150. 3D PLASMA BLOB MODELING

Department: Mechanical & Aerospace Engineering
Faculty Advisor(s): Sergei Krasheninnikov

Primary Student
Name: Justin R Angus
Email: jangus@ucsd.edu
Phone: 304-654-0377
Grad Year: 2012

Abstract
Coherent mesoscale structures having isolated density bumps in the poloidal plane and extended along the magnetic field line, known as plasma blobs, are formed as a result of turbulent fluctuations in the vicinity of the core-edge boundary and propagate to the scrape of layer (SOL). Blobs contribute ~50% of the plasma particle transport around the last closed flux surface and can dominate the transport in the far SOL. Amongst other things, this makes blobs an important player in plasma confinement and plasma-wall interactions. Most of the work to date on plasma blobs is limited to 2D theory and simulations, which ignore the variation of blob parameters along the magnetic field line. However, 3D features, such as drift waves and parallel density profiles, can have an important impact on blob dynamics. In this work, 3D effects on blobs are addressed analytically and numerically using the code BOUT++. It has been demonstrated that the time scale of the convective blob motion is on the order of the time scale of unstable drift waves for a wide range of parameters typical for current and future tokamaks. The density gradients the drive the blob radially outward are depleted during the nonlinear stage of drift wave turbulence resulting in a much more diffuse blob with a greatly reduced radial motion. Furthermore, an initially varying blob density profile along the field line can lead to blob spinning and is yet another 3D mechanism that can reduce the net charge that drives the blob radially outward. This is similar to the spinning that arises due to a radial temperature profile in 2D sheath connected blobs. A nondimensional parameter governing the relative importance of the spinning due to a parallel blob profile is obtained and verified with numerical simulations.

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