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  • 1
    Electronic Resource
    Electronic Resource
    Ponderomotive feedback stabilization of external kinks and disruptions in tokamaks (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 3429-3439 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A novel mechanism for feedback stabilization of external kink modes and disruptions is proposed in which modulated radio-frequency (RF) antennas apply a stabilizing ponderomotive force (PF) to the plasma. The RF power required for n=1 kink stabilization in the Princeton Beta Experiment-Modified (PBX-M) [Phys. Fluids B 2, 1271 (1990)] is estimated, and it is shown that the idea can be tested with the existing antenna system. A simulation of a preliminary n=0 modulation experiment on PBX-M also shows that the PF can balance applied vertical field oscillations. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871448
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  • 2
    Electronic Resource
    Electronic Resource
    Far field sheaths from waves in the ion cyclotron range of frequencies (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 2890-2900 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experimental evidence suggests that unabsorbed wave energy in ion cyclotron range of frequency fast wave (FW) experiments can result in deleterious edge interactions. A model describing the formation of far field sheaths due to FW interaction with material surfaces is presented. Near conductors that do not conform to flux surfaces, an incoming FW causes the generation of a slow wave (SW) component. The E(parallel) of the SW drives an RF sheath, in a manner similar to what has been previously discussed for antenna (near field) sheaths. To assess the importance of the proposed mechanism, a heuristic scaling model of the resultant sheath voltage V is developed and compared with a numerical code. The model illustrates the important dependencies of V on the single pass absorption, edge density, FW frequency, FW cutoff location, and limiter/wall geometries and yields qualitative agreement with the experimental observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870529
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  • 3
    Electronic Resource
    Electronic Resource
    Cross-field blob transport in tokamak scrape-off-layer plasmas (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 222-233 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent measurements show that nondiffusive, intermittent transport of particles can play a major role in the scrape-off-layer (SOL) of fusion experiments. A possible mechanism for fast convective plasma transport is related to the plasma filaments or "blobs" observed in the SOL with fast cameras and probes. In this paper, physical arguments suggesting the importance of blob transport [S. I. Krasheninnikov, Phys. Lett. A 283, 368 (2001)] have been extended by calculations using a three-field fluid model, treating the blobs as coherent propagating structures. The properties of density, temperature and vorticity blobs, and methods of averaging over ensembles of blobs to get the average SOL profiles, are illustrated. The role of ionization of background neutrals in sustaining the density blob transport is also discussed. Many qualitative features of the experiments, such as relatively flat density profiles and transport coefficients increasing toward the wall, are shown to emerge naturally from the blob transport paradigm. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1426394
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  • 4
    Electronic Resource
    Electronic Resource
    Resistive modes in the edge and scrape-off layer of diverted tokamaks (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 4622-4631 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The linear behavior of resistive ballooning modes in the edge and scrape-off layer of diverted tokamaks is explored in the context of a collisional fluid model. It is shown that the large magnetic shear and small poloidal field in the X-point region act to increase the wave number, and hence the importance of resistivity, near the X point. The resulting "disconnection" of the eigenmodes across the X point profoundly influences the unstable spectrum. A new class of modes called resistive X-point (RX) modes exploits this synergism between resistivity and the X-point geometry, giving rise to robust growth rates at moderate-to-low mode numbers. Relative to an equivalent limited plasma, the diverted plasma is shown to be more unstable in the edge (inside the separatrix), and more stable in the scrape-off layer. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1314623
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  • 5
    Electronic Resource
    Electronic Resource
    Effect of neutrals on scrape-off-layer and divertor stability in tokamaks (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 519-529 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The influence of ion–neutral interactions (charge exchange, elastic scattering) on scrape-off-layer (SOL) stability is studied in the eikonal limit for a single-null X-point geometry typical of tokamak divertors. Instability drives due to curvature and to the ion–neutral drag effect are included in the model. The ion–neutral interaction terms are highly localized near the divertor plates; these terms are stabilizing for typical parameters and large enough to affect the SOL ballooning-interchange stability in the absence of resistivity. It is shown that the growth rate of ideal curvature-driven modes is significantly reduced by the ion–neutral interaction terms; the growth rate of resistive ballooning modes is not affected much by the neutrals, because resistivity allows the mode to disconnect from the divertor region. In both cases, the X-point geometry significantly affects the stability. An ion–neutral drag instability localized near the plates is only found in a small region of parameter space. Conditions for the existence of this instability in X-point geometry are discussed. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873196
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  • 6
    Electronic Resource
    Electronic Resource
