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  • 1
    Electronic Resource
    Electronic Resource
    Poloidal rotation in Alcator C (1986)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 57 (1986), S. 2020-2022 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The poloidal rotation velocity in the edge region of ohmically heated Alcator C plasmas was measured using the Doppler shift of impurity emission lines. The measurements were made using a high-resolution, photon counting detector, sensitive to wavelengths from ∼1200 to ∼2000 A(ring), mounted in the exit plane of a 1-m Ebert–Fastie spectrometer. The transitions used were the 2s2p 1P01–2p2 1D2 at 1371.292 A(ring) in O v and 2s 2S1/2–2p 2P01/2 at 1242.804 A(ring) in N v, both of which exist near the limiter radius in Alcator C. The measured rotation velocity is ∼4.1×105 ±2.1×105 cm/s at n¯e∼1.7×1014 cm−3 in the direction of the electron diamagnetic drift and is ∼1.4×105 ±2.5×105 cm/s at n¯e∼2.3×1014 cm−3 in the ion diamagnetic drift direction, although the error bars associated with this last measurement do not preclude the possibility of zero rotation or rotation in the electron diamagnetic drift direction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1138777
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  • 2
    Electronic Resource
    Electronic Resource
    Particle transport in the scrape-off layer and its relationship to discharge density limit in Alcator C-Mod : 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. 2107-2117 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Cross-field particle transport in the scrape-off layer (SOL) of Alcator C-Mod [Phys. Plasmas 1, 1511 (1994)] can be characterized by an effective particle diffusivity (Deff) that increases markedly with distance from the separatrix. As a consequence, recycling onto the main-chamber walls is large compared to plasma flows into the divertor volume. The SOL exhibits a two-layer structure: Steep gradients and moderate fluctuation levels are typically found in a ∼5 mm region near the separatrix (near SOL) where parallel electron conduction typically dominates energy losses. Small gradients and larger fluctuation levels with longer correlation times are found outside this region (far SOL). Deff in the near SOL increases strongly with local plasma collisionality normalized to the magnetic connection length. As the discharge density limit is approached, Deff and associated fluctuation levels become large across the entire SOL and cross-field heat convection everywhere exceeds parallel conduction losses, impacting the power balance of the discharge. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1352596
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  • 3
    Electronic Resource
    Electronic Resource
    Measurement of the current density profile in the Alcator C tokamak using lithium pellets (1989)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 60 (1989), S. 3739-3743 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: High-speed lithium pellets have been injected into Alcator C tokamak plasmas in order to measure the internal magnetic field, and thus current density profiles. In the pellet ablation cloud, intense visible line radiation from the Li+ ion (λ≈5485 A(ring), 1s2s 3S−1s2p 3P) is polarized due to the Zeeman effect, and measurement of the polarization angle yields the direction of the total local magnetic field. A "snap shot'' of the q profile is obtained as the pellet penetrates from the edge into the center of the discharge, in a time of about 300 μs. The spatial resolution of the measurement is about 1 cm. At a toroidal field of BT=10 T, the emission in the unshifted π component of the Zeeman triplet is more than 80% polarized, and q profiles have been obtained. The pellets are perturbative (〈Δne〉/〈ne〉≈1), but the total pellet penetration time is at least a factor of 1000 smaller than the classical skin time. It can thus be anticipated that the current density profile should not be perturbed significantly during the time of the measurement. With some relatively straightforward modifications and refinements, precision approaching 10% for the measurement of q profiles should be achievable. The technique appears viable, using Li, as long as the toroidal field is (approximately-greater-than)4 T.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1140485
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  • 4
    Electronic Resource
    Electronic Resource
