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  • 1995-1999  (19)
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  • 1
    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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  • 2
    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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  • 3
    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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  • 4
    Electronic Resource
    Electronic Resource
    Enhancement of Tokamak Fusion Test Reactor performance by lithium conditioning (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1892-1897 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Wall conditioning in the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Phys. Plasmas 2, 2176 (1995)] by injection of lithium pellets into the plasma has resulted in large improvements in deuterium–tritium fusion power production (up to 10.7 MW), the Lawson triple product (up to 1021 m−3 s keV), and energy confinement time (up to 330 ms). The maximum plasma current for access to high-performance supershots has been increased from 1.9 to 2.7 MA, leading to stable operation at plasma stored energy values greater than 5 MJ. The amount of lithium on the limiter and the effectiveness of its action are maximized through (1) distributing the Li over the limiter surface by injection of four Li pellets into Ohmic plasmas of increasing major and minor radius, and (2) injection of four Li pellets into the Ohmic phase of supershot discharges before neutral-beam heating is begun. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871984
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  • 5
    Electronic Resource
    Electronic Resource
    Characteristics of high-confinement modes in Alcator C Mod (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1992-1998 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The regime of high particle and energy confinement known as the H mode [Phys. Rev. Lett. 49, 1408 (1982)] has been extended to a unique range of operation for divertor tokamaks up to toroidal fields of nearly 8 T, line-averaged electron densities of 3×1020 m−3, and surface power densities of nearly 0.6 MW/m2 in the compact high-field tokamak Alcator C Mod [Phys. Plasmas 1, 1511 (1994)]. H modes are achieved in Alcator C Mod with Ion Cyclotron Resonant Frequency (ICRF) heating and with Ohmic heating alone without boronization of the all molybdenum tiled first wall. Large increases in charge exchange flux are observed during the H mode over the entire range of energies from 2 to 10 keV. There appears to be an upper limit to the midplane neutral pressure, of about 0.08 Pa above which no H modes have been observed. The plasmas with the best energy confinement have the lowest midplane neutral pressures, below 0.01 Pa. There is an edge electron temperature threshold such that Te≥280 eV ±40 eV for sustaining the H mode, which is equal at L–H and H–L transitions. The hysteresis in the threshold power between L–H and H–L transitions is less than 25% on average. Both core and edge particle confinement improve by a factor of 2–4 from L mode to H mode. Energy confinement also improves by up to a factor of 2 over L mode. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871995
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  • 6
    Electronic Resource
    Electronic Resource
    Comments on particle and energy balance in the edge plasma of Alcator C-Mod (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 3373-3376 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Particle balance is examined in a large set of representative Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] diverted plasmas using measurements of neutral gas pressures and ionization source strengths in the main chamber, and parallel plasma flow into the divertor. It is inferred that plasma flow in the scrape-off layer (SOL) is dominated by transport to the main chamber walls rather than by flow into the divertor. It follows that (i) the effective diffusion coefficient for anomalous cross-field particle transport in the SOL must rapidly grow with distance from the magnetic separatrix to account for the measured density profiles, (ii) a significant fraction of the power from the core plasma can be convected rather than conducted into the SOL, and (iii) the neutral pressure at the outer midplane is governed more by cross-field plasma transport than by wall–plasma separation or divertor/limiter geometries. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873051
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  • 7
    Electronic Resource
    Electronic Resource
    High confinement dissipative divertor operation on Alcator C-Mod : The 40th annual meeting of the division of plasma physics of the american physical society (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 1899-1906 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] has operated a High-confinement-mode (H-mode) plasma together with a dissipative divertor and low core Zeff. The initially attached plasma is characterized by steady-state enhancement factor, HITER89P [P. N. Yushmanov et al., Nucl. Fusion 30, 1999 (1990)], of 1.9, central Zeff of 1.1, and a radiative fraction of ∼50%. Feedback control of a nitrogen gas puff is used to increase radiative losses in both the core/edge and divertor plasmas in almost equal amounts. Simultaneously, the core plasma maintains HITER89P of 1.6 and Zeff of 1.4 in this nearly 100% radiative state. The power and particle flux to the divertor plates have been reduced to very low levels while the core plasma is relatively unchanged by the dissipative nature of the divertor. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873447
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  • 8
    Electronic Resource
    Electronic Resource
    The role of particle sinks and sources in Alcator C-Mod detached divertor discharges : The 40th annual meeting of the division of plasma physics of the american physical society (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 1907-1916 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Detailed measurements of the magnitude and location of volumetric recombination occurring in the detached divertor of Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are presented. The drop in divertor plate ion current during detachment is due to two mechanisms: (1) volumetric recombination in the divertor plasma; and (2) reductions in the divertor ion source. Depending on plasma conditions, each of these can be the primary mechanism for the observed ion current reduction in detachment. The ion source during detachment is inferred and its magnitude is consistent with the measured divertor power flow. A scaling of the density in the divertor recombining region for L- (low confinement) mode plasmas is found, ne,r∝n¯e〈sup ARRANGE="STAGGER"〉0.8⋅PSOL2/7. A model based on pressure variation along a flux surface during detachment is consistent with the main features of this scaling. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873448
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  • 9
    Electronic Resource
    Electronic Resource
    Radiative and three-body recombination in the Alcator C-Mod divertor (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 2555-2566 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Significant recombination of the majority ion species has been observed in the divertor region of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] under detached conditions. This determination is made by analysis of the visible spectrum from the divertor, in particular the Balmer series line emission and the observed recombination continuum, including an apparent recombination edge at ∼375 nm. The analysis shows that the electron temperature in the recombining plasma is 0.8–1.5 eV. The measured volume recombination rate is comparable to the rate of ion collection at the divertor plates. The dominant recombination mechanism is three-body recombination into excited states (e+e+D+⇒D0+e), although radiative recombination (e+D+⇒D0+hν) contributes ∼5% to the total rate. Analysis of the Balmer series line intensities (from nupper=3 through 10) shows that the upper levels of these transitions are populated primarily by recombination. Thus the brightnesses of the Balmer series (and Lyman series) are directly related to the recombination rate. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872234
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  • 10
    Electronic Resource
    Electronic Resource
    Enhanced performance of deuterium–tritium-fueled supershots using extensive lithium conditioning in the Tokamak Fusion Test Reactor (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 4252-4256 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Phys. Plasmas 2, 2176 (1995)] a substantial improvement in fusion performance has been realized by combining the enhanced confinement due to tritium fueling with the enhanced confinement due to extensive conditioning of the limiter with lithium. This combination has resulted in not only significantly higher global energy confinement times than have previously been obtained in high current supershots, but also in the highest central ratio of thermonuclear fusion output power to input power observed to date. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871050
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