Scattering of radio frequency waves by blob-filaments

Abstract

Radio frequency waves used for heating and current drive in magnetic confinement experiments must traverse the scrape-off-layer (SOL) and edge plasma before reaching the core. The edge and SOL plasmas are strongly turbulent and intermittent in both space and time. As a first approximation, the SOL can be treated as a tenuous background plasma upon which denser filamentary field-aligned blobs of plasma are superimposed. The blobs are approximately stationary on the rf time scale. The scattering of plane waves in the ion-cyclotron to lower-hybrid frequency range from a cylindrical blob is treated here in the cold plasma fluid model. Scattering widths are derived for incident fast and slow waves, and the scattered power fraction is estimated. Processes such as scattering-induced mode conversion, scattering resonances, and shadowing are investigated.

Filament-assisted mode conversion in magnetized plasmas

“…At density filaments in magnetized plasmas, electromagnetic waves are guided along the filament ("wave-filament bound states"). Several wave-filament bound states exist beyond those predicted in 1 . The new bound states occur under experimentally relevant conditions, and are especially common under typical NSTX conditions, where they are a plausible mechanism for the experimentally observed power losses in the edge plasma.…”

“…In experimental magnetic confinement fusion devices, density filaments occur [1][2][3][4][5][6] , with which electromagnetic waves intended to heat the plasma interact. Such interactions are usually studied from the point of view of wave scattering 1,[7][8][9] : one wishes to ensure that not too much useful power will be scattered in unforeseen directions by the turbulent edge plasma. The possibility of mode conversion at density filaments in the edge plasma was briefly raised in 1 , and is the central topic of this paper.…”

Propagation of radiation in fluctuating multiscale plasmas. II. Kinetic simulations

“…[12][13][14] Other examples occur in edge plasmas in laboratory devices, where radio frequency heating beams have to travel several meters through turbulent inhomogeneities on scales of cm, a scale ratio of up to 10 3 : 1. [15][16][17] Direct simulation of situations like those above is impractical because of the need to simultaneously resolve fine scales while following radiation propagation over much larger distances, especially in multidimensional situations. In Paper I, 18 we formulated a kinetic description of the smallscale processes to obtain a Fokker-Planck equation for the overall large-scale evolution; similar approaches have been used by others, particularly in treating edge plasmas in fusion devices.…”

“…We also included wave growth, damping, spontaneous emission, and scattering of the radiation in the geometric optics approximation. 19,20 Each of the analyses cited above [15][16][17]21 included scattering and some of these other effects for use in laboratory situations. However, in Paper I, we also included LMC 18 which was not incorporated in the other studies cited, but which is needed for the solar-terrestrial applications that we particularly envisage.…”

“…In Paper I, 18 we formulated a kinetic description of the smallscale processes to obtain a Fokker-Planck equation for the overall large-scale evolution; similar approaches have been used by others, particularly in treating edge plasmas in fusion devices. [15][16][17] We expressed the evolution of the radiation in terms of the probability distribution function of the radiation in space, time, and wave vector. The distribution function can be used to calculate quantities such as the wave intensity and polarization via the Stokes parameters, as discussed in Paper I.…”