Scattering of radio frequency waves by blobs in tokamak plasmas

Ram, A. K.; Hizanidis, Kyriakos; Kominis, Yannis

doi:10.1063/1.4803898

Cited by 45 publications

(52 citation statements)

References 9 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The model advanced in this paper is appropriate for magnetized plasmas for which the permittivity is given by a tensor, rather than a scalar. It follows previous studies on the scattering of RF waves by spherical blobs [14,15], but pertains to scattering off a cylindrical filament with its axis aligned along the magnetic field line.…”

Section: Introductionmentioning

confidence: 85%

Scattering of radio frequency waves by cylindrical density filaments in tokamak plasmas

Ram

Hizanidis

2016

Physics of Plasmas

Self Cite

View full text Add to dashboard Cite

In tokamak fusion plasmas, coherent fluctuations in the form of blobs or filaments are routinely observed in the scrape-off layer. Radio frequency (RF) electromagnetic waves, excited by antenna structures placed near the wall of a tokamak, have to propagate through the scrape-off layer before reaching the core of the plasma. While the effect of fluctuations on the properties of RF waves has not been quantified experimentally, it is of interest to carry out a theoretical study to determine if fluctuations can affect the propagation characteristics of RF waves. Usually, the difference between the plasma density inside the filament and the background plasma density is sizable; the ratio of the density difference to the background density being of order one. Generally, this precludes the use of geometrical optics in determining the effect of fluctuations since the relevant ratio has to be much less than one; typically, of the order of 10% or less. In this paper, a full-wave, analytical model is developed for the scattering of a RF plane wave by a cylindrical plasma filament. It is assumed that the plasma inside and outside the filament is cold and uniform, and that the major axis of the filament is aligned along the toroidal magnetic field. The ratio of the density inside the filament to the density of the background plasma is not restricted. The theoretical framework applies to the scattering of any cold plasma wave. In order to satisfy the boundary conditions at the interface between the filament and the background plasma, the electromagnetic fields inside and outside the filament need to have the same k ∥ , the wave vector parallel to the ambient magnetic field, as the incident plane wave. Consequently, in contrast to the scattering of a RF wave by a spherical blob [A. K. Ram, K. Hizanidis, and Y. Kominis, Phys. Plasmas 20, 056110-1-056110-10 (2013)], the scattering by a field-aligned filament does not broaden the k ∥ spectrum. However, the filament induces side-scattering leading to surface waves, and can also couple some power to the cold plasma wave different from the incident wave. The changes induced by a filament in the propagation of electron cyclotron waves and lower hybrid waves are illustrated by numerical results displaying the properties of the Poynting vector. The Poynting flux in the wake of the filament, and directed towards the core of the plasma, develops a spatial structure due to diffraction and shadowing. Thus, the fluctuations affect the uniformity of power flow into the plasma.

show abstract

Section: Introductionmentioning

confidence: 85%

Scattering of radio frequency waves by cylindrical density filaments in tokamak plasmas

Ram

Hizanidis

2016

Physics of Plasmas

Self Cite

View full text Add to dashboard Cite

show abstract

“…Recent attempts at quantifying scattering-induced beam broadening, based on a Fokker-Planck modelling of the diffusion of rays [17,46] or on ray tracing in the presence of fluctuations that satisfy WKB ordering [18] suggest an increase of the beam width at the absorption layer in ITER applications of the order of 100%. The scattering process has also been addressed through a solution of the the full wave equations in the presence of simplified blob geometry [47].…”

Section: Wave Scattering From Density Fluctuationsmentioning

confidence: 99%

On recent results in the modelling of neoclassical-tearing-mode stabilization via electron cyclotron current drive and their impact on the design of the upper EC launcher for ITER

et al. 2015

View full text Add to dashboard Cite

Electron cyclotron wave beams injected from a launcher placed in the upper part of the vessel will be used in ITER to control MHD instabilities, in particular neoclassical tearing modes (NTMs). Simplified NTM stabilization criteria have been used in the past to guide the optimization of the launcher. Their derivation is reviewed in this paper and their range of applicability clarified. Moreover, possible effects leading to a deterioration of the predicted performance are discussed. Particularly critical in this context is the broadening of the EC deposition profiles. It is argued that the most detrimental effect for ITER is likely to be the scattering of the EC beams from density fluctuations due to plasma turbulence, resulting in a beam broadening by about a factor of two. The combined impact of these effects with that of beam misalignment (with respect to the targeted surface) is investigated by solving the Rutherford equation in a form that retains the most relevant terms. The perspectives for NTM stabilization in the Q = 10 ITER scenario are discussed.

show abstract

“…The dispersion relation is independent of the azimuthal mode numbers m. It is easy to show that, in spherical coordinates, for a given angle of propagation θ in (9), n 2 k is positive [4,5].…”

Section: Electromagnetic Fields Inside and Outside The Blobmentioning

confidence: 97%

“…The fluctuations modify the propagation properties of the waves through reflection, refraction, and diffraction. We have been studying the scattering of RF waves by blobs using a full-wave theory [4,5]. The theoretical approach is similar to that for Mie scattering of electromagnetic waves by dielectric particles [6].…”

Section: Introductionmentioning

confidence: 99%

Scattering of ECRF waves by edge density fluctuations and blobs

Ram

Hizanidis

2015

EPJ Web of Conferences

Self Cite

View full text Add to dashboard Cite

Abstract. The scattering of electron cyclotron waves by density blobs embedded in the edge region of a fusion plasma is studied using a full-wave model. The full-wave theory is a generalization of the usual approach of geometric optics ray scattering by blobs. While the latter allows for only refraction of waves, the former, more general formulation, includes refraction, reflection, and diffraction of waves. Furthermore, the geometric optics, ray tracing, model is limited to blob densities that are slightly different from the background plasma density. Observations in tokamak experiments show that the fluctuating density differs from the background plasma density by 20% or more. Thus, the geometric optics model is not a physically realistic model of scattering of electron cyclotron waves by plasma blobs. The differences between the ray tracing approach and the full-wave approach to scattering are illustrated in this paper.

show abstract

Scattering of radio frequency waves by blobs in tokamak plasmas

Cited by 45 publications

References 9 publications

Scattering of radio frequency waves by cylindrical density filaments in tokamak plasmas

Scattering of radio frequency waves by cylindrical density filaments in tokamak plasmas

On recent results in the modelling of neoclassical-tearing-mode stabilization via electron cyclotron current drive and their impact on the design of the upper EC launcher for ITER

Scattering of ECRF waves by edge density fluctuations and blobs

Contact Info

Product

Resources

About