    High spectral resolution, photon counting detector for Doppler temperature measurements in magnetically confined plasmas (1987)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 58 (1987), S. 520-529 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: This paper describes the design, construction, and performance of a single-stage microchannel-plate image intensifier used as a photon counting detector over the wavelength range from 1150 to ∼2000 A(ring). The intensifer incorporates three high strip current (∼300 μA) microchannel plates, constructed with 12-μ-diam channels and 15-μ center–center spacing, in a "Z'' configuration. The use of high strip current MCPs requires gating the power supply to protect the plates from thermal runaway of the strip current. The output pulses are proximity focused onto a P-46 phosphor screen, which is fiber-optically coupled to a linear, self-scanning photodiode array. Maximum frame rates for the photodiode array are ∼ 125 000 frames/s, with maximum count rates of ∼25 000 photoevents/s. The detector was placed at the focal plane of a 1-m focal length Ebert–Fastie spectrometer and the performance characteristics of the spectrometer-detector system were evaluated using a hollow cathode Pt lamp. The linewidths measured during this evaluation demonstrate that the spatial resolution of the detector is better than 50 μ. The spectrometer-detector system was then used to determine ion temperatures from Doppler broadened impurity lines emitted from plasmas of the Alcator C tokamak. This detector demonstrated more than an order of magnitude increase in sensitivity compared to a photon-counting photomultiplier tube with a vibrating mirror previously used for these measurements with the same spectrometer. This permitted a determination of the central ion temperature of the Alcator C tokamak using the "forbidden'' line of Fe xii at 1354.1 A(ring) which was not detected with the previous system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1139263
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  • 5
    Electronic Resource
    Electronic Resource
    Impurity toroidal rotation and transport in Alcator C-Mod ohmic high confinement mode plasmas : The 41st annual meeting of the division of plasma physics of the american physical society (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 1825-1830 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Central toroidal rotation and impurity transport coefficients have been determined in Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] Ohmic high confinement mode (H-mode) plasmas from observations of x-ray emission following impurity injection. Rotation velocities up to 3×104 m/sec in the co-current direction have been observed in the center of the best Ohmic H-mode plasmas. Purely ohmic H-mode plasmas display many characteristics similar to ion cyclotron range of frequencies (ICRF) heated H-mode plasmas, including the scaling of the rotation velocity with plasma parameters and the formation of edge pedestals in the electron density and temperature profiles. Very long impurity confinement times (∼1 sec) are seen in edge localized mode-free (ELM-free) Ohmic H-modes and the inward impurity convection velocity profile has been determined to be close to the calculated neoclassical profile. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874004
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  • 6
    Electronic Resource
    Electronic Resource
    The effect of divertor baffling on Alcator C-Mod discharges : The 41st annual meeting of the division of plasma physics of the american physical society (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 1894-1903 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of divertor baffling on Alcator C-Mod [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] discharges has been investigated using a novel divertor bypass. The bypass allows the in situ variation of the mechanical conductance from the divertor plenum to the main chamber between or even during discharges. The results indicate a strong dependence on baffling for the compression of deuterium and recycling impurity gases in the divertor plenum in a variety of C-Mod discharges, indicating that tight baffling would be desirable in a future tokamak reactor. There is no apparent effect of the divertor baffling for the Ohmic, L-mode and EDA (Enhanced Dα) H-mode discharges in this study on main chamber neutral particle populations, scrape-off-layer flows, nonrecycling impurity screening, global energy confinement or H-mode threshold powers. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874013
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  • 7
    Electronic Resource
    Electronic Resource
    Effects of neutral particles on edge dynamics in Alcator C-Mod plasmas : The 41st annual meeting of the division of plasma physics of the american physical society (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 1919-1926 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Neutral particle densities and energy losses have been measured in the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. Their effect on the formation and evolution of the edge barrier which accompanies the enhanced confinement regime are discussed. The neutrals can enter the edge dynamics through the particle, momentum, and energy balance. Neutral densities of up to 5×1016 m−3 have been measured in the edge barrier region. Neutrals enter the local dynamics around most of the periphery, not just at the X-point. High resolution measurements of the ionization profile have been obtained for the region near the separatrix. The profile shifts inside the separatrix as the plasma is making a transition from low-to high-mode confinement (H-mode) regimes, partly accounting for the dramatic rise in edge density. The measured neutral density is large enough to affect the bulk ion momentum by charge exchange, and thereby introduces a negative radial electric field at the edge. At the same time, significant edge heat flux, carried by the neutrals, contributes to the measured power loss. At very high edge densities, this loss mechanism could contribute to quenching H-modes. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874016
