Effect of the magnetic field geometry on the flute-like perturbations near the divertor X point

Farina, D.; Pozzoli, R.; Ryutov, D. D.

doi:10.1088/0029-5515/33/9/i06

Cited by 85 publications

(112 citation statements)

References 1 publication

Supporting

Mentioning

107

Contrasting

Order By: Relevance

“…The disconnection occurs because a blob (flux tube) that has a circular cross-section at the midplane is stretched into a thin ellipse (even approaching an ion gyroradius in thickness) near the X-point by the magnetic field line mapping. 39,254 Consequently, a perpendicular current can easily flow across the thin dimension of the flux tube, dropping the effective resistance of the blob circuit as depicted in Fig. 22. (The elliptical distortion of the blob cross-section has been observed on CMod by GPI imaging at the X-point.…”

Section: Blob Equivalent Circuitmentioning

confidence: 99%

“…39,47,76 The mathematical description of the X-point fanning effect is carried out by means of an X-point BC analogous to one originally used in linear theory. 111,112,254 A generalized BC that includes both types of cross-field currents has also been discussed. 74 These theoretically predicted trends (collisionality and X-point effects) have been seen in seeded blob simulations 51 and full turbulence simulations 53 using the electrostatic tworegion model.…”

Section: Theoretical Predictionsmentioning

confidence: 99%

See 1 more Smart Citation

Convective transport by intermittent blob-filaments: Comparison of theory and experiment

2011

View full text Add to dashboard Cite

A blob-filament (or simply “blob”) is a magnetic-field-aligned plasma structure which is considerably denser than the surrounding background plasma and highly localized in the directions perpendicular to the equilibrium magnetic field B. In experiments and simulations, these intermittent filaments are often formed near the boundary between open and closed field lines, and seem to arise in theory from the saturation process for the dominant edge instabilities and turbulence. Blobs become charge-polarized under the action of an external force which causes unequal drifts on ions and electrons; the resulting polarization-induced E × B drift moves the blobs radially outwards across the scrape-off-layer (SOL). Since confined plasmas generally are subject to radial or outwards expansion forces (e.g., curvature and ∇B forces in toroidal plasmas), blob transport is a general phenomenon occurring in nearly all plasmas. This paper reviews the relationship between the experimental and theoretical results on blob formation, dynamics and transport and assesses the degree to which blob theory and simulations can be compared and validated against experiments.

show abstract

Section: Blob Equivalent Circuitmentioning

confidence: 99%

Section: Theoretical Predictionsmentioning

confidence: 99%

Convective transport by intermittent blob-filaments: Comparison of theory and experiment

2011

View full text Add to dashboard Cite

show abstract

“…What happens to perturbations in the transition zone is that the cross-field current becomes non-negligible because of a rapid increase of a perpendicular wave number along the field line and the corresponding increase of a crossfield current. The cross-field wave number grows, roughly, as exp(s/L * ) [1], with s being the distance along the field line, and L * being some characteristic length determined by the details of the divertor design and being of order of a few meters for most of the medium-size tokamaks [17]. So, a potential perturbation of the form exp(ik ⊥ · r) imposed on the one side of the X-point decays when one moves into the X-point region because of the finite parallel plasma resistance.…”

Section: X-point Boundary Conditionsmentioning

confidence: 99%

“…In order not to overload our largely conceptual analysis by the unnecessary details we use in Sec. 3, 4 simply the heuristic boundary condition (1). Possible uncertainties are taken into account by introducing the adjustment factor G in the r.h.s.…”

mentioning

confidence: 99%

Theory and fluid simulations of boundary-plasma fluctuations

et al. 2007

Self Cite

View full text Add to dashboard Cite

Abstract. Theoretical and computational investigations are presented of boundaryplasma microturbulence that take into account important effects of the geometry of diverted tokamaks -in particular, the effect of x-point magnetic shear and the termination of field lines on divertor plates. We first generalize our previous "heuristic boundary condition" which describes, in a lumped model, the closure of currents in the vicinity of the x-point region to encompass three current-closure mechanisms. We then use this boundary condition to derive the dispersion relation for low-beta flutelike modes in the divertor-leg region under the combined drives of curvature, sheath impedance, and divertor tilt effects. The results indicate the possibility of strongly growing instabilities, driven by sheath boundary conditions, and localized in either the private or common flux region of the divertor leg depending on the radial tilt of divertor plates. We re-visit the issue of x-point effects on blobs, examining the transition from blobs terminated by x-point shear to blobs that extend over both the main SOL and divertor legs. We find that, for a main-SOL blob, this transition occurs without a free-acceleration period as previously thought, with x-point termination conditions applying until the blob has expanded to reach the divertor plate. We also derive propagation speeds for divertor-leg blobs. Finally, we present fluid simulations of the C-Mod tokamak from the BOUT edge fluid turbulence code, which show main-SOL blob structures with similar spatial characteristics to those observed in the experiemnt, and also simulations which illlustrate the possibility of fluctuations confined to divertor legs.

show abstract

“…2b, the target plate biasing scheme alternates polarity φ t (ψ) = cos (k x k y ψ/RB p ) and drives a series of convection cells. The figure also shows that the perpendicular wavelengths are both stretched k y ∝ exp (−ζ/q * ) and contracted k x ∝ exp (+ζ/q * ) by the action of magnetic shear near the X-point [24]. In order to define the wavenumber across flux surfaces, we define k r = (k x k y ) 1/2 , which is invariant along the field line, and…”

Section: Spatial Optimization Of Sol Perturbationsmentioning

confidence: 99%

Generation of non-axisymmetric scrape-off layer perturbations for controlling tokamak edge plasma profiles and stability

et al. 2012

View full text Add to dashboard Cite

A critical requirement for tokamak fusion reactors is the control of the divertor heat load, both the time-averaged value and the impulsive fluxes that accompany edge-localized modes. We propose driving toroidally varying currents through the scrape-off layer (SOL) plasma both to broaden the SOL by inducing radial convection and to control the edge pressure gradient by inducing resonant magnetic perturbations. The generation of additional convective transport via steady-state convective cells or increased turbulence drive requires that the electric potential perturbations exceed a threshold in amplitude that depends on wavelength. The generation of a coherent magnetic perturbation is optimized by choosing the appropriate width and phasing of the biasing region at the target plate in order to optimize the profile of the SOL current. Longer wavelength modes produce a larger effect because they are not sheared as strongly by the magnetic X-point. Generation of the necessary currents is challenging due to the possibly substantial power requirements and the possible need for internal insulators. We analyze passive current-drive mechanisms that rely on puffing and pumping of neutral gas in a toroidally asymmetric fashion using the UEDGE code to model the ITER divertor.

show abstract

Effect of the magnetic field geometry on the flute-like perturbations near the divertor X point

Cited by 85 publications

References 1 publication

Convective transport by intermittent blob-filaments: Comparison of theory and experiment

Convective transport by intermittent blob-filaments: Comparison of theory and experiment

Theory and fluid simulations of boundary-plasma fluctuations

Generation of non-axisymmetric scrape-off layer perturbations for controlling tokamak edge plasma profiles and stability

Contact Info

Product

Resources

About