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  • 1995-1999  (8)
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Material
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Year
  • 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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    Paper (German National Licenses)
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  • 2
    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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    Paper (German National Licenses)
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  • 3
    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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  • 4
    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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  • 5
    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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  • 6
    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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  • 7
    Electronic Resource
    Electronic Resource
    Alfvén waves and wave-induced transport near an X point (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 659-664 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The behavior of Alfvén waves and the corresponding variation of the wave-induced transport coefficients along a field line including the divertor X-point region are examined. It is shown that several competing effects exist and can be quantified using a quasilinear diffusion model that takes the magnetic geometry of the X point into account. To address the issue of mode behavior and the validity of the eikonal approximation near the X point, an exact analytical solution of an equation describing Alfvén waves in the X-point region is obtained. The results suggest that the X-point region can only dominate Alfvén wave-induced transport on flux surfaces that are very close to the separatrix. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872770
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  • 8
    Electronic Resource
    Electronic Resource
    Role of ponderomotive density expulsion in ion Bernstein wave coupling to the core plasma (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 743-751 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: When the density at the ion Bernstein wave (IBW) antenna is relatively low, mode transformation of the electron plasma wave to the IBW is sensitive to the density gradient scale length, and hence to ponderomotive effects. A second-order nonlinear ordinary differential equation that describes mode transformation at the lower-hybrid layer, including self-consistent ponderomotive density profile modification, is solved for the rf electrostatic potential in front of the IBW antenna, for the particular case of heating just below the second harmonic of the deuterium cyclotron frequency. The complex antenna impedance and a local reflectivity are calculated, assuming vacuum within the antenna box. These calculations reveal diminished antenna coupling to the IBW with increasing ponderomotive density expulsion, as compared to the linear prediction. The ponderomotive force increases the density gradient in the edge plasma, thus enhancing reflection and lowering the loading resistance. The model also describes the direct launch of IBWs in high edge density regimes, lacking a lower-hybrid layer, where the impedance is found to be much smaller than in the low density regime. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872772
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