoptimized set of measurements. Thus it is important that an integrated set of diagnostics (engineered along with the antenna design), capable of fully diagnosing the interaction, propagation, and absorption of the ICRF be available for TPX. The parameters of interest include: coupling of the ICRF antenna to the plasma, launched spectrum, wave propagation, edge plasma interactions, electron heating, and current drive, and fast ion power deposition and losses. The diagnostic set should be designed so it can be upgraded for control of loading and spectrum by providing feedback information for adjusting phase, power level, fuel gas injection, and plasma position. The proposed set focuses on wave propagation, launched spectrum, absorption, and measuring and comparing the plasma edge properties in the near vicinity of the antenna with that measured at a large toroidal distance from them; thus many would be installed as toroidally separated indentical sets. In addition to the planned core diagnostic set, the ICRF specific diagnostics proposed for TPX include: edge density reflectometers, RF fluctuation reflectometers, fixed and moveable Langmuir probes, visible and IR cameras with spectroscopic line filters, rf magnetic loop probes, local ion energy analyzer, direct ion loss detectors, charge exchange analyzers, fast response ECE, and edge temperature ECE. The anticipated approach is one in which individual diagnostics could be provided by industry, universities, or laboratories but overall coordination, driven by specific ICRF issues, would be required. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
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  • 2
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Detailed measurements in two dimensions by probes and Thomson scattering reveal unexpected local electric potential and electron pressure (pe) maxima near the divertor X point in L-mode plasmas in the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)]. The potential drives E×B circulation about the X point, thereby exchanging plasma between closed and open magnetic surfaces at rates that can be comparable to the total cross-separatrix transport. The potential is consistent with the classical parallel Ohm's law. A simple model is proposed to explain the pressure and potential hills in low power, nearly detached plasmas. Recent two-dimensional edge transport modeling with plasma drifts also shows X-point pressure and potential hills but by a different mechanism. These experimental and theoretical results demonstrate that low power tokamak plasmas can be far from poloidal uniformity in a boundary layer just inside the separatrix. Additional data, although preliminary and incomplete, suggest that E×B circulation across the separatrix might be a common feature of low confinement behavior. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
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  • 3
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Density fluctuations in low-collisionality, low-beta (β∼0.1%), currentless plasmas produced with electron cyclotron heating (ECH) in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] have been studied using a 2 mm microwave scattering diagnostic. Pulsed gas puffing is used to produce transient steepening of the density profile from its typically flat shape; this leads to growth in the density fluctuations when the temperature and density gradients both point in the same direction in the confinement region. The wave number spectra of the fluctuations that appear during this perturbation have a maximum at higher k⊥ρs (∼1) than is typically seen in tokamaks. The in–out asymmetry of the fluctuations along the major radius correlates with the distribution of confined trapped particles expected for the ATF magnetic field geometry. During the perturbation, the relative level of the density fluctuations in the confinement region (integrated over normalized minor radii ρ from 0.5 to 0.85) increases from ñ/n∼1% when the density profile is flat to ñ/n∼3% when the density profile is steepened. These observations are in qualitative agreement with theoretical expectations for helical dissipative trapped-electron modes (DTEMs), which are drift-wave instabilities associated with particle trapping in the helical stellarator field; they suggest that trapped-electron instabilities may play a role in constraining the shape of the density profile in ATF, but have little effect on global energy confinement. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
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  • 4
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 4470-4482 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The most important mechanisms for eroding plasma-facing components (PFCs) and introducing carbon into tokamak divertors are believed to be physical sputtering, chemical sputtering, sublimation, and radiation enhance sublimation (RES). The relative importance of these processes has been investigated by analyzing the spectral emission rates and the effective temperatures of CI, CD, and C2 under several operating conditions in the DIII-D tokamak [Plasma Physics Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159; Proceedings of the 18th IEEE/NPSS Symposium on Fusion Engineering, Albuquerque (Institute of Electrical and Electronic Engineers, Piscataway, 1999), p. 515]. Discrimination of chemical sputtering from physical sputtering is accomplished by quantitatively relating the fraction of CI influxes expected from dissociation of hydrocarbons to the measured CD and C2 influxes. Characteristics of sublimation are studied from carbon test samples heated to surface temperatures exceeding 2000 K. The shielding efficiency of carbon produced at the divertor target is assessed from comparison of fluxes of neutral atoms and ions; approximately 95% of the primary influx appears to be redeposited before being transported far enough upstream to fuel the core plasma. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
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  • 5
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The Advanced Toroidal Facility (ATF) [Fusion Technol. 10, 179 (1986)] is the world's largest stellarator. It was designed and built to demonstrate high beta, steady-state operation in a toroidal confinement system. During its final operating period ATF achieved pulse lengths of over one hour (4667 s). The objectives of these experiments were (1) investigation of plasma performance at times that are long compared to the plasma/wall equilibrium time; (2) determination of plasma control and wall conditioning techniques; and (3) adaptation of plasma diagnostic and data acquisition systems to long-pulse operation. Other experiments have also extended earlier studies of dimensionless-parameter plasma confinement scaling. By employing two discrete electron cyclotron heating (ECH) frequencies (28 and 35 GHz), and by simultaneously modulating the ECH power, magnetic field, and plasma density, it has been possible to maintain fixed plasma beta and collisionality while modulating the normalized gyroradius. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
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  • 6
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Recent experiments in the Advanced Toroidal Facility (ATF) [Fusion Technol. 10, 179 (1986)] have been directed toward investigations of the basic physics mechanisms that control confinement in this device. Measurements of the density fluctuations throughout the plasma volume have provided indications for the existence of theoretically predicted dissipative trapped electron and resistive interchange instabilities. These identifications are supported by results of dynamic configuration scans of the magnetic fields during which the magnetic well volume, shear, and fraction of confined trapped particles are changed continuously. The influence of magnetic islands on the global confinement has been studied by deliberately applying error fields which strongly perturb the nested flux-surface geometry, and the effects of electric fields have been investigated by means of biased limiter experiments.
