Resonance in Fast-Wave Amplitude in a Low-Density Peripheral Plasma

Authors:   R. J. Perkins and J. C. Hosea and N. Bertelli and G. Taylor and J. R. Wilson

Abstract:  Wave propagation across inhomogeneous plasma is a critical issue for high-power plasma heating systems using waves in the ion cyclotron frequency range. Efficient coupling across the low-density scrape-off layer (SOL) has been especially challenging on the National Spherical Torus eXperiment (NSTX), where a large fraction of the wave power is lost to the divertor along SOL field lines. Using a cylindrical cold-plasma model, we demonstrate a special class of modes that carries a large fraction of the wave energy along the peripheral plasma. The modes occur when the radial fast-wave phase difference across the SOL is roughly /2, leading to a pronounced increase in wave amplitude. Such modes could be important in explaining the loss of fast-wave power on NSTX. They also demonstrate how a small layer of diffuse plasma can drastically alter the global wave solution.
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Submitted to:  Physical Review Letters
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Download PPPL-5235 (pdf 2.8 MB 8 pp)
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