Fokker-Planck description of the scattering of radio frequency waves at the plasma edge
- School of Electrical and Computer Engineering, National Technical University of Athens, Athens GR 15773 (Greece)
- Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (United States)
In magnetic fusion devices, radio frequency (rf) waves in the electron cyclotron (EC) and lower hybrid (LH) range of frequencies are being commonly used to modify the plasma current profile. In ITER, EC waves are expected to stabilize the neoclassical tearing mode (NTM) by providing current in the island region [R. Aymar et al., Nucl. Fusion 41, 1301 (2001)]. The appearance of NTMs severely limits the plasma pressure and leads to the degradation of plasma confinement. LH waves could be used in ITER to modify the current profile closer to the edge of the plasma. These rf waves propagate from the excitation structures to the core of the plasma through an edge region, which is characterized by turbulence--in particular, density fluctuations. These fluctuations, in the form of blobs, can modify the propagation properties of the waves by refraction. In this paper, the effect on rf due to randomly distributed blobs in the edge region is studied. The waves are represented as geometric optics rays and the refractive scattering from a distribution of blobs is formulated as a Fokker-Planck equation. The scattering can have two diffusive effects--one in real space and the other in wave vector space. The scattering can modify the trajectory of rays into the plasma and it can affect the wave vector spectrum. The refraction of EC waves, for example, could make them miss the intended target region where the NTMs occur. The broadening of the wave vector spectrum could broaden the wave generated current profile. The Fokker-Planck formalism for diffusion in real space and wave vector space is used to study the effect of density blobs on EC and LH waves in an ITER type of plasma environment. For EC waves the refractive effects become important since the distance of propagation from the edge to the core in ITER is of the order of a meter. The diffusion in wave vector space is small. For LH waves the refractive effects are insignificant but the diffusion in wave vector space is important. The theoretical model is general enough to study the effect of density blobs on all propagating cold plasma waves.
- OSTI ID:
- 21347121
- Journal Information:
- Physics of Plasmas, Vol. 17, Issue 2; Other Information: DOI: 10.1063/1.3304241; (c) 2010 American Institute of Physics; ISSN 1070-664X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
COLD PLASMA
ELECTRIC CURRENTS
FLUCTUATIONS
FOKKER-PLANCK EQUATION
ITER TOKAMAK
PLASMA CONFINEMENT
PLASMA PRESSURE
PLASMA WAVES
RADIOWAVE RADIATION
SCATTERING
TEARING INSTABILITY
WAVE PROPAGATION
CLOSED PLASMA DEVICES
CONFINEMENT
CURRENTS
DIFFERENTIAL EQUATIONS
ELECTROMAGNETIC RADIATION
EQUATIONS
INSTABILITY
PARTIAL DIFFERENTIAL EQUATIONS
PLASMA
PLASMA INSTABILITY
PLASMA MACROINSTABILITIES
RADIATIONS
THERMONUCLEAR DEVICES
THERMONUCLEAR REACTORS
TOKAMAK DEVICES
TOKAMAK TYPE REACTORS
VARIATIONS