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  • 8
    Electronic Resource
    Electronic Resource
    Modeling of particle and energy transport in the edge plasma of Alcator C-Mod (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 2791-2796 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In the present study recycling and transport in the edge plasma of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] is modeled and analyzed with the multi-fluid code UEDGE [T. D. Rognlien et al., J. Nucl. Mater. 196–198, 347 (1992)]. Matching the experimental plasma density profiles in the scrape-off layer (SOL) requires a spatially dependent effective anomalous diffusion coefficient D⊥ growing rapidly towards the wall. The midplane pressure of neutral gas, Pmid, is a key parameter that reflects the magnitude of anomalous transport of plasma from the core. Recycling of plasma on the main chamber wall appears to be quite significant, especially in the case of high Pmid∼0.3 mTorr when the main wall provides ∼70% of recycling neutrals in the main chamber. In the upper SOL (well above the x point) draining of particles by the poloidal flow is weak and thus the particle balance is predominantly radial. For the radial heat transport it is found that energy flux carried by radial plasma convection and by charge-exchange (CX) neutrals is quite significant in SOL. In the high Pmid case, heat conduction by CX neutrals along with radial heat convection by plasma carries most of the power flux (∼75%) across the last closed flux surface. Even in the low Pmid case, heat conduction by CX neutrals dominates the radial heat flux far out in the SOL. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873236
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  • 9
    Electronic Resource
    Electronic Resource
    Volume recombination and opacity in Alcator C-Mod divertor plasmas : The 39th annual meeting of division of plasma physics of APS (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1759-1766 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Volume recombination within the Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] divertor plasma is measured and found to be a significant fraction of the total ion sink under detached divertor conditions. The recombination occurs in regions where Te∼1 eV and ne∼1021 m−3. Measurements of the spatial distribution of the recombination are presented. The determinations of the recombination rates are made by measuring the D0 Balmer spectrum and by using a collisional radiative model describing the level populations, ionization, and recombination of D0. The concept of "recombinations per Balmer series photon" is developed to simplify the determinations. Measurements of the opacity of Lyβ emission are presented. It is observed that up to 50% of the Lyβ emission is trapped, indicating that Lyα is strongly trapped in some cases. The effects of Lyα,β trapping on the "recombinations per photon" curves are calculated and considered in the recombination rate determinations. Observations indicating the presence of molecular activated recombination are discussed. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872845
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  • 10
    Electronic Resource
    Electronic Resource
    Impurity transport in Alcator C-Mod plasmas : 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. 1605-1609 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Trace non-recycling impurities have been injected into Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] plasmas in order to determine impurity transport coefficients. Subsequent impurity emission has been observed with spatially scanning x-ray and Vacuum Ultra-Violet (VUV) spectrometer systems. Measured time-resolved brightness profiles of helium- and lithium-like transitions have been compared with those calculated from a transport code which includes impurity diffusion and convection in conjunction with an atomic physics package for individual line emission. During Low-Confinement-Mode (L-mode) plasmas, the transport can be characterized by pure diffusion, with coefficients ∼5000 cm2/s, reflecting the ∼20 ms decay in the x-ray and VUV line brightnesses. During High-Confinement-Modes (H-modes), the impurity confinement times are much longer, and the modelling requires that there be a strong inward convection (of order 1000 cm/s) near the plasma edge, with greatly reduced diffusion (of order 100 cm2/s), also in the region of the edge transport barrier. These edge values of the transport coefficients during H-mode are qualitatively similar to the neo-classical values. Nitrogen has also been injected, and after the H- to L-mode transition, the inner shell satellite lines of lithium-like nitrogen dominate in intensity the resonance line of helium-like N5+ in a thin shell near the plasma edge. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872291
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