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  • 11
    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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  • 12
    Electronic Resource
    Electronic Resource
    Interchange instabilities in a partially ionized plasma (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 2217-2231 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In a partially ionized plasma, the charged particle population is coupled to the neutrals through charge exchange, ionization and recombination. An interchange instability is identified in which the driving factor is the neutral pressure gradient and the dominant collisional process is charge exchange. Regimes are considered in which the mean free path for neutral collisions with ions λni is small in comparison to the macroscopic length scales. The instability is analogous to a gravitational interchange mode with an effective gravity geff=νinVn where νin is the collision frequency of ions with neutrals. The neutral flow Vn=−(Mniνin)−1dpn/dx results from a balance between the neutral pressure gradient and collisional friction with the ions. An arbitrary kyλni dispersion relation is derived using fluid equations to describe the ions and the Boltzmann equation to describe the neutrals. This dispersion relation contains viscous and inertial effects and is substantially altered in the presence of realistic parallel wavelength due to a coupling between the unstable interchange mode and a stabilizing shear Alfvén mode. Reasonable conditions under which the modes may exist are examined and the possible relevance to divertor plasmas is considered. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872895
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  • 13
    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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  • 14
    Electronic Resource
    Electronic Resource
    Two-body similarity and its violation in tokamak edge plasmas (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3191-3193 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Scaling laws found under the assumption that two-body collisions dominate can be effectively used to benchmark complex multi-dimensional codes dedicated to investigating tokamak edge plasmas. The applicability of such scaling laws to the interpretation of experimental data, however, is found to be restricted to the relatively low plasma densities (〈1019 m−3) at which multistep processes, which break the two-body collision approximation, are unimportant. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871624
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  • 15
    Electronic Resource
    Electronic Resource
    Plasma–neutral interaction in tokamak divertor for "gas box'' neutral model (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 2280-2285 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Plasma flow through the gas cloud in a tokamak divertor for "gas box'' divertor geometry and Knudsen regime of neutral transport is investigated. It is shown that similar to the neutral models that have considered previously, (i) plasma parameters near the target is sensitive to the energy flux into the hydrogen recycling region and can change rapidly, resulting in bifurcation-like behavior, which might be interpreted as a transition to detached regime, (ii) plasma flux onto the target starts to decrease at a very low plasma temperature near the target, while a strong pressure drop already occurs. At low plasma temperature near the target the recombination processes can significantly alter the plasma flux onto the target. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871910
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  • 16
    Electronic Resource
    Electronic Resource
    The influence of a radial electric field on neoclassical orbits and ion prompt losses from tokamak edge plasmas (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1186-1194 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of the radial electric field on neoclassical orbits in a tokamak using the integrals of drift motion is considered. It is shown that for the most interesting case of high radial electric field shear, due to a dramatic change of the particle orbit topology at a distance of the order of one particle orbit width, it is not correct to apply local theory to describe neoclassical transport processes. For step-like electrostatic potentials typical for the tokamak plasma periphery in the high (H) mode, the trajectories of the particles can be either squeezed or expanded depending on the location of the potential jump with respect to the unperturbed particle trajectories. The widening of the orbits decreases the threshold energy of suprathermal particles which can be promptly lost and enhances the ion prompt losses which might increase the amplitude of the potential step. It is shown that these effects are more pronounced for a diverted tokamak with the X point located at the inner side of the torus. Simultaneous growth of prompt losses and potential magnitude might be the reason for the fast formation of the narrow electrostatic barrier during the "low''–"high'' (L–H) transition. Toroidal plasma rotation induced by the radial electric field can strongly enhance the destabilizing effect of the step-like potential.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870715
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  • 17
    Electronic Resource
    Electronic Resource
    Plasma rotation and electrostatic potential formation at the edge due to suprathermal particle lossesf| (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 1583-1591 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Generation of the edge radial electric field and corresponding plasma rotation is studied for toroidal confinement systems with a separatrix configuration of magnetic surfaces. It is shown that classical acceleration of plasma by ion orbital losses can explain the electric field and the plasma rotation observed near walls when plasma flow to the neutralizing wall is included.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870710
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  • 18
    Electronic Resource
    Electronic Resource
    On the sink/source effects in two-dimensional plasma turbulence (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 5091-5095 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Ad hoc sinks and sources of turbulence are added in the two-dimensional (2D) magnetized plasma model to mimic excitation and damping of the modes. Depending on linear growth and damping rates and sink and source localizations, two kind of the turbulence and large scale flows evolution regimes were found: quasisteady state and dynamic. The quasisteady states with saturated averaged energy and enstrophy are characterized by strong large scale flows generated due to the inverse cascade in the nonlinear mode coupling. It is shown that for the adopted model there is not complete stabilization of high-k harmonics (microturbulence) by the generated zonal flows and convective cells. The dynamic regimes are characterized by nonvanishing energy and enstrophy fluctuations around averaged values and the energy spectrum has maximum, which corresponds to the turbulence source. The inverse cascades in these regimes are suppressed by the mode damping in the k-space sink region. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1415750
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  • 19
    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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  • 20
    Electronic Resource
    Electronic Resource
    Radiation fronts in tokamak divertor plasmas (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 2297-2304 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The impurity radiation from a tokamak divertor can be significantly enhanced due to the formation of a V-shaped impurity radiation front and the effects of perpendicular plasma heat conduction. For some conditions, the transition to a V-shaped radiation front can have a bifurcation character, such that the impurity radiation region jumps from the divertor targets to the entrance into the divertor. The divertor geometry can play a significant role in the formation of a V-shaped impurity radiation front. The analysis of the energy transport and impurity radiation suggest that divertor geometries with "vertical" target, neutral gas recirculation where sidewalls, or their combination are the most attractive from the point of view of the formation of a V-shaped radiation front and the plasma energy loss. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872903
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