    Materialart: Digitale Medien
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  • 7
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Low-aspect-ratio tokamaks offer both the economic advantage of smaller size and a number of physics advantages which are not available at conventional aspect ratio. The Small Tight Aspect Ratio Tokamak (START) [Fusion Technology 1990, edited by B. E. Keen, M. Huguet, and R. Hemsworth (North-Holland, Amsterdam, 1991), Vol. 1, p. 353] was conceived as a first substantial test of tokamak plasma behavior at low aspect ratio. It has achieved plasma currents up to 200 kA, peak densities of ∼2×1020 m−3 and central electron temperatures of ∼500 eV at an aspect ratio of 1.3–1.5. Central beta values of ∼13% have been measured and the volume-averaged beta 〈β〉 can approach the Troyon limit. Plasmas are naturally elongated (κ(approximately-less-than)2.0) and are vertically stable without feedback control. Major disruptions have not been observed at low aspect ratios (A≤2.0).
    Materialart: Digitale Medien
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  • 8
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 2249-2259 
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The steady-state nature of the ELMO Bumpy Torus (EBT) [J. Vac. Sci. Technol. 18, 1088 (1982), and references therein] plasma implies that the neutral density at any point inside the plasma volume determines the local particle confinement time. This article describes a Monte Carlo calculation of three-dimensional atomic and molecular neutral density profiles in EBT. The calculation has been done using various models for neutral source points, for launching schemes, for plasma profiles, and for plasma densities and temperatures. Calculated results are compared with experimental observations—principally spectroscopic measurements—both for guidance in normalization and for overall consistency checks. Implications of the predicted neutral profiles for the fast-ion decay measurement of neutral densities are also addressed.
    Materialart: Digitale Medien
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  • 9
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 29 (1986), S. 1659-1668 
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Plasma equilibrium in the ELMO Bumpy Torus (EBT) [in Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Tokyo, 1974), Vol. 2, p. 141; Plasma Phys. 25, 597 (1983)] was studied experimentally by measurements of the electrostatic potential structure. Before an electron tail population is formed, the electric field is found, roughly speaking, to be in the vertical direction. The appearance of a high-energy electron tail signals the formation of a negative potential well, and the potential contours start to nest. The potential contours are shifted inward with respect to the center of the conducting wall. The electric field between the plasma and the conducting wall forces the plasma inward, balancing the outward expansion force. This force balance provides a horizontal electric field that cancels the concentric radial electric field locally at the separatrix of the potential contour and leads to convective energy loss.
    Materialart: Digitale Medien
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  • 10
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 60 (1989), S. 795-797 
    ISSN: 1089-7623
    Quelle: AIP Digital Archive
    Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik
    Notizen: A method of accurately supporting and positioning an electron source inside a large cross-sectional area magnetic field which provides very low electron beam occlusion is reported. The application of electrical discharge machining to the fabrication of a 1-m truss support structure has provided an extremely long, rigid and mechanically strong electron gun support. Reproducible electron gun positioning to within 1 mm has been achieved at any location within a 1×0.6-m2 area. The extremely thin sections of the support truss (≤1.5 mm) have kept the electron beam occlusion to less than 3 mm. The support and drive mechanism have been designed and fabricated at the University of Wisconsin for application to the mapping of the magnetic surface structure of the Advanced Toroidal Facility torsatron1 at the Oak Ridge National Laboratory.
    Materialart: Digitale Medien
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