    Nonlinear density expulsion and electron heating in the nonadiabatic regime, and application to Ion Bernstein Wave heating (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 3187-3193 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In the limit where a strong parallel electric field has short parallel scale lengths, the parallel electron motion becomes nonadiabatic and highly nonlinear, and the usual ponderomotive treatment of the slow time scale behavior of electrons is invalid. Here, a new nonadiabatic model for describing the resulting heating and expulsion of electrons from regions of a strong electric field is developed. The model shows that a typical electron is heated to a value characterized by the "quiver" velocity in the applied field. A nonlinear density expulsion still occurs in this nonadiabatic strong rf field limit, but exhibits an algebraic dependence on the wave amplitude in contrast to the exponential dependence that occurs in conventional ponderomotive theory. Results are applied to electrons in the edge plasma, near a high-power Ion Bernstein Wave heating antenna. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872458
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  • 7
    Electronic Resource
    Electronic Resource
    Generalized ballooning and sheath instabilities in the scrape-off layer of divertor tokamaks (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 1330-1341 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The stability of the scrape-off layer to high toroidal mode number ballooning-type instabilities is considered. The equilibrium includes a simple model of the X-point geometry, and parallel (as well as cross-field) equilibrium variations of temperature, density, and potential. The latter are computed numerically from the Braginskii form for Ohm's law. The stability analysis includes the effects of curvature, resistivity, parallel variation of the E×B drift frequency, and sheath boundary conditions at the divertor plate. Importantly, the equilibrium model assures consistency among the possible instability drives; i.e., the pressure weighting of the curvature, the plasma potential (E×B drift), and the conditions at the divertor plate are coupled by the equilibrium model. Numerical solutions indicate two modes: (i) the curvature-driven mode with growth rate enhanced by the sheaths; and (ii) the E×B shear mode driven by equilibrium variations in the region between the X point and the plate. The latter mode is shown to be partly driven by the X-point geometry. The effect of finite Larmor radius, resistivity, and electron inertia on these modes is investigated. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872309
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  • 8
    Electronic Resource
    Electronic Resource
    Scrape-off layer profile modifications by convective cells (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 699-701 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Convective cells (CC's) are important in understanding density profile modifications induced by ion cyclotron range of frequencies (ICRF) antennas. This has motivated the present work in which the effect of CC's on transport in the scrape-off layer is studied, in the regime where the density gradient scale length Ln and the cell size L are comparable. Monte Carlo simulations show that closed cell convection acts to flatten the density profile, and that open cells enhance the particle flow to the wall, depleting the density and yielding profiles similar to those measured near ICRF antennas. A new one-dimensional, two-branch model of CC transport is shown to agree well with the simulations. The model gives rise to two characteristic scale lengths, only one of which is retained in the enhanced diffusion models that are applicable for Ln(very-much-greater-than)L. The two-branch model is expected to be useful in analyzing ICRF experiments. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871871
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  • 9
    Electronic Resource
    Electronic Resource
    Low-power fast wave antenna loading as a radio-frequency sheath diagnostic (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 420-426 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Radio-frequency (RF) sheaths induced by the RF E(parallel) component are a ubiquitous feature of Ion Cyclotron Range of Frequency (ICRF) heating and current drive. An important consequence of RF sheaths is the power dissipation, Psh, caused by ions accelerated in the rectified sheath potential and flowing into material boundaries, such as the antenna structure and nearby limiters. It is shown that the RF sheath-power dissipation yields an antenna loading resistance RL larger than the usual fast wave (FW) loading at very low RF power PRF (typically below 100 kW), because Psh scales as ||Erf||, whereas the FW coupled power scales as ||Erf||2. The curve RL(PRF) has a maximum at PRF=0 and rapidly decreases with power until asymptoting at the usual FW loading. The ratio RL(0)/RL(∞) is a measure of the average RF sheath voltage on the antenna and nearby limiter surfaces. It is suggested that this technique could be used to measure the RF sheath properties of different antennas or operational regimes (e.g., different phasings) before attempting high-power operation. A generalized loading model is applied to interpret recent measurements on the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. I, p. 69] for a four-strap antenna with and without a Faraday screen. The possible use of this effect as a diagnostic for the scrape-off layer density is also discussed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871813
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  • 10
    Electronic Resource
    Electronic Resource
    Effect of ion-neutral charge exchange on radial electric fields in the edge plasma (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 853-860 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A fluid treatment of the perpendicular plasma conductivity is extended to include the effects of arbitrary ion-neutral collision frequency ν and large perpendicular drifts. It is shown that ion-neutral friction (charge-exchange) can give a nonmonotonic relationship between the nonambipolar radial current and the radial electric field, leading to a bifurcated plasma equilibrium. This bifurcation may influence the properties of the low-to-high-confinement (L-H) transition in the presence of neutrals. The physical origin of this effect is an interplay between the surface-averaged equilibrium drifts and the small poloidal variations in the pressure and parallel flow around the flux surface generated by the geodesic curvature. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1447919
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